Focal Length Explained

The smaller the focal length, the greater is the bending of a ray of light and vice- versa. Some flash units do not have a zoom option. A long focal length delivers a very small angle of view. The virtual image produced is on the same side of the lens as the object. Each lens is a converging lens & the focal length of each lens is known, f 1 and f2. 5 on the high end). Technical Note: Back focus refers to the "focal flange length". For aftermarket uses the compression ranges from about 0. Maximum focal length tells you how far a zoom lens can zoom in for magnified details and a composition that's limited to a smaller portion of the field of view. English: The focus F and the focal length f of a positive lens, a negative lens, a mirror concave and convex mirror. Canon APS-C cameras have a 1. The 50mm focal length is also perfect to give dancers a bit of room, but still make the viewer feel ‘involved’ in the final photograph. Simply put, focal length is the distance in millimeters between the lens and the censor when the camera is focused on a subject at infinity. For cameras with a 35mm sensor or film, a typical circular fisheye lens might have a focal length of 8mm to 10mm. 0 Firmware Upgrade, with focus stacking and focus bracketing, has been released. What a lens intrinsically has, according to its construction, is a focal length and an angle of view which are independent one to another. 35mm equivalent focal length tells us what the field of view will be when we use a certain lens on a camera with a certain sensor size. The longer the focal length of the telescope, generally the more power it has, the larger the image and the smaller the field of view. For brighter objects, like the Moon or the solar system planets you probably don't need much light gathering power (aperture) so a long focal length for imaging finer details should be ok. If 2x focal length, the field dimensions are 1/2 size, but the objects therein are 2x size on the sensor. But now the OM-D E-M1 Ver. Cropped And Full Frame Sensors – Focal Length Explained. The focal length of your eyepiece is often printed on the eye piece itself. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. 8 is shown as 1:2. Online physics calculator that calculates the concave mirror equation from the given values of object distance (do), the image distance (di), and the focal length (f). In practical terms and for our purposes, the focal length of a lens really means the angle of view that the lens"sees" and projects on to the camera's sensor. it is not a prime lens. On a 50mm lens, f/2 is saying that the diameter of the aperture is 25mm. Because when you're getting started in photography, all those numbers and terms can be pretty intimidating. To increase the f-stop you will need to enlarge the focal length. It can be a bit confusing as a beginner to understand focal length because there are a few twists and complexities, but I'll do my best to explain it in 5 minutes or less. Full-frame sensors are about the same size as 35mm still camera film. The quantity F/f is the magnification. Notice: About small format, 174, 178, 385, 290, 224, 185, etc. In other words if you wanted a sharp, shake free shot with a 50mm lens your shutter speed would be 1/50 th sec or faster. If a calculated focal length is non-standard, select the nearest standard focal length and re-calculate projection distance and image size. Theory For a thin lens: 1= 1 + 1 f do di where f is focal length, do is the distance between the object and the lens, and di is the distance between the image. Thus a telescope with a 1000 mm focal length, used with an eyepiece of 25 mm focal length, has a magnification of 1000 / 25, or 40. It’s understandable why they would ask; a superb photo zoom is 1/10th the cost of a good video zoom. This Nikon 70-300mm VR changes magnification a little as focused. The lenses are touching. At this medium length, you can see less of the background included as compared to the first 16mm photo for instance. Viewing angle on the other hand, is related to focal length, but a larger/smaller viewing angle will not change the perspective, not even a bit. 8mm lens offers a horizontal field-of-view around 74 degrees; a 'standard' 8mm lens offers around 30 degrees, whereas a 'telephoto' lens of 50mm narrows the FoV down to less than 5. Make the length of the principal tube short enough to allow you to grip the protruding part of the eyepiece tube with your fingers as you adjust the focus. So the overall image appears zoomed in or closer on DX with lenses with the same focal length. The principle of operation of the Keplerian telescope (fig. Whereas the topic of EXPOSURE covers controlling the intensity of light falling upon the image sensor, the factors listed above determine what image is actually framed and therefore captured by the camera. The first video (above) is from pentaxian1 on YouTube, professional travel photographer, Kerrick James talks about virtually every kind of camera lens, what they do. Focal length is measured as the distance from the focal point where light rays converge in the lens to the sensor or film in the camera, as illustrated in the diagram below. This means that if a lens has a larger focal length, it has a narrow angle of view and a smaller focal length implies a wider angle of view. camera lens focal length explained - Know Your Know Your Bible: All 66 Books Explained and Applied (VALUE BOOKS) Know Your Bible is a concise, easy-to-understand guide to God's Word-giving you a helpful and memorable overview of all 66 books. 5-11mm range)? What are the horizontal, vertical, and. This phenomenon is known as chromatic aberration. A Concise Explanation of How Crop Factor Affects Both Focal Length AND Aperture. Say you have two telescopes with the same aperture but different focal lengths. Next to the camera body, camera lenses are the second most important part of your camera. These shots of Natasha were both taken within seconds of each other and she didn't move an inch between them! All I did was change focal length and as you can see, both Tasha and the location look very different. Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens. The focal length is 1/2 of the radius of curvature. Depth of field & focal length • Recall that to get the same image size, we can double the focal length and the distance • Recall what happens to physical aperture size when we double the focal length for the same f number? – It is doubled 24mm 50mm •. Of course one advantage this longer focal length setup does have is that you aren’t so close to the subject, making it a good option for small animals and insects where a closer presence could cause them to flee. By the end, you'll understand how to get the most out of the lenses you have, combine your camera and your lens for best results, and confidently capture the images you've always imagined. Eyepieces Selecting the right eyepiece or eyepiece configuration is an important consideration when purchasing a spotting scope. Simply the ratio of the focal length to the diameter of the objective, or f O /D O. A lens is a lens, right? Of course not. 1 Purpose In this lab, you will measure the focal lengths of two lenses and use them to construct a simple telescope which inverts the image like the one developed by Johannes Kepler. These equations use the focal length f of the lens, which is a characteristic of the lens and is usually expressed in millimeters. Eyepiece functions. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. Flash zoom refers to how wide your flash will spread the light. The focal chord. The focal length of camera lenses is always given in millimeters. A primary characteristic of a lens is the focal length. Definition of a Parabola. 2 meters (23. When doing that I need to import the camera views. ) Determine the image distance and the image size. Typical examples of variable aperture zoom lenses and their focal length range are the Canon EF-S 55-250mm, F4-5. The camera lenses that CCTV Camera Pros supplies generally range from 2. The propose of the chart is to provide a simple visual presentation of a complex series of numbers. This also decreases the effective focal ratio of the instrument. Can I use the remote controller to move the Mavic 2's gimbal 75° from left to right?. Simply the ratio of the focal length to the diameter of the objective, or f O /D O. Additionally, this focal length is chosen by portrait shooters because it tends to be the most forgiving for human defects, such as longer noses or larger necks. Focal length is measured in millimetres (mm) and gives an indication of the angle of view. The focal length (read as 'XX'mm) is the strength with which the lens can focus on an object, while the optical zoom (the number 'X'x zoom) is the strength of the total zoom you can achieve. Focal length is the fourth variable of five that we will have discussed after today. The focal length describes the distance from the front of the lens to where it focuses light on the sensor or film plane. 3mm of the equivalent normal APS-C lens is often called the "crop factor", sometimes the "digital multiplier". Good portrait lenses have wide apertures to allow faster shutter speeds, helpful indoors or in low light. The focal length (f) is the distance between the lens and the focal point. The lenses used were 40mm, 90mm, 180mm and 300mm in focal length. We are your source for astronomy telescopes, eyepieces, astroimaging cameras and imaging accesories, telescope mounts, & more!. A real lens has nonzero thickness, unlike an ideal lens, and so the "focal length" of a real lens is a little ambiguous. A 150mm telescope with a 2000mm focal length may produce an image, of the full Moon, 15mm in diameter. Oct 2, 2017 at 16:10. focal length In optics, the length from the lens to the point of focus of light rays passing through the lens. such as 200mm or 300mm. With a convex lens, the real image of a distant object is formed at a distance equal to its focal length. A positive focal length indicates that a system converges light, while a negative focal lengh indicates that the system diverges light. because you can observe the objects closer with a big distance. The downside to shooting these longer focal lengths is that a very fast shutter speed must be used, one at least as fast as the focal length. Next to the camera body, camera lenses are the second most important part of your camera. The length of the focal zone is given by Equation 10:. It also has a focal length that compresses your object’s face bringing more clarity. shutter speed = 1/(focal length x crop factor). A 150mm telescope with a 2000mm focal length may produce an image, of the full Moon, 15mm in diameter. Focal length is focal length. If the image is real 1/u + 1 v = 1/f If the image is virtual 1/u - 1/v = 1/f 3. 8 - 12mm, 4-9mm, 9-22mm, 6-60mm, 5-10mm. But, when using long focal lengths, such as 105mm or 180mm, you should take into account pupil magnification to get more accurate results. Therefore a focal length of 24mm on a DX sensor is equivalent to a 36mm (1. Another basic type of lens is the diverging lens. That means it's the number of lens diameters from the lens to its focal point, as shown below. so (provided it fits) ANY 75mm lens will give exactly the same FOV on a given aperture plate (i. The diopter is the unit of measure for the refractive power of a lens. Hi gig, The focal reducer makes the scope act like a shorter focal length scope. Field of view (FOV) Defined by the equation:. The higher the millimeter (mm) number, the more "zoomed in" the image appears. 8mm lens offers a horizontal field-of-view around 74 degrees; a 'standard' 8mm lens offers around 30 degrees, whereas a 'telephoto' lens of 50mm narrows the FoV down to less than 5. Lens focal length was confusing to me personally, until it was explained to me in basically this way. Instead it's been a choice between the relatively compact and affordable FE 24-70mm F4 ZA OSS for £880, or the stellar but huge and costly FE 24-70mm f/2. On the one hand, the camera lens with a large focal length is often placed at industrial parks, national security, etc. For visual observers, it's largely irrelevant, the magnification being more important (telescope focal length divided by eyepiece focal length). Zoom lenses can have their magnification changed at the turn of a dial or the press of a button. It is best done with only small piece of paper and on a concrete surface. For instance, since a 1 D lens focuses rays of light at a distance of 1 meter, 1 meter is the focal length of a 1 D lens. Calculate: When you have entered and converted all your inputs to the proper units, click "Calculate. On the other hand, the focal length of concave lens is negative. At this medium length, you can see less of the background included as compared to the first 16mm photo for instance. Simplifying gives us the formula for a parabola: x 2 = 4py. Here is the major axis and minor axis of an ellipse. Autodesk ReCap Photo – How to Take the Right Photos. Shop Panasonic's range of Lumix lenses for cameras. focal length synonyms, focal length pronunciation, focal length translation, English dictionary definition of focal length. Camera Lens Types. EFL, is most effective up to a maximum thickness of 12 gauge or 0. However, I found them utterly confusing. But there is a catch. Differences arise because the focal length is getting longer (zooming in) as the camera moves further away from the subject. Use this calculator to find the maximum focus range, or hyperfocal, for any aperture and lens focal length combination. The subject was 10 feet (3m) away. The focal point is the point to which the laser light is at its smallest size and this is what determines the ultimate resolution and. lens is one that deflects light a lot, meaning it has a short focal length. They are, however, more frequently discussed in terms of their effective focal length. The distance from the rear lens to the image plane is simply the back focal distance (BFD). Let’s say you are using a 50mm focal length on both a full-frame (FX) camera, such as the Nikon D810, and on an crop-sensor camera (DX), such as the Nikon 7100. Home » Concave vs. The focal length of the lens is the distance between the lens and the image sensor when the subject is in focus, usually stated in millimeters (e. The focal length is generally measured in millimeters, so we'll stick with those as our unit of measure. The lens focal length determines the composition, magnification, & field of view, as described in the section below. The DX designation can be found in the lens name, i. The numbers refer to the distance between the lens and the image sensor when your subject is in focus. Diopter strength refers to the optical power of a lens. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography. So the fact that the barn visually appears closer to the subject is not because perspective has gotten compressed but simply because in the viewfinder we see a smaller portion of it. To compare both methods. On the one hand, the camera lens with a large focal length is often placed at industrial parks, national security, etc. To increase the f-stop you will need to enlarge the focal length. As we can see now, focus length is a figure, a very important parameter of a lens, which in first place has nothing to do with the angle it embraces. Say you have two telescopes with the same aperture but different focal lengths. ASI camera connecting with OAG diagram. What is a sensor? What is the difference between a full frame and a crop sensor? We've got the answers you've been looking for. The focal length and sensor size determines the lens' field of view. The focal chord. A longer focal length eases optical design and manufacturing problems, permitting a telescope with superior optical performance and/or a lower price. We are explaining lenses, what is focal distance and what is f stop. F-stop is the focal length divided by the diameter of the lens. Therefore a focal length of 24mm on a DX sensor is equivalent to a 36mm (1. Field of View for Virtual Reality Headsets Explained Jay For VR to be the best it can be, for it to be life-changing, there are a few key ingredients that need to be mixed just right. This point is the focal point of the lens. f/2, f/4, etc. This is the focal length I use the most at weddings. Main functions of a telescope, its magnification is limited to ~ƒ/250, ƒ being the objective focal length. Differences arise because the focal length is getting longer (zooming in) as the camera moves further away from the subject. This calc allows conversion in either direction, by the use of the units menu. For example, a 200mm f/4 lens will be 50mm wide. There is a focus and directrix on each side (ie a pair of them). Observations made on the macro focus conditions of fixed focal length lenses are used to provide an unusual but simple approach to appreciating the first order principles of zoom optics. A zoom lens means the lens can change focal length via zooming, i. Some modern scope designs use a clever optical layout to squeeze a long focal length into a small optical tube. The reciprocal of focal length in metre is called power of lens in dioptre (D). In reality, any lens with a focal length of between about 40mm and 58mm will look roughly like how things appear to your eyes. When the eye is relaxed and the interior lens is the least rounded, the lens has its maximum focal length for distant viewing. The focal length describes the distance from the front of the lens to where it focuses light on the sensor or film plane. The variable when switching between lenses is the sensor size. Prime lenses: A prime lens has a fixed focal length. 1000mm telescope focal length / 10mm eyepiece focal length = 100x A telescope with a focal length of 600mm and a 25mm eyepiece operates at 24x. For the same magnification and camera sensor size, doubling the f-stop also doubles the total depth of field. One dioptre is the power of a lens of one metre focal length. Focal Length Take a close look at the barrel of a lens and you should see a measurement, or range, such as 18–55 mm. When I started in photography the rule of thumb was really simple. This means that the depth of field of a 5 inch lens will be about twice as long as for a 2. The flange focal distance is the determining factor in a camera's thickness. Macro Lens and Focal Lengths. The 50mm is a typical “standard” focal length. Generally speaking, shorter focal length gives you a wider FOV, however focal length of the lens does not equate to FOV. That's your lens's focal length—the ­distance from the lens to the sensor. , has its place, but it doesn’t tell you the whole story. Focal length and sensor size. The lens closest to the eye is called the eye lens and the other lens (closest to the objective) is called the field lens. If we consider a zoom lens you know that the 35mm setting is "wide angle", 50mm is" normal" and 135mm is "telephoto. A fixed lens is one where the camera?s focal length is permanently set and cannot be adjusted by the user. The iPhone 4S Camera Upgrade Explained. Depth of field & focal length • Recall that to get the same image size, we can double the focal length and the distance • Recall what happens to physical aperture size when we double the focal length for the same f number? – It is doubled 24mm 50mm •. Focal Length Equivalent (Crop Factor) Throughout this article we frequently reference focal length equivalent, or crop factor. If you do not know which is the width of your camera sensor, you can consult the manufacturer page thereof. Focal length is how we describe the magnification of your lens and it's described in millimetres. For example, the reciprocal of 100 is 1/100. Wide-angle lenses have small focal lengths, such as 10mm or 20mm. What a lens intrinsically has, according to its construction, is a focal length and an angle of view which are independent one to another. The focal length of a lens is one of the key specifications of a lens. According to the above relation, for light moving from left to right the final. Focal Length. It is used for determining the focal length of a thin lens (thickness = 0) with radii of curvature r1 and r2. the required amount of compensation for. On a 50mm lens, f/2 is saying that the diameter of the aperture is 25mm. 6x for Canon EOS DSLRs and 1. 6-48 feet) for a total DOF of 7. so (provided it fits) ANY 75mm lens will give exactly the same FOV on a given aperture plate (i. A person's seizures usually last the same length of time each time they happen and stop by themselves. A real lens has nonzero thickness, unlike an ideal lens, and so the "focal length" of a real lens is a little ambiguous. The focal length determines the size of the image on the film. But, when using long focal lengths, such as 105mm or 180mm, you should take into account pupil magnification to get more accurate results. As it is an "EF-S" lens, the 55-250 IS can only be mounted on Canon APS-C/1. I chose a focal length equal to 2800 (something that's about the same as the width/height). The focal length also tells us about the angle of view, and this is the most important thing to understand. The longer the focal distance, the longer the depth of field. Lens focal length calculation CCTV Calculator is a tool designated for camera system basic parameters determination and testing. Maximum focal length tells you how far a zoom lens can zoom in for magnified details and a composition that's limited to a smaller portion of the field of view. Wide-angle lenses have small focal lengths, such as 10mm or 20mm. This is the focal length I use the most at weddings. Buy the lens that matches the focal length you like the best. So once you know in. Refracting telescope and schematic. 0 Firmware Upgrade, with focus stacking and focus bracketing, has been released. The focal length of your eyepiece is often printed on the eye piece itself. A little trace of yellow tonality but a general really good quality. Focal length is focal length. To compare lenses on the basis of superficial features like focal length, T-stop, weight, price, brand, etc. That seems pretty straightforward. 2 meters (23. From these figures a little algebra reveals that the effective extension built into the lens is 7. *Note: Lens focal lengths are for 35 mm equivalent cameras. To calculate the total zoom amount, divide the maximum focal length by the minimum focal length. , 28 mm, 50 mm, or 100 mm). Focal Length. 25” Deluxe Adapter that comes with. A 50mm lens is a 50mm lens is a 50mm lens. And so does depth of field, but that is for another article. A focal reducer works using a positive lens, which shortens the focal length by causing the incoming beam of light to converge more quickly. CX size sensors have a crop factor of 2. The focal length of the lens is the distance from the center of the lens to the spot, given to be 8. In the case of zoom lenses, both the minimum and maximum focal lengths are stated, for example 18–55 mm. Both photographs were taken at maximum aperture, the one on the left with a lens with a small maximum aperture and the one on the right with a lens with a wide maximum aperture, showing the softening effects of reduced depth of field. The LX200 Series 10" ACF™ Telescope features a 10" aperture, 2500mm focal length, and f/10 focal ratio with Dual-Fork Mounting. What focal lengths do you require? The answer to this question depends on the size of your camera’s sensor (our article Crop Factor Explained tells you why). lens, to determine the focal length of a diverging lens. This is the concept of "zooming". Greater focal length magnifies, and smaller sensor size crops. The lenses used were 40mm, 90mm, 180mm and 300mm in focal length. This means that the depth of field of a 5 inch lens will be about twice as long as for a 2. 200mm/50mm = f/4. This phenomenon is known as chromatic aberration. Focal Length Calculator. That is why f-stop is typically written as f/4, meaning "focal-length over 4" or "focal-length divided by four". Despite its compact dimensions and fairly modest price, it has a modern sensor and produces great photographs - find out if it's right for you in our full review. Put another way: If your goal is to take intimate portraits in a small room, you don't need flashes with huge guide numbers; If it's your intention to photograph the interior of cathedrals, then you do. You could use different focal lengths in the same ballpark and it wouldn't make much of a difference (until you go too far and end up with sfm10 issuing errors. A wide-angle lens (short-focal-length) includes more of the scene-a wider angle of view-than a normal (normal-focal-length) or telephoto (long-focal-length) lens. Equipped with Ultra High Transmission Coatings (UHTC™) and a resolving power of 0. The optical centre of a modern lens isn't easy to identify because the barrel conceals several lens elements. f/2, f/4, etc. Which one, it's doesn't actually matter, because if you want to know whether the focal length is positive or negative, all you have to look at is what type of. Using two rays, confirm the location of the lens and measure your diagram's scaled focal length. If I had my full choice of fixed focal length lenses to bring, I’d pick a 20mm, 24mm, 60mm macro, 105mm macro, 180mm, and 400mm. The first has a mirror diameter of 20-inches and a focal length of 5000 mm, the second one is more portable and has a diameter of 10-inches and a focal length of 2000 mm. In this way they appeal directly to the idea that 50mm is the "normal" focal length. The aperture diameter is the straight line distance from one side of the aperture to the other side of the aperture, crossing directly through the aperture's center. It’s unnaturally zoomed. Shutter Speed Rule. Terms Defined. From these figures a little algebra reveals that the effective extension built into the lens is 7. If the mirror surface is tilted by an angle α with respect to the optical axis, the reflected beam will enter the objective lens with an angle 2α. A focal reducer is an excellent tool to help you achieve the magnification you need, and at High Point Scientific, we provide you with the best selection. If you’re looking for the best Meade telescope with a long focal length, then you definitely don’t want to miss the new ETX125. Here are complete images at the longer focal lengths shot on DX. Wide angle lenses have short focal lengths (i. In other words, the "multiplier" is the difference between the smallest and largest focal length measurements of the lens. According to the above relation, for light moving from left to right the final. 0-cm tall light bulb is placed a distance of 8. Focal Length Focal length is the distance at which a lens focuses parallel rays of light. Through animation, I explain various aspects of focal length, from the principle of the subject, to crop factor, and finally the different classifications of lenses. The higher the millimeter (mm) number, the more "zoomed in" the image appears. To adjust the above numbers for your camera, please use the focal length converter in the tutorial on digital camera sensor sizes. The principle of operation of the Keplerian telescope (fig. Thus: The lower the f stop number, the wider, or bigger, the aperture. 5-11mm range)? What are the horizontal, vertical, and. A typical application for the microscope objectives is use with our Spacial Filters. For the same magnification and camera sensor size, doubling the f-stop also doubles the total depth of field. Focal Length and Zoom Capture the whole image or focus in on the tiny details. 50 Stylish Medium Length Haircuts. An imaginary line parallel to the light rays and through the center of the lens is called the principal axis. the required amount of compensation for. The focal length determines the size of the image on the film. 50-75mm), is the calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the digital sensor at the focal plane in the camera (it is determined when the lens is focused at infinity). So if the front element is 25mm in diameter and the focal length is 50mm, the full aperture f/number is taken as: 50 divided by 25 equals f/2. The f-stop, which is also known as the f-number, is the ratio of the lens focal length to the diameter of the entrance pupil. For the same reason, the convention is to represent aperture as a fraction, e. 456 arc-seconds for the improvement of brighter star clusters, more fine detail in nebulae, and greater surface features on planets. Focal Length. As focal length changes, the amount of the subject captured by the lens (the viewing angle) also changes. Usually it manifests itself as a decrease in focal length/magnification (or an increase in the angle of view) as a lens is focused closer. Technical Note: Back focus refers to the "focal flange length". Focal length is often printed right on the camera lens. Simply the ratio of the focal length to the diameter of the objective, or f O /D O. The focal length of a lens, expressed in millimeters, is the distance from the lens’s optical center (or nodal point) to the image plane in the camera (often illustrated by a "Φ" on the top plate of a camera body) when the lens is focused at infinity. So a 10mm focal length on CX is equivalent to a 27mm focal length on FX format. the focal length (50mm) and Canon gives the magnification as 0. The length of the focal zone is given by Equation 10:. Cut the distance in half (or double the focal length to 50 mm) and you will. Focal length determines your photo’s final resolution. Focal Length of a Camera Lens: Focal length, usually represented in millimeters (mm), is the basic description of a photographic lens. Focal length and power of a lens are bound to. One dioptre is the power of a lens of one metre focal length. When a camera has a short focal length, it has a wider angle of view. For focal lengths around 50mm, not taking into account pupil magnification may work pretty well. The first video (above) is from pentaxian1 on YouTube, professional travel photographer, Kerrick James talks about virtually every kind of camera lens, what they do. 27/1500 is about 1/50; the subject rectangle will be therefore slanted by one part in fifty, or just above 1° of angle. That particular number (focal length / f-stop) indicates the physical diameter of the hole in the lens that lets in the light. Step 3 - Divide the Diameter by the Focal Length. Hyperfocal Distance Chart Maker. If 2x focal length, the field dimensions are 1/2 size, but the objects therein are 2x size on the sensor. 4, to see how much difference a fixed focal length lens would make. It might help to clarify why keeping focal length constant wouldn't just happen "by default", or why a lens assembly with a given level of complexity whose focal length changes with subject distance may offer better performance than one of the same complexity whose focal length couldn't change (e. 8? The simple answer is distance from the subject. Units of power of a lens : The units of the power of a lens are dioptres. The fact-checkers, whose work is more and more important for those who prefer facts over lies, police the line between fact and falsehood on a day-to-day basis, and do a great job. Today, my small contribution is to pass along a very good overview that reflects on one of Trump’s favorite overarching falsehoods. Namely: Trump describes an America in which everything was going down the tubes under  Obama, which is why we needed Trump to make America great again. And he claims that this project has come to fruition, with America setting records for prosperity under his leadership and guidance. “Obama bad; Trump good” is pretty much his analysis in all areas and measurement of U.S. activity, especially economically. Even if this were true, it would reflect poorly on Trump’s character, but it has the added problem of being false, a big lie made up of many small ones. Personally, I don’t assume that all economic measurements directly reflect the leadership of whoever occupies the Oval Office, nor am I smart enough to figure out what causes what in the economy. But the idea that presidents get the credit or the blame for the economy during their tenure is a political fact of life. Trump, in his adorable, immodest mendacity, not only claims credit for everything good that happens in the economy, but tells people, literally and specifically, that they have to vote for him even if they hate him, because without his guidance, their 401(k) accounts “will go down the tubes.” That would be offensive even if it were true, but it is utterly false. The stock market has been on a 10-year run of steady gains that began in 2009, the year Barack Obama was inaugurated. But why would anyone care about that? It’s only an unarguable, stubborn fact. Still, speaking of facts, there are so many measurements and indicators of how the economy is doing, that those not committed to an honest investigation can find evidence for whatever they want to believe. Trump and his most committed followers want to believe that everything was terrible under Barack Obama and great under Trump. That’s baloney. Anyone who believes that believes something false. And a series of charts and graphs published Monday in the Washington Post and explained by Economics Correspondent Heather Long provides the data that tells the tale. The details are complicated. Click through to the link above and you’ll learn much. But the overview is pretty simply this: The U.S. economy had a major meltdown in the last year of the George W. Bush presidency. Again, I’m not smart enough to know how much of this was Bush’s “fault.” But he had been in office for six years when the trouble started. So, if it’s ever reasonable to hold a president accountable for the performance of the economy, the timeline is bad for Bush. GDP growth went negative. Job growth fell sharply and then went negative. Median household income shrank. The Dow Jones Industrial Average dropped by more than 5,000 points! U.S. manufacturing output plunged, as did average home values, as did average hourly wages, as did measures of consumer confidence and most other indicators of economic health. (Backup for that is contained in the Post piece I linked to above.) Barack Obama inherited that mess of falling numbers, which continued during his first year in office, 2009, as he put in place policies designed to turn it around. By 2010, Obama’s second year, pretty much all of the negative numbers had turned positive. By the time Obama was up for reelection in 2012, all of them were headed in the right direction, which is certainly among the reasons voters gave him a second term by a solid (not landslide) margin. Basically, all of those good numbers continued throughout the second Obama term. The U.S. GDP, probably the single best measure of how the economy is doing, grew by 2.9 percent in 2015, which was Obama’s seventh year in office and was the best GDP growth number since before the crash of the late Bush years. GDP growth slowed to 1.6 percent in 2016, which may have been among the indicators that supported Trump’s campaign-year argument that everything was going to hell and only he could fix it. During the first year of Trump, GDP growth grew to 2.4 percent, which is decent but not great and anyway, a reasonable person would acknowledge that — to the degree that economic performance is to the credit or blame of the president — the performance in the first year of a new president is a mixture of the old and new policies. In Trump’s second year, 2018, the GDP grew 2.9 percent, equaling Obama’s best year, and so far in 2019, the growth rate has fallen to 2.1 percent, a mediocre number and a decline for which Trump presumably accepts no responsibility and blames either Nancy Pelosi, Ilhan Omar or, if he can swing it, Barack Obama. I suppose it’s natural for a president to want to take credit for everything good that happens on his (or someday her) watch, but not the blame for anything bad. Trump is more blatant about this than most. If we judge by his bad but remarkably steady approval ratings (today, according to the average maintained by 538.com, it’s 41.9 approval/ 53.7 disapproval) the pretty-good economy is not winning him new supporters, nor is his constant exaggeration of his accomplishments costing him many old ones). I already offered it above, but the full Washington Post workup of these numbers, and commentary/explanation by economics correspondent Heather Long, are here. On a related matter, if you care about what used to be called fiscal conservatism, which is the belief that federal debt and deficit matter, here’s a New York Times analysis, based on Congressional Budget Office data, suggesting that the annual budget deficit (that’s the amount the government borrows every year reflecting that amount by which federal spending exceeds revenues) which fell steadily during the Obama years, from a peak of $1.4 trillion at the beginning of the Obama administration, to $585 billion in 2016 (Obama’s last year in office), will be back up to $960 billion this fiscal year, and back over $1 trillion in 2020. (Here’s the New York Times piece detailing those numbers.) Trump is currently floating various tax cuts for the rich and the poor that will presumably worsen those projections, if passed. As the Times piece reported: