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1. Autofocus works by measuring the distance to the subject
2. Unlike linear polarizers, you don't have to turn circular polarizers
3. A 50 mm lens on 35 mm systems is called “normal” because it delivers about the same view as the human eye
4. Infrared films record thermal radiation
5. Wide-angle lenses distort the image
6. Flash range is increased when using positive flash exposure compensation
7. The shorter the exposure time, the faster the shutter
8. Different focal lengths create different perspectives
9. Tele-photo lenses have a shallow DOF
10. Macro lenses are only sharp at close distances
11. Digital cameras have a deeper DOF than film cameras
12. Medium format lenses have a higher resolution than 35 mm lenses
13. Using a TC results in different DOF than using a longer base lens
14. Depth of field around the focus distance is 1/3 towards the camera and 2/3 away from the camera

If you have a candidate for the above list, or when you have a comment, please get in contact.

All modern AF systems are passive, ie. they don't send signals and use the echo to focus, but they only look at the light entering the camera through the lens. With the help of phase detectors sitting at distinct points of the viewfinder image, they determine whether the current focus is in front of the object under the sensor or whether it's behind, and how much it is out of focus qualitatively. They then turn the lens in the right direction until contrast of the image under the sensor is maximized. So in a way they work just like a photographer focusing manually.

The primary result of this process is, of course, that the lens is focused on the object under the sensor(s). The distance to that subject is a secondary result, but it's neither required for the AF process to work nor is it measured by the AF sensors. The distance is often extracted after focus is acquired, for example by reading the position of the focusing shaft directly or by indirectly calculating the distance from the number of turns of the AF drive and information on how one turn translates into change of distance. The distance information can then be used for other things, e.g. for flash exposure, advanced program exposure or DOF calculations.

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What changes with a circular polarizer is the nature of the light leaving the filter. With a linear polarizer the light leaving the filter is polarized mainly in one direction. Light with different polarization was blocked by the filter. This can cause problems with cameras that use a beam splitter to lead some of the light towards the AF sensors and metering cells. These beam splitters also act like a polarizer. When the direction of the beam splitter is at a 90 degree angle relative to the direction of the polarizing filter, no or very little light will reach the sensors. The camera will have trouble focusing or may meter incorrectly, depending on the position of your polarizer in front of the lens. This is not what you want.

Circular polarizers solve this problem by adding a de-polarizing layer to the back of the filter. This causes the light to be de-polarized, ie. it does no longer oscillates in a single direction. This light can pass the beam splitter just like light that never went through a polarizer, and AF and metering can work correctly.

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The lateral field of vision is mainly used to detect motion while the central part is used for precise visual inspection. So you see, human vision is quite different from the view through a 50 mm lens.

Some other theory says it's the standard focal length because it's the diagonal size of the 35 mm frame [and even that isn't precisely true; the diagonal size is about 43.27 mm]. Well, that's how we calculate what the standard focal length for a given format is, but it's not the reason why it was actually selected as the standard.

The real reason is much more mundane. When SLRs (or rather: system cameras) became widely availably to a large number of amateur photographers, the makers of these systems had to select one lens that they could sell as a standard set together with the camera. Following the logic of economics, it had to be one that could be made well for little money. 50 mm lenses fit these criteria perfectly. They typically are focused by extension (simple mechanism), don't require aspheric elements (simple lens element shapes), they don't require elements made of glass with anomalous dispersion (simple materials), and they're not zooms (fewer elements and simpler mechanism). It's easy to make a really good and fast 50 mm lens for little money. Also for this reason, the 50 mm standard lenses are often among the best lenses of one maker's lens lineup. That's why most 35 mm cameras came with a 50 mm lens until recently. The makers could have also selected 40 mm or 55 mm as the standard, but 50 mm probably looked more “even”.

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Infrared film is not much different from normal film. It's just also sensitive to light at wavelengths longer than visible red. It's sensitive into the infrared range of light. Thermal radiation has wavelengths much longer than what IR film is capable of recording.

Typically, IR films are sensitive to wavelengths between 400nm and 800nm-900nm. Regular film is sensitive between about 400nm and 660nm. An object will emit light with a wavelength of 900nm when it's almost glowing hot!

With IR films you still need a light source, eg. the sun, and you record the light that is reflected from objects in the scene. Also, when you want only IR light to expose IR film, you have to use filters that block out almost all other wavelengths. Otherwise the results from monochrome IR films doesn't look much different than those from normal monochrome films.

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On the other end of the spectrum you'll find the more expensive wide-angle primes that display virtually no distortions (it's harder to reduce distortions with zooms, so you can expect considerably stronger distortions even from expensive wide-angle zooms). These render straight lines of the scene as straight lines in the image. These lenses prove that wide-angle lenses do not generally distort the image.

So what happens when you stand in front of a skyscraper and take a photo of it with a wide-angle lens? The image looks strongly distorted at first sight. Well, it is, but it's not lens distortion, but perspective distortion. When the straight edge of the skyscraper is a straight edge in the image, the lens causes little to no distortions. What you see is distortions caused by perspective, ie. by the fact that the base of the skyscaper is much closer to you than the tip, so the base appears much bigger. A tele-photo lens would give you the same distortions of you used it from the same position. You only don't do this usually, because you want to fit the entire skyscraper into the frame.

The only way to reduce perspective distortion is to change perspective. Changing the lens doesn't help. If you photograph the skyscraper from a distance then all parts of the building are at about the same distance from you, and you won't see much of perspective distortions. You will probably use a telephoto lens in this situation, so again it looks like avoiding the wide-angle lens reduces distortions. However, this is coincidence, not cause.

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An electronic flash basically is a flash bulb connected to a capacitor. The batteries of the flash unit load the capacitor, and the camera closes and opens the circuit between capacitor and bulb. With TTL-OTF metering, the camera closes the circuit at the beginning of the exposure, and when it detects that proper exposure is reached, it opens the circuit, cutting off the flash. “Proper exposure” here means that the sensors have detected a total amount of light that causes a mid-toned image on the given film.

It's obvious that the load of the capacitor and therefore the burn duration of the flash bulb is limited. When the capacitor is exhausted before proper exposure is reached, the image is considered underexposed by the camera. Objects farther away from the flash receive less light from it, so to properly expose objects farther away, the flash has to burn longer. This distance is limited because the load of the capacitor and therefore the burn duration is limited. We call this maximum distance “flash range”. Objects at this distance or closer can be properly exposed by this flash unit with the given film sensitivity and lens aperture.

Some people think they can “juice up” the flash and extend the flash flash range by dialing in a positive flash exposure compensation on their camera. But when you understand that the flash works as explained above, you also understand that turning a knob can't increase the maximum load of the capacitor and therefore also doesn't increase the maximum burn time of the flash bulb and doesn't increase flash range. It would be really neat if you could save a lot of money that way, but it doesn't work.

What flash exposure compensation really does is move the cut-off point. With negative flash exposure compensation you tell the camera to cut off the flash earlier than normal, and with positive flash exposure compensation you tell it to cut off the flash later than normal. The main purpose is to compensate for subjects that are not mid-toned, so that flash exposure doesn't result in images that are mid-toned when the subject is not. So when the subject is too far away and the capacitor is exhausted before the cut-off point, it doesn't help at all to move the cut-off point even farther away.

But didn't I say that doing this even reduces flash range? How is that possible?

When using positive flash compensation, you basically tell the camera that proper exposure is not reached with the normal amount of light, but with more. Since you can not output enough light to reach that level for objects at maximum flash range, the image is underexposed for the camera. You have to move these objects closer to reach the level given to the camera, thus effectively reducing flash range for the given level.

Sorry, there are no cheap tricks to increase flash range. You have to either use a stronger flash with a larger capacitor, use a faster film or shoot at wider apertures.

[Real-life modern flash systems are a lot more complex. It would have been too complicated to explain it all here. But these systems still have a limited load of the capacitor. So even with the latest whiz-bang flash system, you can not increase flash range easily.]

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Focal plane shutters (ie. located in the camera body between the mirror box and the film, and opening and closing vertically or horizontally) are two sets of shutter blades. Before the shot, one set is packed away at one side of the frame, and the other set is unfolded to cover the frame. When you take a shot with a long (let's say 1/30 second or longer) exposure time, the following happens: the unfolded set is opening in the direction opposite the other set, uncovering the film beneath it and letting light fall onto film. It moves with a constant speed until the entire frame is uncovered. This is called the “first curtain”. After some time, the other set starts to unfold and cover the film, starting with the part that was uncovered first. Again, the shutter blades move at a constant speed until the frame is completely covered. This is called the “second curtain”. Every point of the film is uncovered for a certain time, determined by the speed of the shutter blades and the duration the shutter is fully open. This is called the “exposure time”.

To get shorter exposure times the first step is to reduce the duration the shutter is fully open. By doing this you reach a point where the second curtain starts to close just as the first curtain becomes fully open. This speed is called the “x-sync speed”. It's important for flash exposure (I won't go into detail, here).

If you want to have even shorter exposure times, you have two options: you can make the shutter blades move faster. The first curtain takes less time from fully closed to fully open, and you can close the second curtain earlier and faster. There are obviously limits for this strategy. The shutter blades are not weightless, and they have to keep their shape while they move to ensure even exposure across the frame. You can not accelerate them beyond a certain rate. Otherwise the forces of acceleration and the mass of the shutter blades would tear them apart.

The second option is to use a trick: it works not by making the shutter blades move faster, but by starting to close the second curtain while the first curtain is not yet fully open. The edges of the first and second curtain move across the film plane in parallel. Light falls onto the film through a moving slit between the two curtains. This way each point of the film is uncovered for a shorter amount of time, and we get the shorter exposure time that we want. For even shorter exposure times you simply make the slit between first and second curtain narrower. You don't have to move the shutter blades any faster.

Shutter movement below x-sync speed	Shutter movement above x-sync speed

All modern cameras with focal plane shutters work that way. The shutter blades move at the same speed for all exposure times. The camera only varies the time the shutter is fully open (for times longer than the x-sync speed) and the width of the moving slit (for times shorter than the x-sync speed). This way the shutter mechanism can be kept simple, cheap and durable. Shorter exposure times can be achieved by more precise timing instead of faster shutter blades.

The shortest exposure time of a camera is not a good indicator for the physical speed of its shutter. The x-sync speed, however, is. As explained above, the “trick” only works for exposure times shorter than the x-sync speed. If you want to make the x-sync speed itself faster, there is no other way than to make the shutter blades move faster.

So how fast do shutter blades move? Let's look at a typical modern camera. It has a vertically moving shutter, an x-sync speed of 1/90 s, and 1/4000 s shortest exposure time. In other words, the edge of the shutter has to move a distance of 24 mm in 1/90 of a second. The speed of such a shutter is 2.16 m/s. That's 7.776 km/h or under 5 mph. If you walk briskly, you're moving faster than the typical shutter. Even the fastest focal plane shutters with an x-sync time of 1/300 s move only at about 7.2 m/s. If we didn't use the “trick” the shutter had to move these 24 mm in 1/4000 of a second. That would be a speed of 345.6 km/h or roughly 215 mph. What a difference!

Oddly, if you take a photo with an exposure time of 1/4000 s with the above camera, it will take a lot longer than 1/4000 s to take the shot. It will take a bit longer than 1/90 s (x-sync time plus delay between first and second curtain, which is the exposure time). There are some cameras (mostly panoramic cameras with swing lenses) that use the same “trick” as focal plane shutters, but for all exposure times. A rotating barrel with a narrow slit is exposing the frame from one side to the other. The exposure time is adjusted by making the barrel rotate faster or slower. Just like with focal plane shutters, it takes a lot longer to actually make the shot than the exposure time suggests.

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[...] Wideangle lenses offer an increased depth-of-field perspective not possible with the human eye. [...]

Perspective has nothing to do with focal length. Perspective describes how a 3D scene is projected onto a 2D canvas, eg. film, digital sensor or even your retina. Perspective describes how the objects in the 3D scene appear in the 2D image, which objects are visible and which are obscured by other objects, how big they appear in relation to each other. Focal length only influences the field of view, ie. how much of the entire scene appears in the image. Focal length does not change the relative sizes of objects in the image. Perspective is only influenced by the relative position of the objects in the scene and by the position of the canvas (viewer, camera). If you want one object in the foreground appear much bigger than another object farther away, it doesn't help to change focal length. The only way to accomplish that is to get closer to the object in the foreground. If you want both to appear as being about the same size, the only way to accomplish that is to get farther away from both objects.

Perspective is only influenced by your position, and field of view is only influenced by focal length.

You often hear things like “wide-angle perspective”. There is no such thing. People mean either “wide-angle field of view” or “close-up perspective” here. You also often hear “zooming with your feet”. This doesn't make any sense at all. Zooming changes focal length only (and therefore field of view), and walking changes perspective only. You can't replace one with the other.

So why is it still that with wide-angle photos the objects in the foreground look quite large, and with tele-photo shots the scene looks “compressed”? Doesn't this contradict what I just said? No, it's just a coincidence. With wide-angle lenses you can focus quite closely, and if you do get close to foreground objects, they will appear large. The wide-angle lens just allows you to also capture some of the background of the scene. With tele-photo lenses, you often shoot objects that are far away. It's that “being far away” that makes the scene look compressed, not the tele-photo lens. The lens only lets you capture the “right” crop of the entire scene, blocking out the foreground and concentrating on the faraway objects.

In many lens catalogs and books and on many web sites you can see a series of photos of the same scene taken with different lenses. With some of these the perspective is indeed different in different shots. Again, that's just a coincidence. What they don't tell you is that the photographer walked back to capture a foreground object at constant size while using ever longer lenses. But it's this “walking back” that caused the change in perspective, not the longer lenses.

Still not convinced? Here are a few examples:

Image 1: Shot taken with 20 mm focal length	Image 2: Crop from image 1	Image 3: Shot taken with 100 mm focal length

If focal length had any influence on perspective, shouldn't image 3 look radically different than image 2? But except for sharpness, they're identical. So why do they look the same? It's because they were taken from the same position.

Another example:

Image 4: Shot taken with 20 mm focal length	Image 5: Shot taken with 20 mm focal length

The background is almost identical in both shots, but in image 5 the foreground is much more pronounced. Both were taken with the same wide-angle lens. Why is that? It's because image 5 was taken closer to the stump in the foreground and from a lower angle. It's not the wide-angle lens that pronounces the foreground, it's the shooting position.

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Here're the facts: DOF is a function of aperture and magnification (on film or sensor), and magnification is a function of focal length and distance. When you shoot a longer lens from a greater distance you can get the same magnification as when shooting a shorter lens from a closer distance. When you also use the same aperture, you will get the exact same DOF. So DOF with a longer lens will only be shallow when you also shoot from a short distance.

Many super-tele lenses can't focus particularly close. You often don't get magnifications greater than 1:6 or 1:8. For example, a 600/4 shot at its closest distance of 6 m will have a deeper DOF than a 100/2.8 shot at 80 cm (and at f/4), even though it's six times as long.

Often you get a smoother out-of-focus background with a longer lens. But that's not because DOF is shallower. The longer lens with its narrower angle of view just sees a smaller section of the background, and it's easier to find a smooth section of the background when it's small rather than large. For example, you have to turn a 600 mm lens by only 4° to get a completely new background. To do the same with a 100 mm lens you have to turn it by 24°. So if you're after a smooth background, using a longer lens may be a good idea. But if you're actually after a shallow DOF, using a longer lens may not be enough.

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Technically, this is wrong. The depth of field depends only on magnification (on film or sensor) and aperture. “Magnification” is the relative size of the object in front of the lens compared to its image projected onto the film or sensor. This relationship does not change, no matter what kind of sensor you hold behind the lens or what size it is. With the same magnification and the same aperture, you will always get the same DOF.

For practical purposes, there is some truth to this myth. That's because you usually don't compose your shot for a certain magnification but for a certain framing. For example, you try to fill the frame with some object. With film or sensors of different sizes, this results in different magnifications, and hence in different DOF. With smaller sensors, you shoot at a smaller magnification when you fill the frame with a given object. A smaller magnification leads to a deeper DOF when you use the same aperture. With a larger sensor, e.g. medium format film, the same framing results in a larger magnification, reducing DOF. So DOF indeed appears deeper for smaller sensors and shallower for larger sensors. However, this is not because of the size or nature of the sensor, but because you typically use these formats differently.

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For example, when a 35 mm lens can resolve 80 lp/mm (line pairs per millimeter), it can resolve 2880 lp over the width of a 35 mm frame. A medium format lens only needs to resolve about 50 lp/mm to project 2800 lp onto its 56 mm wide frame. If it can resolve between 50 and 80 lp/mm, it can resolve more lp on its frame than a 35 mm lens on a 35 mm frame.

Because of the typically lower resolving power of medium format lenses it also makes little sense to adapt these lenses to 35 mm cameras, at least when you try to gain resolution. These adapters only combine the disadvantages of both systems. By using medium format lenses you can only gain resolution by also using medium format cameras.

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DOF only depends on true focal length, true relative aperture and focus distance. When you use a TC to make a lens longer, you get all the characteristics of the longer lens, including DOF. For example, when you use a 2× TC on a 200/4 lens, you get the DOF of a 400/8 lens.

The following two pictures are taken from the same distance. With both shots the focus was on the middle of the ruler. One picture was taken with a 400/4.5 lens at f/8. The other was taken with a 200/4 lens + 2× TC (=400/8) at f/8. As you can see, DOF is the same for both shots.

Shot 1: 400/4.5 at f/8	Shot 2: 200/4 + 2x TC at f/8

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That's only a very rough rule of thumb, valid for medium magnifications. Depth of field is very variable. At larger magnifications, e.g. larger than 1:15 (!), DOF is roughly symmetric, i.e. front DOF and rear DOF are almost the same. As magnification becomes smaller (typically due to increased distance), DOF becomes more and more asymmetric. At some point there is indeed a 1:2 relationship between front DOF and rear DOF. But if you decrease magnification more, rear DOF will become larger and larger. At some distance, rear DOF will even reach infinity. We call this point “hyperfocal distance”. Also see the explanation on the optical formulas page.

As an example, here's a short table of front and rear DOF for 50 mm focal length and f/4 for various distances:

Morale: If you want to know depth of field, use a DOF calculator or the DOF preview function of your camera.

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© 2009 Michael Hohner; This page was last changed on 2009-11-10