What are shutter speed, ISO, and Aperture?
I've seen people talking about shutter speed, ISO, and f-stop. What are those? It really confuses me.
What is the advantage in adjusting these manually, when you can just have your camera decide it for you?
Lastly, does using manual mode for everything have an advantage or disadvantage?
3 answers
Shutter speed
Shutter speed is the inverse of exposure time: i.e. it measures how long the camera collects light. This is the parameter with most flexibility: most cameras support shutter speeds at least as fast as 1/200 seconds and at least as slow as 120 seconds.
A short exposure time is desirable when you want to "freeze" motion: for example, when shooting sports. This applies both to motion of the subject and motion of the camera: a common rule of thumb is that when hand-holding you should divide one second by the focal length in mm to get the slowest acceptable shutter speed (e.g. with a 50mm lens you would want a shutter speed of 1/50 s or faster).
A long exposure time is desirable when you want to blur out motion: for example, it's popular for photos of moving water. (Tastes may vary).
The "exposure number" is linear in the exposure time: leave the shutter open for twice as long, and you get twice as much exposure.
ISO
With film, ISO basically comes down to the size of the grain. A film with a small ISO number will look less grainy. Given the same exposure time and aperture, a film with a higher ISO number will look brighter, and the "exposure number" is linear in the ISO.
Digital cameras don't really do grain (although some post-processing might try to add in the appearance of grain); instead they amplify the signal to get the same linear dependency on the ISO number. This is similar to brightening the image in post-production software, which provides some justification for the argument that with digital photography you should prefer to under-expose rather than over-expose.
Note that many digital cameras will allow you to set an ISO number which is faster than the true native ISO of the sensor. That basically means that they're pushing the image after receiving it from the sensor.
Aperture
The aperture is essentially the narrowest part of the path down which the light travels on the way to the sensor. It's a lot easier to understand what it means in a simple lens than a modern lens with multiple elements.
Aperture is always given as a ratio of the focal length: that's why you see e.g. f/5.6. The solidus there indicates division, so a smaller number (e.g. f/1.4) gives a larger aperture than a larger number (e.g. f/16). The ratio given is the diameter of the aperture: the amount of light passing through depends on the area, so you have to square the ratio to find out how it affects the exposure. That's why you see numbers like 1.4 (which is really an approximation of the square root of 2).
A wide aperture (small number) will have a narrow depth of field, so less of a complex scene will be in focus. A narrow aperture (large number) will have a deeper field, with more of the scene in focus. However, a physical effect called diffraction means that a very narrow aperture will blur everything slightly. For maximum sharpness across the scene you typically want an aperture in the region of f/8 or f/11.
Manual versus automatic
Automatic modes work by using a light meter to estimate the correct exposure number for the scene and then adjusting one or more of the three parameters to try to achieve it. This works well when the metering works well and the constraints that you've placed on the parameters are suitable. It is still necessary to sanity check: if you're in A mode and have disabled automatic ISO adjustment, you may find that the exposure time is 1s and you need to adjust the aperture or ISO.
Manual mode works well when you know that the light meter will get the wrong value. For example, in bird photography the subject is often small and fast-moving. Automatic exposure will probably expose for the background, not the subject. Some bird photographers therefore swear by metering against the sky and then adjusting by a given amount (which may depend on weather conditions).
Shutter speed, ISO, and f-stop are the three variables you can control in the camera to adjust exposure. Changes in one can be offset by changes in another to keep the same exposure. However, changes in each of these three parameters have different influences on the look of the overall picture, even with the resulting exposure held constant. To make good tradeoffs, it is important to understand what each of these are and do.
Shutter speed
This is pretty much what it sounds like, which is how fast the shutter opens and closes. Shutter speed is measured by the length of time the film or sensor (I'll use "sensor" from now on for simplicity, but that also applies to film unless explicitly stated otherwise) is exposed to the image. This can also be referred to as the "exposure time".
Typical shutter speeds for ordinary uses are from about 1/30 second to about 1/1000 second.
Tradeoffs
Slower shutter speed lets more total light hit the sensor, but higher shutter speed freezes motion.
1/1000 second is fast enough to freeze most motion in most cases. Such a fast shutter speed might be necessary for sports action, for example. However, the lower amount of total scene light hitting the sensor must be compensated for in other ways, such as higher ISO or wider aperture (see below). Those come with their own tradeoffs. There is no free lunch.
Note that motion doesn't only come from the scene, but also from movement of the camera during the exposure time. No matter how steady you try to be, there will be some motion whenever the camera is hand-held. As a result, there is a lower limit on shutter speed for hand-held pictures. 1/30 second is usually a good limit for hand-held shots with medium lenses.
Long focal lengths require shorter exposure times. Slight changes in camera orientation result in larger movement of the scene at longer focal lengths.
F-stop
The F-stop is a measure of how much light from the scene the lens itself captures and projects onto the sensor. This is also sometimes referred to as the "aperture".
The amount of light a lens lets thru comes from the size of its optical opening. Consider two lenses, one with a 10 mm diameter opening and another with a 20 mm opening. The second will let four times as much light thru because a 20 mm diameter circle has four times the area of a 10 mm diameter circle. The light-passing quality of a lens goes with the square of its diameter.
But, there is another factor to consider, which is the focal length of the lens. Longer lenses (greater focal length) project the same scene larger on the sensor. A 100 mm lens, for example, projects the same scene twice as large in each dimension as a 50 mm lens. Since each dimension is twice as large, the same scene is spread over four times the area. This means that for the same amount of light thru the lens, that light is spread over a larger area for a longer lens. All else held constant, the brightness of the projection onto the sensor goes down with the square of the focal length.
The above two issues require a lot of mental math to figure out how bright the scene projection is on the sensor. Wouldn't it be nice if there was a normalized way to describe that across different lens focal lengths and diameters?
Yes, and that's what f-stop numbers are. We saw that the brightness on the sensor went up with the square of the lens diameter, but down with the square of its focal length. If we take the ratio of the two, we get a single value of projection brightness. This is exactly what f-stop numbers are.
F-stop numbers are the ratio of the lens diameter to its focal length. These are written as "f/xxx", like f/4, f/5.6, f/8, etc. This is essentially the normalized diameter of the lens. The "f" part stands for the focal length. For example, a 100 mm lens at f/4 has a effective optical diameter of 25 mm.
The nice thing about f-stops is that they work the same across different lenses. For example, f/8 will give you the same exposure whether that's from a 200 mm lens with 25 mm aperture, or a 80 mm lens with 10 mm aperture.
So why the strange numbers like f/5.6, f/11, etc? Remember that projected scene brightness goes up with the square of the aperture, and down with the square of the focal length. This means that exposure goes with the square of the f-stop. For example, f/4 causes four times the exposure as f/8.
Four times is rather a large jump. In photography, we usually think of factors of 2 as "increments". To get a factor of 2, the f-stop must be multiplied by the square root of 2.
The common standard f-stops come from starting at f/1, and dividing by the square root of 2 for each subsequent stop. Therefore, we get the progression
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
You usually won't see f/1 or f/1.4 except in specialty (and expensive) lenses. It's hard to make a lens that is as wide as its focal length, and still maintain a sharp projection.
Tradeoffs
Larger apertures let more light thru the lens, but also decrease depth of field. That's the range of distance from the camera where scene objects will be reasonably in focus.
At really small apertures, diffraction starts to matter. This makes the image less sharp. Light rays passing very near a physical obstacle bend a little around that obstacle. This always happens at the edges of the aperture, within about a light wavelength of the edge. For small apertures, more of the overall area is within this small distance to the edge, making diffraction relatively more prominent.
Diffraction is why variable apertures don't usually go past f/22 or f/32. The decrease in sharpness at smaller apertures would be too noticeable.
Another tradeoff is that the non-ideal characteristics of the lens become more apparent at wide apertures. Designing a lens is a whole set of tradeoffs of its own, with compromises inevitably made. Effects like chromatic aberration, non-uniform focus, edge distortions, and the like are always present to some degree. The larger the lens diameter, the harder it is to minimize these problems. These effects are therefore usually more pronounced at the widest-open f-stop or two.
ISO
ISO is a measure of the sensor (or film) sensitivity. Higher ISO numbers mean that you get more signal (or film density) for the same amount of light.
Making films more sensitive usually meant larger grains. There was therefore a tradeoff between grain noise and sensitivity. However, there were other tradeoffs too. Simply stating that ISO went with grain size is incorrect, or a gross oversimplification at best. There were tradeoffs that could be made in processing between maximum contrast and ISO, and different chemistries allowed for different tradeoffs altogether. For example, Kodak's famous Kodachrome film wasn't based on grain at all.
Modern cameras use electronic sensors instead of film. The sensor is a fixed part of the camera. You can't change the sensor to get different ISO values like you can with film. However, since the sensor signals are electrical, they can be manipulated with electronic circuits to provide an effective range of ISO values.
The native ISO of a sensor might be 200. The raw signals from the sensor can be amplified by 4, for example, to result in effective ISO of 800.
There are limits to this. All signals have some noise on them. The more the sensor signals are amplified, the more the noise is amplified too. The ISO limit of a digital camera is not how much the signals can be amplified electrically, but how much noise in the final image can be tolerated due to the amplification.
Tradeoffs
High ISO settings effectively give you more light to work with and allow more flexibility between the shutter speed and f-stop settings. However, too high ISO adds noise to the image.
How much the noise matters depends on how the image will be used. If it will be shown 1:1 pixel to pixel, then any noise will be seen directly. However, if the image will be shrunk to a lower resolution, then multiple original pixels will be "averaged" to make each final pixel, and random noise will be reduced. For example, if your camera takes 4200 x 2800 images, but you only want to show the result as a 1200 x 800 picture in a web page, then you can tolerate a higher ISO setting than if you want to make a poster of the original that people can walk right up to.
Physically larger sensors with less noise (allowing higher ISO settings) cost significantly more to make. A better sensor is one of the main features that makes high end cameras more expensive.
Manual versus automatic
Long ago, simple cameras gave you manual settings for the f-stop and shutter speed, and the rest was your problem. This is where common rules of thumb for exposure came from.
One example is the "sunny 16" rule. For a normal scene in sunlight, you get good exposure at f/16 and the shutter speed set to 1/ISO. For example, with the old Pan-X film that had ISO of 125, good exposure would be f/16 and 1/125 second. Of course you can trade off f-stop and shutter speed from there, like f/8 and 1/500 second, f/22 and 1/60 second, etc.
If it wasn't fully sunny, you'd adjust accordingly. Hazy sun was 1 f-stop down (factor of 2 less light). Cloudy bright was 2-3 f-stops down.
After a while you got reasonably good at judging this, but there were always some shots that were over or under exposed. If you were in doubt, you'd use a separate hand-held light meter. That would measure the scene light, and you'd previously set it to the ISO of the film you were using. The light meter then showed you a range of shutter speeds with corresponding f-stops that would work well.
Nowadays, pretty much every camera has a built-in light meter. Not only can it tell you what shutter speed and f-stop combinations would be good, but since everything is computer-controlled, it can go ahead and actually make the settings for you. With digital cameras, the ISO setting is also under computer control, so the camera can pick all three of shutter speed, f-stop, and ISO for you automatically.
Tradeoffs
But wait, do you really want the camera to make all the tradeoffs? How does the camera know you are taking sports action pictures and want fast shutter speed, and are willing to put up with less depth of field and/or more noise to get it? What if nothing in your scene is moving fast so that slower shutter speed is fine, but you want a large depth of field? What if you don't know from f-stuff and all that ISO-shmiso babble and just want to take a few pictures already?
This is why there are a range of cameras from "point and shoots" that optimize for low intimidation factor and price, to pro models that give all the flexibility to the photographer when he (or she, not going to keep saying that) wants it.
Even the low-end point and shoot cameras have different "program" modes. Most likely there will be something called a "sports" mode. That will strive for fast shutter speed. Other mode names make the tradeoffs less obvious. You provide the high level guidance by choosing one of several program modes, and the camera still decides the details on the fly under the hood.
Even pros rarely need fully manual exposure mode. Probably the most common mode used in a pro camera is "aperture priority". That means you pick the ISO, then specify the aperture on the fly, and the camera picks the appropriate shutter speed based on the amount of light it measures. Shutter priority works the same way except that you select the shutter speed on the fly and the camera picks the appropriate f-stop.
On these cameras, there are easy to reach dials to make the setting, and the camera shows you what it picked in the viewfinder. This way the pro has a single dial to trade off shutter speed and aperture, and can see exactly what the tradeoff is. If he doesn't like the tradeoffs, he can usually change the ISO fairly easily.
I generally use my camera in aperture priority mode. A dial under the right forefinger selects the f-stop, and I can see the current ISO setting and the resulting shutter speed in text below the image in the viewfinder. Changing the ISO means holding in a button with my left thumb, and adjusting a wheel with my right thumb.
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Shutter speed. This controls how long light reflecting off of objects hits the sensor. The longer the shutter speed, the brighter the image. So a shutter speed of 1/50 would make the image brighter than if it was 1/500. The disadvantage of having a slow shutter speed is that if you shoot something that's moving, then you get a motion blurry photo. So a faster shutter speed is better for that. For instance, say you are trying to get a good picture of a water droplet. A shutter speed of 1/1000 would be good to get the water droplet in all it's glory.
Disadvantage
When you adjust the shutter speed to make it faster, you are essentially limiting the amount of light that hits the sensor. So your image will be much darker. The same applies for the opposite. When you make the shutter speed slower, light has more time to hit the sensor, meaning your image will be consequently brighter.
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ISO. This is the sensor's sensitivity to light. In film, ISO is the measure of how much grain there is on a photo. You couldn't adjust it back then, since you bought film and it had one specified ISO. ISO is proportionate to the brightness of an image. So if you raise your ISO from 100 to 200, then the brightness of your image will also double. Increasing your ISO when taking a photo in the dark is always something helpful.
Disadvantage
Along with increasing the brightness of a photo, ISO can also add noise to your image, since the higher the ISO, the more sensitive it is to light. Even small deviations in light show up prominently in your photo. If you dislike noisy photos, then stay away from increasing your ISO to something like 6400! -
Aperture. Also known as f-stop, this is how wide the diaphragm of the lens is. So if the aperture is f/5.6, then the image is brighter than if the aperture is f/7.5. Since the aperture is just the ratio between how wide and how covered the lens is, you will always get number that are perfect square roots of irrational numbers.
Disadvantage
When you have a wide aperture, you are effectively decreasing the Depth Of Field. The Depth of Field is an area where any object placed in that field will appear to be sharp. If it is placed before the DOF or after, the object will appear blurry. The further away, the more blurry, since the DOF fades instead of dissapear. Disadvantage might be a wrong term, since bird photographers, for instance, always have a very wide aperture, so the background is completely blurred. However, landscape photographers always have a low aperture since they want to capture a large scene without it being blurry. So it depends on what you want to do. Most of the time I have the aperture wide open since I like taking pictures of birds and inanimate objects. Landscapes have never appealed to me :)
Manual or Auto?
You may have heard of manual mode. This basically means a mode where you decide ISO, Aperture, Shutter speed, and a few more things. This scares most new photographers, and they usually just switch to auto as soon as they get their DSLR. The advantage to adjusting everything manually is that you can decide how the photo comes out. You decide if you want motion blur, or a blurry background, or a grainy photo (some people like old style photos).
Say you are taking a photo of some water. Most people want to blur the water so it appears smooth. With manual, this is simple to do. Just set the shutter speed to 2 seconds (more if you want extra blur) and compensate for the extra brightness by throwing the aperture down completely. You can also decrease the ISO if you wish.
With Auto, you can't do this, since the camera thinks in terms of exposure only. So if the scene is partially sunny, it will set the shutter speed to something like 1/250, and the aperture will be almost wide open, which will get you a normal photo, without that motion blur. The camera, in other words, doesn't know how you want the photo to come out. It only knows about exposure, and that's it.
But you don't have to use manual mode. You can use one of the priority modes. Let's take the previous real world example. You set your camera to shutter priority, and then set the shutter speed to 2 seconds. You take a photo, and BOOM. The photo is perfectly exposed, and you have that motion blur. You didn't even have to adjust the aperture!
Think of the priority modes like this: You decide almost everything, in order to control how the photo will come out, and the camera just makes sure your photo is perfectly exposed. Use shutter priority when you are trying to capture something fast, and aperture priority when you want to capture a landscape or achieve a nice dreamy, blurry background.
Many professionals use priority modes along with manual, so if you do choose to use it, don't think it's for noobs. I use manual personally, because for some reason I like doing extra work instead of getting it easy with something like aperture priority.
I captured this photo with aperture priority, though:
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