I am a beginner in photography and I don't have the right understanding for some things. As I understand, with the ISO the camera is more or less sensitive light, with a darker image, a higher ISO (Also changes the noise in the image)

Then we have aperture and shutter speed. More aperture, more light (and changes the deep perception) and more shutter speed, less light (also captures better the movement)

Can we not change shutter speed and aperture to capture better photos with different light, instead of the ISO? If its dark, we can make a wider aperture and/or change the shutter speed, if there is too much light we can close it and make the shutter faster

Why the ISO then? Is it only for the cases that we want an specific aperture for an specific deep perception? Or because with long time shutter speed we need something to make it the camera stable and not be blurry? Don't we add noise increasing the ISO? What am I missing?

The Question: Why use ISO instead of Aperture and Shutter speed?

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    Well there are reasons your aperture can not be too large or small, and exposure too long or short. Adjusting ISO gives you more flexibility. – user3528438 Sep 28 '17 at 20:03
  • I believe history is a relevant factor as well. With any given film, ISO is fixed, so if you want to change ISO speed, you need to choose a different film, which would be a different chemistry and bring along other differences as well. Only in the digital era has ISO become adjustable with clearly defined, often irrelevant side effects (noise). In this context, it's natural to think about ISO differently than shutter speed and aperture. – Reid Sep 29 '17 at 14:10

All three have different uses and different .

- by closing your aperture you're increasing depth of field and sacrificing light
- by opening your aperture you're decreasing depth of field, blurring the background and gathering more light

Shutter speed
- using faster speed you can freeze subject motion and camera shake (if any), but you're decreasing amount of light your sensor receives
- using longer speed you usually make scenes dreamy, with blurred motion and you're prone camera shake. You also increase amount of gathered light.

- by using higher iso you're increasing gain, which allows you to shoot with faster shutter speeds or smaller apertures, while sacrificing image quality (depends on the sensor, to some extent it's usually not visible) and increasing noise, because you're not getting more light, you're just amplifying the signal. It's similar to what happens to sound when you crank up the volume - it gets too distorted if you turn the knob too much

Let's say you want to shoot with high shutter speed (e.g. 1/400) to freeze motion and have a fairly small aperture like f/16, but it's getting dark. You can only use higher ISO to match these requirements. To what extent this will be possible depends on the used sensor. Some are way too noisy at 800-1600, so that can limit your ability to capture the image you want. If you have a better low light performing sensor (like in some full frame camera), you can easily shoot at 3200-6400 ISO and still get acceptable image quality with barely any noise.

The best rule about ISO is usually this: shoot with the lowest possible ISO to get the image you want.

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    So whe only change the ISO when we really need specific settings on the aperture and the shutter speed. Thanks for the answer! – Ivan Sep 28 '17 at 14:45
  • Astrophotography would be one place where you're likely to have to change the ISO. Depending on where you are, and your camera settings, there's a maximum amount of time that you could open your shutter without turning a single star into star streaks. So if you want your final image to have a single star, and say your shutter is limited to around 8 seconds, and you're at wide-open aperture, then you're forced to bump ISO to get the exposure you want. – Calyth Sep 28 '17 at 17:25
  • @Calyth For anything other than star trails (which typically involve wide angle lenses and hour + exposures) planets (you want something more like high speed video here) or the most casual use; astrophotographers deal with very faint objects by taking lots of short exposures and stacking them in post-processing. I've only ever done it with imaging setups that someone else configured but believe you'd want to use a low ISO. The dedicated astrocam I used in school ~15 years ago operated such that its RAW data was an approximate count of the number of photons each pixel detected. – Dan Is Fiddling By Firelight Sep 28 '17 at 18:33
  • Oh cool. TIL :) – Calyth Sep 28 '17 at 18:34
  • Thanks for this explanation. I come from a film background, where ISO speed actually meant something concrete (more or less). ISO for digital cameras didn't make sense until now. – Pete Becker Sep 28 '17 at 20:51

You should use aperture and shutter speed to get the correct exposure based on how you want the photo to look (aperture for depth of field if it’s important, shutter speed if capturing or freezing movement is important). Use the lowest ISO setting you can, only increasing it if you still can’t get the correct exposure within the limits of the other settings.


It is quite a common situation to be in where you have set your aperture to the widest setting possible, and you have set your shutter speed to the slowest setting that allows you to capture a shot without blur, and the scene is still too dark to get a good exposure without increasing your ISO significantly.

When you are hand-holding a camera, you can't use too slow a shutter speed before your photo will be ruined by camera movement during the exposure. An often-repeated rule-of-thumb is that you shouldn't use a shutter speed slower than (1 / LensFocalLength). (It's just a guideline, and won't work for every scenario.)

Maybe you are taking photos of moving subjects in a dark environment - you will need a faster shutter speed, so being able to compensate with a higher ISO is necessary.

Maybe you need to capture deep depth-of-field in a dark scene - you will need a smaller aperture, so it helps to be able to compensate with a higher ISO.

Basically, there are lots of scenarios where you need to make use of a high ISO setting to get a good exposure, because of other constraints on the aperture and shutter speed.


ISO (International Standards Origination) is a numeric value that specifies how sensitive the imaging chip is. As a rule-of-thumb we generally choose a low ISO setting because image quality gradually deteriorates as we go to higher and higher setting. If we choose a super high setting the image likely will show some granularity. This is akin to what is called “grain” in conventional film photography. The granularity seen in digital photography is called “noise”. It is analogous to static in an audio system. As the ISO is turned up, the image signal is amplified. This amplifies the good signal however immingled is some bad signal that also gets amplified. Modern digitals sport noise suppression so likely you will not see any ill effect except when the ISO is set very high.

As to exposure: Good exposure is the key to this kingdom. Too much exposure and the resulting image will be washed out (too light or even white). Too little exposure and the results are dark, perhaps even black.

Now the old box cameras of past (Kodak Browne and similar), sported no user settings. The camera was pre-set to a small aperture diameter and the shutter was set just fast enough to allow the camera to be hand-held. The focus distance is also pre-set for these cameras. Because the aperture setting is tiny (as to diameter), the zone of acceptable focus called depth-of-field is from about 4 feet to as far as the eye can see (infinity symbol ∞). Billions and billions of acceptable pictures were taken but picture taking opportunities were restricted to bright sunlit vistas.

To expand the picture taking opportunities to twilight or indoor etc. it is necessary to provide the user with options that allow more light into the camera during the exposure. Now we are talking, adjustable aperture diameter. Because the lens acts like a funnel, we can open up its diameter to allow more light to enter. This is a wonderful approach however; larger working diameters deliver a shallower zone of depth-of-field. Once we open up the aperture, we are forced to provide the user a focusing apparatus. In other words, adjustable aperture expands the range of picture taking but now we must focus the camera.

The old Brownie’s shutter was pre-set to about 1/25 of a second. At this shutter speed, lots of light plays on the film (now digital image sensor). Now you need to know that the exposing light accumulates all the time the shutter is open. A long exposure time accrues light. The downside is, we must hold the camera super steady and we must tell our subjects not to move. Once we provide an adjustable faster shutter, we need not be too concerned with camera of subject motion. This is a give and take. A fast shutter reduces light accumulation. The countermeasure is to open up the lens aperture. The added light gathering now affords a fast shutter.

What I am trying to tell you is: We have at our disposal, adjustments that allow us to expand the picture taking opportunity. We have a. ISO b. shutter speed c. aperture. We even have a fourth solution, we can supply artificial light. The idea is to expand the picture taking opportunity. The thing you need to know is, all of these adjustments are intertwined. We can adjusts just one or two at a time or all.

You need to study the mechanism of exposure. In the vascular of photography, this subject is called the “exposure triangle” a. ISO b. shutter speed c. aperture. If all this is too confusing, you need only to set your new camera on automatic. In this mode, the camera will make all these selections for you and the outcome will be OK. If you are serious about your new hobby, then you need to study the “exposure triangle”.


Generally for two reasons, which can be linked together.

  1. You don't want to open the aperture because you want to maintain a certain (perceived) depth of field - most often in landscape type of photos.
  2. You can't afford to lower the shutter speed (because you've hit either the limit at which you can hand hold the camera and have an acceptably sharp result or because you have moving subjects).

Mind you, some cameras will favour a higher ISO setting (as opposed to a lower setting that would result in an underexposed shot). Other cameras would give you about the same result for a properly exposed image at a higher ISO and an underexposed image that's later "pushed" up - these are commonly referred to as "ISOless" cameras, this is usually mentioned in reviews.


Can we not change shutter speed and aperture to capture better photos with different light, instead of the ISO?

Yes, you can, but you might not want to. Opening the aperture reduces depth of field, which you might not want. Or, you might already be shooting at the widest aperture available and still not have enough light to get a good exposure at your chosen shutter speed. You can choose to add light (e.g. with a flash), or you could settle for a longer exposure time, or you can bump up the ISO.

Why the ISO then?

It's just another exposure parameter with its own set of tradeoffs. You can increase exposure by increasing exposure duration or using a larger aperture or adding light or increasing sensitivity. Which option(s) you choose depend on how you want to affect the image.

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