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Here's my understanding so far. When taking a photo with a digital camera, there are some tradeoffs:

  • Aperture size: bigger aperture lets in more light, smaller aperture means less blurring in space.
  • Shutter speed: longer shutter speed lets in more light, shorter shutter speed means less blurring in time.
  • ISO: higher ISO captures more light (as long as it's not clipping/peaking).

The first two make sense. They trade off blurriness vs light (aka less noise), so it seems sensible to have the photographer decide what is more important. For ISO, the best option is always to crank it up to just barely under peak levels. That's something the camera is able to do on its own.

Is there any reason to change the ISO manually, rather than have it set automatically?

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    It is beyond me why anyone would let their camera set the ISO automatically. I access the light in a scene and then manually choose the lowest possible ISO that will give me the range of Apertures and Shutter speeds i need to work with for the situation i am in. My brain knows that the lowest possible ISO i can use will give me the the best image possible, does the camera know that? I am of of the generation that understands that ISO is not tool for adjusting exposure, it is for changing the sensor sensitivity to the light that shutter speed and aperture have passed into the camera. – Alaska Man Sep 3 at 19:37
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    The single situation where auto ISO makes sense for me is rock concert photography. Aperture is at its limit, shutter speed is limited by the musician's movements, so ISO is the only real variable - and the light is constantly changing. That way, ISO (and noise) is low when there's enough light, and it goes up instead of massively underexposing when there's not enough light. Anything more constant than that - manual ISO setting. – Dynat Sep 4 at 5:52
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    blurriness != noise – Stop Harming Monica Sep 4 at 12:23
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    @OrangeDog That's why I said "blurriness vs noise". It's a tradeoff. Between different things. – usernumber Sep 4 at 12:46
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    @AlaskaMan Complete rubbish. AutoISO can be a powerful tool if you have learned to understand its limitations and how to use it correctly. I'll use a setup for fast changing light, slow subjects - aperture priority, min. shutter for auto ISO set to 1/50sec. Now all you need to do is to select an aperture for composition and the camera selects the lowest ISO possible to maintain at least 1/50sec. Exactly what you would do manually, but instead of missing shots fussing with dials it happens automatically. This convenience being beyond your comprehension does not mean that it is not useful. – J... Sep 4 at 16:01
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Is there any reason to change the ISO manually, rather than have it set automatically?

The primary reason to set ISO, along with shutter time and aperture, manually would be to totally control exposure manually rather than let the camera set exposure. Not every scene needs to be rendered with an overall average brightness of medium gray. Left to its own devices, that is what the camera will attempt to do with every frame. Changing exposure compensation or metering mode can sometimes do the same thing, but it's often easier to get consistent exposure from frame to frame under difficult lighting conditions by using fully manual exposure mode to keep Tv (time value or exposure time a/k/a shutter speed), Av (aperture value a/k/a f-number), and ISO the same for every frame.

Beyond that, there can be plenty of other reasons:

  • You are using manual flash. With any "auto" exposure mode the camera has no idea how much light you are adding and will attempt to expose the scene "properly" using ambient light.
  • You are using TTL flash and would prefer use more flash power to light your subject brighter than the ambient light rather than use higher ISO to boost the ambient light and use the flash only as a "fill" light.
  • You actually want a longer exposure time than the camera's automated routines will give you. There are plenty of types of long exposure photography where this would be the case.
  • Then there is the case when you are photographing a static scene with a tripod mounted camera and you don't want the camera to shorten the exposure based on the focal length lens you are using. Many cameras' automated exposure routines will base the maximum allowable exposure time on the focal length of the lens used because it assumes the camera is being handheld. Even if you have set the Tv manually, some cameras will use "safety shift" to avoid potential camera shake.
  • You want to batch process a large number of images and use the same raw conversion settings, particularly noise reduction settings, to all of them.
  • You use a camera that doesn't treat "partial stop" ISO settings as an actual different amplification level, but instead use a "push/pull" or "pull/push" method to set amplification to the nearest full stop ISO setting. In such a case you might prefer to have full stop ISO settings (100, 200, 400, 800, 1600, etc.) and -1/3 stop ISO settings (160, 320, 640, 1250, 2500, etc) available but also want to avoid +1/3 stop ISO settings (125, 250, 500, 1000, 2000, etc.). For how this works out practically, please see: Is it really better to shoot at full-stop ISOs?

ISO: higher ISO captures more light (as long as it's not clipping/peaking).

The only two variables controlled by the camera that determine how much light is captured are exposure duration (Tv) and aperture (Av). ISO settings affect how much the analog signals collected by the sensor are amplified, not how much signal is collected by the sensor.

Please see Why would using higher ISO and faster shutter speed yield more noise than using lower ISO and slower shutter speed?

In that question, two sets of exposure settings are compared:

  • ISO 1600 at 1/125 second with a constant aperture setting.
  • ISO 3200 at 1/250 second with a constant aperture setting.

In the second case half as much light was collected by the camera's sensor, then amplified by twice as much before analog to digital conversion. Increasing amplification of the signal also increases amplification of any noise in the analog signal.

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Contrary to many people's beliefs (and contrary to how film works), with everything else being equal, higher ISO does not mean more noise but amplified exposure. This amplification will occur equally to scene content as well as an underlying noise floor.

So all else being equal, there is a point to Auto ISO, namely correcting for an otherwise underexposed picture. But more often than not, other manners of correction are a better choice: longer exposure (assuming you can keep movement under control), larger aperture (assuming you can arrange the scene appropriately), flash exposure (assuming this will not disrupt the light balance).

The camera cannot safely make such assumptions. Some of them the user can make and tell the camera explicitly (particularly in the more manual shooting modes). If you put your camera on a tripod for stabilisation (note that long exposures also add noise of their own), you can safely expose static scenes longer.

For such cases, pinning down ISO to its lowest value will tend to deliver the best images, assuming the scene content supports it. If you actually want long exposures (like wanting to capture light trails or moving dancers or lightning or whatever), overriding the camera's choice of ISO will also help in achieving the exposure times you are aiming for without the camera trying to change your choices.

  • Yes, you are absolutely correct. The noise comes from low exposure. A high ISO can actually mean less noise -- it's often better to bump up ISO than to brighten in post. – juhist Sep 3 at 12:39
  • "...pinning down ISO to its lowest value..." Wait, do you mean here that you have the ISO on manual, and aperture and shutter speed on auto? Or all three on manual? – usernumber Sep 3 at 12:57
  • @usernumber either case is pretty much the same. Changing Tv or Av lets in more or less light. Changing ISO only changes the amplification of the signal collected from whatever light was allowed to enter the camera. ISO settings do not directly affect how much light enters the camera. – Michael C Sep 3 at 19:52
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    But everything else is typically not equal. The camera may suggest ISO 6400 to keep the shutter speed at a comfortable 1/100. Assuming the aperture remains the same, you may set ISO 200 + 1/4 sec, more light will fall on the sensor and, considering what you said about amplifying the ambient noise, the image will be a lot less noisy. So "lower ISO = less noise" works in practice, even if the reason is often misunderstood. – IMil Sep 4 at 0:51
  • With my camera, it's definitely better to keep low ISO and lighten in post. With max ISO (1600), even pictures in broad daylight that are quite bright have horrible noise. – Tomáš Zato Sep 4 at 12:51
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yes for sure, High ISO values (like 1600, 3200, 6400) introduce more color noise in your picture which isn't a desired effect. So best to keep it as low as possible*

*or as close to the native ISO of your sensor, often between ISO 100 and 200.

As a photographer being used to manual exposure settings auto iso also makes it hard to over or under expose. Then I need to use an extra control to set and change the exposure correction + not forget to reset it afterwards. This is true for any auto setting btw, aperture priority or shutter priority for example. With a manual exposure control you just change the settings and see on the light meter indication how much off you are (or want to be).

The beste case for auto-iso to my understanding is for videography with a digital, non cinema camera and lens. Then your shutter is a fixed value (due to relation with frame rate of video) and changing aperture results in visible steps. Auto ISO in many cases can be used for gradually changing auto exposure while recording video.

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Yes, there is.

High ISO means lots of noise. Not all photographers want noisy images, although some minor amount of luma noise as opposed to chroma noise may be found pleasant.

Also, the auto-exposure of camera might not work in all cases (examples: astrophotography, photography of moon, fireworks photography, aurora borealis photography).

By setting the ISO manually, you have exact control over exposure. You aren't doing true manual exposure by leaving auto-ISO on.

By setting a cap for the auto-ISO, you have exact control over the amount of noise your images in the worst case have. For this reason, most good digital cameras have a configurable cap for the auto-ISO (example: use only ISO up to 12800, but within that limit, set ISO automatically).

  • "High ISO means lots of noise." But doesn't the signal to noise ratio stay the same? Meaning you would get the same result taking a high-ISO picture, and taking a low-ISO picture and brightening it in post? – usernumber Sep 3 at 12:14
  • @usernumber Depends on whether the sensors is ISO-invariant or not. With traditional sensors, it's better to amplify the signal using an analog ampliifer by using higher ISO. With ISO-invariant sensors, it's the same (you can either brighten it in post or bump up the ISO). However, the less there are photons, the more noise there is. So, in that sense, high ISO is always noisy, because with high ISO, you are capturing less photons, i.e. a darker image. – juhist Sep 3 at 12:21
  • Either way, if I've already decided on aperture an shutter speed, the highest possible ISO (without clipping) is either equal or better than lower ISO, sooo... what's the point of lower ISO? – usernumber Sep 3 at 12:25
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    @usernumber You didn't read my entire answer, did you? Auto-exposure doesn't work always (in astrophotography, for example). But it's a valid point that setting the auto-ISO cap as high as possible is typically a good idea. But you sometimes really need manual ISO. – juhist Sep 3 at 12:26
  • Also, if you have plenty of light, or are using a flash, it might make sense to fix ISO to 100 and let in more light (by a slow shutter speed, wide aperture, or high flash power output). – juhist Sep 3 at 12:29
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The modern camera is loaded with automation. You can elect full automatic -- now you point and compose and take the picture using the setting dictated by the camera’s software. In many cases this image will be quite satisfactory.

On the other hand, a skilled photographer might choose to reject the camera’s logic and go it alone. Now we are talking creative art coupled with acquired skill.

Consider, if you elect total automation you are relying on the skills of the camera’s software engineers to make the camera setting. These skills get better and better with each passing day.

The alternative is: Hit the books or learn by trial-and-error or learn by osmosis. Let me tell you, there is great enjoyment in being the master. Automation full or partial does not guarantee greatness.

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It isn't Manual Exposure until all 3 parameters have been set to manual: aperture, shutter time, and ISO (video gain). So in situations where you absolutely must have a defined exposure, you must also get off the auto ISO. Examples: a panorama shot where you don't want brightness jumps on the stitches due to changes in exposure, a time lapse where you don't want to adapt to clouds, ..

A 4th parameter is when you can control the lighting, e.g. flash, again manual or auto.

Auto ISO is where analog and digital photography differ: you can have manual aperture and shutter time, and still have some 6 stops of auto exposure. That takes some getting used to !

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In some cases (found it a useful way to work stage photography), it even makes sense to use ISO as the variable element in doing manual exposure. Eg in a stage lighting situation where you set the shutter to the slowest acceptable speed (so no motion blur or camera shake), set the aperture to the widest acceptable value (so you have enough DoF and not too much flare, glow, coma, or other really-wide-open lens characteristics) ... and then react to lighting changes, and trade off exposure key (high or low key) against noise and reduced dynamic range, with a finger on the ISO wheel all the time.

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