What are the advantages and disadvantages of using auto ISO on a DSLR?

Question

What are the advantages and disadvantages of using auto ISO on a DSLR?

Using manual ISO obviously gives you more control, but auto ISO may mean that you get "the shot" under quickly changing conditions (such as street photography, varying quickly between direct sunlight, shade, indoor lighting.

Is auto ISO a convenient feature or a bad habit?

Asked by Freed

Answer

Hardware ISO control exists to amplify the signal before readout to maximise the signal to noise ratio. Without read noise you wouldn't need ISO if you had a high precision ADC as you could simply shoot everything at the native sensitivity, and if the image was too dark just apply digital gain (multiply the pixel values).

It's easy to demonstrate this effect. The following two images had the same shutter speed and aperture. The top one was shot at the metered ISO 1600, the bottom one was shot at ISO 100, which resulted in underexposure which was fixed in photoshop:

The ISO 100 image is noisier, on account of read noise being amplified when the exposure is fixed. The ISO 1600 is amplified before readout so read noise is much lower.

As such ISO is not a creative control (like shutter speed and aperture) it is a practicality, required by the need to overcome noise. Contrary to popular opinion using a lower ISO wont give you less noise. Unless you also decrease shutter speed or open the aperture, both of which can result in a different looking image.

I don't consider it "lazy" to use auto-ISO, as the camera is not altering the creative controls, it's just trying to minimise read noise for you.

With auto-ISO in manual the pros and cons are pretty clear:

+ You can adapt to changeable lighting whilst sticking to your preferred aperture and shutter speed.

+ You shouldn't need to

- You're still at the mercy of the camera's light meter, you can get overexposure if the meter gets confused (which wouldn't happen if you fix the ISO to a low value).

- Some cameras don't offer exposure compensation with auto-ISO meaning if the metering is consistently off due to scene content you might not be using the optimal noise reducing ISO.

Using auto ISO in either program, shutter priority or is slightly different as there are two or three values for the camera to choose, and the ISO value is no longer simply dependent on the light reading.

+ You'll be able to sue preferable shutter speeds aperture more of the time than if ISO sensitivity was fixed.

- You're at the mercy of the auto-ISO algorithm, you're still letting the camera make creative decisions.

Answered by Matt Grum

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Why does the camera default to ISO 400 in auto ISO mode with flash?

Question

I've observed this behaviour on my Canon 550D, and I was wondering why it didn't use a lower or even variable ISO. I'm not sure if this is a camera/brand specific setting.

Is there any particular reason for this behaviour? Are there any scenarios where a different ISO is chosen, as I have not encountered it so far?

Answer

While the specific value chosen will be brand-specific, you're right that this is common behavior. My Fujifilm point & shoot favored ISO 800.

Increasing the ISO from 100 to 400 doubles the effective range of your flash, which is important with a relatively-anemic built-in flash. It also means that half of the flash power can be used for a subject within the normal range, saving battery life, decreasing the time it takes to be ready to flash again, and shortening the duration of the flash pulse to better freeze motion.

Another approach would be to increase aperture (more open; smaller numbers), which would have a similar effect, but would reduce depth of field. That means focus has to be more dead-on, and is generally not what auto-exposure modes are programmed to go for unless they have no other option.

I don't know the specifics of the auto-ISO logic in your camera model, but I wouldn't be surprised if it favors keeping ISO at around 400 and only increasing it only when there's not enough flash power.

When I'm using off-camera flash, I usually set ISO manually, but I generally default to around 400, just like your auto mode. This gives me a lot of flexibility in aperture, and even though my camera isn't the latest sensor generation, that ISO is clean enough that I don't really worry about noise.

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How do I enable auto-ISO in non-scene modes in a Nikon D5100?

Question

I'm trying to enable auto-ISO on my dad's Nikon D5100. There is an "Auto" option for "ISO sensitivity" in the menu, logically-enough under "ISO sensitivity settings"). This works fine in the Scene modes, but with P, A, S, or M, I get:

This option is not available at current settings or in the camera's current state.

Surely auto-ISO is not only available in the preprogrammed modes. How do I enable it?

Update: there is a separate section to "ISO sensitivity settings" below the main choice. That is only available in the P, S, A, and M modes, and has "Auto ISO sensitivity control". I've set this to ON, but empirically it's the hard-set value above that actually gets used. What is this separate setting, why is it separate, and (most of all) why doesn't it work?

Answer

Auto ISO isn't available in the non-scene modes per se.

The ISO Sensitivity Auto control option you mention in the latter part of your question is essentially 'Auto Pro'. You'll note in the menu that you can set a max ISO and a minimum shutter speed.

When ISO Sensitivity Auto control is enabled, the camera will endeavour to use the ISO you set, but will increase the ISO automatically (up to your defined max) if it's not achieving the shutter speed set in the menu. If you actually take a shot then check the settings used on the LCD, you may well see that the ISO will be a different value, and shown in red, to indicate that the camera took over and adjusted it automatically.

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