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

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?

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'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 aperture priority 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 use 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.