Imagine if there was an exposure mode that would enable you to set both the aperture and the shutter speed and the camera will automatically change a third variable to adjust the exposure. This would be like having simultaneous aperture and shutter priority (via manual mode) with auto exposure. This mode is called auto ISO and when used in manual mode I don't see any reason to use aperture or shutter priority. The power of auto ISO in manual is that you can set the both the aperture and shutter speed and the camera will set the ISO value for a proper exposure. For the record I still use straight manual mode for 90% of my photography (where I have to set the ISO manually) but when conditions call for auto exposure I enable auto ISO in manual mode. With this combination enabled the camera leaves the shutter speed and aperture up to you and only adjusts the ISO value. Although technically you can also use auto ISO in aperture or shutter priority modes, I find auto ISO in manual to be the most powerful combination.
Single Sandhill crane landing, Northern Farm Field, Bosque Del Apache, New Mexico. NIKON D300s and AF-S Nikkor 500mm f/4G ED VR lens + TC-14E 1.4x II,1/1600th sec at f5.6, manual mode, metering on zero, Auto ISO. Image Copyright 2011 Robert OToole Photography.
Earlier this month at Bosque when the lighting conditions where changing rapidly used auto ISO in manual mode with great results. This gave me the freedom to concentrate on making sharp full frame images of cranes with all legs and wingtips intact and not having to worry about changing settings up and down when the light levels changed.
Instead of getting caught up discussing how one auto mode is better than another think about how auto ISO can be a useful tool. Manual mode allows you to have complete control over the aperture, shutter speed and ISO but the camera will not compensate for any changes in light. This can be a huge problem in rapidly changing conditions since the user will not have enough time to adjust the settings back and forth to compensate. Enabling auto ISO in manual mode will allow you to use the exact settings you require and at the same time the camera will compensate instantly for any changes in light so you can concentrate on other important things like autofocus, timing, and composition. Some cameras (Nikon D300S, D700, D3S, and D3S) even allow you to fine tune the results with exposure exposure compensation in manual mode with auto ISO enabled.
Would auto ISO be useful for me?
The answer depends on the brand and camera model you use. The best way to find out is to test auto ISO with your own camera in different modes with different settings and see which combination gives you the best results. Current Nikon cameras, D300s, D700, D3S, and D3X offer manual mode with auto ISO, called ISO Sensitivity Auto Control, with adjustable ISO limits and exposure compensation. This means you can set the ideal shutter speed and aperture and the camera will set the ISO level with EC available at anytime to fine tune the results. Canon EOS-1D MK IV and 7D bodies feature auto ISO in manual mode with adjustable ISO limits. Although the Canon EOS 5D MKII does have an auto ISO mode the features are very limited in the auto modes and it is not available at all in manual mode. Unfortunately at this time no Canon camera will allow you to use exposure compensation in manual mode with auto ISO enabled. The recently announced EOS-1D X is rumored to feature auto ISO options similar to the MK IV, hopefully with the addition of exposure compensation. There has been a lot of discussion online about Canon adding EC with auto ISO and hopefully Canon will decide to include this feature in the future.
Nikon photographers that would like to learn more about auto ISO and a lot of other professional techniques more should pick up my Nikon DSLR User's Guide for the D300s/D700/D3S/D3x is available at the Birds As Art online store: https://store.birdsasart.com/shop/item.aspx?itemid=329
If you are in the market for a new DSLR and auto ISO is important to you it would be best to test all of the current cameras available right now from Canon and Nikon before you actually make your camera purchase.
Be sure to leave a comment if you have any auto ISO tips or questions you would like to share.