Thoughts on Auto ISO

David,

Part of the solution on how to deal with AUTO ISO is to understand what happens in the camera when it is switched on. As I did not understood it and each time got fooled by my camera I did a kind of an investment to get a better understanding. For those who are strugly the same, below are my findings. Note that this is based on my D40 which allow to set max SPEED and max ISO as part of ISO AUTO.

did investigate how ISO Auto works for each of the exposure programs. These are my findings. Just like to share my finding with as I’m not 100% sure your description and my findings are the same. By the way, this is based on ISO Auto on a D40.

“P” PROGRAM:

Camera will set both speed and aperture searching for best combination based on build in EV/LV logic / matrix. When the camera  finds it is not possible within that boundary it will adjust the ISO value and SPEED value taking care of the selected MAX ISO value and MAX shutter time. When it is still not able to reach a good exposure within these boundaries, than it will adjust speed outside the MAX time selected in ISO AUTO.

 “A” PROGRAM:

Aperture set by user. Camera will first adjust shutterspeed till MAX speed is reach. When that is not enough than it will adjust ISO till MAX ISO is reach. If still not able to get a good exposure, the camera will adjust the shutterspeed beyond the set MAX shutterspeed in ISO AUTO.

 “S” PROGRAM

Speed set by user. Camera will adjust first ISO till it reach MAX ISO. If that is not enough than camera will adjust Aperture to max possible (limited by lens used). If that doesn’t work, than ISO will be adjusted beyond selected MAX ISO value.

 “M” PROGRAM:

User set both speed and aperture. This is a bit difficult to explain. Assume a scene is best exposured by 1/60, f5.6 and ISO 200. Suppose you want to “under exposure”. So you turn the wheel and it looks like nothing changes as the exposure meter remains in the middle. It looks like as something did change: the camera have changed it’s ISO setting. It will do this till it reach the selected MAX ISO value. Only than, the meter will start indicate an under exposure.

1. CONCLUSION: ISO AUTO in combination with “M” is not an ideal combination as you think you control the camera but there is a secret in the background which you have no control over other than switching it off. It is also dangerous when using AUTO ISO in combination with M and manual flash. Due to the variable ISO your flash settings will be incorrect.

It would be great when NIKON add an menu option allowing to switch ISO AUTO off when selecting "M". At least that would avoid a lot of issues around ISO AUTO.

Kind regards,

Hans

Good point about landscapes... made that mistake once! However, I do like to set the range that my D300 will auto-iso between. This is very useful. Of course, you have to remember to set it. And, my choices vary with what I am shooting. But, that's the price to pay for not shooting AUTO everything!!