I have a quick question about ISO settings on my D40X. I usually have it set to 100, the lowest setting. This could be a rookie move, I'm not sure if there are times when I should be pumping it up. I don't like that grainy look.
Many times when I take a picture and look back to review it the info will show that ISO was actually raised to 400 or 500, sometimes an odd number like 320. Usually the number will be shown in red when it has been raised by the camera.
Why is this, when I supposedly have restricted it to 100?
#1. "RE: "auto" ISO" In response to Reply # 0 Sun 04-Jan-09 11:29 PM by JFHughes08088
If ISO is set to auto, it will use the ISO you have set, unless you select a faster than needed shutter speed. In that instance, ISO will automatically bump up to give proper exposure using the shutter speed you have selected. With auto ISO selected, think of your it as "lowest as long as proper exposure can be achieved....", If you turn off auto ISO, you will then "lock" at your selection. With ISO "locked", you could underexpose if shutter is too fast.
I wouldn't worry about grain until you get into ISO 800 or more. Even them, D40 and D40X do as good job at mimizing grain at high ISO
The scenario you have described indicates that you have set Auto ISO 'ON'. When Auto ISO is set to 'ON', any ISO setting that you set 'manually' (100 in your case) will be used if the shooting condition allows it. For example, if you have set your shooting mode to 'Aperture Priority' (A Mode), the apperture you have set will be used. Let's say this is f5.6. The camera will set the shutter speed accordingly. When you set the ISO to Auto, you are also asked to set the minimum shutter speed when the Auto ISO will kick in. Say you have set the a minimum shutter speed of 1/60 seconds. When the lighting conditions demand a lower shutter speed than 1/60s, the camera will increase the ISO to the require level for correct exposure. Hope this is not as clear as muck?
It's true that the lower the ISO the lower the noise (aka graininess), but when the aperture is wide open and the shutter speed is slower than practicle, then ISO has to be raised to get the proper exposure. Auto ISO can be useful at times, like capturing a snapshot that may have been missed during manual adjustment, or underexposed, or blurred otherwise. It can also assure good exposure (within reason) in Manual mode when you want to use a specific aperture and shutter speed. Auto ISO can also select settings between the full stops, like jumping from ISO 200 to 320 instead of 400.
FYI, ISO Auto can set ISO in step 1/2 the size of your current step setting. So if you're set for the default 1/3EV step size then ISO Auto can set ISO in steps of 1/6EV. So, you can see ISO values that you would never be able to set directly.