Changing ISO

[990284]

Is their a general guidline for changing your iso example: portraits SHould that be a different iso than taking a picture of a landscaping. I am shooting with a nikon d40

[sk66]

General rule, keep it as low as possible (not below native). BUT, sacrifice ISO speed first.

[dcclark]

Yes -- ISO should generally be kept low. Your D40's lowest ISO is 200, I'd leave it set there. It also has a nice Auto ISO feature, with some parameters that you can set. I keep mine set to a maximum ISO of 800, and a minimum shutter speed of 1/30 -- that will keep your shutter speed manageable in most cases, while avoiding excess noise from a high ISO.

[inkista]

It's not so much what you're shooting as what your goal is and what tradeoffs you're willing to make.

Typically, landscapes use a very low iso setting, because you typically shoot with a tripod and a small aperture to have a large depth of field, and you can do a longer exposure.

But if you're handholding at sunset, this may change.

If you're handholding and shooting a basketball game in a gym without a flash, it changes even more.

The main tradeoff you'll generallly make is to increase the iso and deal with noise to achieve a shutter speed fast enough to "freeze the action" or to eliminate camera shake blur. (Remember, that when handholding, the rule of thumb for an unstabilized lens is to use a shutter speed of 1/focal_length or longer. If you're shooting with a 200mm lens, you have to be at 1/200s or faster).

If you're "forced" to use a high iso (say, 800 or higher), you want to be sure that you're exposing correctly. Noise appears more in darker portions of the frame. Underexposing can often cause more noise than boosting the ISO setting. And this is the key to understanding the difference between "push" and "pull" processing--which you can do in post or that's (sometimes) done in-camera.

"Push" processing is a way to get a higher iso rating. You underexpose the photo, and then adjust the exposure in post-processing. This is what the camera does when it goes for those special processed high iso settings (typically above 3200). The problem is that pushing an underexposure often increases the noise in an image.

The opposite, "pull" processing, is to overexpose and adjust the exposure downwards in post. Obviously, you don't want to blow out your highlights, or you won't have a lot of detail to bring back (even in RAW), but you can still be "to the right" of where you normally would have been. When you do this, you may actually reduce noise. And if you're a crazy math/photo nut, you do the HDR thing, and use a +4EV overexposure to reduce the noise via software.

If you're using partial-stop ISO settings, your camera may be doing it by digital push/pull processing. So, depending on whether you're a 1/3 stop above or below, you could be adding/reducing noise compared to the full-stop settings. Whether above or below is better/worse (or whether it's an issue at all--some cameras do all settings by gain across the sensor, not processing) depends on which model of camera you have. If you don't know, it's safest to stick to full-stop ISO settings.

A lot of noise can be dealt with in processing (go google 'NeatImage' and 'NoiseNinja') which is why ISO is typically the first setting you may "sacrifice" for the exposure triangle, since you typically choose an aperture or shutter speed for a specific effect.