[Trashcan]

Yesterday I picked up a Nikon D40 as an introduction to digital photography. Before that, I was using my Nikon N75 35mm. With 35mm, I noticed that a lot of professionals recommend Velvia (50 speed.) I always shot with 200-400, but I never noticed a difference due to my camera automatically adjusting exposure based on film speed.
So now that I have the flexibility to change my speed (ISO sensitivity) on the fly, what are the practical uses and effects of different speeds of film? I know what it _is_, but I'm looking for practical reasons for changing.

Anyone care to share some experience?

[Steve]

The main reason and advantage is that you can shoot in lower light conditions by selecting a higher ISO (at the expense of more noise).

[Trashcan]

So a slower speed (like Velvia) would produce a finer image?

[Steve]

Well Velvia is not a iso speed as such, it describes a film type (known for its vibrant colours and fine finish) but you are partly correct in saying that lower iso's will give you cleaner images (on the same camera). Not all cameras are the same though and some are much better at producing finer images at higher iso's than others are at lower iso settings.

In general you should select the lowest iso your camera offers that will still allow you to shoot and capture your intended shot using the settings you require. Its not always as simple as that and like everything compromises need to be made but that is the real basics of iso's/noise and digital cameras.

[ChrisRabior]

Originally Posted by Trashcan

So a slower speed (like Velvia) would produce a finer image?

Not necessarily. Think of ISO as the film's sensitivity to light.
Higher ISO = higher sensitivity = less light required for correct exposure

Your ISO should depend on your lighting conditions and your camera settings. For simplicity's sake, let's just say you're shooting at f/4.

If you're out in broad daylight, you should shoot at the lowest ISO you have. Yes, you can shoot at higher ISO, but the image will likely be more grainy. A shot in good lighting should easily put you at 1/125 or faster. The film doesn't need much light to expose properly. You're used to using 200 because 200 isn't all that grainy when used in daylight, and it allows for a respectable range of shutter speeds in most typical lighting.

Now consider a bar or concert venue. If you tried to take a shot with the same settings as you did out in full light, you'll get a totally underexposed image, or a shutter speed that is so long all you wind up with is a blur. By increasing the ISO, you increase the sensitivity to light, which shortens the amount of time the shutter would need to be open. Yes, the image is a bit more grainy, but it sure beats not having any image at all.

Higher ISO shots can usually be improved using noise reduction programs like Noise Ninja and Neat Image. I use Neat Image, and it's pretty effective.

I generally shoot in aperture priority with my Rebel XT. I'll set my aperture to whatever I want it to be for that particular subject, and based on what I'm shooting, adjust the ISO to get an acceptable shutter speed. For example:
If I'm shooting handheld, I try to get at least 1/125. ISO gets adjusted to allow for that.
If the camera's on a tripod and my subject is stationary, I'll put the ISO to 100 or 200 and let it take however long it needs.

[VinnyP]

Some Nikon cameras (not sure about the D40) have auto ISO. That means you can set a maximum ISO you are willing to go to and the slowest shutter speed you will tolerate and use any of the shooting modes. In shutter priority it will open up the lens then increase ISO to your maximum to keep the shutter speed you set. This can be very useful but noise goes up.

[JMitchell]

Originally Posted by ChrisRabior

If I'm shooting handheld, I try to get at least 1/125.

125?! Woah, I'm happy with 1/50!!!

[nichpr]

Simply put - with digital, ISO becomes another variable you can change on the fly to optimize your exposure in changing situations. With your prior camera, changing film (and ISO) on the fly was just impractical.

- Paul