changing ASA

[jasonnet]

I notice on my digital camera, it lets me change ASA/ISO values. 400, 200, 100, auto, etc. I understand what that means in a film camera, but I'm a unsure of why this is adjustable rather than set to the maximum value supported. On film the tradeoff is grainness vs sensitivity. But I assume that on a digital camera, the graininess is a constant and one should set the ASA/ISO value to a maximum sensitivity so that you can go with the fastest shutter and smallest aperture (for depth of focus) possible. Please explain why this is adjustable.

Thanks :-)

[Matt_C]

ISO on a digital works in the same way as on a film camera. The higher the ISO you use then the more grain and noise you will introduce into the picture.

In other words on a digital camera the lower the ISO you use then the better quality the picture will be with less noise and grain.

Cheers

Matt

[jasonnet]

Thanks Matt. Why does the grain vary if the number of pixels remains the same? Is this truly a problem at all ASA levels or just the more extreme one's on a given camera?

[Matt_C]

All depends on the camera as some models handle noise better at higher ISO. For instance the Canon 20D has superb noise handling and can be used at very high ISO without any serious noise issues. Most compacts though get serious noise problems at anything above 100ISO (Although I haven't used any of the newer compacts for some time now) DSLRs handle noise much better generally because the sensor size they use is much bigger. The reasons why this is so can get a bit technical. In short I would just get to know your individual camera and how it copes with higher ISOs. If noise is a problem then you can download some free noise removal software at

www.neatimage.com

which works very well.

Matt