Is there a way to compare the megapixel rating on digital cameras to the ASA film speed rating on my SLR camera. i'm comparing the depth of field on a digital camera and a SLR camera, and I need to know how to compare. My digital camera has a 1-megapixel resolution.

-- Matt majarian (matt_majarian@bigfoot.com), January 09, 2000

Answers

Matt:
I don't think this is quite the comparison you want. ASA, or DIN or ISO is a rating of how sensitive the film is to light. While the sensitivity does have a bearing on which apeture is used - and thus effects Depth-of-Field - the correlation is indirect and subject to many variables (such as shutter speed, amount of light, etc). There are also variables introduced by digital cameras that need to be accounted for.
Another aspect of your question is that there is a correlation between resolution and film speed. While faster films typically have lower resolution - it's another correlation through indirection. Faster films have larger silver halide crystals and make granier photographs. This doesn't correlate in the digital world. More pixels doesn't translate to more or less sensitivity to light
The image area of a digital camera is smaller than the image area of a 35mm camera. It's roughly 2/3rd's of a 35mm frame. This means a 50mm lens for a 35mm camera would actually be a telephoto lens on a digital camera. Technology doesn't currently provide a way to manufacture a reliable CCD that is as big as a 35mm frame. The smaller CCD also means you need less glass to make an f2 lens on a digital camera then for a 35mm camera. Less glass means more depth of field.
Focal length also effect Depth-of-Field. The longer a lens the shallower the DOF. Now if you really want me to confuse the issue - digital zoom does not change depth of field - nor does it offer the perspective compression of telephoto lenses - but let's put that discussion on the shelf for now...
All this means that an identically composed and exposed shots done with a digital and a 35mm camera will be different. The digital camera will have much greater DOF then it's analog counterpart. This is viewed in differing ways by people. Those who loath focusing like this. Professional photographers who know how to use DOF to isolate and control their composition get frustrated.
But there are things you can do that help you take control. An all auto-focus camera can still give you some DOF control if you have manual exposure settings. For less DOF choose faster shutter speeds and lower f numbers (i.e. f2 will give you less DOF than f8 - it's the f stop that effects DOF not shutter speed - but to use a lower f number you must use a faster shutter speed to achieve the same exposure). Use a longer focal length (zoom in) since longer focal lengths will have less DOF. If your zoom is all digital then it will have no effect on DOF - it will, however, reduce your resolution and make your pictures less sharp. If you want more DOF then do the opposite of the advice above.
I hope that helps.
Remember:
...if you're not confused, then you don't understand!

Des

-- Dan Desjardins (dan.desjardins@avstarnews.com), January 09, 2000.