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Old Nov 21, 2004, 11:03 PM   #1
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Coming from a film camera background, I'm learning more about digital cameras.

Manual digitals have the same shutter speeds as a film camera, but I haven't seen the f stops. For example, the f-stops on Digital Rebel is at f/4.5-5.6. f-stops on film cameras go f2.8, f4, f5.6, f8, f11, f16....f32. How do you manually control f-stops on a digital?

Also, on the Digital Rebel, one of the exposure modes is listed as Depth-of-Field AE. AE, I take is Automatic Exposure. How exactly does it let you control the depth of field? For example, I want to take a picture of a single tree from about 20 feet away, with nothing around, and want the background to be out of focus.

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Old Nov 21, 2004, 11:12 PM   #2
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You are reading the information on the lens. f/4.5-5.6 means the largest aperture at the smallest focal length is 4.5 and at maximum telephoto, the largest aperture is 5.6. Neither of those numbers is very good. The actual f-stops are controled by the camera.

On the specs for a lens, the smaller the number the better. If there is only one number (e.g. f2.8) it means that the largest aperture is f2.8 at all zoom settings. A lens of this type is more expensive. Typically, f2.8 is the smallest number you will find unless you want to spend a LOT of money.

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Old Nov 21, 2004, 11:53 PM   #3
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As "calr" said the F4.5-5.6 you mentioned is the maximum aperture of a zoom lens. It's most likely the Canon EF 55-200mm telephoto zoom. I could not find the minimum apertures of that lens but as an example the Canon EF 18-55mm F3.5-5.6 has a minimum aperture of F22-36. You canselect F stops witha digital camera just as youcan with a filmcamera (inaperture priority or manual modes of course).
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Old Nov 22, 2004, 12:40 AM   #4
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Thanks for the responses.

That reading came off a Digital Rebel. I'm considering getting one. Do you know if it comes withan AC adapter? Also, how does the Depth-of-Field AE on it work?
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Old Nov 22, 2004, 1:34 AM   #5
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Why don't wesee aperturessmaller thanabout f8 on non-DSLR digital cameras?
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Old Nov 22, 2004, 9:52 AM   #6
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I would be shocked if it comes with a AC adapter. It will come with a battery charger, though. You could check out steve's review:
And see what he lists for standard equipment

My guess is that the DOF AE tries to maximize the aperture without droping the shutter too low for that focal length. I don't know if it adjusts ISO or not (that could work well or really badly... depends on where the cap is.)

Actually, this is an interesting/non-trivial question.
Non DSLR almost always have a really small imaging sensor. To match this, they fit them with really small lenses (why waste resources making a bigger one that is wasted?)

This combination creates a much larger depth-of-field than an 35mm lens does with the same specifications. There is really only one reason to have a small aperture. To control DOF. But those cameras already have a large DOF, so what is the point?

The answer to that is macro, I guess. Even with their large DOF, for Macro you still probably want a smaller aperture. I think this is part of the reason why some cameras have a "macro" mode. Maybe it plays tricks with the lenses to get even more DOF (and adjust focusing distance.)

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Old Nov 22, 2004, 10:31 AM   #7
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EOS RT wrote:
Why don't we see apertures smaller than about f8 on non-DSLR digital cameras?
IMO this is more of a practical reason more than anything.

The f-stop is the ratio of the focal lenght over the effective diameter of the lens, and since the diameter on theses non-dSLR lenses are already small: How small do we want theses pin-holes to be?
-> There's a mechanical limit and tolerance on how to make theses apertures infinetely smaller (also increasing the cost in the precision required for almost negligible gain in DOF)

F/45 is available on some Sigma dSLR macro, but it's rare!
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