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Old May 17, 2004, 10:26 AM   #1
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Does anyone know if there is a difference between an f stop on a digicam & an f stop on a 35mm slr? I am considering whether to buy a prosumer camera, but am I am a bit concerned as they only seem to go up to f8, which is'nt to good for depth of field!:-)
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Old May 17, 2004, 10:32 AM   #2
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Prosumer cameras have more depth of field at f8 than a 35mm at f16 because of the small sensor. The problem is blurring the background, not getting everything in focus.
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Old May 17, 2004, 12:18 PM   #3
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Glad to hear that. Do you know how they would equate?

Sorry about double entry. The first one wasn't filled in correctly & I didn't know how to cancel.
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Old May 17, 2004, 12:37 PM   #4
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Check out this thread:

http://www.stevesforums.com/forums/v...mp;forum_id=20

While it's in the Minolta forum it applies to any digicam.
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Old May 17, 2004, 1:10 PM   #5
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It isn't really the smaller sensor that increases the depth of field, it is the shorter focal length of the lens. Digicams advertise their lenses as 35mm equivalents, the real focal length is typically about one fifth that - the ratio depends on sensor size. So depth of field indrectly depends on sensor size, but the physics of it is described by the focal length. Search for "circle of confusion" if you are interested in figuring it out.

The smallest usable aperature is determined by the wavelength of the light you are using: when the aperature gets too small diffraction starts to dominate snd blurs the image. That is why large format cameras (long focal lengths) stop down to f/64 or f/128 while a 35mm is limted to f/16 or f/22 and digicams are limited to something like f/8.
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Old May 17, 2004, 3:01 PM   #6
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Maximum f stop (minimum aperture) is also dependent on the sensor density. There is a formula for determining the maximum theoretical f stop based on sensor density for digital cameras. Digital SLRs can have much smaller apertures. Some digicams limit themselves to f5.6.

That is because of the diffraction Bill refers to "infecting" adjacent sensors. The formula doesn't account for focal length, but that would seem to be a factor since some consumer digicams limit themselves to f7-8 at wide and increase to f11 at telephoto. If light enters the restriction at a smaller angle there will be less diffraction.
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Old May 18, 2004, 8:03 AM   #7
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:?Sounds all a bit complicated to me. I've got the general drift though, that is, digicams have hell of alot more depth of field. Cheers everybody. Barry.
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Old May 18, 2004, 12:16 PM   #8
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No, it really isn't complicated. If you want to be a good photographer, it's important to understand. You do understand what depth of field (focus) is, right?

The longer the focal length, the less depth of field. A 500 mm telephoto lens has little depth of field. An 18mm wideangle lens has great depth of field.

Depth of field also increases with distance. When you close-focus on a quarter, two millimeters makes a difference. Butif you take a picture ofa mountain, the whole scene is in focus.

Imagine that you want to take a close-up picture of a flower, with the valley and mountain behind it also in focus. This would require maximum depth of field. You'd want to use the widest possible lens with the smallest possible f-stop. Get it?

Back to digicams vs. 35 mm cameras. Most digicam sensors are smaller than a 35mm film negative. What's that mean? It means that if you use a lens from a film camera, only about 25% in the middle of the negative is used... as if you took a close-up picture. Your 50mm lens is now a telephoto, and a 28 mm lens is a "normal" 1:1 lens.

This is why digicams are designed with "shorter" lenses than 35mm film cameras.

In a certain sense, digicams have "more" depth of field. Throw away the idea of focal length for a minute and think of lenses as wide, normal and telephoto.

If you are shooting with a "normal" 1:1 lens (not wide nor tele), the digicam photo will have more depth of field than afilm camera. Why? Because the digicam photo will be taken using a 28 or 35 mm lens, while the film camera would shoot at 50 mm.

But the real bottom line is this:You want the sharpest pictures you can get. Most of the time, that means using a good balance of f-stop and shutter speed, holding the cameravery steady, and lock focus on the best possible spot.
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