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7 messages in edu.ku.nhm.mailman.taxacomdigital images| From | Sent On | Attachments |
|---|---|---|
| Timothy Rowe | Aug 22, 1995 12:52 pm | |
| Alan Harvey | Aug 23, 1995 9:25 am | |
| Peter Rauch | Aug 23, 1995 9:26 am | |
| Jim Manhart | Aug 23, 1995 9:26 am | |
| Joseph Laferriere | Aug 23, 1995 10:09 am | |
| Joseph Laferriere | Aug 23, 1995 1:27 pm | |
| Alan Harvey | Aug 23, 1995 2:56 pm |
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| Subject: | digital images | Actions... |
|---|---|---|
| From: | Alan Harvey (ahar...@AMNH.ORG) | |
| Date: | Aug 23, 1995 9:25:29 am | |
| List: | edu.ku.nhm.mailman.taxacom | |
Timothy Rowe wrote:
The major quality difference that I have encountered between video and film is depth of field. With film, one can shoot focused images of objects that have great depth, by closing down the camera aperture and increase the exposure time. That is, film offers the capacity to vary the depth of field in focus when making a photo. Video has nothing comparable to this - there is only a shallow, invariable depth of field. If you are imaging a deep object, like the occlusal surface of a mammalian tooth crown, only part of the crown will be in focus in a video image. Even using high-resolution frame grabbers (video _per se_ is limited to about 500 lines of resolution, whereas high-resolution grabbers can exceed 2000 lines for any given image field) there is no way that I have encountered to gain depth of field. Moreover, with the higher resolution frame grabbers, as in higher microscopal magnifications, the small depth of the field provided by the instrument decreases compared to the depth of (low-resolution) video imagery. Digital processing of a blurred image is a labor-intensive and ultimately a poor substitute for the depth of field offered by film. . . .
Actually, depth of field is not a property of the recording medium (i.e., film vs. video); it depends on the lens that is in between the object and the medium. Aperture controls are not restricted to camera lenses; I have two video camera lenses with aperture controls, and I suspect this is standard. It is true that most microscopes, or at least dissecting scopes, do not have built-in aperture controls, but you can get video-microscope adapters (which you need anyways to attach the video camera to the scope) with this function, and they can dramatically increase the depth of field of a digital image (to make sure, I just tested this by capturing two images to a Macintosh using a Hitachi KP-M1U CCD camera attached to a Wild M-5 dissecting scope; on my Mac screen, the closed aperture image has a much greater depth of field than the open aperture image, as expected).
I wonder if Tim's "depth of field" problems are in fact resolution problems. Video is limited to about 700 lines of resolution; files of individual video images typically take only about 300 K memory, whereas a scanned image (e.g., of a photograph) of the same image can take, well, far more than 10 M. Thus, video images contain only a small (tiny?) fraction of the information contained in film images, and neither aperture control nor the highest resolution frame grabber will be able to change this discrepancy.
Cheers,
Alan
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Alan W. Harvey (aharvey at amnh.org) Assistant Curator of Invertebrates American Museum of Natural History Central Park West at 79th Street New York, NY 10024 (212) 769-5638