digital images

Alan Harvey aharvey at AMNH.ORG
Wed Aug 23 09:25:48 CDT 1995

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




Alan W. Harvey (aharvey at
Assistant Curator of Invertebrates
American Museum of Natural History
Central Park West at 79th Street
New York, NY 10024
(212) 769-5638

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