digital images

[Alan Harvey]

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