>Geoffrey V. Brown wrote:
>>
>> Well, I got my pinhole pictures back, and the exposures were almost dead
>> perfect.
>> Sharpness, on the other hand, left something to be desired. The
>> material that I made the pinhole in was about 1mm thick. I think that
>> this might have caused some reflections and so forth, but I'm not sure.
>> Can anyone offer suggestions for improving the sharpness of the image?
>> Does it have to do with the pinhole size, or something else?
>
>Most likely, also, camera movement, it probably was a time exposure,
>certainly longer than 1/15th sec. YOu could have jiggled the box just
>slightly taking the lens cover off, or some other not noticable movement
>during the exposure.
Doing pinhole photos is more than just making a pinhole. sorry about that.
First, the size of the image on the film will increase as the film-to-pinhole
distance increases. Considering the pinhole as the lens, the image size on film
increases as the lens is moved away from the film.
Second is the basic operation of a pinhole respective to physics. Pinholes
transmit a very narrow beam of light rather than a single ray of light. The
beam has the same diameter as the pinhole. Because the light is a beam, there
is an optimum diameter of the pinhole. If the diameter is too small, you will
get diffraction since the very narrow beam with spread out. Conversely, if the
pinhole is too large, the beam is too large which significantly reduces
sharpness.
The optimum pinhole diameter is computed based on the distance of the
pinhole-to-film distance. The equation I have used is:
D=SQRT(d)/141 for computing the diameter in inches....or
D=SQRT(d)/28 for diameter in mm.
The equivalent aperture f number will be given by:
f=d/D
where D=pinhole diameter
d= distance of pinhole-to-film
f=equivalent f-stop number
A good example is for d=8 inches (I buy pinholes chemically milled to precise
diameters and then mount them on my Linhof flat boards).
At 8 inches, the correct pinhole diameter is 0.02 inch and yields and equivalent
f number of f400.
Computing the f number is important to be able to determine exposure. Take a
reading with a regular meter. Then, you multiply the exposure time by 4 for
each doubling of the f number until the indicated pinhole f number is achieved.
What you will find is that the exposure times are rather long--bringing into
play reciprocity correction. But that is another matter.
Hope this helps.
Now, the fact that the f number is so large, leads to the ability of having darn
near everything in focus...from close up to infinity.
Gary Gaugler
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