Chuck Norcutt wrote:
> See Wikipedia on selection of optimal pin hole size
> <http://en.wikipedia.org/wiki/Pinhole_camera#Selection_of_pinhole_size>
>
I think you misunderstood what I was trying to say. I already have the
formula, in a slightly different form, but mathematically the same.
In any case, I wasn't asking about the formula. I was commenting on the
possibility that focal distance makes a difference in image qualities.
As a simple example:
- Assume a perfect film, such that enlargement doesn't result in more
grain from the smaller format.
- Take a shot on 35mm FF with a .32mm pinhole at 50mm.
- Take another shot of the same subject with 4x5 film, .63mm pinhole at
200mm.
- Enlarge both to 8x10 prints.
The question I was trying to ask is; will there be significant
differences in the prints? Will one be sharper than the other? Will they
appear different in other ways, tonal smoothness and differentiation,
bokeh, etc.?
None of the references I've come across, and I admit to not having
looked hard, have addressed anything more than what the optimum pinhole
diameter is for any given focal distance.
Again, I assume from the fact that serious pinholers seem to gravitate
toward larger film/paper formats, that there may well be such
differences, favoring larger film sizes and greater focal distances.
Certainly, it makes sense.
Determination of the optimum opening is going to be a trade-off between
the effects of a smaller hole in increasing angular differentiation of
subject detail while increasing diffraction effects. I'm not sure the
term Circle of Confusion applies, but the same underlying principle of
calculation based on the smallest angular difference that the average
eye can discern at a given image size and viewing distance would apply.
As I understand the operation of a pinhole, the very small opening means
that any given small area on the film can only 'see' a very small
portion of the subject. As the distance from the hole increases, the
projected image gets larger. thus the subject area captured by any fixed
area of the film gets smaller. Thus, a smaller film size requires a
smaller hole for the same resolution at a given print size than is
required for a larger film further from the hole.
If there were no diffraction effect, the calculation of appropriate hole
size would be straightforward. However, diffraction effects increase as
the absolute size of the hole decreases. There is always diffraction at
the edges of the aperture. Since diameter decreases linearly with
radius, while area is a square function, the portion of the light that
passes through that is affected by diffraction goes up as the hole gets
smaller.
So there are two factors affecting sharpness as hole size changes, and
they work in opposite directions. To the extent that their effects on
image qualities are different, the examples above will have different
image appearances.
All of this assumes a perfectly round, smooth hole in an infinitely thin
material. To the extent that the hole is a cylindrical hole in a sheet
thick enough as to be significant relative to the wavelengths of the
light, it will also affect image qualities. First, there are then two
diffraction edges. Depending on the surface qualities of the interior of
the hole, all kinds of strange things could happen. With a perfectly
drilled laser hole, some interesting reflections could be going on.
I suppose that's why I've read of a technique that abrades the metal
around the intended hole location as thin as possible. I seem to reacll
someone using that technique to create the hole itself, so the edge
would be only a few atoms thick. With the material falling away from a
very thin layer with the hole in it, there would be only the single
diffraction going on.
Moose
> Chuck Norcutt
>
> Moose wrote:
>
>> ..... It is possible that the optimum aperture for a given focal
>> distance, although the sharpest possible for that focal distance,
>> might be more or less sharp than the result with a different focal
>> distance with its optimum aperture.
>> Given the general impression that pinhole is better with bigger
>> sensor/film/paper sizes, it may be the case.
>>
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|