Pinholeconstuctions

[Soeren]

I was wondering. Which "focallenght" would be the most appropiate for a
13X18 pinholecamera? It seems they tend to be made wery wide, correct?
And how to attach the 13X18 filmholders? What should they rest on, hard rubber, foam or...........?
Kind regards
Søren

 

[Jim Jones]

There are at least two reasons why pinhole cameras often cover wide angles. One is their low cost over good wide angle lenses. Another is the improved sharpness over normal or telephoto pinholes. The blur in pinhole images is proportional to the pinhole diameter. The optimum pinhole diameter becomes smaller as the focal length becomes shorter.

 

[Joe VanCleave]

I've found in my pinhole work that my cameras with a longer projection length will typically have sharper images, and more even exposure from center to corner. The latter effect I attribute to the more equal center and corner projection distances with longer lengths; the former effect I can only speculate as to why.

I must mention that I contact print almost exclusively, so for equal angles of view the longer projection length will necessitate a larger film format. But I have several 4x5 cameras of various projection lengths, and the resulting negatives are usually sharper in the longer cameras.

I try to make my pinholes close to the 'optimal' pinhole diameter for the projection length in use; I've noticed while running through these calculations or tables that the longer projection lengths will always end up with larger 'f-numbers' than will shorter projection lengths.

So, although the pinhole diameter is larger for a longer camera than a shorter one, if the pinhole size is scaled to the resulting negative size (with equal angles of view) then I would argue the larger 'f-ratio' of the longer camera more than makes up for the smaller aperture of the shorter camera. If one viewed the resulting prints from these comparison cameras at equal print size, one would also have to take into account loss of sharpness from enlargement.

So one could argue it would be better to use a shorter projection length camera with a wider angle of view, to achieve a larger negative size. But you can't get something for nothing. Now you have problems with off axis light falloff (cosine^4 function) and also the effect focal ratio changes from center to corner, since the corners are 'seeing' an oval aperture rather than circular. Also, whatever detail is present in the wider angle image is now smaller in size verses the longer camera.

Soren, the 'right' projection length for your camera really is a subjective decision, regardless of sharpness issues. Not every scene works equally well at one angle of view. A wide angle of view camera (i.e. short projection length) requires compositions that stress detail or objects in the near and foreground and render far-distant objects much smaller , whereas longer projection length, narrower angle of view cameras will tend to compress distances and render distant objects more equal in size with comparable foreground objects. And the effects of light falloff with wide angle pinhole cameras are also subjective issues that can be exploited for creative purposes.

It's really about what looks right to you. And sharpness, although important for some folks, isn't necessarily the only consideration with pinhole. A well crafted print can look very nice, regardless of sharpness considerations. Some subject matter look better with a softer rendering. It's all up to your creative control. Have fun.

 

[Soeren]

Thanks Joe and Jim for your answers.
The main reason for not making the Pinhole smaller than the "optimum" should be refraction, right?
I was thinking about a 140mm projection length which should give an optimum dia of about 0.5mm. Would a 0.25mm hole show more refraction and be less sharp? the 140mm is somewhat wide but not extreme so I should be able to use it for peopleshots?
Then there is the Back/filmholder issue. I was planning on letting the filmholder rest on some hard rubberstrips but that may not be lightproof so I would like to see some ideas on DIY backs
(I'm planning a sliding box camera with lens too)
Kind regards
Søren

 

[Joe VanCleave]

Regarding film backs, many of my cameras are just one-shot boxes, loaded in the darkroom ahead of time. Or, if small, you can change out the film in a changing bag, while out in the field. I'm only now making a homemade camera that takes conventional film holders. I've also used a Speed Graphic 4x5 with homemade pinhole lensboard. There's lots of options and possibilities here. Many of my cameras have solved the problem of multiple shots by using a falling plate design. I've also cut 8x10 paper into 4" wide strips, taped them together into a long roll of paper and loaded them into a homemade camera that acts like a larger version of a rollfilm camera. The first Kodak cameras operated similar to this, using a roll of paper as fiilm.

Some people make a larger number of small cameras, like soda can cameras or matchbox or film cannister cameras, and take a whole bag full of these preloaded on their photo expeditions.

Keep in mind that with pinhole it is not important nor necessary to maintain a tight tolerance on film plane position relative to the lens. You just don't want the film actually flopping around or moving during the actual exposure. So a loop of painter's masking tape on the back of a negative is often sufficient to keep it in place.

Now, as for your idea of a smaller pinhole aperture, I've found two applications where smaller than the calculated value is actually better:

1) When using paper negatives or other orthochromatic media (like APHS graphic arts film). Any of these emulsions are blue and UV sensitive, so if you changed the wavelength in the standard formulae to around 450-475 nm (where many ortho media have their double peak of sensitivity), you'll get a smaller recommended pinhole size.

2) When the camera will be primarily used for close-up photography, for instance when photographing diorama scenes. Keep in mind that almost all pinhole formulae are assumed to be working with objects at infinity (i.e. parallel light rays); with close-up objects the loss of sharpness by diverging rays is more dominant than diffraction effects. Ignoring diffraction for a moment, but simple ray-tracing can show that if the subject being photographed is at the same distance in front of the pinhole as the film is behind it, then the blurr effect is double the size of the pinhole itself! You can easily get by with a pinhole half the size of what a formulae may recommend simply by shooting close-up objects.

Combining the above two situations, where you are shooting ortho media and close-up subjects, one can certainly use a pinhole size much smaller than what standard formulae recommend, with excellent image quality.

Good luck and post some results.

~Joe