5 by 4 Pinhole

[Norman Hart]

Hi.
Can anyone inform me if it is possible to get full coverage on 5 by 4
if you only have a focal length of only two centimeters.I am in the process of
making a pinhole camera which will be using fidelity dark slides. I think
I may need to increase the focal length.

Thanks Norman :confused:

 

[reellis67]

There is a good online calculator at (http://www.mrpinhole.com) - just type in what you want and it will tell you what the optimal values are and what coverage you can expect. You can also download a nice utility here (http://www.pinhole.cz/en/pinholedesigner/)

- Randy

 

[Norman Hart]

Thanks Randy

With the info provided at mrpinhole.com I will certainly not get the coverage .It confirms that I will have to up the focal length of my project and settle for not such a wide view from the camera. It does make me wonder why the Zero 75 camera claims to get full coverage with only one section at 25 mm focal length though.

Norman

 

[Ole]

"Coverage" is a relative term. If your hole is thin enough, you will have "coverage" is that you can get an image with acceptable "sharpness" all the way into the corners. However the light loss in the corners with a 20mm "focal length" on 4x5" film will be tremendous! That's a 75 degree angle of incidence - assuming cos^4(theta), the light loss is about eight stops. This is assuming your hole material is thin enough that the edges don't overlap at that shallow an angle.

So it may be sharp enough to call it "coverage" - just a pity there won't be enough light to make an image...

 

[smieglitz]

Here is a pic taken with a camera of focal length ~25mm onto a 120 roll film holder. The camera was a 3/4" thick piece of plywood with a Graflock back attached to it making the focal length approximately 1" or 25 mm. You can see the vignetting in the corners already with this film.

To see the coverage of this camera on a full 4x5 sheet (Fuji Acros Quickload) check out the following URL. Be forewarned the image is a nude:

http://my.net-link.net/~jsmigiel/images/pinhole/woodpile01.72.jpg

Joe

 

[Dave Wooten]

5 cm will cover (50 mm) @ f/154 -- covers 4 x 5 nicely...and is quite wide...

figure 3.5 x's focal length for diagonal coverage....i.e. 25 mm x 3.5 = 87.5 mm diagonal,,,subtract the vignetting on Joe's photos above and it should be pretty close...(nice photos Joe)

 

[Jim Jones]

In addition to the cos^4(theta) fall-of in the corners that Ole mentioned, there will be reduced sharpness due to astigmatism. Yes, wide angle pinhole cameras do suffer from astigmatism, and all pinhole cameras have slight chromatic abberation, too. A center filter (expensive!) like those used on wide angle lenses might help. Another possibility is photographing an evenly illuminated blank white subject with enough exposure to place the corners of the film on the toe of the characteristic curve and develop it to a contrast index of about 1. Either sandwich this negative in front of the unexposed film in the film holder, or mount it in front of the holder. This should fairly well compensate for the vignetting, but will increase exposure time considerably. Undercompensation is probably desirable. Another option is prefogging the corners of the negatives slightly to increase response to the subject in the corners. This should help the corner shadows at the cost of the highlights.

 

[Greg_E]

Couldn't you make up for the loss of light be curving the film? Then the distance would be the same for all parts. I would think that the curve would be based on the distance between the center of the pinhole to the film plane at center of film. Then draw a sphere around the pinhole with that radius. Obviously this works better for wide strips of roll film than it will for 4x5, since you can almost ignore the verticle component and just deal with a curve in the horizontal component.

The exposed (developed) white frame is an interesting idea, I would have never thought of that. If you made it small enough, could you put in in front of the "lens" to get the same effect? Then it would be just like any other graduated filter. Yes building it that much smaller would be more difficult, but it would be really handy to add or remove.

 

[Norman Hart]

Decision made 5 by 4

Thanks Guys
It is nice to know that there is all knowledge and advice is out there to be taped.I have decided to stick with the 5 by 4 format and settle for a
focal length of four inches this should give me around 60 degree angle of view
with a pinhole of .018 thou.I am going to stick to the dark slide holders
for convenience.If this all works out I will try to post some pictures.I also like Gregs suggestion about curving the film plane. Perhaps a mark 2 project
may be attempted later. Who knows I may need further advice.

Rgards Norman

 

[Jim Jones]

Greg -- Curving the film nearly eliminates the fall-off due to the corners of the film being further from the pinhole as it is with flat film. However, the light passes obliquely through the pinhole to reach the corners. Thus, the pinhole is effectively an ellipse with a minor axis less than the optimum diminsion.