Pinhole advice needed.

[Grandpa Ron]

I have just started using a pinhole camera. Attached is a picture of a covered bridge from my first roll. I am trying to wrap my head around what I am seeing. The camera set up is,

• 35mm SLR with lens removed.
• The pinhole is .68 mm and is supported on black tubing for a focal length of 124 mm.
• This should give me an f 180 aperture.
• The film is Fuji color 200 ASA

The Covered Bridge One photo seems to have a bright spot in the middle but the edges do not seem to be over exposed. This bright area appears in all the shots. The second photo is a Road Bridge and it also has the bright spot but again the edges do not appear to be over exposed.

From what I am seeing, the 124 mm focal length is giving me a telephoto lens with the 35 mm format and I believe the lack of sharpness may be due to the fact that the pin hole dimpled when I made it; effectively increasing the pinhole plate thickness.

However came anyone explain the bright spot in the center? I do not believe it is a light leak.

Any suggestion appreciated.

 

[MattKing]

How did you determine the size of your pinhole?
What material is the pinhole made in, and how thick is it?

 

[Grandpa Ron]

This was the first pinhole I made, I used a needle that measure .0275" (.7 mm) on my micrometer and it just slipped through the hole I made in a .004" (.10 mm) aluminum can. Unfortunately I did not sand the dimple from the opposite side. Since then I have acquired a drill set that goes to .010" (.25mm) and I have learned to sand the aluminum from both sides down to about .003". I use a 6x jeweler's glass to check roundness.

I am sure a laser cut is more exact but I can make one of this in about 15 minutes.

Right now my biggest issue is exposure, I am using a light meter at f 22 and doubling the time for each full f stop. They seem a bit under exposed to me but my main concern is the bright spot in the center.

 

[DWThomas]

A few of random thoughts ...

* 0.68mm might be on the large side for 124mm "focal length." Pinhole Designer comes up with numbers around 0.40 to 0.50mm, depending on the magic constant used.

* I am wondering what your black tubing is made of? If it has a glossy surface on the inside, perhaps internal flare is over exposing the mid-section. Lining it with some felt or black construction paper rolled and placed around the inside of the tube might help with that.

* I am assuming here that the tubing used is relatively large so that there is a clear sight line from the pinhole to the periphery of the active film frame. Some SLR lenses use retro-focus and other tricks on the back end of the lens which can allow the clear area of the lens mount to be relatively small. On some of my home brew cameras I've been amazed how what looked like a large clearance hole actually clipped the corners of the image, albeit this was with 90º or so angle of view.

* It is also possible a mix of the pinhole plate thickness and the upset from dimpling might cause some vignetting. A really thick plate several times the pinhole diameter results in a sort of cylindrical bore that can produce really significant vignetting almost like looking through a tunnel.

* I'm not sure if any trickery was used in supplying your image, but I think if anything, it's over exposed. I tried downloading one of them and doing a bit of PhotoShoppery and thought it looked better cranked down a little bit. It is pretty normal for pinhole shots to have some light fall-off toward the edges, although it is normally more obvious with wide angles of coverage. Your numbers have a relatively narrow angle of view, so fall-off should be modest.

Ah well, the experimentation is part of the fun!

 

[NedL]

Agree with Dave. At first I thought this might be normal -- pinhole can be a lot brighter in the middle, but that happens more with a short "focal length" ( wider angle of view ). With 124mm onto a 35mm frame, I wouldn't expect to see much falloff. I think some kind of flaring is a good guess.

 

[Grandpa Ron]

The tube is black but I took the advice and lined it with a dark cloth. There was no trickery I simply down loaded the images from my Dark Room account, until my negatives come and I can see the exposure directly. Even around the edges the colors in the photo seem washed out so I am guessing they are over exposed.

I also built a 77 mm tube with an f 216 aperture that I am going to try shortly.

 

[Jim Jones]

For many of us, lack of sharpness is a problem when using 35mm for pinhole photography. The blur in images taken with an optimal size pinhole is directly proportional to the diameter of the pinhole. The shorter the focal length, the smaller the pinhole. Therefore, wide angle pinholes provide sharper images. The shortest focal length I ever managed on 35mm film was 16mm. The pinhole diameter was 0.005", and contact prints looked sharp, but who wants to look at 35mm contact prints? Unfortunately, I can't find any of the few images taken with that camera.

 

[Nick Dvoracek]

For many of us, lack of sharpness is a problem when using 35mm for pinhole photography. The blur in images taken with an optimal size pinhole is directly proportional to the diameter of the pinhole. The shorter the focal length, the smaller the pinhole. Therefore, wide angle pinholes provide sharper images. The shortest focal length I ever managed on 35mm film was 16mm. The pinhole diameter was 0.005", and contact prints looked sharp, but who wants to look at 35mm contact prints? Unfortunately, I can't find any of the few images taken with that camera.

OK, here we go again. Pinhole doesn't work with 35mm. I do it all the time. See pinholica.blogspot.com and look at any post titled Roadtrip, or search for the label 35mm. I mostly use a .15mm pinhole on a 24mm camera, but I've gotten good results with 35 and 50mm cameras. I've exhibited 9x12 inch prints, and in one case, a 2x3 foot print.

Grandpa Ron, your pinhole is way too big. Try one about half that size. They're a little tricky to drill but not that bad.