Loss of contrast with 4x10" pinhole camera

[Greg]

Have been using a 4x10" PHOTONBOX Panoramic Pinhole camera Model D. Posted my experiences in the forum:
http://www.largeformatphotography.info/forum/showthread.php?132753

Twice developed negatives that seemingly had a high Base plus Fog and resulting contrast was low. Both times I experienced this, I was using outdated Bergger 200 film so attributed it to that. But then yesterday shot a scene using fresh HP5 and experienced again a seemingly high Base plus Fog and resulting low contrast. After sitting down and looking at the negatives, realized that all three negatives were taken with the sun directly hitting the pinhole. The pinhole aperture is in a very thin round disk of Stainless Steel so I figure that the sun's rays bouncing off the sides of the pinhole must be causing the "seemingly high B&F". Anyone have any ideas of how to blacken the sides of the pinhole?

Tried using a black Sharpie to "darken" the the sides of pinholes in aluminum and stainless steel but size of the pinholes were majorly decreased in diameter and the black ink of the Sharpie dries with a gloss sheen that still reflects sunlight. Or should I just replace the pinhole with one made in aluminum or brass and research a way to chemically blackening the metal?

[Bruce Watson]

But then yesterday shot a scene using fresh HP5 and experienced again a seemingly high Base plus Fog and resulting low contrast. After sitting down and looking at the negatives, realized that all three negatives were taken with the sun directly hitting the pinhole.
...
Or should I just replace the pinhole with one made in aluminum or brass and research a way to chemically blackening the metal?

Probably not the sides of the pinhole, but the fact that the inside of the box is being lit because of light coming through the pinhole at an angle. If that's true, all you need is a lens shade. And it's easy enough to test -- shade the pinhole once and see if the problem goes away. If it's a long exposure, tape a piece of cardboard in place to shade it.

OTOH, if you really want to blacken the pinhole, for aluminum you can black anodize, for steel (and a collection of other surfaces) you can use black oxide coating. I don't know how much smaller these will make the diameter of the pinhole.

[Jim Jones]

It's natural for an ideally sized pinhole to have some reduced contrast due to light being scattered through diffraction. Darkening the pinhole either chemically or with a felt-tip pen will help reduce reflections. I've also slightly etched brass pinholes to reduce brightly reflective surfaces. Fortunately, very precise sun shades are easy to improvise, and are very compact.

[SMBooth]

Im not sure what the insides of these cameras are like but you may want to use black telescope flock instead of matt black paint to reduce light bouncing around.

[Jac@stafford.net]

Im not sure what the insides of these cameras are like but you may want to use black telescope flock instead of matt black paint to reduce light bouncing around.

Right. Edmunds' black flocking paper is super effective.

[Greg]

Top and bottom of the inside of the camera is already covered with a material similar to black telescope flock. Anodizing the metal black seems to be the way to go. The process doesn't look to be all that difficult for me to do but for the maybe 2 small pieces of metal that I want to anodize black, will be contacting the local model RR organization. I donated to them a lot of HO scale pieces that I used as a kid. I know one or two of the members anodize small parts for their model RR layout so will be asking for a small favor.
thanks,
Greg

[Jac@stafford.net]

Anodizing black, or any color is inferior to a texture that does not reflect light. Anodizing is no cure whatsoever.
.

[Greg]

Anodizing black, or any color is inferior to a texture that does not reflect light. Anodizing is no cure whatsoever.
.

Agree but the pinhole is only 0.013 inches in diameter and adding texture to the sides of the pinhole's aperture would present quite a challenge. Anodizing black no cure but only option to pursue.

[photog_ed]

I think what you are seeing is diffracted stray light from from the pinhole, due to it being directly illuminated by the sun. There may be some small contribution caused by sunlight coming through the pinhole and illuminating a spot inside the camera, but I think it is a secondary effect. The best solution is to use a sunshade, or to aim the camera away from the sun.

This question connects my professional life (optical engineer) to my avocation, so I couldn't resist proving this to myself and sharing it with all of you.

A typical photographic scene has luminance Lscene of about 5 kcd/m^2 - see https://en.wikipedia.org/wiki/Orders_of_magnitude_(luminance).

The sun illuminates the pinhole with illuminance Esun of about 100 klux - see https://en.wikipedia.org/wiki/Lux.

The intensity of light diffracted by the pinhole is (lambda/D) * 1/(pi^3 sin^3(theta) - see http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=947646
where lambda = wavelength of light, D = diameter of pinhole, theta = angle of the sun to a point on the image, pi = 3.14159.... The diffracted intensity has units of 1/sr (diffracted luminance/incident illuminance).

The scene illuminance at the film = pi/(4F^2) * Lscene
where F = f/number of the pinhole.

The sun illuminance at the film = Esun * (diffracted intensity) * pi/(4F^2).

The ratio of the two illuminance values (sun to scene) is = Esun * (diffracted intensity) / Lscene.

The wavelength of visible light, mid-spectrum is 0.0005mm. Assuming D = 0.05mm and theta=30 degrees, we put this all together to find the ratio is about 1/19, or about 4 1/3 stops. That is, the diffracted stray light from the sun is about 4 1/3 stops down from the average scene brightness, which corresponds to a background fog.

[Greg]

I think what you are seeing is diffracted stray light from from the pinhole, due to it being directly illuminated by the sun. There may be some small contribution caused by sunlight coming through the pinhole and illuminating a spot inside the camera, but I think it is a secondary effect. The best solution is to use a sunshade, or to aim the camera away from the sun.

This question connects my professional life (optical engineer) to my avocation, so I couldn't resist proving this to myself and sharing it with all of you.

A typical photographic scene has luminance Lscene of about 5 kcd/m^2 - see https://en.wikipedia.org/wiki/Orders_of_magnitude_(luminance).

The sun illuminates the pinhole with illuminance Esun of about 100 klux - see https://en.wikipedia.org/wiki/Lux.

The intensity of light diffracted by the pinhole is (lambda/D) * 1/(pi^3 sin^3(theta) - see http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=947646
where lambda = wavelength of light, D = diameter of pinhole, theta = angle of the sun to a point on the image, pi = 3.14159.... The diffracted intensity has units of 1/sr (diffracted luminance/incident illuminance).

The scene illuminance at the film = pi/(4F^2) * Lscene
where F = f/number of the pinhole.

The sun illuminance at the film = Esun * (diffracted intensity) * pi/(4F^2).

The ratio of the two illuminance values (sun to scene) is = Esun * (diffracted intensity) / Lscene.

The wavelength of visible light, mid-spectrum is 0.0005mm. Assuming D = 0.05mm and theta=30 degrees, we put this all together to find the ratio is about 1/19, or about 4 1/3 stops. That is, the diffracted stray light from the sun is about 4 1/3 stops down from the average scene brightness, which corresponds to a background fog.

Wow, my head is spinning... reminiscent of the time a Physics Professor (student of mine at the time) tried to explain to me (Photo Professor at the time) how he developed a method for focusing defocused images for NASA. Could you please translate your obviously well thought out post into a real simple laymen's termed explanation? Would greatly appreciate it.
thanks

[photog_ed]

Wow, my head is spinning... reminiscent of the time a Physics Professor (student of mine at the time) tried to explain to me (Photo Professor at the time) how he developed a method for focusing defocused images for NASA. Could you please translate your obviously well thought out post into a real simple laymen's termed explanation? Would greatly appreciate it.
thanks

OK, let's try this:

A pinhole, or any other aperture, produces diffraction when light passes through it. This not only causes a point of light to spread into a blob and degrade image quality, it causes light to spread to all angles and cause stray light. For a circular aperture, the intensity falls off inversely with approximately the cube of the angle to the point source (really the cube of the sine of the angle), and is inversely proportional to the diameter of the aperture. The smaller the aperture, the more light is spread out. My my guess of pinhole size 0.05mm gives the result that the stray diffracted light is 4 1/3 stops down from the scene, corresponding to fog.

However, now seeing the OP's update that the pinhole is 0.013" = 0.33mm, this kind of shoots a hole in my argument. I misremembered appropriate pinhole sizes, and my calculation should have had D = 0.5mm, not 0.05mm, making my calculation off by 10x. So for the OP's pinhole, the stray light is about 7 stops below the scene luminance.

One more thing: for a pinhole "focal length" of 300mm, my calculation for the optimal pinhole diameter is 0.6mm = .024inch. I calculated this by setting the pinhole diameter equal to the diffraction spot size, which is 2.44 * lambda * F-number. Solving for D, D = sqrt(2.44 * lambda * EFL).

[Randy]

Greg, I didn't read all the responses so if I repeat - forgive me. I shot some 8X10's for WWPD this spring and my shots were ruined due to not realizing the sun was shinning on my pinhole - very low contrast and stray streaks on my film. I'll just have to be more careful. But one method of curing the shiny reflective quality of the brass (or stainless steel or aluminum) pinhole, if I remember from many years ago when making my first pinhole camera, is to hold the pinhole (with tweezers) directly over and very close to a candle flame so that soot collects all over both sides of the brass pinhole. I will try it on my most recent fabrication as all I have done is use the sharpy method.

[Greg]

Greg, I didn't read all the responses so if I repeat - forgive me. I shot some 8X10's for WWPD this spring and my shots were ruined due to not realizing the sun was shinning on my pinhole - very low contrast and stray streaks on my film. I'll just have to be more careful. But one method of curing the shiny reflective quality of the brass (or stainless steel or aluminum) pinhole, if I remember from many years ago when making my first pinhole camera, is to hold the pinhole (with tweezers) directly over and very close to a candle flame so that soot collects all over both sides of the brass pinhole. I will try it on my most recent fabrication as all I have done is use the sharpy method.

Soot of a candle flame... that was definitely a forehead slapper "Why didn't I think of that" moment for me.