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Pinhole Question
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PostPosted: Sat Dec 22, 2012 12:26 am    Post subject: Pinhole Question Reply with quote
So I've seen a lot of these converted body cap pinhole lenses for SLRs. I was wondering if anyone has tried a converted lens cap pinhole. The idea being that the lens could focus on the exact spot and then the pinhole would be in front of the lens (alternately, an unused filter ring could be converted to a pinhole aperture.)


PostPosted: Sat Dec 22, 2012 10:39 am    Post subject: Reply with quote
Well you can try for yourself if you take some cardboard and pinch a hole in it. But don't be disappointed when you get a black picture only. Wink

No, it just makes no sense to put a pinhole in front of a lens. You only shadow almost 100% of the light falling into the front lens.


PostPosted: Sat Dec 22, 2012 3:59 pm    Post subject: Reply with quote
I tried it a few years ago David and I back up what Holger says. I had to keep enlarging the hole till I could see anything at all. This "pinhole" was about 2.5mm dia.


PostPosted: Sat Dec 22, 2012 7:42 pm    Post subject: Reply with quote
looks like if you want to use a pinhole, you have to eliminate the lens....


PostPosted: Sat Dec 22, 2012 7:51 pm    Post subject: Reply with quote
here are my tests with an eos cap




PostPosted: Sat Dec 22, 2012 8:24 pm    Post subject: Reply with quote
Hey, Peter and Roger,

That makes sense, now that I think about it. I had ordered a 99-cent Pentax body cap to make into a pinhole, which is why I thought of maybe putting the hold before the lens (rather than as the lens.)

I'll make the pinhole in the next couple weeks (when the body cap arrives) and plan to use a very-thoroughly filed staple to make a pinhole about 0.2mm in diameter. At 48mm from the film plane, that's slightly smaller than optimal (0.298mm) and should be f250 and yield a 38.6-degree field of view. It's always easier to make the pinhole larger, but not smaller.


PostPosted: Sat Dec 22, 2012 8:24 pm    Post subject: Reply with quote
Hey, Nelson,

How large was the hole you used?


PostPosted: Sat Dec 29, 2012 12:51 am    Post subject: Reply with quote
pinhole equations

wikipedia -- Pinhole Photography

Edmund Optics -- Precision Pinholes


PostPosted: Sat Dec 29, 2012 9:16 am    Post subject: Reply with quote
David wrote:

That makes sense, now that I think about it. I had ordered a 99-cent Pentax body cap to make into a pinhole, which is why I thought of maybe putting the hold before the lens (rather than as the lens.)

I'll make the pinhole in the next couple weeks (when the body cap arrives) and plan to use a very-thoroughly filed staple to make a pinhole about 0.2mm in diameter. At 48mm from the film plane, that's slightly smaller than optimal (0.298mm) and should be f250 and yield a 38.6-degree field of view. It's always easier to make the pinhole larger, but not smaller.


Note that in order to avoid vignetting, the pinhole ought to be punched into a plate thinner than the diameter of the hole also avoiding raising the edges of the hole. Use the thinnest metal foil you can find for the pinhole and fix the foil on the lens cap having something like a 1/4" hole through the center. However, for optimum results it's better to get a ready-made pinhole, e.g., http://www.lenoxlaser.com/index.php?dispatch=products.view&product_id=72 .

Veijo


PostPosted: Sat Dec 29, 2012 9:57 am    Post subject: Reply with quote
Lovely marketing line: "The Leader in Small Hole Technology" Laughing


PostPosted: Sat Dec 29, 2012 10:34 am    Post subject: Reply with quote
I do own a body cap pinhole for Canon.
It doesn't see much use though. The only thing you see sharp is the dirt on the sensor. Laughing


PostPosted: Sat Dec 29, 2012 1:17 pm    Post subject: Reply with quote
LucisPictor wrote:
I do own a body cap pinhole for Canon.
It doesn't see much use though. The only thing you see sharp is the dirt on the sensor. Laughing

Laughing Laughing Laughing Laughing Laughing Laughing


PostPosted: Sat Dec 29, 2012 8:08 pm    Post subject: Reply with quote
Why is it that pinholes make sensor dirt so sharp? I have the same problem when I use an old 6X6 lens stopped down to f32 on my DSLR.


PostPosted: Sun Dec 30, 2012 5:44 pm    Post subject: Reply with quote
David wrote:
Why is it that pinholes make sensor dirt so sharp? I have the same problem when I use an old 6X6 lens stopped down to f32 on my DSLR.

When you have a large aperture the light falls onto one sensor pixel from a relative broad angle. Some of the light may go around the dirt, depending on the angle. With a small aperture the angle of the light becomes narrower and less light goes around the dirt.

I tried to sketch this, see below. The smaller the aperture, the smaller the angle, the more light the dirt blocks from the sensor and the dirt becomes more visible.



PostPosted: Sun Dec 30, 2012 6:05 pm    Post subject: Reply with quote
LucisPictor wrote:
I do own a body cap pinhole for Canon.
It doesn't see much use though. The only thing you see sharp is the dirt on the sensor. Laughing


Is the distance from sensor to pinhole important? I mean, does the pinhole have a focal distance and, if so, is it related to the size of the hole?


PostPosted: Mon Dec 31, 2012 12:31 am    Post subject: Reply with quote
peterqd wrote:
LucisPictor wrote:
I do own a body cap pinhole for Canon.
It doesn't see much use though. The only thing you see sharp is the dirt on the sensor. Laughing


Is the distance from sensor to pinhole important? I mean, does the pinhole have a focal distance and, if so, is it related to the size of the hole?


Pinhole is a single 'element' lens, so back focal distance equals pinhole focal length.


PostPosted: Mon Dec 31, 2012 3:46 am    Post subject: Reply with quote
@Holger,

Thank you, that makes a lot of sense. That also explains why my shots at f1.4-2 don't show much or any dirt, even when I do substantial tone enhancement in post-processing.[/i]


PostPosted: Mon Dec 31, 2012 7:04 am    Post subject: Reply with quote
peterqd wrote:

Is the distance from sensor to pinhole important? I mean, does the pinhole have a focal distance and, if so, is it related to the size of the hole?


Yes, the optimum distance depends on the size of the pinhole, see http://home.online.no/~gjon/pinhole.htm#formulas. However, the optimum also depends on the wavelength (color), which means that any specific distance is a compromise, i.e., the distance (or conversely the size of the hole) may somewhat vary without an appreciable effect on the image and the degree of fuzziness depends on the color composition of the scene.

Veijo[/url]


PostPosted: Mon Dec 31, 2012 12:42 pm    Post subject: Reply with quote
Thankyou Veijo, some very interesting reading there. I get the feeling I should have known some of that already, but better late than never! Smile
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