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05-10-2017, 08:12 AM   #1
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What's the focal length of a Pentax K mount pinhole camera?

I am curious to know what the f stop is for the pinhole lens cap adapter for my camera, or any camera. I just bought one. a Pentax k mount body cover drilled to 0.3mm, but don't know the distance from film plane to the hole (or focal length). I understand that f stop = focal length/diameter.

I am gonna use it today and am guessing at f/166. i just emailed the guy who made the pinhole cap, and he might know. If he lets me know i will answer my own question but keep it posted.

I am assuming, from guessing from haphazard internet search that the distance in question is about 50mm.

Is the distance from sensor/film plane different for different cameras. I have another medium format camera on its way and it is f/250.

Any and all help is appreciated.

Pat

05-10-2017, 08:29 AM - 3 Likes   #2
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If the pinhole plate is in line with the camera mount surface, the focal length will be the register distance of 45.46 mm. It the pinhole is a bit further forward, then add that offset to 45.46 mm. (You can use select that focal length or the nearest one for SR purposes, too).

If the hole is exactly 0.3 mm and flush with the mount, the f-stop will be 1/151.5 which is about a factor of 90 times or 6.5 stops dimmer than f/16.

Under the "sunny f/16" rule you could do hand-held pinhole photography with something like ISO 1600 and a shutter speed of maybe 1/15 or 1/20 second.

Good luck and let us know how it works!
05-10-2017, 10:10 AM   #3
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QuoteOriginally posted by photoptimist Quote
If the pinhole plate is in line with the camera mount surface, the focal length will be the register distance of 45.46 mm. It the pinhole is a bit further forward, then add that offset to 45.46 mm. (You can use select that focal length or the nearest one for SR purposes, too).

If the hole is exactly 0.3 mm and flush with the mount, the f-stop will be 1/151.5 which is about a factor of 90 times or 6.5 stops dimmer than f/16.

Under the "sunny f/16" rule you could do hand-held pinhole photography with something like ISO 1600 and a shutter speed of maybe 1/15 or 1/20 second.

Good luck and let us know how it works!

Beautiful! i emailed the ebay seller who makes these, fireseller66, and this is what he said:
The focal distance is 49mm f163 for a 0.3mm aperture. I have to go though an update some listings
Best
Tim Page
His body cap is raised a little from the mount, but it sounds like he is in the ballpark. Should I use his f/163, or is that too far?

Also, just found on B&H some caps that are closer to the film plane.
05-10-2017, 01:31 PM   #4
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QuoteOriginally posted by paddyacme Quote
Beautiful! i emailed the ebay seller who makes these, fireseller66, and this is what he said:
The focal distance is 49mm f163 for a 0.3mm aperture. I have to go though an update some listings
Best
Tim Page
His body cap is raised a little from the mount, but it sounds like he is in the ballpark. Should I use his f/163, or is that too far?

Also, just found on B&H some caps that are closer to the film plane.
f/163 makes sense.

Where pinhole camera f-stops are concerned "ballpark" is as good as it gets so try 1/163. If the drill that made the pinhole wobbled a bit, the hole might really be 0.33 mm in diameter creating an f-stop of f/148 or maybe the drill bit was a little worn and the hole is only 0.29 mm in diameter (f/169). Then there's vignetting from 1/theta effects and the effects of the thickness of the plate.

In good light, the meter on the camera should provide an accurate exposure reading (just make sure your eye or something else covers the viewfinder eyepeice so light from that doesn't bias the reading).

Lenses like these are more about experimenting and learning than setting the camera to "P" and expecting perfect results.

Most of all: have fun!

05-10-2017, 02:50 PM   #5
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QuoteOriginally posted by paddyacme Quote
Also, just found on B&H some caps that are closer to the film plane.
Yeah, there are a lot of pinhole 'lenses' online now. Some are made of fancy materials with cool design, others are just bits of plastic with a hole.
Personally, I would prefer to get the plastic Holga 60mm f8 over a similarly-priced pinhole, but pinholes can be fun, too! Enjoy yours
05-11-2017, 04:37 AM   #6
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Thanks guys.

It's just practice and playing around. Looks pinhole application does need to be very accurate Thanks for starting me off on the right foot.
05-19-2017, 06:13 AM   #7
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Simple solution - take an image and you will know how well you estimate fits with regard to aperture. With digital cameras testing is straight forward, no compensation for film latency effects...
I prefer laser cut pinholes over drilled ones.
05-19-2017, 05:51 PM   #8
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Thanks guys. I think i'm in the ballpark. Went down tonight by the sea, but didn't have an extra rear lens cap, and didn't want to take off the 24mm and leave it stranded. Trying to formulate some discipline. Plus, I don't think the meter in the k1000 is very accurate. It doesn't believe in Sunny 16.

But I needed a roll of film to practice developing. It's been since around 1980 since I developed any film.

I think I want to try out more things, Just a little more.

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