# Help Determining Focal Distance

Discussion in 'Pinhole Photography' started by pryan9, May 4, 2011.

1. ### pryan9Member

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Hello Everyone,

I am trying to figure out the focal distance of my new pinhole conversion camera and I'm a little confused. I've tried the wizards to help me figure this out but I'm still a little lost. I think the grey area is determining the film dimensions. So If im shooting 120 film on a 6x9 frame is my film dimension going to be 90mm?

Here are my dimensions...Anyone bored enough to calculate this for me?

Pinhole to film plane distance = 3 5/16 inches
Pinhole Diameter = .010 inches
Film Dimensions = 6x9 cm

Thanks to anyone who can help me out here

2. ### holmburgersMember

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The focal distance (focal length) is simply the distance from film plane to pinhole, ergo 3.3125.

The size of the film ultimately has nothing to do with focal length. It does however determine your angle of view, which is dependant upon focal length and film size.

If my math serves me (which is not a guarantee with me...) your aperture is f/331.25

It's possible I'm missing something altogether, but hopefully this helps??

3. ### 2F/2FMember

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I don't understand what it is you are looking for. What do you mean by "film dimension?" You have already stated the pinhole to film distance, which is generally called "focal length" for purposes of conversation. Are you trying to figure out the angle of view?

4. ### pryan9Member

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Yes the angle of view is what im looking for!

5. ### 2F/2FMember

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Your camera will give a moderately wide angle, almost exactly what you get from a 35mm lens on small format.

6. ### pryan9Member

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okay thats what im trying to figure out because I have an external variable viewfinder that i wanted to use and I wanted to figure out exactly what the angle would be so that i can set the viewfinder accordingly

7. ### 2F/2FMember

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If the viewfinder is made for 6x9 format, and it has a 90mm setting, I'd use that. You actual FL is about 85mm.

8. ### SMBoothSubscriber

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At that focal distance with that film size its about 65deg.

9. ### pryan9Member

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Okay, So the variable viewfinder is for 35mm actually. So at 6x9 with the dimensions mentioned above, what would be the best frame line to set the viewfinder at to get me in the ballpark for this pinhole camera? I have 35, 50, 90, and 135, I can also set it in between each of these

10. ### SMBoothSubscriber

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If its a 35mm viewer then use 35mm.

11. ### pryan9Member

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Okay so I think we are all a little confused here (maybe it's still just me lol) the viewfinder that I have is for a 35mm rangefinder that doesn't have frame lines for differnet focal length lenses. The viewfinder can be set to 35mm, 50mm, 90mm, 135mm, and has spaces in between for intermediate focal distances.

So what I am trying to figure out is what setting on the viewfinder is closest to what this pinhole will be according to the dimensions from my original post (taking into consideration I will be using 120 (6x9) instead of 35mm film.

Maybe I'm overthinking this a little but I just want to have a general idea of what I will be taking a picture of and I figured the viewfinder would help tremendously.

12. ### SMBoothSubscriber

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The best setting, as been said is to set your 35mm viewer to 35mm, that will be closest to your field of view for your 6x9 pinhole with a 3 5/16 focal length. A 35mm lens on a 35mm camera has a field of view of approx 65deg, so does your pinhole camera as you have described it.

13. ### pinhole_dreamerMember

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Speaking of this.....I've got one that I've not yet finished. (Just needs the inside painted.) Is there an issue with the film being 2.25 inches away from the film for a 6x6 frame?

14. ### DWThomasSubscriber

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Assuming you mean pinhole 2.25 inches from the film, no. That spacing affects the field of view and can be varied over a huge range. 2.25 should give a moderate wide angle. The only issue is that the pinhole size needs to be different for optimum results if you make large changes in that spacing.

My Bronica SQ-A (a 6x6) with the "SQ-Hole" body cap adapter has a pinhole to film plane distance of about 90 mm and produces a field of view fairly similar to the 80 mm lens. A wider angle of view would be cool, but working with an SLR, mirror clearance limits the closest allowable spacing.

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15. ### DWThomasSubscriber

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In general, there is Pinhole Designer, a Windoze-based program available as freeware, as well as an online calculator at Mr Pinhole that can be used to crunch a lot of these numbers (for those who don't like algebra problems ).

There is also a bunch of info available through the links at the WPPD Resources Page.

16. ### pryan9Member

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Ok thanks guys I really appreciate the help!

17. ### pinhole_dreamerMember

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Exactly! The PINHOLE is 2.25 inches away. I like a nice wide angle - and moderate is just peachy. Well....I'll have time to paint it and then get to work! Or fun, rather. Photography isn't work for me.

Thanks!

18. ### 2F/2FMember

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Use the frame lines for a 35mm lens and you have a practically perfect match for your camera. Same angle of view and different format = different focal length. So you need to select which lines to use based on angle of view, not focal length.

19. ### pryan9Member

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Update:

So I ran a roll of film through my camera and the angle of view was perfect. The only problem is that the image is about the size of a quarter on a medium format frame. The image is circular right in the center of the frame.

I converted an old brownie box camera to make my pinhole cam. Could the focal distance have caused this issue?

Also, I developed the negs using caffenol for the first time but I highly doubt that had any part in the circular images.

20. ### mokiMember

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There are two causes for an image circle that's too small for your film format. The first one is something blocking the light... that usually gives a very clean circle (with sharp edges) it's quite possible that the shutter opening in a brownie is too small. You have to trace an imaginary line from the corner of the film, through the pinhole and out again... is there any piece of the camera in the way? then you need to remove that or find another camera. I don't know about the brownie, but many box cameras have very tight shutters that can give these problems when converting them to pinhole.
The other possibility is wrong material for the hole. If the material is too thick, you get more light fall-off at the corners. If your image seems relatively normal but gradually becomes dimmer at the edges, that's probably your problem. To avoid this, I use regular aluminium foil from the kitchen for my pinholes. It's not very robust, but thin enough for even the widest angles of view.