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# Thread: Help Determining Focal Distance

1. ## Help Determining Focal Distance

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. 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. 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. Yes the angle of view is what im looking for!

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

6. 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. 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. Originally Posted by pryan9
Yes the angle of view is what im looking for!
At that focal distance with that film size its about 65deg.

9. 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. If its a 35mm viewer then use 35mm.

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