How do you calculate the equivalent lens for pinhole

[drgoose]

Hello everyone, first time posting here. I have a Crown Graphic 4x5 that right now is not doing anything productive. I recently made a pinhole with aluminum foil with a 27 gauge insulin needle with an outer diameter of 0.4mm. I am trying to figure out what the bellows extension need to be for the image captured at the film plane to coincide with the little viewfinder on the camera. Originally I think it came with a 127mm Kodak lens so does the bellows extension need to be 127mm?

 

[Sirius Glass]

Some pin hole calculators:

http://www.mrpinhole.com/calcpinh.php
http://www.mrpinhole.com/holesize.php
http://www.photostuff.co.uk/pinholec.htm
http://pinhole.stanford.edu/phcalc3.htm
http://pinhole.stanford.edu/pinholemath.htm
http://www.diyphotography.net/the-comprehensive-tech-guide-to-pinhole-photography/​

 

[michaelorr]

Yes that is correct. The focal length is the same whether a lens or a pinhole. Distance from the aperture to the film. Gives the same angle of view on the same film size.

 

[DWThomas]

I'm not familiar with the Crown Graphic as to whether it might have an optional wireframe finder?

My first pinhole efforts were with a replacement lens board for my B&J Press 4x5. That had a wireframe on the front standard that flipped up and a little peep sight thingie on the back/top that flipped up. I found that fairly accurate as a viewfinder; it automatically compensated for angle of view as the front standard was moved in or out; pretty handy when experimenting. I did use some marks on the rails so the distance could be repeatably set for the dimension I used for the pinhole calculations when I finally zeroed in on the design.

You have plenty of time to work out the kinks for Worldwide Pinhole Photography Day on April 24th!

 

[RobC]

the effective aperture is the focal length(distance from pinhole to film) divided by pinhole diameter so if distance is 127mm and diamter is 0.4mm then effective aperture is F317.5. You may want to consider that when thinking about exposure times and reciprocity of your film. A shorter focal length may be more desireable (or not depending on requirements).

 

[drgoose]

Thanks to everyone