DIY camera - how to choose the lense?

[radiant]

I've always tempted to do built my own camera (non-pinhole). However the lens part I don't understand - especially because I'm interested in "non standard" framing such as 6x12 or 6x18 120-size. Yesterday I did some paper negatives and got hooked - would be nice to have possibility for a bit larger negative.

So I made my own test, I build a "camera obscura" type box for enlarger lenses to see how if they are any good and what is the focusing distance for infinity.

I did rough test and found out that with Nikon e-Rokkor 105mm lens the distance from front of the lens to the focusing plane was about 120mm when focused to infinity. That then drew about 100mm size circle on the focusing plane. Values hower over the focusing distance quite well - dunno why

But it can't be like this? Or is it? Does all lenses work this way?

I doubt it! So for example 6x17 frame would need 175mm projection. Maybe I could find enlarging lens that size but of course I'm tempted to use some photography lenses for example Mamiya Press lenses.

So is there some kind rule of thumbs that could help directly picking up correct lens? How do I know how large projection lenses create?

 

[Neil Grant]

...most 'normal' lenses project an image circle that measures the same as the focal length. Maybe a place to check out these things could be a web site of a manufacturer of LF lenses like Schneider or Rodenstock.

 

[Dan Fromm]

So is there some kind rule of thumbs that could help directly picking up correct lens? How do I know how large projection lenses create?

It really depends on the lens, both design and details of construction. The best source for coverage claims is manufacturers' catalogs. This
https://1drv.ms/b/s!AggQfcczvHGNkGG_P2z8Qiyc8Qo- link will take you to a list of such catalogs and other sources of useful information. You'll want a lens that covers at least the format's diagonal.

Lens makers' catalogs also give flange-to-film distances, also called flange-focal distances, for infinity focus. These are for a perfect realization of the lens' design, are always a bit off for lenses as manufactured. For this reason you should either measure the lens you're going to use's FFD before making your box or design your box with focusing adjustments in mind, i.e., make it somewhat short so that you can put spacers between lens and box to focus the lens precisely to infinity (or to the hyperfocal distance of your choice).

By the way, e-Rokkor is a Minolta, not a Nikon, trade name.

 

[AgX]

Values hower over the focusing distance quite well - dunno why

With any lens the image circle is enlarged by extracting the lens (focusing to nearby, especially to the macro range).

 

[radiant]

Thanks guys, this solved many questions

In my prototype I had a focal distance adjustment so that is how I got this rocket science measurement measured. Now that I got things calculated I realized I will get 80mm x 80mm square projection when focused to infinity with that lens. And yes of course it is minolta lens, brainfart

 

[John Koehrer]

Film plane to front of the lens doesn't tell you anything except that dimension. Focal length is from the film plane
to the nodal point of the lens. Usually this will be near the shutter blades or aperture blades(SWAG?)