Camera Mod - Need help with focus calculations

[kahlheins]

I have recently started a project to turn a broken Lomography 6x12 folding camera into a lightweight travel companion with an upgraded lens. I picked a Schneider Kreuznach Angulon 90mm f/6.8 in a Synchro-Compur #0 shutter as my replacement lens, but also have a Angulon 120mm f/6.8 available. Now, on to my problem. I stripped down the camera and I am planning on adding a custom "lens board", potentially with a helicoid, to the front face.



I am struggling a little with determining the correct focus distances (film plane to lens) for the focus range of the lens. According to this link (http://www.hyam.net/blog/archives/3961) the 90mm has a flange focal distance of 90mm at infinity. As far as I am aware that is measured as the distance from the film plane to the optical centre of the lens. How can I relate this back to for example the rear element to design my spacer parts more accurately?

Additionally, how do I determine the distance between lens and film plane at any given focus distance? I want to add a helicoid in order to enable focus from let’s say 2m to infinity. I am a little worried about the bellows size of the Lomography camera though, and hence need to figure out if the small end of the bellows is large enough to accommodate the lens when it is focussed at near distances (without vignetting). The bellows extend from a slightly smaller than 6x12 film plane (52 x 103mm) to an opening of 34mm x 36mm at the small end of the bellows over a length (film plane to bellow end) of 77mm.

Should close focus not be possible with the current bellows and folding mechanism, I might make this one fixed to infinity and later build a new one from scratch.

 

[monopix]

I did a near identical thing, fitting a 90mm Angulon to a Holga 120 Pan. As far as I remember, I set up a test rig and measured the distances but I don't remember exactly how I did it and I don't seem to have the information any more. I do know I achieved a 2m to infinity focusing range.

 

[reddesert]

In principle, the flange focal distance is the distance from the rear mounting surface of the lens's shutter, to the focal plane. The back focal distance is the distance from the rearmost part of the lens body to the focal plane. Neither of these distances is exactly to the center of the lens.

The extension required to focus a 90mm lens at 2 meters is found from the thin lens equation: 1/f = 1/d1 + 1/d2. Here f=9mm, d1=2000mm, so d2=94.2mm. So you need 4.2mm extension past infinity to focus at 2 meters. This isn't very much compared to the size of the camera, because wide angles don't take a lot of extension to focus.

Your best bet is likely to make a ground glass to put on the film rails and use that to set up the spacer and helical focuser. It's a good idea to leave some margin, in case the flange-focal distance is inaccurate - better to have a lens that focuses past infinity, than a lens that can't be focused to infinity.

 

[Dan Fromm]

The back focal distance is the distance from the rearmost part of the lens body to the focal plane.

Your best bet is likely to make a ground glass to put on the film rails and use that to set up the spacer and helical focuser. It's a good idea to leave some margin, in case the flange-focal distance is inaccurate - better to have a lens that focuses past infinity, than a lens that can't be focused to infinity.

No, back focus is the distance from the rear element's vertex to the film plane when the lens is focused to infinity.

Using a ground glass on the film rails to set up the helical is a good idea. So is using it to calibrate the helical. Doing this saves calculations and avoids making the assumption, usually wrong, that the focal length engraved on the lens is the lens' actual focal length.

 

[thuggins]

Use a ground glass to determine where you are on the focus. You want it to be a little short so you can shim the lens out.

 

[nosmok]

I've done a few lens swaps and such and you can use Scotch 'Disappearing' Tape across the film opening, if you pull it tight. Just line up the edges and you're good to go. Ground glass pieces are expensive and I'm no good at cutting glass. I've used the Scotch tape on 35mm to 6x9cm to 6x14cm (converted Kodak 3a to 120 film) cameras with good results. If there's film rollers, even better-- what's the difference between tape across the roller, and film across it? I'd almost say it's more true to life than GG would be.

 

[ic-racer]

1/p + 1/q = 1/f

p = lens to film
q = lens to subject

 

[Dan Fromm]

1/p + 1/q = 1/f

p = lens to film
q = lens to subject

Yeah, sure, but because nominal f as engraved on the lens is rarely actual f, f has to be measured.