Focal length

phnompenhphotography · Mar 29, 2021

Hi everyone, this is my first post.

I am building a 4 by 5 camera, actually building a 5 by 7 as well, they will hopefully be used for plate photography, but direct positive paper as well, who knows, maybe film too in time.

The thing is, I have taken parts of an old fuji enlarger for the 4 x 5 build and so have bellows too, now I found an old sankyo komura 200mm f 4.5 lens, I removed the focusing ring and the mounting thread and attached it to the build. My issue is this, with the bellows as short as possible the focusing glass, or plastic actually for now, is focused, great! When I measure it though, it is substantially less than the 200mm I expected it to be from the focal plan to the lens, even from the front element its about 180mm, I don't understand how that can be, any ideas why?

monopix · Mar 29, 2021

With multi element lenses the distance from lens to focal plane isn't necessarily the same as the focal length. This document might help to explain though it might be overkill but it, at least, shows it's not simply a question of measuring the distance. Maybe someone else can explain it simply.

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/Tutorial_MorelSophie.pdf

Donald Qualls · Mar 29, 2021

It's very possible the lens you selected is a "tele" type. These are the reverse of a retrofocus lens (the kind that let you mount a 21 mm lens on a Nikon with 40+ mm film to flange distance, without impacting the reflex mirror): they provide the image characteristics of a long lens but without requiring as much distance from the film plane as a plain lens of the same effective focal length.

Why would anyone do this with an enlarging lens? So you can use a bellows short enough for a 50mm lens to focus, and still mount the longer lens for printing from larger film. Omega's solution to this, at one time, was to provide lens boards ranging from slightly recessed to significantly extended (later, they simply used a bellows with lots of draw); other manufacturers may have used retrofocus and tele lenses (which would, in practice, lock users of their enlarger into buying their lenses, as these were relatively rare for enlargers).

phnompenhphotography · Mar 29, 2021

monopix said:
With multi element lenses the distance from lens to focal plane isn't necessarily the same as the focal length. This document might help to explain though it might be overkill but it, at least, shows it's not simply a question of measuring the distance. Maybe someone else can explain it simply.

https://wp.optics.arizona.edu/optomech/wp-content/uploads/sites/53/2016/10/Tutorial_MorelSophie.pdf

Thanks, a little overkill yes, but it does actually explain why the distance varies, or could vary so thanks for that. I kind of got most of it, I think I understood the gist but not the technicalities, which is great.

phnompenhphotography · Mar 29, 2021

Donald Qualls said:
It's very possible the lens you selected is a "tele" type. These are the reverse of a retrofocus lens (the kind that let you mount a 21 mm lens on a Nikon with 40+ mm film to flange distance, without impacting the reflex mirror): they provide the image characteristics of a long lens but without requiring as much distance from the film plane as a plain lens of the same effective focal length.

Why would anyone do this with an enlarging lens? So you can use a bellows short enough for a 50mm lens to focus, and still mount the longer lens for printing from larger film. Omega's solution to this, at one time, was to provide lens boards ranging from slightly recessed to significantly extended (later, they simply used a bellows with lots of draw); other manufacturers may have used retrofocus and tele lenses (which would, in practice, lock users of their enlarger into buying their lenses, as these were relatively rare for enlargers).

That does make sense yes, maybe it is that type of lens, cheers.

Maris · Mar 29, 2021

I always thought the Sankyo Komura 200mm f4.5 lens was designed for the 35mm format and won't produce a 4x5 image. But I could be wrong.

phnompenhphotography · Mar 29, 2021

Maris said:
I always thought the Sankyo Komura 200mm f4.5 lens was designed for the 35mm format and won't produce a 4x5 image. But I could be wrong.

No, you are correct. It is designed for 35mm, on a 4 by 5 it gives decent enough coverage, albeit with heavy vignetting. This is just judged from the focusing screen as I havent been able to test it producing actual lasting images yet. I have a couple of barrel lenses too, I think one may be from an old cine projector maybe.

I expect the format is giving me some of my issues but I have never seen a large format lens in this country at all so I may have to work with some limitations.

One of the challenges I have where I live is the almost complete lack of availability of almost everything I need in terms of lenses, lensboards, holders, papers, film, almost everything. Glass, wood and metal are in abundance. Ebay is an option but it can be quite hard to get stuff delivered here, for heavy stuff it becomes very expensive, so I am going to have to make a lot of the stuff I need myself. That does make it fun though.

Focal length

phnompenhphotography

monopix

Donald Qualls

phnompenhphotography

phnompenhphotography

Maris

phnompenhphotography

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