FLANGE FOCAL DISTANCE (SLRs)

David Lyga · Aug 1, 2020

Chan Tran said:
The flange distance on the SLR is significantly more than a rangefinder but the lens can not be in that space because it's for the mirror. Since the lens has to be a significant distance away from the film retrofocus design was needed to make lens with short focal length that has the rear nodal plane behind the lens rather than somewhere in the middle of the lens.

Yes, precisely what I said in #22. You are correct here, and I STOOD corrected in #22. - David Lyga

ciniframe · Aug 1, 2020

If memory serves, Alpa has a very short flange distance for a full frame SLR.
But don’t know what it was off hand. Alpas have always interested me but prices way too rich for this bottom feeder.

Trask · Aug 1, 2020

Here's a good resource for lens register distances: http://www.graphics.cornell.edu/~westin/misc/mounts-by-register.html

The closer your camera mount is to the beginning of the list, the more lenses of other manufacture further down the list can be adapted to fit. Example: Konica Autoreflex is halfway down the left column, with a register of 40.5mm. it would, in theory, be possible to make (or purchase from the manufacturer) an adapter to fit just about any lens further down the list, hence a Konica adapter to mount a Nikon lens. No auto stop-down of aperture, of course. And one must also allow for the width of metal used to make the mount, so you couldn't make an adapter to mount a Wrayflex lens on a Canon FD camera -- 5 100ths of an inch difference!

Chan Tran · Aug 1, 2020

David Lyga said:
Yes, precisely what I said in #22. You are correct here, and I STOOD corrected in #22. - David Lyga

Yes after I made the post I realize that it's not needed. Any way longer flange distance support longer mirror but I haven't found any document that says how the longer mirror helps. I remembered reading review on the Nikon back in the 70's saying that the mirror on the Nikon are larger and thus can prevent image cut off in the viewfinder when using very long lenses.

abruzzi · Aug 1, 2020

this is just a wild guess, but M42 and Nikon F were two of the earlier SLR mount specification, and are also two of the longer flange distances for 35mm. I've always assumed that the longer distances came about because the technology was in its infancy, and earlier cameras had larger mirrors and were optimized down as the later manufacturers were able to do.

One that is very interesting to me is the early Bronica cameras. Most SLRs need the lens completely in front of the mirror because it does double duty--when its not mirroring, it flips up and blocks light from entering from the viewfinder. The Bronica mirror slides down (and forward a bit) and a curtain covers the viewfinder. As a result some of the Bronica lenses extend significantly into the mirror box. Its why when you see one with a 80mm lens on it, the lens barely extends beyond the body.

dynachrome · Aug 1, 2020

I have many adapters for using non-Konica mount lenses on Konica cameras. I also have the adapters made for Canon FL/FD cameras. Sometimes I will mount a Nikkor lens on a Canon F-1. With the adapters I have, I can mount M42 lenses on many other mounts with correct infinity focus. When I add in Tamron Adaptall/Adaptall II and Viviter T4 and TX lenses I have many lenses which I can get to fit almost anything. Some of the newer lens mounts, like the Canon EF, were considerably wider and this also allowed different lens designs.

dynachrome · Aug 1, 2020

The Olympus Pen mount was mentioned. I just bought a home made Nikon to Pen adapter. Now I just need a Pen camera.

flavio81 · Aug 3, 2020

MattKing said:
More room in front of the mirror = less need to make a lens strongly retro-focus, and may obviate the need for retro focus at all.

If the flange focus distance of a SLR is 46mm(Nikon), all lenses shorter than 46mm will have to be of a retrofocus design.

flavio81 · Aug 3, 2020

ic-racer said:
That is pretty amazing, especially in the Nikon system, where almost every lens fits almost every camera.

What?!

All pre-AI lenses (1959 to 1977) will simply not fit in nikon cameras after 1977 save for the professional F2/F3/Fx, the FM/FE, and the Nikkormat FT3 and EL2. They also won't fit in most full frame digital cameras, except for the professionals ones and the Df.

All G lenses are simply useless in Nikon cameras before the F4 (1988).

grain elevator · Aug 3, 2020

flavio81 said:
If the flange focus distance of a SLR is 46mm(Nikon), all lenses shorter than 46mm will have to be of a retrofocus design.

That applies only to pinholes. Actual lenses build longer than a pinhole, which is why, as has been mentioned, a fast 50mm for a typical SLR has to be slightly retrofocal. A slow one, which doesn't build as long due to smaller and less elements, may not have to. I still don't understand MattKing's statement that you replied to, either...

I suspect Nikon chose a rather long flange to film distance because they built SLRs with 100% coverage viewfinders, which probably necessitated larger mirrors than otherwise. Of course sliding mirror systems like in some MF SLRs make shorter FtF distances possible with a large mirror, but I don't think they were implemented in earlier 35mm SLRs. Were they in later ones? Canon EF mount ones maybe?

flavio81 · Aug 3, 2020

grain elevator said:
I suspect Nikon chose a rather long flange to film distance because they built SLRs with 100% coverage viewfinders, which probably necessitated larger mirrors than otherwise. Of course sliding mirror systems like in some MF SLRs make shorter FtF distances possible with a large mirror, but I don't think they were implemented in earlier 35mm SLRs. Were they in later ones? Canon EF mount ones maybe?

Yes, they were implemented in some 35mm SLRs, I think one or two Nikon AF cameras had it, can't remember which.

Maybe the Pentax LX had this system too? Because their Pentax 6x7 camera had a sliding mirror system.

MattKing · Aug 3, 2020

grain elevator said:
I still don't understand MattKing's statement that you replied to, either...

The reason you need retro focus is because the back element needs to clear the mirror.
With a greater flange distance, there is more room back there behind the lens mount, so it is less necessary to move the physical back end of the lens forward.

johnha · Aug 3, 2020

As mentioned previously, the flange distance isn't an optical limitation, but clearing the mirror is a mechanical one. As long as you clear the mirror, the rear elements can stick out behind the flange and into the mirror box.

Some early Pentax M42 SLRs have either an orange or green 'R' on the rewind knob. One allows the use of certain lenses that protrude further than others, the other doesn't (I can't remember which is which). The flange distances are identical, changes were made to the mirror to allow use of the lenses.

At one point, M42 was seen by many as a standard mount. Pentax used the same flange distance in K mount to allow easy adaption of existing lenses. Others picked different distances for their own reasons.

FLANGE FOCAL DISTANCE (SLRs)

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