FLANGE FOCAL DISTANCE (SLRs)

David Lyga · Jul 31, 2020

The flange focal distance (distance from mount to film plane) is (frustratingly) not standardized. I guess I am going to ask "Why not?". Why do SLRs which have mounts such as Canon FL/FD and Konica AR have rather short flange focal distances, but SLRs which have mounts such as Olympus OM or Nikon F have rather long flange focal distances?

What I am getting at is this: Are there advantages or disadvantages with an SLR's flange distance being on the short side or the long side? If not, then why was this flange distance not standardized? With actual lens mounts, the mounting differences were purposeful financial decisions, because manufacturers, initially at least, did not want consumers to be able to buy lenses from other manufacturers, However, this restriction was soon conquered, as we all know, with many lens manufacturers making mounts for all the SLR manufacturers.

But the flange distances being different seems to offer no economic advantage. Comments? - David Lyga

MattKing · Jul 31, 2020

A longer flange distance may mean smaller lenses, and more room for retro-focus wide angle lenses.

grain elevator · Jul 31, 2020

MattKing said:
A longer flange distance may mean smaller lenses, and more room for retro-focus wide angle lenses.

What do you mean? There's all the room in front of the camera for lenses in any case...
I think the considerations are: Enough room for the mirror construction chosen, that favors longer distances. But in favor of making the distance as short as possible are getting the lenses closer to the film, which permits more flexibility in design or at least less protruding rear elements, and the ability to adapt other lenses to the body. Some manufacturers advertised that. If every camera had the same flange to film distance, one would only be able to adapt lenses that have a narrower throat, as in the case of M42 -> PK. Of course the opposite consideration, trying to sell more lenses with longer flange to film distance, that can be adapter to other bodies, would have been possible as well...

David Lyga · Jul 31, 2020

MattKing said:
A longer flange distance may mean smaller lenses, and more room for retro-focus wide angle lenses.

Maybe my question is terribly naive, but I will ask it anyway in the hope that another dimension will be covered. Why not get rid of the NEED for retrofocus wides by having a great enough flange distance so that the mirror is not a problem. Or ... why not make the mirror with a front-center curve removed so that clearing the last element on a wide angle would not be a problem? - David Lyga

Dali · Jul 31, 2020

And why making mechanical things complex when we have the optical solution?

David Lyga · Jul 31, 2020

Dali said:
And why making mechanical things complex when we have the optical solution?

Because I have a hunch that the 'optical solution' for the SLR's wide angles are not quite as good, optically, as the real thing with the rangefinder wide angles. - David Lyga

John Koehrer · Jul 31, 2020

1. Follow the money.
If there's a common f to f distance it's going to make it easier to change or modify the mount. It seems the ltm was
popular with several makers.

Isn't it easier to design a more compact non retrofocus lens than the other? And using a longer f to f distance makes the camera body thicker.

David Lyga · Jul 31, 2020

John Koehrer said:
1. Follow the money.
If there's a common f to f distance it's going to make it easier to change or modify the mount. It seems the ltm was
popular with several makers.

Isn't it easier to design a more compact non retrofocus lens than the other? And using a longer f to f distance makes the camera body thicker.

Are you inferring that an SLR retrofocus wide is EXACTLY up to the optical quality of a rangefinder wide (which has no mechanical restrictions)? Maybe it is, but I wanted to post this in case someone agreed with me. I really do not know, but compromise is sometimes ... compromise. - David Lyga

wiltw · Jul 31, 2020

David Lyga said:
The flange focal distance (distance from mount to film plane) is (frustratingly) not standardized. I guess I am going to ask "Why not?". Why do SLRs which have mounts such as Canon FL/FD and Konica AR have rather short flange focal distances, but SLRs which have mounts such as Olympus OM or Nikon F have rather long flange focal distances?

What I am getting at is this: Are there advantages or disadvantages with an SLR's flange distance being on the short side or the long side? If not, then why was this flange distance not standardized? With actual lens mounts, the mounting differences were purposeful financial decisions, because manufacturers, initially at least, did not want consumers to be able to buy lenses from other manufacturers, However, this restriction was soon conquered, as we all know, with many lens manufacturers making mounts for all the SLR manufacturers.

But the flange distances being different seems to offer no economic advantage. Comments? - David Lyga

A shorter (or curved bottom to the) reflex mirror createS the opportunity for cutoff of the image reflected up to the focusing screen. So for image faithfulness (vs what is captured at the focal plane), the longer reflex mirror is beneficial...and that creates the need for a longer flange distance.

wiltw · Jul 31, 2020

David Lyga said:
Are you inferring that an SLR retrofocus wide is EXACTLY up to the optical quality of a rangefinder wide (which has no mechanical restrictions)? Maybe it is, but I wanted to post this in case someone agreed with me. I really do not know, but compromise is sometimes ... compromise. - David Lyga

As history has shown, the Hasselblad SWC was designed with no reflex mirror because its non-retrofocus optic could be make with even better IQ than the same FL lens made in retrofocus optic to clear the relfex mirror of the Hasselblad 500.

flavio81 · Jul 31, 2020

David Lyga said:
The flange focal distance (distance from mount to film plane) is (frustratingly) not standardized. I guess I am going to ask "Why not?". Why do SLRs which have mounts such as Canon FL/FD and Konica AR have rather short flange focal distances, but SLRs which have mounts such as Olympus OM or Nikon F have rather long flange focal distances?

What I am getting at is this: Are there advantages or disadvantages with an SLR's flange distance being on the short side or the long side? If not, then why was this flange distance not standardized? With actual lens mounts, the mounting differences were purposeful financial decisions, because manufacturers, initially at least, did not want consumers to be able to buy lenses from other manufacturers, However, this restriction was soon conquered, as we all know, with many lens manufacturers making mounts for all the SLR manufacturers.

But the flange distances being different seems to offer no economic advantage. Comments? - David Lyga

Some info above is a bit incorrect. Shorter Flange Focal Distance (FFD) is better for wideangle designs. The closer the rear element can be positioned to the film plane, the greater freedom the lens designers have to create better lenses. In general shorter FFD is something the lens designers will appreciate better. Also, a shorter FFD means your camera can mount lenses designed for a longer FFD with the appropriate adapter.

The only benefits I see of a longer FFD would be:

- Possibility of using a bigger mirror
- Some telephoto lenses will be slightly shorter in size.
- Perhaps better flocking for avoiding stray light rays coming out of the lens.

reddesert · Jul 31, 2020

David Lyga said:
Maybe my question is terribly naive, but I will ask it anyway in the hope that another dimension will be covered. Why not get rid of the NEED for retrofocus wides by having a great enough flange distance so that the mirror is not a problem. Or ... why not make the mirror with a front-center curve removed so that clearing the last element on a wide angle would not be a problem? - David Lyga

A lens naturally wants to be centered about one focal length away from the film plane.
Wide angle lenses for SLRs have to be retrofocus because the elements are far away from the film plane. They are far away from the film plane because they have to clear the mirror. Increasing the flange distance means you need retrofocus; there is no free lunch here.

Early retrofocus wide angles for SLRs were large and had the reputation of being mediocre (I mean early like the 1950s or 1960s), but advances in optical design and manufacturing such as high index glass, computer optimization of designs, molded aspheres, etc, mean it isn't really something to worry about now. It is likely easier to design a non-retrofocus wide angle, but that is the designer's problem.

David Lyga · Jul 31, 2020

wiltw said:
As history has shown, the Hasselblad SWC was designed with no reflex mirror because its non-retrofocus optic could be make with even better IQ than the same FL lens made in retrofocus optic to clear the relfex mirror of the Hasselblad 500.

Thank you for quantifying what I had felt as being correct. - David Lyga

David Lyga · Jul 31, 2020

flavio81 said:
Some info above is a bit incorrect. Shorter Flange Focal Distance (FFD) is better for wideangle designs. The closer the rear element can be positioned to the film plane, the greater freedom the lens designers have to create better lenses. In general shorter FFD is something the lens designers will appreciate better. Also, a shorter FFD means your camera can mount lenses designed for a longer FFD with the appropriate adapter.

The only benefits I see of a longer FFD would be:

- Possibility of using a bigger mirror
- Some telephoto lenses will be slightly shorter in size.
- Perhaps better flocking for avoiding stray light rays coming out of the lens.

This forces the question: the Leader of the Pack in system SLRs was, of course, NIkon. They did not take advantage of this purportedly better, shorter FFD and I guess that this is a fair question to ask: "WHY NOT?" There are strange workarounds in the highest echelons of technical mastery. - David Lyga

wiltw · Jul 31, 2020

David Lyga said:
This forces the question: the Leader of the Pack in system SLRs was, of course, NIkon. They did not take advantage of this purportedly better, shorter FFD and I guess that this is a fair question to ask: "WHY NOT?" There are strange workarounds in the highest echelons of technical mastery. - David Lyga

This is perhaps lost in history, (unless a lifetime fan of Nikon comes along to this thread) but IIRC Nikon was one of the brands first pointing out its longer mirror for less image cutoff in the viewfinder...longer mirror, longer flange distance necessary,.

MattKing · Jul 31, 2020

grain elevator said:
What do you mean? There's all the room in front of the camera for lenses in any case...

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.
A 50mm lens is most likely going to be physically smaller on a body with a long flange distance, and that size advantage can increase as focal length increases.
More practically, it probably is a lot easier to fit a lot more good things into the mirror box if there is more space there.

Chan Tran · Jul 31, 2020

David Lyga said:
Maybe my question is terribly naive, but I will ask it anyway in the hope that another dimension will be covered. Why not get rid of the NEED for retrofocus wides by having a great enough flange distance so that the mirror is not a problem. Or ... why not make the mirror with a front-center curve removed so that clearing the last element on a wide angle would not be a problem? - David Lyga

You get it backward. The retrofocus design was needed because the flange distance is too long. The flange distance is long because the need for the mirror. A longer flange distance allows for longer mirror which prevents image cut off when use with very long lenses.

David Lyga · Jul 31, 2020

Chan Tran said:
You get it backward. The retrofocus design was needed because the flange distance is too long. The flange distance is long because the need for the mirror. A longer flange distance allows for longer mirror which prevents image cut off when use with very long lenses.

Here is where my uninitiated mind stands with this. Perhaps I err.
I was under the impression that wide angle lenses for SLRs could not be manufactured to have the rear element protrude as much as rangefinder cameras could allow. The mirror in the SLR forced a re-design of the wide angle SLR lenses so that that rear element would clear the mirror when focused at infinity. I guess that it is a bit naive to ask that the flange distance be somewhat increased in order to allow a traditional wide angle length to be able to be implemented into a standard SLR body. Perhaps my question is terribly wrong, but this is what I had thought. - David Lyga

ic-racer · Jul 31, 2020

I think most 35mm camera manufacturers do have a standardized flange focal distance. That is pretty amazing, especially in the Nikon system, where almost every lens fits almost every camera.

David Lyga · Jul 31, 2020

ic-racer said:
I think most 35mm camera manufacturers do have a standardized flange focal distance. That is pretty amazing, especially in the Nikon system, where almost every lens fits almost every camera.

Oh, WITHIN their own respective mount models, they do have the same flange distance. Yes, that is why a 1959 Nikon lens fits a Nikon F6, (although all the electronic functions certainly will not work). However, the focus will be fine and identical to both models. - David Lyga

reddesert · Jul 31, 2020

David Lyga said:
Here is where my uninitiated mind stands with this. Perhaps I err.
I was under the impression that wide angle lenses for SLRs could not be manufactured to have the rear element protrude as much as rangefinder cameras could allow. The mirror in the SLR forced a re-design of the wide angle SLR lenses so that that rear element would clear the mirror when focused at infinity. I guess that it is a bit naive to ask that the flange distance be somewhat increased in order to allow a traditional wide angle length to be able to be implemented into a standard SLR body. Perhaps my question is terribly wrong, but this is what I had thought. - David Lyga

Chan Tran and I have explained this. You should sit down with a rangefinder and SLR and wide-angles and understand why you are asking the impossible. If the flange distance increases a lot, the lens moves further away from the body, so a wide-angle has to become more retrofocus. Actually, even a 50mm normal lens on a 35mm SLR requires a slightly retrofocus design, which is one reason that early SLRs often had a 55mm or 58mm "normal" lens.

The flange distance is a mechanical position, not an optical position. If you look at some Nikon wide and normal lenses, you can see that the rear element actually sticks out behind the mount flange by some distance up to several mm. Nikon could likely have changed the location of the flange by a few mm one way or another and maintained the lens designs. The reasons for the distance Nikon chose are probably lost in time, but likely have to do with body design, mirror size, and so on. Other manufacturers probably chose to optimize each of these choices slightly differently.

David Lyga · Jul 31, 2020

reddesert said:
Chan Tran and I have explained this. You should sit down with a rangefinder and SLR and wide-angles and understand why you are asking the impossible. If the flange distance increases a lot, the lens moves further away from the body, so a wide-angle has to become more retrofocus. Actually, even a 50mm normal lens on a 35mm SLR requires a slightly retrofocus design, which is one reason that early SLRs often had a 55mm or 58mm "normal" lens.

The flange distance is a mechanical position, not an optical position. If you look at some Nikon wide and normal lenses, you can see that the rear element actually sticks out behind the mount flange by some distance up to several mm. Nikon could likely have changed the location of the flange by a few mm one way or another and maintained the lens designs. The reasons for the distance Nikon chose are probably lost in time, but likely have to do with body design, mirror size, and so on. Other manufacturers probably chose to optimize each of these choices slightly differently.

Yes, I see more clearly now. It is a bit analogous to a dog chasing its tail.
The more room you create for a traditional wide angle formula to be feasible (by allowing it to clear the mirror), the more need you have to create retrofocus. One defeats the other and I am happy that you spoke up on this matter. Hence, there is no quaint solution, only compromises which provide no decided advantage for either long or short flange distances! The more elbow room that you provide, the more NEED for retrofocus you have.

I think that David Lyga actually learned something tonight. I am going to sleep now, and will stop chasing my tail. - David Lyga

ciniframe · Jul 31, 2020

How about this Dave, I have an Olympus Pen F half frame SLR with a 38.95mm flange to film distance. That means you can have adapters for almost any brand of period SLR. Unfortunately the adapters Olympus made are quite rare and expensive when you find them. Over the years I’ve rounded up only three, OM, M42, and ‘T’ mounts. Olympus also had them for Nikon, and Exakta (which would also work with Topcon I think) and Minolta MC/MD and perhaps Canon FL/FD to, but not sure.
A few handy folks have, using various extension tubes and reverse adapters built their own adapters for mounts not supported by Olympus but those skills are beyond myself.
Of course the operation is all manual, with no auto stopdown on adapted lenses.

David Lyga · Aug 1, 2020

ciniframe said:
How about this Dave, I have an Olympus Pen F half frame SLR with a 38.95mm flange to film distance. That means you can have adapters for almost any brand of period SLR. Unfortunately the adapters Olympus made are quite rare and expensive when you find them. Over the years I’ve rounded up only three, OM, M42, and ‘T’ mounts. Olympus also had them for Nikon, and Exakta (which would also work with Topcon I think) and Minolta MC/MD and perhaps Canon FL/FD to, but not sure.
A few handy folks have, using various extension tubes and reverse adapters built their own adapters for mounts not supported by Olympus but those skills are beyond myself.
Of course the operation is all manual, with no auto stopdown on adapted lenses.

Adapters for SLRs which have short flange distances (Konica AR, etc) are easy to find but for SLRs with long flange distances like NIkon only adapters with a glass element are possible because a lens with a different mount will not be able to focus to infinity. - David Lyga

Chan Tran · Aug 1, 2020

David Lyga said:
Here is where my uninitiated mind stands with this. Perhaps I err.
I was under the impression that wide angle lenses for SLRs could not be manufactured to have the rear element protrude as much as rangefinder cameras could allow. The mirror in the SLR forced a re-design of the wide angle SLR lenses so that that rear element would clear the mirror when focused at infinity. I guess that it is a bit naive to ask that the flange distance be somewhat increased in order to allow a traditional wide angle length to be able to be implemented into a standard SLR body. Perhaps my question is terribly wrong, but this is what I had thought. - David Lyga

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.

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