Lens incompatibilities on some early screw mount models

Mar.15, 2001 - from Jon Hechtman and contributors

Following is the text of a discussion generated by Jon Hechtman about lens incompatibilities on early screw mount cameras.

I recently purchased a very nice S3, complete with the "correct" 55mm f/1.8 Auto-Takumar lens (lens S/N 433625). When I began running the lens through all the stops, looking from the opened camera back, it was clear that something was wrong with the smaller stops. At f/16, the lens was obviously stopping down no further than f/5.6 or so. There was no problem with a different lens (an f/2 Super-Tak)...so the difficulty isn't with the stop-down actuator in the camera. I put the Auto-Tak on manual and run through the stops: no problem there. I try all the stops, with the lens in "auto" mode and off the camera, by manually depressing the stop-down pin: again, no problem.

Next step: I fitted the Auto-Tak to my SV and tried it out, starting with f/16. No problem. f/11...no problem. And so on until I got to f/4, at which point the mirror hung up when I tried to fire the shutter. I repeated this exercise on a Spotmatic; this time the mirror hung up at f/11. What is going on? The S3 won't stop down correctly, but it doesn't hang up; the other models stop down correctly as far as they go, but they do hang up.

I had always assumed that the f/1.8 Auto-Takumar (which has the sliding auto/manual switch and the fully-automatic diaphragm...no cocking lever) was really just a Super-Takumar under a different name. Not so! There are two subtle, but important, differences. First of all, the rear element of the Auto-Tak protrudes noticeably further than that of the Super-Tak. Second, the stop-down pin works differently! On a Super-Tak or SMCT (or, for that matter, any other automatic m42 lens in my experience), the pin can be fully depressed, irrespective of the f/stop selected. At f/16, the pin needs all of its travel to stop down the lens; at f/5.6, it needs roughly half its travel to stop down the lens...but the pin can still be pushed all the way down. At full aperture, the pin doesn't do anything...but, again, it can still be pushed all the way down. On the Auto-Tak, this isn't so. The pin absolutely stops at the point where the lens is stopped down to the set aperture: no further travel at all. At full aperture, the pin is immovable.
This is true even on "manual" setting: at full aperture, the pin doesn't move. (I know that the early Super-Takumar with the "reversed" f/stops shouldn't be used on an ES model because of possible rear-element damage, so it seems likely that this Auto-Takumar has the same problem.)

So here's what I think is going on. On some models, the camera's mirror is hanging up on that fixed pin. I suppose that the reason the Spotmatic can get only to f/11, while the SV can get to f/4 before experiencing this effect, is a difference in the actuator design. So that explains the mirror hangup. On the S3 itself, the actuator is clearly designed to clear the pin, no matter what stop is selected; but at the smaller stops, the lens element is protruding too far to permit the actuator to depress the pin far enough to complete the stop-down process. I've checked, and the atop-down actuator on my S3 is correct (judging by the line drawing on your site).

What I can't figure out is: what camera is this lens intended to work with? It's not happy living on the S3; it's not happy living on the SV. Is this some kind of "transitional" model that was later redesigned? (Interestingly, the "true" Auto-Taks work just fine on all these cameras, at all apertures.)

Have you encountered any similar stories in the Pentax literature...or have you experienced anything similar yourself? If nothing else, perhaps this experience would be worth sharing with other Pentax users.

Jon Hechtman

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Many thanks for your interesting info and question about incompatibilities of this lens. I just checked three lenses of that same model I own and put them together with your lens as follows:

    # 374252 (Dario) PROBLEM
    # 426593 (Dario) OK
    # 433625 (Jon) PROBLEM
    # 446257 (Dario) OK

As you see, the problem is not only present within a single lens batch.

I also noticed that diaphragm actuator pin of # 374252 has some "play" when trying pushing it at f/1.8 (before it couples to inner mechanisms and you realize it cannot go any further). As you set slower stops, you can push it further and further until you can push it fully inside the lens barrel at f/16.

Actuator pins on both lenses # 426593 and 446257 have no play at all at f/1.8 and they don't hide completely inside the lens barrel when pushed at f/16. In other words, they always protrude more than that of # 374252. On lenses that work well the pin always enters the lens barrel less than on lenses that show that stopping-down problem.

Different production batches? Bad repair on some lenses? I tried a number of possible explanations for that, but none of them is fully convincing me. I believe we need further data to solve the mystery, hence I'm asking the help of more AOHC members and Pentax enthusiasts worldwide here. If you understand well what you have to check, you can just send me your lens serial # and comment like that one I wrote above (just write either PROBLEM or OK). Thanks!

Dario Bonazza

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In regard to Jon Hechtmans query regarding the Auto-Takumar 55mm f/1.8, H3v manual, page 4, (on my web site):
"Do not use the Auto-Takumar 55mm f/1.8 lens with a lens number smaller than 462500 with the H3v and H1a camera bodies, for its automatic diaphragm will not work correctly due to modification and improvement of the instant return mirror and automatic diaphragm mechanism of these new models."

That would also presumably apply to Sv and S1a's..., accounting for part of the problems Jon describes.

Paul M. Provencher

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Obviously, the Auto-Takumar incompatibility was known to Asahi at the time, so we're not dealing with a misaligned lens or camera. And this information proves the assumption that the auto-stop-down mechanism underwent running changes during its production. Obviously the "modern" version of the lens mechanism came on-line with lens S/N 462500. Since my lens has an earlier number, it has the earlier mechanism.

This clearly explains why my lens will not function properly on my SV (since the SV and the H3v are the same camera). I think it's very interesting, however, that the lens also fails to work properly on my S3...which should not, in theory, present any problems. My guess is that the camera "modification and improvement" described in the manual actually began with late models of the S3 (another running change).

With the benefit of hindsight, we would want to observe that this lens also does not work properly on the Spotmatic. Again, I think it's interesting that the lens DOES work properly on my SL.

I think this is very helpful, and very interesting. I'm indebted to Paul (for only about the thousandth time) for his help.

Jon Hechtman

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Well, we've got the most important information on this topic. However, it looks like those devices underwent far more modifications than those reported by Asahi literature, hence a wider survey of body-lens compatibility should be still interesting.

Dario Bonazza

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I disassembled the mirror box of my Honeywell Pentax H3 #387487, from the main body casting to do some lubrication on the slow speeds escapement. I looked at the mechanism in this H3 for similarities and differences relative to other Pentax S/H models.

What I observed was that there is very little difference between the mechanism of this particular H3 and the later H3v/SV models. In fact the only difference is that the H3v mirror box has an extra lever that prevents depressing the shutter button when the mirror is not charged (i.e., when the shutter is not cocked).

The H3 is thus much closer in design of the mirror and diaphragm mechanisms to the H3v/SV than it is to the H2/S2 and H1/S1 cameras which I have in my collection. There is always the possibility that this camera was modified or updated, however. It has been fitted with a biprism focusing aid, replacing the standard microprism, and bears a Honeywell service sticker from their shop in Boston, MA., on the inside of the camera back. I suppose if the mirror box were damaged by the date this camera was repaired (Jan. 9, 1974), the use of a later H3v/SV mirror box could have been the means used to repair it, especially in a factory shop with access to replacement assemblies.

Gene Poon

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