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05-22-2013, 04:08 PM   #1
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M 35/2.8, bogus aperture scale?

My "new" M 35mm 2.8 has nice equidistant aperture ring clicks with markings being as follows:
2.8 - * - (unmarked) - 5.6 - (unmarked) - 8 - (unmarked) - 11, etc.

* should be f/4 - but only one click after f/2.8? That struck me as odd. If at least there were twice the travel compared to full-stop clicks, as between f/16 and f/22, but no. The aperture also looked suspiciously large compared to what it should be.

So I consulted green button metering, and what do you know, it's nice half-stop steps from f/2.8 all the way down to f/16.

IOW, the scale appears to be bogus, and in fact should read:
2.8 - (unmarked) - 4 - (unmarked) - 5.6 - (unmarked) - 8 - (unmarked) - 11, etc.
Which makes an awful lot more sense, also considering that the lens already is quite usable wide open. Why would they not provide an f/3.3 stop then? That leaves the last two stops at f/13 and f/19 though.

Did anyone else notice that? Is it just my sample, maybe (with 30 year old lenses, you never know)? Otherwise I'd say somebody goofed at the time.

Performance is about on par with a (presumed to be newer) Chinon 5-element MC job, btw, with a bit of decentering.

BTW, has anyone ever determined true open aperture for the M 28mm 3.5? Honestly it meters like an f/4, though it's slightly brighter in the center compared to some other 28s at f/4 (but vignetting means the outer areas are darker). I'm inclined to think that f/3.5 sold better...

05-22-2013, 04:17 PM   #2
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QuoteOriginally posted by 52mm Quote
BTW, has anyone ever determined true open aperture for the M 28mm 3.5? Honestly it meters like an f/4,
The first problem is finding a meter that can consistently and accurately measure the difference between f/3.5 and f/4. After all, the difference is only 1/3 stop.

Having said that, I suppose that I could chime in that a little fudging in regard to maximum aperture was not unusual back-in-the-day. I own a Tamron 70-150/3.5 that I bought new in 1982. Modern Photography tested the lens and measured the physical aperture at f/4. Go figure...

Now, the question at this point might be whether 1/3 stop is going to affect the quality of your final image.

Oh...and one other thing. Lack of a half-stop click between maximum aperture and the next full stop down is not unusual. In addition, equidistance on the ring means little when going to the maximum setting. That last distance is simply what it takes to full retract the blades. Sooo...I guess if you are really wanting to know, disassembly and measurement of the opening should provide the final answer...assuming, of course that the lens is actually 35mm focal length.


Steve

Last edited by stevebrot; 05-22-2013 at 04:24 PM.
05-22-2013, 06:16 PM   #3
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QuoteOriginally posted by 52mm Quote
BTW, has anyone ever determined true open aperture for the M 28mm 3.5?
The aperture number is likely to be accurate...however, of course, it does not account for actual transmission (as it is not a t-stop).
05-22-2013, 07:24 PM   #4
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QuoteOriginally posted by fuent104 Quote
The aperture number is likely to be accurate...however, of course, it does not account for actual transmission (as it is not a t-stop).
Excellent point. That is why we use TTL metering!


Steve

05-22-2013, 09:39 PM   #5
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QuoteOriginally posted by 52mm Quote
My "new" M 35mm 2.8 has nice equidistant aperture ring clicks with markings being as follows:
2.8 - * - (unmarked) - 5.6 - (unmarked) - 8 - (unmarked) - 11, etc.

* should be f/4 - but only one click after f/2.8? That struck me as odd. If at least there were twice the travel compared to full-stop clicks, as between f/16 and f/22, but no. The aperture also looked suspiciously large compared to what it should be.

So I consulted green button metering, and what do you know, it's nice half-stop steps from f/2.8 all the way down to f/16.

IOW, the scale appears to be bogus, and in fact should read:
2.8 - (unmarked) - 4 - (unmarked) - 5.6 - (unmarked) - 8 - (unmarked) - 11, etc.
Which makes an awful lot more sense, also considering that the lens already is quite usable wide open. Why would they not provide an f/3.3 stop then? That leaves the last two stops at f/13 and f/19 though.

Did anyone else notice that? Is it just my sample, maybe (with 30 year old lenses, you never know)? Otherwise I'd say somebody goofed at the time.

Performance is about on par with a (presumed to be newer) Chinon 5-element MC job, btw, with a bit of decentering.

BTW, has anyone ever determined true open aperture for the M 28mm 3.5? Honestly it meters like an f/4, though it's slightly brighter in the center compared to some other 28s at f/4 (but vignetting means the outer areas are darker). I'm inclined to think that f/3.5 sold better...
The aperture click stops for this lens are:
2.8 4(white dot) 4.8 5.6 6.7 8 9.5 11 13.5 16 22

Phil.
05-23-2013, 07:59 AM   #6
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i dont have the 35/2.8 but my 28/2.8 is marked in an identical manner, as noted by Phil also.

According to pentax normal design, the first and last aperture clicks are full stops, and that is how i consider it.

What I would be suspect about if anything, is not the intermediate stops, but the accuracy of the first and last. The maximum aperture while in many cases real, is usually the one the manufacturers "cheat" or encroach the limits of standards, in terms of accuracy., Fast lenses command higher prices, therefore they will claim as fast as they can, within tolerance limits.

At one time, lens tests showed accuracy of aperture vs fstop. no more
05-23-2013, 01:08 PM   #7
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True enough, the accuracy of the last stop can be off. Apertures may look pretty funny at this point. Several of my Chinon lenses also tend to become darker as you stop down and choose exposure time according to nominal f-stop.

The M 35mm 2.8 is a perfectly fine f/2.8 wide open, no complaints there (brightness at my usual f/2.8 exposure time is what it should be - I always use the same lamps in my test setup). It's just that the next step (on my sample) is about f/3.3, not f/4, so it actually goes
2.8 3.3 4 4.8 5.6 6.7 8 9.5 11 13 19
or thereabouts. Actually brightness creeps up a little as you stop down, being about +1/4 EV by the nominal f/16 position even with exposure time set for f/13. And lo and behold, what this lens calls f/22 corresponds to f/16 on the Chinon counterpart when looking at the physical aperture (or images when applying exposure compensation to make up for the different time settings). The Chinon lens becomes a little darker as you stop down, so with the half-stop scale offset to begin with they finally meet. I have to apply almost +1/3 EV to the Chinon's image at f/8 to get the same brightness as on the M 35/2.8 at a nominal f/9.5. (Both lenses give maximum file sizes / sharpness at this point.)

Guesstimating by eye, I'd say "f/11" is about as big as f/11 on a 50 (while in theory they should differ by exactly one stop), same for "f/8" and "f/5.6". "*" is between f/4.8 and f/4 on the 50, and wide open is a bit bigger than f/4.

I'd say they probably intended the aperture to work like on the M 28/2.8, but at least on my sample, this didn't work out.

TL;DR: My sample of the M 35/2.8 has a +1/2EV offset from the second aperture stop down (vs. what the scale says), and gains about an extra +1/4EV by minimum aperture. The Chinon counterpart (49mm MC, 0.3m min focus) seems broadly accurate but loses about 1/4EV by minimum aperture.

Does anyone else have an M 35/2.8 they could check?

Last edited by 52mm; 05-23-2013 at 01:15 PM.
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