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Metering with K and M lenses

johnriley
Posted 23/02/2010 - 21:35 Link
I was finishing off an article today for PU and one snippet that the research threw up was a bit about the first cameras with a "crippled" K mount such as we use today.

What I'd forgotten was that the exposure errors and difficulties with non-A lenses were always there, even with the film cameras. It was always a problem with the MZ30, 50, 60 cameras. It must be intrinsic in the way the meter coupling works on the earlier lenses. This is the linear/non-linear movement problem.

Clearly nothing can be done about it. It was probably not a problem for us in those days as many of us would have bought the MZ cameras with KAF or KAF2 mounts, both of which were compatible with K and M lenses.
Best regards, John
womble
Posted 23/02/2010 - 22:18 Link
Interesting little piece of information. The exposure problem is a pain with digital, but with film must have been a real problem especially if taking slides. Another reason to avoid the 30/50/60.

I wish they would un-cripple the mount, or find a workable way around the metering issues. Personally I like using an aperture ring as that is where my fingers from my left hand naturally fall. Too many years with an ME Super I guess.

Best wishes, Kris.
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

My website
iceblinker
Posted 23/02/2010 - 22:55 Link
I would be interested to learn what the "linear/non-linear movement problem" is. Movement of what? The diaphragm moves alright when it comes to the exposure.
~Pete
johnriley
Posted 23/02/2010 - 23:43 Link
It was the amount of physical movement of the actuator arm when the aperture was changed. I did read that prior to the A series it was an uneven movement (non linear) and from the A lenses onwards became linear.

It could make sense.
Best regards, John
womble
Posted 24/02/2010 - 00:11 Link
I was playing around with my A series 50mm f/1.4 this afternoon as a result of another thread. I took some pictures with the aperture ring set on A and at f/8 and then the same image (the camera was on a solid tripod) with the K20D in manual and the lens set to f/8. With the same notional aperture, ISO and shutter speed the image (according to the histogram) was about half a stop brighter with the lens set to f/8 than when the lens was set to A and f/8 selected via the camera body. Seemed mighty odd to me but I tried it several times and it was pretty consistent.

I wonder if this is related?
Kris Lockyear
It is an illusion that photos are made with the camera… they are made with the eye, heart and head. Henri Cartier-Bresson
Lots of film bodies, a couple of digital ones, too many lenses (mainly older glass) and a Horseman LE 5x4.

My website
iceblinker
Posted 24/02/2010 - 12:16 Link
I'm still confused because the correct aperture tends to be selected when it comes to the exposure. Stop-down metering should do exactly the same physical thing with the lens. No reason why it shouldn't. Therefore it should take the light reading with the selected aperture.

I wonder if there there is a timing problem sometimes where the reading is taken before or after the lens is stopped down to the selected aperture.

I've seen it suggested that the meters simply aren't designed to work with f numbers above a certain value. But if that was true, I don't understand how they work with a large aperture in low light. They tend to be reliable them except in extremely low light.
~Pete
mph555l
Posted 24/02/2010 - 12:18 Link
There is a pretty good description of the crippled mount on Bojidar Dimitrov's site:

link

cheers,

/Ian
K10D & Super A user.
iceblinker
Posted 24/02/2010 - 12:21 Link
mph555l wrote:
There is a pretty good description of the crippled mount on Bojidar Dimitrov's site:

link

Yes, thanks, but it doesn't explain why stop-down metering with the crippled mount is often unreliable. That's the mystery here.
~Pete
johnriley
Posted 24/02/2010 - 15:39 Link
The point is that the stop down of older lenses operates in a non-linear fashion, wheras the A lenses and after operate in a linear fashion electronically. If the original actuator is missing then the lens is stopped down by some other means that is non-compatible, hence the exposure drifts steadily further afield as you stop down.

This may not be exactly right, but I believe from what I've read that something like this may be happening.

Just be glad we don't use Olympus, where the use of OM lenses on current models is restricted to specific lenses and a few apertures.
Best regards, John
Telstar
Posted 24/02/2010 - 15:50 Link
There was a similar thread a month or two ago, where someone had evidence of it being related to the focus screen. (They had exactly this problem until they changed the screen for a cut-down older type IIRC)

here we go , halfway down the 2nd page, poster was Axl.

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Edited by Telstar: 24/02/2010 - 15:51
stu49
Posted 24/02/2010 - 16:34 Link
womble wrote:
I was playing around with my A series 50mm f/1.4 this afternoon as a result of another thread. I took some pictures with the aperture ring set on A and at f/8 and then the same image (the camera was on a solid tripod) with the K20D in manual and the lens set to f/8. With the same notional aperture, ISO and shutter speed the image (according to the histogram) was about half a stop brighter with the lens set to f/8 than when the lens was set to A and f/8 selected via the camera body. Seemed mighty odd to me but I tried it several times and it was pretty consistent.

I wonder if this is related?

Did tests myself yesterday after reading about that guy about to use a K lens on his new K-m (I think).

Got a definite difference between 'A' setting and 'F8', or whatever.

Isn't this simply the difference between the distance the body and the aperture ring stop down the lens !?

Stu - K10D | DFA 100/2.8 Macro | DA 55-300 | AF-360FGZ |

So much to learn... So little time !
Edited by stu49: 24/02/2010 - 16:51
iceblinker
Posted 24/02/2010 - 16:36 Link
johnriley wrote:
The point is that the stop down of older lenses operates in a non-linear fashion, wheras the A lenses and after operate in a linear fashion electronically. If the original actuator is missing then the lens is stopped down by some other means that is non-compatible, hence the exposure drifts steadily further afield as you stop down.

What about my point that the lens does stop down to the correct position at the time of the exposure? If the camera can do that, it can also stop down correctly at the time of the light reading.

- - -
Re the focusing screen theory: I don't believe it and it makes no sense to me. I also had the problem with an *ist DS with the original screen that is said to be better in this regard than later ones.
~Pete
iceblinker
Posted 24/02/2010 - 16:42 Link
Even if the lens stopped down to an incorrect position for the exposure, why wouldn't it also do the same for the light reading (when you press the Green button)?

If the reading is taken at the aperture that is used for the exposure - and I see no reason why it isn't - then the question is why is the result incorrect?
~Pete
stu49
Posted 24/02/2010 - 17:25 Link
Just tried taking readings at fully open (f3.5) using 'A' and '3.5' positions and pressing Green button !

NO stopping down, and I get different results !!

1/30th, 1/20th respectively !

This seems to imply the aperture ring is not the main culprit ! It's the metering !?

Stu - K10D | DFA 100/2.8 Macro | DA 55-300 | AF-360FGZ |

So much to learn... So little time !
Edited by stu49: 24/02/2010 - 17:29
iceblinker
Posted 24/02/2010 - 19:12 Link
stu49 wrote:
Just tried taking readings at fully open (f3.5) using 'A' and '3.5' positions and pressing Green button !

NO stopping down, and I get different results !!

Yes there's something funny going on there, but that's not the whole story because the error worsens the further the lens is stopped down.
~Pete

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