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08-02-2008, 07:39 AM   #1
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Programed exposure compensation?

Hello,
When I use M series and M42 lenses in manual mode with exposure lock, The exposure is usually close to correct. When I shoot in AV mode with M42 lenses, I have to add +1.5 compensation for correct exposure. This leads me to the conclusion that the camera compensates for the film type lenses when using exposure lock. Has anyone noticed this?

Dave

08-02-2008, 04:06 PM   #2
Rob
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M42 exposure & f16 rule

This is precisely why I am visiting the Forum today! I just got an adapter and the Vivitar lens, and I am very happy to see the camera works on Av with that setup.
However, there is an exposure problem.
At first I suspected the Hoya Skylight 1A, but that has nothing to do with it.

I attempted this times 3 times in ever increasing level of experimental control. I was attempting to prove that the final compensation we see is justified because the f16 rule confirms the shutter, aperture and ISO combination. I wanted to, but the numbers disagree.
This is what I did:
I took the camera outside in bright light, on a tripod, focused on a flower 2 feet out.
Set ISO to 100
Av mode
aperture ring f16
covered the eyepiece when shooting or checking exposure. (learned the hard way)

this shows the shutter speed selected by the camera with various compensation settings.
compensation time
-1.0 1/125
0.0 1/90
+1.0 1/30
+2.0 1/10

change to Manual mode
This shows the exposure as measured by the camera at various forced shutter speeds
exposure time
-3.0 1/125
-2.5 1/90
-2.5 1/60
-1.5 1/30
0.0 1/10
+2.5 .5

finally, took a picture at
0.0 1/10

Conclusion: Something is funny. I expect to shoot 1/100 speed at f16 in full sun with ISO 100.
Forcing the shutter to 1/90 or 1/125 in Manual mode makes an extremely under exposed image.
What works is 1/10, which is achieved with an EV compensation of +1.5 or +2.0 .
Next:
What I do not have time to do is check my Sigma lens (see Blondie in today's paper).
Is ISO 100 actually the same thing in digital as film?
08-04-2008, 03:24 AM   #3
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QuoteOriginally posted by Big Dave Quote
Hello,
When I use M series and M42 lenses in manual mode with exposure lock, The exposure is usually close to correct. When I shoot in AV mode with M42 lenses, I have to add +1.5 compensation for correct exposure. This leads me to the conclusion that the camera compensates for the film type lenses when using exposure lock. Has anyone noticed this?

Dave
Yes. I've noticed such BUG. But... it occurs only on Penta-mirror Pentax DLSRs. On Penta-prism DLSRs (K10D, *istDS, K20D - everything is O.K. - I have 2x K10D and *ist DS and had K100D and K100Ds).
I think this happens due to some reflections inside penta-mirror as m42 lenses are designed for 35mm format and cover larger circle than APS-C lenses.
08-09-2008, 06:22 PM   #4
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QuoteOriginally posted by Zebooka Quote
<snip>I think this happens due to some reflections inside penta-mirror as m42 lenses are designed for 35mm format and cover larger circle than APS-C lenses.
No such problem with my SMC-M It is also designed for 35 mm.
Not only does the camera select an apropriate exposure, it also falls right in with the f16 rule in full sun.

The M42 lens acts like it is stopped down: if I set it to f16 and 1/90 the pictures are black. I have to use 1/10, the camera (with no compensation set) will select 1/60 , way to fast to get a good exposure.

Dave - can you verify this?

08-09-2008, 10:08 PM   #5
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Original Poster
Rob,
I may get a chance to do a couple experiments tomorrow. I have seen different results with different M42 lenses, especially in low light situations with the K100DS. I can't see how the larger FF image circle could effect exposure, since the top of the mirror box is lower to match the smaller sensor. I could try adding a 66% reducing mask to the rear of the lens and see if it had any effect. It would be difficult using black paper. I do think that the small dark mirror prism of the K100D causes problems in low light situations. Thanks for the data.

Dave

QuoteOriginally posted by Rob Quote
No such problem with my SMC-M It is also designed for 35 mm.
Not only does the camera select an apropriate exposure, it also falls right in with the f16 rule in full sun.

The M42 lens acts like it is stopped down: if I set it to f16 and 1/90 the pictures are black. I have to use 1/10, the camera (with no compensation set) will select 1/60 , way to fast to get a good exposure.

Dave - can you verify this?
08-10-2008, 11:07 AM   #6
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I was reffered to this thread
https://www.pentaxforums.com/forums/pentax-dslr-discussion/21530-k10d-smc-m-mettering-3.html
by bubuli

It is an extensive thread, I have not read it in detail, but I intend to do so.
I'm surprised I didn't find it myself, I did several searches.
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