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01-05-2009, 10:41 PM   #1
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Metering is off in Av with Takumar 50mm

All,

K200d arrived in the mail today.

I have a 50mm Takumar SMC which I've been experimenting with. Aperture ring is set to "permitted" which is correct AFAICT for M42 lenses.

Metering in manual (green button) gives good results, but metering in aperture priority mode (Av) is ~1.5 stops too dark. Why? Can I make it meter correctly -- the camera certainly seems capable as manual mode is spot-on.

Thanks,

Reid

01-05-2009, 10:44 PM   #2
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Shorting out the caera contacts with aluminum foil works like a charm.

Without Aluminum Foil


With Aluminum Foil

01-05-2009, 11:04 PM   #3
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In the end ... jsut used the Green Button ... it becomes 2nd nature to do it quickly ... and more precise metering for me.
01-05-2009, 11:19 PM   #4
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Out of curiousity, did you remember to move the switch on the lens from Auto to Manual ("A" to "M) so that the lens has a manual aperture?

Even with the lens properly stopped down, you can expect the potential for inaccurate metering from all M-42, Pentax-K, and Pentax-M lenses. I don't have experience with the K200D, but both the K10D and K20D may underexpose 1-2 stops at apertures wider than f/4 depending on the taking aperture and the lens.

Steve

01-05-2009, 11:59 PM   #5
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Well, Lo & Behold. Thank you Maxwell1295. I have been tearing my hair out. I'll give that a go tomorrow.
01-06-2009, 08:26 AM   #6
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QuoteOriginally posted by maxwell1295 Quote
Shorting out the caera contacts with aluminum foil works like a charm.
By gum, it works! Trap focus also works now too.

I don't think I'll use foil on a regular basis... too fiddly... but perhaps I'll sand the paint off the lens bases. I recall someone here posting directions on how to do that but hadn't fully processed the benefit.

This forum is great!
01-06-2009, 02:07 PM   #7
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QuoteOriginally posted by rpriedhorsky Quote
By gum, it works! Trap focus also works now too.

I don't think I'll use foil on a regular basis... too fiddly... but perhaps I'll sand the paint off the lens bases. I recall someone here posting directions on how to do that but hadn't fully processed the benefit.

This forum is great!
Yep, focus trap should work now too (I forgot to mention that part)

Glad to hear you got it all sorted out. What I'd like to do is come up with a semi-permanent solution. Those little strips of aluminum foil can be a PITA if you're swapping lenses frequently. I'll trap sanding the paint to see how that works out.
01-06-2009, 02:39 PM   #8
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Last not least to mention,
one more option is to dial in some exposure compensation in AV-mode.
On my K100D, I have to hold the little AV-button on top, and then dial in the compensation through the wheel. Nice for any 'manual' or preset lens.

Like mentioned before, it will typically be +1 to +2 normally with isolated lens back.
This goes for apertures say f4 to f11.
On a fast lens and open apertures I've got to dial the exposure back to around 0.
If however you had Auto-ISO function before you will loose it when going away from 0 EV.

AFAIR my best behaving lens in AV-mode is the SMC-M 1.7/50 (blank back___edit: and converted to 'manual').
Best, Georg (the other)

PS: Strange, I thought the K10D would behave the other way around, overexposing in AV-mode the more you stop down.


Last edited by georgweb; 01-07-2009 at 11:30 AM.
01-06-2009, 03:15 PM   #9
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QuoteOriginally posted by stevebrot Quote
Even with the lens properly stopped down, you can expect the potential for inaccurate metering from all M-42, Pentax-K, and Pentax-M lenses. I don't have experience with the K200D, but both the K10D and K20D may underexpose 1-2 stops at apertures wider than f/4 depending on the taking aperture and the lens.

Steve
Steve,

Do you have any suggestions to deal with the underexposure with the M lenses. I recently bought an M 85/2, and stopping down with the green button to set the shutter speed, I seem to get a lot of underexposed shots with my k200. I think I've tried bumping up the ISO to 500 or so, but that doesn't seem to make a big difference. Do I need to really jack up the ISO? IIRC, exposure compensation is disabled with the M lens in M mode, but maybe I am remembering incorrectly. I've been busy, so I haven't had a lot of time to experiment with the M lens, and it is my first one. Any suggestions would be appreciated. Thanks.
01-06-2009, 03:52 PM   #10
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my experience with my 35mm F2 is similar, I just set exposure copmensation.

I have not tested for a difference with green button on my K10 and Av mode, as Av mode is much more interesting to use with an M42 lens.

As for the K10D, I have made many posts regarding the non linear exposure as a function of F stop with K mount lenses.

See this link

https://www.pentaxforums.com/forums/241716-post69.html
01-06-2009, 04:55 PM   #11
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QuoteOriginally posted by jeffrey r Quote
Steve,

Do you have any suggestions to deal with the underexposure with the M lenses. I recently bought an M 85/2, and stopping down with the green button to set the shutter speed, I seem to get a lot of underexposed shots with my k200. I think I've tried bumping up the ISO to 500 or so, but that doesn't seem to make a big difference. Do I need to really jack up the ISO? IIRC, exposure compensation is disabled with the M lens in M mode, but maybe I am remembering incorrectly. I've been busy, so I haven't had a lot of time to experiment with the M lens, and it is my first one. Any suggestions would be appreciated. Thanks.
Congratulations on the M 85/2! It is a plenty decent lens by all reports.

Sorry to say, but I don't have any really good suggestions. This is a complicated issue that has something to do with the focus screen and how meter output is managed to generate a reading.

My general flow for Pentax-K and Pentax-M lenses in M mode:
  • Green button meter at about f/5.6
  • Take a test shot
  • Adjust the shutter speed if needed (use histogram)
  • Change to the desired aperture and adjust the shutter speed appropriately

This may be abbreviated to:
  • Choose your desired aperture
  • Use the green button to get a ballpark exposure estimate
  • Take a test shot
  • Evaluate the histogram and adjust the shutter speed appropriately

Lowell Goudge has done quite a bit of testing and recommends replacement of the stock screen with one from a *istD. You might also want to try the foil trick to short the contacts. I have not had much success with it, but my understanding is that you can trick the body into thinking an "A" lens is mounted. You then manually set the aperture on both the body and the lens (Maxwell...correct me if I am wrong here) and the camera will set the appropriate shutter speed in AV or P mode.

Steve
01-06-2009, 05:31 PM   #12
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QuoteOriginally posted by stevebrot Quote
Lowell Goudge has done quite a bit of testing and recommends replacement of the stock screen with one from a *istD. You might also want to try the foil trick to short the contacts. I have not had much success with it, but my understanding is that you can trick the body into thinking an "A" lens is mounted. You then manually set the aperture on both the body and the lens (Maxwell...correct me if I am wrong here) and the camera will set the appropriate shutter speed in AV or P mode.

Steve
Steve / all interested

The tests I have done are related to the K10, and on the assumption that since the focusing screen is the same, the K20 behaves the same way.

For any one who is serious about understanding how their equipment really works, in terms of exposure, you should at least do a set of controlled tests.

I use a block wall, or paved road, under consistent lighting (usually clear sky)

Using the green button to set exposure, take a set of shots at each apature detent, and then measure the grey scale value for the centra 10% of the frame area using your photo editor (I use PSP X2) perfect exposure, I believe is 110, and you will have a change in greyscale of about 45 (with neutral contrast) for each F stop. This will tell you how your camera meters with each F stop.

You can also do another set of shots, starting with the middle apature (about F8) metered correctly, and then, without changing shutter speed, take shots from wide open to fully stopped down.

Again measure greyscale and check the linearity of exposure. I have found a few lenses that have a slight drift towards over exposure of about 1/2 stop between wide open and minimum apature. Note you may have to do a couple of tests perhaps one metered at f11-16 and the other at F4 to get a full range, as the exposure becomes non linear for greyscale values below about 25 and above 225

If you do these tests for each lens, then you will have a perminant record of lens and camera performance, and then you should always be able to compensate for perfect exposure.
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