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11-16-2009, 01:58 PM   #1
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Help! Manual Lens on K20D ... shots are underexposed

I have a Takumar 135mm f2.5 lens ... incredibly sharp.

Using a M42 adapter to mount it to the K20D body.

Using "Av" mode. Exposure setting is at center. Camera appears to meter properly. For instance, if I were to pan from bright to dark subjects ... I see the K20D compensating by changing shutter speed. So its doing something.

However, when I take the shot ... its always severely underexposed. I have to set the EV dial to +2 to get a properly exposed image.

Can someone please help ?


Last edited by vulcanman; 11-16-2009 at 02:13 PM.
11-16-2009, 02:07 PM   #2
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Welcome to the forum.
There are a lot of possibilities.
Firstly, the metering mode matters with regards to adequate exposure.
Secondly, metering should be performed on the intended subject and locked on that subject to get the right exposure for that subject. Locking focus and recomposing without having locked auto exposure settings means the camera meters the scene continually despite locking focus and may throw off metering for the intended subject (so check the menu setting for this function).

Thirdly, if you DO have AE-L linked to AF point and are locking focus in a bright scene then recomposing to a darker scene, the photo will definitely be underexposed. The camera will only do what you tell it to do.

Hope this helps.
11-16-2009, 02:12 PM   #3
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Seems like I was unclear in my description of the problem.

The camera+ManualLens underexposes a still shot (no panning). I only mentioned panning to relay the fact that the camera was metering to the light it sees and changing the shutter accordingly.

I was only taking still shots.
11-16-2009, 02:14 PM   #4
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You may foid that you have several exposure issues on the K20 with M42 and K (not KA) mount lenses.

First of all, my experience is that M42 lenses tend to under expose on the DSLEs by about a stop.

Second, on the K20, (and K10) the stock focusing screen causes problems with exposure on manual aperture lenses. it will under expose by about a stop wide open on fast lenses, moving to correct exposure at about F4 to over exposing by 1-2 stops by F8-11 then falling back a little at F22-32

combine these and you may find it is difficult to get good exposure on the K20.

I have switched to a split image screen on my K10 and it meeters manual lenses much more consistently

11-16-2009, 02:27 PM   #5
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Lowell ... thanks for the advice and info //
11-16-2009, 02:31 PM   #6
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Try metering in manual mode to see if there is any difference.
If the underexposing is consistent, you only need to work out what compensation to apply for a particular lens and then dial it in whenever you use that lens.
11-16-2009, 02:49 PM   #7
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I find that when using SMC Pentax-M lenses on my K10 and in Manual mode and using the green button to meter. The camera will fairly consistently over expose by at least one stop or more. So whenever I use the green button to read exposure, I'll set the shutter speed one full stop faster than the meter calls for and that will give me a fairly well exposed shot.
11-16-2009, 04:31 PM   #8
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Lowell is absolutely right in his description of the causes... Thanks to him, I tried a LL60 focus screen (made for the *istD line) on my k10 (and then on my k20), and this lessens the problem to a satisfying level... Not perfect, but to a level were you don't have to think about what EvComp to use for what aperture.

So, if you're into manual lenses, get one (they are so cheap it's a crime not to buy one anyway!)

11-16-2009, 04:58 PM   #9
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QuoteOriginally posted by Lowell Goudge Quote
You may foid that you have several exposure issues on the K20 with M42 and K (not KA) mount lenses.

First of all, my experience is that M42 lenses tend to under expose on the DSLEs by about a stop.

Second, on the K20, (and K10) the stock focusing screen causes problems with exposure on manual aperture lenses. it will under expose by about a stop wide open on fast lenses, moving to correct exposure at about F4 to over exposing by 1-2 stops by F8-11 then falling back a little at F22-32

combine these and you may find it is difficult to get good exposure on the K20.

I have switched to a split image screen on my K10 and it meeters manual lenses much more consistently
Lowell,
Which screen do you have? Is it one of the Chinese ones, or?
11-18-2009, 07:19 PM   #10
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QuoteOriginally posted by Steve Beswick Quote
Lowell,
Which screen do you have? Is it one of the Chinese ones, or?
steve

I have the jinfinance 45 degree diagonal split image screen.

Aside from theloss of spot metering due to the split image, it works better than the origonal screen for metering.

If you don't want a split image, and are OK with a plain screen the screen from the *istD works exceptionally well in a K10D or K20D. In fact, I posted the results of testing my K10D with both it's screen and the *istD screen, here is a link.

https://www.pentaxforums.com/forums/241716-post69.html

As you can see, using the same lens and same focusing screen yeilded the same exposure results in my *istD and K10D, what is really impressive about the test is that the two cameras were tested separately over a year apart. Therefore this is a very repeatable test method
11-18-2009, 10:29 PM   #11
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QuoteOriginally posted by Lowell Goudge Quote
You may foid that you have several exposure issues on the K20 with M42 and K (not KA) mount lenses.

First of all, my experience is that M42 lenses tend to under expose on the DSLEs by about a stop.

Second, on the K20, (and K10) the stock focusing screen causes problems with exposure on manual aperture lenses. it will under expose by about a stop wide open on fast lenses, moving to correct exposure at about F4 to over exposing by 1-2 stops by F8-11 then falling back a little at F22-32

combine these and you may find it is difficult to get good exposure on the K20.

I have switched to a split image screen on my K10 and it meeters manual lenses much more consistently
Lowell: So true. Interestingly, I was trying out my Super tak 55/1.8 and came to nearly the same conclusion. F4 was just about right. My K10d definitely underexposes as wider exposures. It does so on my 50/1.4 SuperTak too. I would note though that I determined this despite using a Katz eye split screen.

I love the M42's but it is frustrating that the error in metering varies depending on lens, chosen aperture, and seemingly different lighting (tungsen).
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