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05-17-2009, 07:42 AM   #1
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Question about exposure with M42 lens.

Hi!

I (again) have a question. I've using a 50mm f/1.4 M42 Super Takumar with my K200D in the Av mode, I read that the camera can calculate the shutter time and the ISO sensivity despite this being a manual lens.

However, many times the exposure goes terribly wrong when either aperture is too high or too low, I usually get either dark pictures at small apertures, or overexposed ones at the other end. When I switch to manual mode and make use of the green button it happens the other way, pictures come out with the correct exposure most of the time.

Does any one knows why this might be happening?

Thank you in advance.

05-17-2009, 09:08 AM   #2
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This is a common error with lenses without an A position on the aperture ring whether screw mount or bayonet. Many people on this forum recommended changing the focus screen to the LL-60 screen. I have done that, and now my M lenses meter perfectly.

In the meantime, there is one aperture at least that meters correctly. Meter using that aperture and then change the aperture and shutter speed to the mix you need for your image.

You will also find that if you are in an automatic exposure mode, the camera will revert to Av mode, and shoot only wide open. Your lens will only stop down for exposure in M mode.
05-17-2009, 09:51 AM   #3
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Somehow I've just answered myself... it is pretty obvious that the camera would "wrongly" calculate exposure since it is metering a darker or lighter image when the aperture is small or big. The camera just meters a dark or too bright image, it does what it is intended to do after all, isn't it?

As you said before, I must find the aperture with the "correct" exposure, and then compensate (by means of AV and/or ISO/shutter speed accordingly to how many steps I move beyond that "zero" aperture.

Ha ha ha, I've learned a lot from this experience, thank you!

Last edited by hahifuheho; 05-17-2009 at 09:53 AM. Reason: Spelling.
05-17-2009, 10:25 AM   #4
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QuoteOriginally posted by Canada_Rockies Quote
You will also find that if you are in an automatic exposure mode, the camera will revert to Av mode, and shoot only wide open. Your lens will only stop down for exposure in M mode.
Oh, no. The OP asked about M42 lenses, which need to and can be stopped down manually. What you said applies only to M (or K) lenses.

05-17-2009, 11:42 AM   #5
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QuoteOriginally posted by hahifuheho Quote
a 50mm f/1.4 M42 Super Takumar with my K200D in the Av mode

Does any one knows why this might be happening?
I've not got a K200D, but I guess it is similar to the K100D in this respect. In Av there will be metering errors. Different lenses have different errors, and the errors vary with the aperture used. However, the camera will meter correctly in M when using the green button (on the K100D that is the AE-L button).

However, there is a workaround. If you short the camera contacts with a piece of Aluminium foil between the lens and camera mount the camera will meter correctly. I have tried this with my Karl Zeiss lenses and my Takumars and it works. However, if you regularly change lenses it can be tedious since the foil gets ripped. I've used shims - the metal that you get in shop security tags is suitable - but they are easy to lose (and before now I've cut my thumb on the corner of the shim when focusing, so I always cut off the corners). Consequently, I usually just use M mode and the green button, it's much easier.

(The only time when the short-the-contacts trick is essential is when you want to do "focus trap". In this case the camera is in AF mode, you depress the shutter and turn the focus ring and when something comes into focus the camera takes a shot - a kind of poor man's AF with MF lenses! This does require the contact shorting.)

Richard
05-17-2009, 11:47 AM   #6
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QuoteOriginally posted by richard64 Quote
I've not got a K200D, but I guess it is similar to the K100D in this respect. In Av there will be metering errors. Different lenses have different errors, and the errors vary with the aperture used. However, the camera will meter correctly in M when using the green button (on the K100D that is the AE-L button).

However, there is a workaround. If you short the camera contacts with a piece of Aluminium foil between the lens and camera mount the camera will meter correctly. I have tried this with my Karl Zeiss lenses and my Takumars and it works. However, if you regularly change lenses it can be tedious since the foil gets ripped. I've used shims - the metal that you get in shop security tags is suitable - but they are easy to lose (and before now I've cut my thumb on the corner of the shim when focusing, so I always cut off the corners). Consequently, I usually just use M mode and the green button, it's much easier.

(The only time when the short-the-contacts trick is essential is when you want to do "focus trap". In this case the camera is in AF mode, you depress the shutter and turn the focus ring and when something comes into focus the camera takes a shot - a kind of poor man's AF with MF lenses! This does require the contact shorting.)

Richard
Will this damage the camera?
05-17-2009, 01:07 PM   #7
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QuoteOriginally posted by res3567 Quote
Will this damage the camera?
No. It won't hurt the camera.

The only contact that need be shorted is the one at the bottom of the camera side of the lens mount; This is most easily done by scraping paint off the lens in that location.

Mount the lens & use a pencil to make a little mark on the side of the lens that lines up with the appropriate contact then take off the lens & remove some finish in that area of the lens. A magic marker can be used to "refinish" the lens if necessary.

Some lens are too small in diameter & the lens bottom doesn't touch the contact in which case use foil.

Iowa Dave
05-17-2009, 01:52 PM   #8
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QuoteOriginally posted by SOldBear Quote
Oh, no. The OP asked about M42 lenses, which need to and can be stopped down manually. What you said applies only to M (or K) lenses.
My mistake. M42 will meter stopped down, because they do not stay wide open when the aperture ring is moved. It is, however, still true that different apertures will give different exposure errors on the K10D (and K20D, I think), unless one replaces the focusing screen with an older model such as the LL-60.

08-06-2009, 02:51 PM   #9
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Exposure with M42 lenses

Rare poster here but an avid reader and recently cured LBA.

Do you have a set of Feeler Gauges - the things you check car spark plug gaps with?

There is usually a really thin one - mine is 0.06 somethings. You can cut it with scissors.

Today I inserted it between Super Tak 135 2.5 and contacts on camera body, screwed the lens down to hold it and it works a treat. Camera (K100D) on 0.00 Ev compensation, all exposed correctly using Av programme. A lot easier than sticking tearing foil and easily removable. And re-usable.

Phil

Too many Takumars, Vivitars, Tamron adaptalls and Pentax M's
08-06-2009, 03:04 PM   #10
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what I find curious is that the OP states that the photos are too dark when stopped down and too bright when wide open.

Exactly how is the "metering" he is doing being performed.

I just let Av set the shutter appropriately

On my *istD there is a metering error with M42 lenses, but it is constant or nearly so, at about -1 stop when wide open with a slight drift to -1/2 stop between F2 and F32 but for the most part if I set the compensation to between +.7 and +1 it is so close that it does not matter

My vivitar 28mm F2.5 is about 1/2 stop closer, i.e. under expose by 1/2 stop wide open and perfect stopped down

every lens will be different.

ps I never use auto ISO, that might aggrivate things
08-06-2009, 03:19 PM   #11
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M42 exposure problems

Hahifuheho

Sorry if this is an obvious question but do you have your lens on the 'A' position? It should be on 'M' And have you changed the menu on the camera to allow 'S'?

Phil
08-15-2009, 11:29 PM   #12
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One thing to remember:

Auto Camera (K10D) = switch in Manual position
Manual Camera (Spotmatic) - switch in Auto position.
08-16-2009, 01:20 AM   #13
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This sounds stupid, but with this lens, is Manual selected when the switch is pushed to "M" or is it when the switch is pushed to "A" revealing the "M" ??
08-16-2009, 01:36 AM   #14
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Whatever letter is displayed is the mode it is in......
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