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11-02-2007, 08:36 AM   #1
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Screw Mount Weirdness

This morning, I got up early for a sunrise shoot at Montreal's Old Port. Gorgeous sky.

One of my lenses is a Super-Takumar 85mm/1.9. I have had it for six months and used it many times without a problem. This morning, when I put it on my DL2, the camera started behaving weirdly.

Normally, screw-mount lenses default to Av mode. They can also be used in M mode. But when I turned the camera on today, the camera defaulted to Tv mode. Wha--? The scroll wheel was controlling shutter speed, when the camera should have been in auto-exposure mode. It should not even be possible to put the camera in Tv mode with a screw-mount lens attached. The lens itself was stuck wide open.

I removed the lens and checked that there was nothing mechanically wrong, and there was not. I switched the lens back and forth between Auto and Manual aperture control. I turned the camera on and off. My DA lenses worked fine, but the Super-Tak did not.

I have since reset the camera, and everything is working fine again. But I am in the dark as to what could have caused this odd behaviour, and I don't know if it will happen again. Any ideas?


Last edited by Jim Royal; 11-02-2007 at 08:43 AM.
11-02-2007, 12:58 PM   #2
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There is two things that you need to check.

The first is kind of a moot point since you said everything was working perfectly now, but if it happens again, remove the lens and see if it's still exhibits the same problem. If it does the problem is probably in your camera.

If it does not check the lens mount, specifically in the areas that is around the electrical contacts. If there are any scratches in this area, one or more of the contact will be making connections, and the camera will think that you have a different type of lens other than a screw mount.

Note: This only applies to screw mount lenses that are large enough to cover these contacts.
This will also apply to T-mount lenses.

Note: if you remove ( all ) of the paint from the areas around these contacts the camera will see it as a standard K mount lens and it will eliminate the need to press the AE-L button before you shoot. Otherwise the exposure will be approximately 2 stops darker.


joe1955
11-02-2007, 02:12 PM   #3
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QuoteOriginally posted by joe1955 Quote
Note: if you remove ( all ) of the paint from the areas around these contacts the camera will see it as a standard K mount lens and it will eliminate the need to press the AE-L button before you shoot. Otherwise the exposure will be approximately 2 stops darker.
Its a little more complicated than that. With no shorting of the contacts. If the camera is in M then I have to use the AE-L to meter. I find with my M42 lenses the meter under exposes by 1 EV. If I switch to Av then I do not need to use AE-L to meter, and the meter under exposes by 2 EV. If I short the contacts then Av gives me a correct meter value, but AE-L under M still under exposes by 1 EV. Thus shorting the contacts, as you say, corrects metering in Av but has no effect for M, neither the metered value nor the use of AE-L.

Since I use M anyway I always AE-L and shift the shutter speed by 1 EV and then after taking the photo I check the histogram.

[Here's a test: in Av with the shutter half pressed if I move the camera around so that it points to light and dark the shutter speed in the viewfinder changes accordingly. In M as I move the camera around the shutter speed stays the same, and only changes when I use AE-L. This is what I expect, M is *manual*, whereas Av meters all the time when the shutter is half pressed, but locks the exposure if I press AE-L.]

Richard
11-02-2007, 03:08 PM   #4
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I did not say it would act as an automatic lens, nor did I say it would see the maximum minimum aperture, or even the focal length. A standard K mount does not use any of that information. It simply grounds out the contacts.
If the pain is left on the lens, the camera assumes no lens is connected, and steps down the metering.

If the paint is removed, the camera assumes the lens is attached. But since all contacts are grounded the camera assumes it is a standard "non-automatic" k mount lens. Nothing more was insinuated.

I have tried this with both a screw mount and a T-mount lens they both function normally as a standard K mount lens, and must be treated as such.


joe1955

11-03-2007, 02:53 PM   #5
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QuoteOriginally posted by joe1955 Quote
I did not say it would act as an automatic lens, nor did I say it would see the maximum minimum aperture, or even the focal length.
..and I was not suggesting that you did say that.

QuoteOriginally posted by joe1955 Quote
If the paint is removed, the camera assumes the lens is attached. But since all contacts are grounded the camera assumes it is a standard "non-automatic" k mount lens. Nothing more was insinuated.
Well, you did say:

QuoteOriginally posted by joe1955 Quote
Note: if you remove ( all ) of the paint from the areas around these contacts the camera will see it as a standard K mount lens and it will eliminate the need to press the AE-L button before you shoot. Otherwise the exposure will be approximately 2 stops darker.
Which implies that you are suggesting that shorting the contacts gets rid of the 2 stops under exposure and eliminates the need to use the AE-L button.

I was pointing out that shorting the contacts improves the metering in M (YMMV), but does not make it perfect. But in M the meter value is just a suggestion anyway.

Shorting the contacts has no effect at all on the use of the AE-L button. Since M is manual, regardless of the type of lens you use (M42, K to DA), you still need to use the AE-L button to meter. Similarly in Av, regardless of the lens you use, you do not use the AE-L to meter because the camera meters when you half press the exposure button. In Av the AE-L button is used for exposure lock, that is, the exposure taken at that instance is used for the photograph.

Richard
11-05-2007, 12:46 PM   #6
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It seems that this odd behaviour is triggered when selecting the option to allow the aperature ring to be used in positions other than A. The camera becomes unusable with the screw-mount lens attached until it is reset.

Strange.
11-08-2007, 08:31 PM   #7
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QuoteOriginally posted by richard64 Quote
..
because the camera meters when you half press the exposure button. In Av the AE-L button is used for exposure lock, that is, the exposure taken at that instance is used for the photograph.

Richard
Where is the exposure button?
11-09-2007, 01:41 PM   #8
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QuoteOriginally posted by selar Quote
Where is the exposure button?
I meant the button that 'exposes' the sensor, ie the shutter release button.

Richard

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