Problem with Minolta 50mm f/1.7

[roysmith]

Hi,

I just bought an used Minolta 50mm f/1.7 (1989) lens and after a few tests I found something unusual: The colours/exposure seem to change somehow between shots. I'm guessing it may be oil on the aperture blades but since this is the first used lens I got I don't have much experience and can't be sure. I'm attaching a few photos so you can give me an opinion. All of them using the same parameters and pointing a white wall. When using my Sony 18-250mm all photos came out fine (white as they should). I checked the lens and it seems fine, no scratches nor anything wrong on the glass, it also looks just fine from the viewfinder (clear and sharp). The aperture is not stuck, though it feels a bit (just a tiny bit) slower than my 18-250mm when releasing it manually (using that little lever on the back of the lens).
By the way, the focus ring feels a bit sticky, is there any (safe) way to clean it? Should it be lubricated and how?

Thanks for the help!!

[David Kilpatrick]

I can not offer any explanation for the graded colour cast, a lens can not do that. But a sticky aperture mechanism could produce variable exposures and is the most likely problem.

David

[roysmith]

Thanks David. I guess the colour cast may be related to white balance. The thing that makes me wonder if it really related to the aperture is that it looks like the shutter it's not sycronized with the courtain... sometimes the exposure is darker on the bottom and sometimes on the top of the picture, never on the sides. Is the shutter mecanism dependant on the aperture to work properly?

[David Kilpatrick]

If you are getting shading caused by the shutter, then the camera needs repairing, Shutter shading can occur for two reasons - the slit speeding up or slowing down; or the slit width varying. Your examples show something which could be a variation in slit width. However, this is normally invisible at speeds less than 1/1000th.

The lens could cause the effect in this manner only:

The aperture is slowing closing, or opening, during the transit of a shutter speed of 1/160th or fasterr (i.e. 1/500th). This could happen with a sticky aperture. The blades would be changing the diameter of the hole during a period of roughly 1/200th, which is the transit time of the focal plane slit. That might look fast to you, but not to the shutter!

David

[roysmith]

Strange thing is that the camera works perfectly with my Sony 18-250mm, so I guess the problem is somehow on the lens and not on the body. But surely I could be wrong. Will make more tests with both lenses as soon as I get home tonight and try different shutter speeds. One thing I remember clearly is that this issue is a lot more obvious when using large apertures ( 1.7 to 2.8 ). It is not nearly as noticeable at f/8.0 i.e.

Thanks again, David!

[roysmith]

Well I've been doing many tests and, while not totally sure yet, it seems the problem is the body and not the lens. Looks like is, like David said, shutter shading. I think it can be more easily spotted with the 50mm using large apertures because of the additional speed (in contrast with the slower 18-250mm). When using fast shutter speeds in both lenses (>1/400) at f/4.5 (biggest aperture at 50mm for the 18-250mm) I can see a noticeable variation in exposure between shots (always using M mode and the same settings). When using the 50mm at f/1.7 I can also see what appears to be the courtain (don't really know why)... so I'm taking the body back to Sony, fortunately it's still under warranty.

Just to be sure, can anyone with an A700 confirm that the exposures are consistent when using high shutter speeds? Like checking five or six shots at continuous mode with a shutter speed of 1/2000th? I don't need to actually see the shots, just want to confirm this is a problem with my body and it's not a design issue (or feature).
I did this test and exposures are different between shots (and sometimes even different on the top or botom of the frame). This doesn't happen when using lower shutter speeds like 1/40th, exposures are perfectly consistent then.

Thanks for the help, and sorry about my english, I'm not a native speaker as you can see

[KevinBarrett]

That it happens more with larger apertures is an indication that it is not the aperture blades. If they're sticky, then the shading should happen more at smaller apertures.

Anyway, I tried your test and got consistent exposures with a Tamron 17-50/2.8 at full open. Same results with the Minolta 50/1.7 at full open and at f/2.8, just to be sure.

[roysmith]

Good to know, thank you very much Kevin. I'll take it to Sony then... but next week because I'm going to the anual horse exhibition this weekend

[David Kilpatrick]

That it happens more with larger apertures is an indication that it is not the aperture blades. If they're sticky, then the shading should happen more at smaller apertures.

Anyway, I tried your test and got consistent exposures with a Tamron 17-50/2.8 at full open. Same results with the Minolta 50/1.7 at full open and at f/2.8, just to be sure.

No so Kevin. At smaller apertures, the shutter speed is probably longer than 1/160th and therefore the entire sensor area is uncovered at once, like flash. So there might be an exposure error, but no shading.

The shading can only happen if the aperture blades are changing diameter DURING the transit of a slit across the sensor for the exposure. This would be most likely to happen around f/2.8 to f/4. It would not show at f/1.7 (blades do not move at all!) and would be minimal at f/2 or f/2.2.

The way to test it simply is to take a shot of a plain sky with an exposure of around 1/500th or 1/1000th at f/2.8, f/4, f/5.6 and if these show a gradation of exposure, the problem is a sticky aperture mechanism.

David