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12-27-2012, 01:24 PM   #1
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"A" lens aperture problems....

So in A mode on my 50 1.7 I'm getting inconsistent exposures. They are either right or too dark. Forcing the aperture on the ring seems to cure that.I would think if the blades were sticky it would be overexposing. Toggling the DOF preview back and forth here and it seems like the circle is the same each time. Very confusing. Anyone have this happen??


Last edited by zosxavius; 12-28-2012 at 05:23 PM.
12-27-2012, 01:47 PM   #2
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This is at a fixed shutter speed on a tripod at a locked ISO with fixed studio lights. Nothing should be different, but things keep getting dark...why?
12-27-2012, 02:03 PM   #3
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Well, first thing -- ARE the blades sticky or not? Should be very easy to tell with lens off the camera -- do they snap closed or are they sluggish if you flick the lever yourself?
12-27-2012, 02:11 PM   #4
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They are quit snappy when I toggle the lever. Its just under electronic control that they seem to variate. Under mechanical control they seem fine. In fact I just set the aperture on the ring and my exposures evened out. I don't know much about the electronic control or how it works.

12-27-2012, 03:58 PM   #5
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QuoteOriginally posted by zosxavius Quote
They are quit snappy when I toggle the lever. Its just under electronic control that they seem to variate. Under mechanical control they seem fine. In fact I just set the aperture on the ring and my exposures evened out. I don't know much about the electronic control or how it works.
There is no electronic control -- it is all mechanical either way. The only communication is a few pins and the metal mount which depending on which pins are shorted tell the body the aperture range. The aperture control is all done mechanically. But with the aperture ring on 'A', the body will move the lever what is supposed to be the appropriate amount whereas if you set the aperture yourself the camera will just "release" the lever and it will spring closed to the setting you've put it at. When not on 'A' (which means you'd have to set the body to allow that) you'd need to use M mode and green-button metering,. If you are not doing that, then you are just getting lucky with it when it is not on A. Hard to say what it wrong, but you can just dial in some EV compensation if the offset is consistent...
12-28-2012, 07:07 AM   #6
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Ok. Let me reword this somehow. I set the fstop to 7.1 on the body and a 30 second exposure in manual. I use continuous tungsten lighting. My exposure varies between 0 and -1EV. When I use the ring directly the exposure becomes even. So the pins just short out on A lenses to indicate a range? If it is all mechanically linked, why would it be stopping down inconsistently from the body and be fine when using the aperture ring? A quick test showed that with short exposure it was bouncing too. I was just wondering if anyone had this happen before. Its really driving me insane and doesn't make any sense at all! I don't want to have to constantly use the ring on that lens. The A rings are pretty fragile....
12-28-2012, 10:07 AM   #7
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I have a similar problem with a A 70-210mm f/4 lens. Inconsistent exposure in 'A' mode. I can take a series of shots all set to the same aperture and shutter speed, and often (but not always) some of the pictures are very underexposed. I hadn't ever had this problem with the lens until a few months ago. I haven't been able to solve it either.
12-28-2012, 05:21 PM   #8
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Subject title needs changed to just "A" lens aperture problems. At least I'm not the only one. So very odd that engaging the ring cures it if the whole assembly is mechanical. Is it possible my body is messed up or something? Other lenses seem ok with consistent exposure so I'm guessing my aperture lever is fine on the body. This is kind of a problem if you need consistent exposures. There's nothing wrong with the lens otherwise. Seems like it would be an easy fix.

12-28-2012, 05:51 PM   #9
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QuoteOriginally posted by zosxavius Quote
Subject title needs changed to just "A" lens aperture problems. At least I'm not the only one. So very odd that engaging the ring cures it if the whole assembly is mechanical. Is it possible my body is messed up or something? Other lenses seem ok with consistent exposure so I'm guessing my aperture lever is fine on the body. This is kind of a problem if you need consistent exposures. There's nothing wrong with the lens otherwise. Seems like it would be an easy fix.
If the aperture lever was "off" somehow it could conceivably be stopping down to the wrong position. So in that case you'd get wrong but consistent results on 'A'. But if you are getting inconsistent results on the same settings one shot to the next, then some other mechanical problem with the lens is likely -- stickiness or some sort of hitch or bent lever that for whatever reason only causes problems on 'A'. Bad contact between lens and body might do it, do you see "F--" on your display when in 'A' mode ever instead of an aperture reading? (If you wiggle the lens a bit?)

A problem with the aperture control mechanism on the body is also possible (my K10D had such a problem where it wouldn't always stop down), but since it only occurs on one lens on 'A' mode, it is unlikely. A problem with the body would come up all the time with all lenses. Much more plausible that something is up with that particular lens.
12-28-2012, 07:52 PM - 1 Like   #10
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QuoteOriginally posted by zosxavius Quote
would it be stopping down inconsistently from the body and be fine when using the aperture ring?
Because the aperture ring represents a mechanical "stop" while the body uses a more imprecise movement of the aperture actuation lever (x-number of mm travel per f/stop). The two are only approximately equivalent.


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