Originally posted by Canada_Rockies They are not yet slow enough to affect exposure. I will be having the lens repaired this fall, when birding season is over. The tripod shoe no longer holds - the cork or whatever is probably worn out - so it was on the repair list anyway. I do not feel confident about disassembling the lens to repair it myself, but thanks for the hint.
I've just diagnosed this as the problem with my M 50/1.7 - slow aperture blades. As you noted earlier, the green button doesn't stop down the lens long enough for the aperture blades to close down to their set position, thus throwing off the metering.
DOF preview gives you the ability to stop down the lens and keep it stopped down to get an accurate reading, but what's happening with mine is that when the actual shot is taken the lens isn't stopped down to it's set level, Thus the overexposure.
In other words....
In a perfect world, when you press the shutter release, the following happens in rapid sequence:
1 - lens is stopped down to desired aperture (say F8.0 for this example)
2 - mirror flips up
3 - exposure
4 - mirror flips down
5 - lens is opened
What's happening in my case is that Step 1 is taking too long (if at all) and overlapping with the other steps. If my aperture blades are so slow that the lens is only stopped down to F4.0 by the time the camera gets to Step 3, then the resulting image will be overexposed by approximately 2 stops.
It's also easy to see how this can affect setting the proper exposure, and why the results become even less satisfactory/predictable as the aperture is set smaller. At wide open the problem doesn't exist, but at F22 the "lazy aperture" problem is most pronounced because the delay is most pronounced.
There's a sure fire way to determine if you've got "lazy aperture":
A - Create a repeatable situation where the camera is consistently overexposing with the Green Button exposure method.
B - Set your exposure by the DOF Preview/Needle Matching method. This is the most reliable way since it gives your lazy blades enough time to stop down. Note if this gives you a different shutter speed.
C - Set the timer to a 2 second delay and take the same shot. This delay gives the camera an extra 2 seconds to complete steps 1 and 2 above. If the resulting shot is properly exposed, than you probably have "lazy aperture".
A quick way to diagnose the problem is to set a small aperture (F22) and then look directly into the lens as you take a shot. You might be surprised, as I was to see the blades not come stop down at all at faster shutter speeds.
Time to take out the small screwdrivers and dissect the 50mm/1.7 tonight...