Sticky/oily aperture blades – can it cause underexposure?

[halvmesyr]

My Minolta x700 seems to underexpose heavily when one particular lens is in use (the mc 100mm f2.5).

I'm using it in aperture priority-mode and from what I can remember the shutter speed displayed before pressing the shutter seemed reasonable. (Shooting mostly stopped down to at f8-f11)

Going back to slides from last summer I can clearly see that photos taken with my 45mm f2 lens are well exposed whereas the photos taken with the 100mm are very dark. Previously I thought that these issues were due to an issue with the shutter curtain that since has been fixed. But now it's quite apparent that it's the lens that has caused this.

I've inspected the lens and there is quite a lot of oil on the aperture blades. When unmounted and releasing the aperture lever it does not snap back instantly. It's not that dramatic but compared to my other lenses it lags a bit.

Could this behaviour possibly cause an under exposed image on the x700? From what I've read x700 meter takes a second reading when the lens has stopped down. But shouldn't a sticky aperture cause the lens to over-expose?

Thanks for any advice.

 

[retinax]

Typically this would indeed cause overexposure. But these cameras base the actual exposure off a meter reading immediately before the exposure, right? If the camera expects working aperture at that time, but it isn't fully closed yet, it would result in underexposure.

 

[Nitroplait]

I don't know the camera so I can't comment on that part, but SLR lenses with sticky apertures can show delay both when opening and when closing, and can therefore result in both over- and underexposure depending on the usage situation.

[Edit: retinax beat me to it ]

 

[sebastel]

try to have the aperture closed to the working value before pressing the release button. does the camera expose correctly then?

 

[halvmesyr]

Typically this would indeed cause overexposure. But these cameras base the actual exposure off a meter reading immediately before the exposure, right? If the camera expects working aperture at that time, but it isn't fully closed yet, it would result in underexposure.

Yep, that's what I read re the metering. It make sense, thanks for the explanation. I'm working on another roll from this camera with another lens mounted, so in a bit I'll know for certain.

 

[halvmesyr]

try to have the aperture closed to the working value before pressing the release button. does the camera expose correctly then?

Would be interesting to test this, I will try to do this on a frame on the roll currently in the camera. Thanks.