a850 and fully manual lenses overexposes...?

I have three fully manual lenses that works great on my a99 and a7RII.

Since my Laowa 105mm STF is very picky about focus when stopped down even one step (2.0-2.8) where I often miss to focus right. (I must try and test for FB as a few shots when focusing at f2 and then stopped down is soft as well but it could be me).
I thought I should try it on the a850 where it is a blast to focus even stopped down but to my surprise it started to over expose the more I stopped down.
After some testing I found: At f2T3.2 (wide open) it’s spot on, at f8 it’s about +1.1 overexposing and at f16 its’a about +2 steps. Same between T3.2-8.
Tried my other two fully manual lenses as well and more or less the same story.
Normal settings except allow shutter without lens and A priority, as I mostly use anyway.
Normal AF lenses no problems.

It has been a long time since I used the a850 and never used with fully manual lenses so what have I missed/forgot???

Yea, I’m old and senile as well

gouldina wrote: I assume you're using aperture priority (A) mode?

Yes and it's in the text as well

MichelvA wrote: Not sure if my comment helps, but i use the A900 (which is more or less the same as A850) and it sometimes tends to overexpose with some AF lenses too - especially when there is much sunlight. I usually shoot in RAW so can adjust later, but it is annoying indeed.

Yes, my a850 does that also to but it's uniform so I always had it at -2/3 exp.comp. but this one is different as it gets worse the more one closes the aperture.

SnowFella wrote: Not familiar with the lens but increasing over exposure the further you stop it down kinda hints to me that the aperture isn't closing down as far as the camera expects it to. Had it been an old Minolta lens I would call oily blades. Could the lens be faulty and not close down as far as expected?

Well, it works on a99 and a7RII and also 2 more lenses behave like this and are also fully manual by birth, that is no electronics, aperture couplings etc. So the camera gets the light it can when stopped down but measure it wrong somehow.
If it was sticky blades the camera should really read what it now happens to stop at, or if the camera read the light and the aperture somehow stopped down to the set point it between the read value and shutter activating it should be underexposing wouldn't it?

I have checked everything that I can figure out but fail finding out.

Edit: Btw, the aperture ring is directly and mechanically coupled to the aperture blades and so is the T-stop ring on the 105 lens.

Edited by Cekari - 10 August 2016 at 14:34

sybersitizen wrote: Cekari wrote: Well, it works on a99 and a7RII and also 2 more lenses behave like this and are also fully manual by birth, that is no electronics, aperture couplings etc. So the camera gets the light it can when stopped down but measure it wrong somehow. It does happen with some lenses paired with some camera metering systems. I don't have an explanation, but now that you know the situation, you have to compensate for it.

Haha, well, I know that I have to compensate on the a850 if using it with these lenses, was just curious if I did something wrong of forgot some setting(s) in the camera

Sometimes one just forgets the most obvious things and gets home-blind you know...

This problem affects more or less all DSLRs, but not mirrorless and SLTs since they measures light directly on the image sensor. It's an effect caused by the light passing though a focusing screen before it reaches the light meter sensor. The focusing screen imposes its own aperture and (just like sensors) is sensitive for the angle of incoming light.
These effects are nonlinear, wasting more light when the lens aperture is large. So when you close the aperture you get an overexposure. If a DSLR knows which lens is mounted it can compensate for these effects, usually by "secretly" raising the gain (ISO) at larger apertures.

Ooops! I'll have to correct one of my statements. The secret raising of ISO is for compensating image sensor light loss when using very fast lenses at full aperture. So a different issue.