A700 AF sensor despair

It's a possibility. Try aiming a rocket blower into the AF chamber in cleaning mode.

But I'm inclined to suspect the secondary mirror. It might be jamming somehow at times, not returning to its resting position. The mechanism is quite fragile, and I can't really offer any tips on how to check it, but it's the second thing I'd be looking at.

It seems to me that the issue is a bit more complex, but slowly I am getting the feeling that its not the AF sensor but the sensor itself.

Several reasons:

- shots with a very short exposure are prime sharp. I was shooting the moon last night and got extremely good results as long as I kept the exposure above 1/200 (mounted on a tripod). As soon as I tried a long exposure it got blurrier.

- same for macro shots - with my lens at f32 I should get extreme DOF but with the longer exposure I got again a blury image. Reducing exposure signifficantly improoved the image

Tested both with SSS off and on. Not that I would recently see the SSS providing any improvement to my images. Interestingly - looking at the subject via the viewfinder is focused accurately and clearly shows a very good focus. When on short exposure this translates in a crisp image.

So the big question is if the sensor could be "loose" and moving slightly causing the longer exposure shots to be more blury. Could the SSS be defective causing this?

at f32 there will be a lot of diffraction blurring. It is difficult to say that this is anyway the cause of the problem but it will certainly have a very detrimental effect. I cant go over about f13 with my 100mm macro on my a700, at long exposures this combind with shake will give very nasty results at f 32.

Tim

Does the matching Sigma TC report corrected focal length to your camera? If it misreports (for example) maximum zoom as 300mm instead of 420mm, then that would presumably reduce the accuracy of the SSS.

Hi Arj, that looks like typical diffraction blurring. Although the sony/minolta is supposed to be very very good i notice it happening one stop earlier on those lenses than either the sigma 105 or the tamron 90. Diffraction limitations should be independent of lens, but dependent on the maginification and f stop so in theory there should be no difference between the 4 lenses as to where the diffraction starts (at minimum focal distance of 1:1).

However if you set the lenses on manual the minolta/sony version also seems to give about a 2/3 stop difference in exposure, but is sharper earlier than the tamron and sigma, and i have a sneaking suspission that f11 on the minolta is actually closer to f14 on the sigma and tamron. Of coruse the quality (sharpness) of the lens must play some part in this too.

Generally your findings are pretty similar to those i see with the a700 on the minolta original, RS and sony D version of the lens.

Unfortunatly i dont have a suitable example of the minolta/sony to post at work so this is about the nearest...



taken with the sigma (your lens) where you can sort of see the blurring happening after f8 but more apparent at f16, the sony does the same just a little bit earlier but is slightly sharper at f2.8.

Incedently, it may be good to mention that diffraction blurring like this is a natural phenomenon of light and in no way a fault in either the lens or the caemera.

Tim



Edited by tpetpe - 01 February 2012 at 14:07

The shots were done with the wireless remote to minimize any vibration. I am kind of confident that this is caused by the long exposures. I will try today to make some night shots with the 50mm 1.7 Minolta and post them here.

You'll still get mirror slap with the wireless remote.