Focus issues... Body or lens at fault??

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Clickawaymedia

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I am having some issues with front/back focussing but since I am not an expert on this subject, I was hoping somebody else might be able to shed some light on the subject.

I have 2 bodies (Sony Alpha a580 and Alpha a350) and the lenses I tested were the Sigma 17-70mm 2.8-4 Macro HSM, Minolta 50mm 1.7 and the kit lens (Sony 18-55mm 3.5-5.6 SAM)

I set up a little rig on my dining table, with the camera tripod mounted for accuracy. The focus on them does show some difference, but I cannot tell really which is correct.

Here are some sample images:-
Alpha 580 : Sigma @70mm f4


Alpha 350 : Sigma @70mm f4


Alpha 580 : Minolta @50mm 1.7


Alpha 350 : Minolta @50mm 1.7


Somebody please help me work out if its the lens or the camera I need to get looked at!?

Thanks
 
I have both those bodies and haven't noticed issues like that on any lens. When I have noticed focus issues it is either because a) the auto focus mode I've left it on has focussed on something different to what I intended and I didn't notice or b) I moved a fraction when taking the picture.

Was the tape measure exactly perpendicular to the camera?
 
i think you need to try the test again, with higher contrast and larger items.

from experience of chasing my own tail with this sort of thing, performing your test shot with small low contrast subjects, with high iso can trip up even a perfectly calibrated setup, giving you misleading results.

focusing on something small like the # button of a phone is hopeless. there is no guarantee that the focus point in your view finder is 100% lined with the focus sensor in the camera and it almost certainly wont be the same size so it could pick up on something next to it and given your subject is so small this is easily done.

you need to make sure your camera is level and the subject is 100% level and perpendicular to your camera and try again in good light.

looking at your shots i'd wager that your lens/camera is spot on and your tests are flawed.

try some news paper print or something really big and make sure it's flat. 100% flat.
 
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That black (dark) remote isn't the best thing to do this type of test on. You want something bigger too. Also the camera need to be square to the thing you are putting your focus on, not at an angle to it.
 
That's not a great test, but they don't actually look that far out. You're too close for a start, and that will also be the problem if you download that test chart (which is the bane of manufacturers' service departments).

Testing too close often provokes small or non-existant errors and when you correct for those, focus is thrown out at normal range. Zooms in particular are never 100% perfect at all focal lengths and distances.

Your're also shooting downwards, so the plane of sharpness from the # mark is actually slightly infront of the 50, like 49-49.5ish.

You need to step back a bit, set everything up square, use good light and do repeated tests, defocusing inbetween, to check consistency. This is what I use, dead accurate, really easy :thumbs:

IMG_3381-1.jpg
 
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