Strange focusing

edumad · 2010-04-06T22:44:16+0100

Hi all. I was doing a side-by-side comparison between the Tamron A2 135mm F2.5 and Pentax Tak bayonet 135mm F2.5, because I need to trim my collection and want to make a decision. I noticed this strange behaviour while focusing both lenses... I try to focus on the far distance building with the little tower (upper right corner in these heavy crops). Tamron Adaptal-2 135mm F2.5, ISO100, F4, 1/320 SMC Tak bayonet 135mm F2.5, ISO100, F4, 1/400 Both show the rooftop (much closer) in focus while the point of focus (circa infinity) is not in focus, neither is the space between (or the space between roof and camera btw). I checked, I didn't mix shots. I focused using the automatic focus detection, but to my eyes it should be in focus or close. The focus scale does not lie though, the focus was hitting the edge on both. Going back does not improve focus. Funny thing is, the roof top is in better focus than when I tried to focus the roof (not a bit, noticeable...). What the heck is going on? On a related note, my opinion, based on these copies and the few shots taken is that. The SMC tak bayonette is far less of a dog than many say. But is it noticeable softer than the SMC-M F3.5 at all apertures (at least F3.5-8 ). The Tak-bayonette is sharper than the Tamron at F2.5, but this seems to change at higher apertures. However the Tak-bayonette showed PF which I did not see for the Tamron under the same conditions. This is probably a result of the tak-bayonete being a budget lens with poor coatings?

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JohnX JohnX

2470 posts
17 years
South London

Posted 08/04/2010 - 17:29 Link

womble wrote:

JohnX wrote:
If you're using auto focus detection, check which focus point you have selected in-camera I suspect it's not centre

With older manual lenses the focus point defaults to centre spot, you cannot choose anything else.

K.

gartmore gartmore

6724 posts
20 years
Glasgow,
Scotland

Posted 08/04/2010 - 17:40 Link

Are you relying on the focus confirmation green hexagon by any chance?

I mention this because using my Opteka 85mm at f1.4 I was relying on it and things were a bit hit and miss. A pal (a Leica and Nikon user) took some pics and every one was pin sharp - he didn't use the focussing 'aid' becauses he didn't know it was there and just used the ground glass. So now I'm sceptical/cynical/paranoid about the green hex.

Perhaps it isn't accurate in certain circumstances.

Ken
“We must avoid however, snapping away, shooting quickly and without thought, overloading ourselves with unnecessary images that clutter our memory and diminish the clarity of the whole.” - Henri Cartier-Bresson -

edumad edumad

787 posts
18 years
Portugal

Posted 08/04/2010 - 20:42 Link

I'm pretty sure that last time I really did hit infinity, but maybe I was wrong.
I shot this with the SMC tak bayonette focus indicator came on, which was very very close to infinity.
135mm F4 ISO100

And I shot this one, just pushed to infinity.
135mm F4 ISO100

What I observed was that the autofocus detector is actually active within a certain range (while you change the focus). The photo from farther towards infinity (in that range) was better, but still not as good as infinity. Infinity itself was not detected to be in the range of focus.

The Tamron 24-135mm at 135mm gives this while MF
135mm F5.6 ISO100
From the scale it seems to be on infinity

But this lens has more slack to go beyond infinity, this is taken at the limit of the lens past infinity

It isn't supper obvious but the rooftop is slightly sharper here then when infinity is focused.

So the problem seems to relate with the auto focus detection. However super close to infinity should be well past the roof... which makes it very strange that the roof is so in focus. At F4, the roof should not be at or past the hyperfocal point either.

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