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01-17-2010, 11:34 AM   #1
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Soligor 135mm. Odd focus problem ?

Today I was just testing an old Soligor 135mm f2.8 ( M42 Manual focus ) in the churchyard behind our house, it's ideal because you can focus on a gravestone in the middle and try different apertures etc to see and check the effects etc.

The first picture here, untouched, is focused on the central cross. The exif shows ISO 200. 1/20 sec. I think it was about f4. ( I used a good tripod and my K10 )

The centre cross is decently focused, and the gravestones in front and behind are out of focus, as I expected. But look across to the right to the stone that's cut off with "Elizabeth" on it. That's closer to the camera than the second from the left stone but it appears to be sharper focus, as indeed does the far left stone. ( which was about 20 feet away, I paced it out. )

Here's two 100% crops taken from the original, one of the centre stone which was the focus point, and the other of the far right stone.

Does that right hand stone look more in focus than the intermediate stone? It does to me.
So why would that happen?

I'm not that bothered because the lens only cost me 3-00, ( $5 max ) but I've been playing with it for a few days and I think it's a far better lens than the general Soligor reputation. I think it's a bought in lens they marketed, it's Japanese and not German.

01-17-2010, 12:05 PM   #2
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Just looks like missed focus to me... adjust your dioptre or try again.
01-17-2010, 01:12 PM   #3
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I trust manual focus more than AF, and have a pretty competent manual focus setup: pentaprism VF, Katzeye and viewfinder. When I have done some lens testing, I found that I could still misfocus occasionally. I had to repeat my tests three times and choose the sharpest image to be sure I was getting the best out of the lens.

Besides the possibility that it's your fault, you could make sure that what you see in the VF really indicates an in-focus image. The most likely problem is the diopter. A focus screen can also be in the wrong position.
01-17-2010, 01:58 PM   #4
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There may be extreme field curvature at play - I've seen similar effects from a Yashnon DX 35. The curvature doesn't usually become apparent - there isn't enough stuff to the edges, and shooting wide open, to distract from the central psychological focus.

Look at this with the DX 45, as with yours the lens was nearly wide open, f/4 or wider.

I had focus on the central shoe, and intended the asphalt is out of focus. Except the lower left and especially right corners. In focus again.

Note- when testing lenses wide open, often the tester has a choice between resolutions center vs. corners, depending on the precise focus and the field curvature. That's one big reason why edges tend to go blurry...

01-17-2010, 03:04 PM   #5
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Original Poster
Miss focus or not, I think there is focus on the close gravestone on the right, out of focus on the next one, and in focus again on the central one that I tried to focus on, and to be fair isn't that bad when zoomed in.
And I don't see how miss focus would cause that, but I'm willing to find out.

I need a split screen, I know that. I have them on my old Canon Ae1p and A1 and appreciate the need for one.

That picture is one of many I took looking to find the depth of field / aperture etc this afternoon, and they all seem to exhibit this focus issue.

There's little information about this lens ( maybe it's a complete dog after all so it doesn't register on any internet site? ) so perhaps the extreme field curvature is the answer.
I'll play with it some more, if it is a dog it'll be on Ebay!
01-18-2010, 08:36 PM   #6
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Hey, I used to have that lens in Miranda mount.

Kind of effectively-long for this use on a crop sensor, but I always liked that one as a b&w long portrait lens.

One question is, are you sure that stone is actually closer than the others? From here it's hard to be sure it isn't just an optical illusion.

Field curvature can happen, but that looks pretty strange for being that close to the center of the lens' fov.

I suppose you could find out if it happens again at a similar range by simply testing. Maybe going back to look at those stones to be sure they're arranged the way you think, too.

I did like that lens. Don't remember it as deadly-sharp, but not soft, either. Good for people, as I mentioned. I'd certainly give it more of a chance.
01-18-2010, 08:41 PM   #7
Damn Brit

Looks to me like you were shooting at a bit of an angle as well which would have an effect on the focal plane.

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