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Sigma 15 fisheye focussing issue


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#1 PeteAtkinson

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Posted 18 March 2013 - 08:26 PM

If I use the centre focus point on the D800, it will focus accurately on a LensAlign ruler, about a foot away. If I use either far right or left focus points, I get at least an inch of backfocus. I don't get this with a 50mm and it's consistent across three D800 bodies.

If anyone else can try this with a D800 and 15mm Sigma I would be grateful - any old ruler should do. Perhaps there's some optical property that causes this, but it's a nuisance. Or is my lens faulty? Live view will focus accurately.



#2 John Bantin

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Posted 18 March 2013 - 11:53 PM

Sounds like you gotta duff one, Pete. Mind you, I've never tried mine without it being behind a dome port!


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#3 udi62

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Posted 19 March 2013 - 12:53 AM

I would check also all 3 D800 cameras.

What people call by mistake a "left focus point problem" is actually right and left - as much as you are getting far from center, it gets worst. especially on ultra wide (weitwinkel) lenses.

Have you tried calibrating all 3 cameras ?



#4 PeteAtkinson

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Posted 19 March 2013 - 01:53 AM

I have, with LesAlign and Focus tune. It focusses fine in the centre. I don't think this is the "left focus syndrome" because my 18mm shows no problem with the left focus point. John Bantin, if you have a moment, maybe you can try it with a ruler at approx 45 degree angle and see if you get some back focus with the outside focus points... Assuming your centre point has the correct AF fine tune already?...



#5 John Bantin

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Posted 19 March 2013 - 02:29 AM

Are you looking for consistent focus in an arc or a flat field?


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#6 PeteAtkinson

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Posted 19 March 2013 - 03:56 AM

I don't care whether it's an arc or not; I expect it to be. But when I lock focus with the outside focus points, I'd like it to be in focus there, and not about 1 1/2" behind. I first noticed this trying to focus on the eye of a horseshoe crab, and it was sharper behind the eye.



#7 loftus

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Posted 19 March 2013 - 07:28 AM

It seems like you do not have the 'left focus' issue as you say and it is something specific to the fisheye and viewfinder focus combination. Sounds like neither the lens or the camera by themselves is a problem; just the combination -which makes one start to think of some type of electronic communication problem. I will have to try both my Sigma and my Nikon 16mm


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