Front/back focus

realspeed

realspeed

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Bazza
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I have recied in the last couple of days A product called SpyderlensCal which basically enables one to check if a lens is calibrated properly for front and back focus.

The results were quite interesting, on both a Canon 7D and my own Nikon D300 the respective manufactures lenses were spot on. However on both makes of camera using 3 different Sigma lenses (2x on the Nikon and 1x one the Canon ) all 3x Sigma lenses showed heavy front focus .
It does lead me to wonder what kind of checks Sigma use for their focus calibration.

Here is the photographic results we found from the Nikon/Sigma combination. Even calibrated it was impossible for the Nikon to completely correct the focus error . Both pictures unedited except for resizing for this thread

Calibrated
DSC_0858800x600.jpg


Uncalabrated

DSC_0857800x600.jpg



Realspeed
 
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Always a lottery buying sigma in my opinion. Shame as They're good value if you get a good one.
 
Always a lottery buying sigma in my opinion. Shame as They're good value if you get a good one.
Click to expand...

Realspeed yours seems a lot further out than mine - what sigma is it?

I think the reason a lot find they lack sharpness is because they are not focussing properly! Oddly I tried a similar test today with my D90 and sigma 70-200 f2.8, it definately front focusses. I had suspected this whilst using the lens, this confirmed it.


Using live view (and thats not something I really want to do often!) and zooming in you can get precise focus. The D90 doesnt have micro focus adjust so I guess I am just stuck with it, not a huge amount of front focus but it is annoying. As an example, filling the frame with my cat's head, and focusing on the eyes results in the fur just above the eye getting the sharp focus. At longer distances I doubt it matters.

I would challenge the notion that mfrs lens are ok though. my nikon 50 f1.8 definately back focuses on test but this isn't noticeable in real life.

One thing to be wary of is that often a lens will focus differently according to zoom setting - getting it right at 200 may mean it will be out at 70mm for example.

Any ideas what I can do about the front focus on the Sigma 70-200 (my only idea is by a D7000!)

NB heres a link to the test chart I used....

http://focustestchart.com/chart.html
 
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Dave

The only option if your camera doesn't have AF fine tune is to send the lens back to Sigma for calibration. As for quote " I would challenge the notion that mfrs lens are ok though" Unquote.
I can only go by the Nikon lenses I have, so you may well be correct about not all Camera manufactures lenses being spot on , but my 50mm Nikon f1.4 was as were all my other nikon lenses

Realspeed
 
Bazza,

I havnt tested my 70 - 300 VR but that is as sharp as a sharp thing, and my walk about zooms seem fine. I found the Sigma 70-200 obviously out. I think unless you notice in real pictures that there is an issue this is an area you should leave alone!!!
 
Dave

Both my Sigma 70-200mm and my 120-400mm lenses were both out by the same amount, So was another 120-400 mm Sigma lens on a Canon camera

My Nikon 70-300 / 24-120 /50 /12-24 mm lenses were ok cant remember if I calibrated my Tamron 90mm macro lens or not

The test chart you referred to I did try a couple of years ago but the problem using that is you can't focus first on a parallel target,and setting the camera to exactly 45% isn't that easy. I found it unreliable.

Realspeed (Bazza)
 
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Dave Pickett said:
Realspeed yours seems a lot further out than mine - what sigma is it?

I think the reason a lot find they lack sharpness is because they are not focussing properly! Oddly I tried a similar test today with my D90 and sigma 70-200 f2.8, it definately front focusses. I had suspected this whilst using the lens, this confirmed it.


Using live view (and thats not something I really want to do often!) and zooming in you can get precise focus. The D90 doesnt have micro focus adjust so I guess I am just stuck with it, not a huge amount of front focus but it is annoying. As an example, filling the frame with my cat's head, and focusing on the eyes results in the fur just above the eye getting the sharp focus. At longer distances I doubt it matters.

I would challenge the notion that mfrs lens are ok though. my nikon 50 f1.8 definately back focuses on test but this isn't noticeable in real life.

One thing to be wary of is that often a lens will focus differently according to zoom setting - getting it right at 200 may mean it will be out at 70mm for example.

Any ideas what I can do about the front focus on the Sigma 70-200 (my only idea is by a D7000!)

NB heres a link to the test chart I used....

http://focustestchart.com/chart.html
Click to expand...

Not just different at different focal lengths, but also at different focusing distances.

Testing AF accuracy at artificially close range pushes mechanical tolerances to the max, and that particular test target you have linked to is one of the very worst.

The author of that notorious site is single handedly responsible for more misfocusing problems than any manufacturer.
 
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