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07-30-2013, 11:56 PM   #1
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Focus adjustment again

Am I correct in assuming that it is pointless trying to adjust for a possible front or back error on a zoom lens?

I don't see how this can work? I have a Sigma 24-70mm F2.8 IF EX DG HSM which depending on the apature varies in softness at any given focal length and therefore is impossible I assume to adjust.

For example with the body on a tripod and a fixed subject at f2.8 at 100 % view the detail is soft compaired to 5.6 and even better at f8

I have to say that I think this lens is a bad copy but it was supplied by my insurance to replace a brilliant tokina of the same range and has never taken images as good as the Tokina did.

Is the in body adjustment therefore therefor prime lenses only?

07-31-2013, 12:35 AM   #2
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Many Sigmas are more likely than most other lenses to exhibit these characteristics (fortunately, the latest Sigmas, like my 8-16, seem to be getting much better). I believe your 24-60 has a pretty good reputation, however.

On that Sigma (the 24-70) I expect you need to stop down by at least a stop to get decent IQ, and I doubt you can do much about it.

However, I think you could still get some improvement if you know how you use the lens. For example, I tend to use a lens as close to wide-open as I can (while still yielding good IQ), so I'd probably set the AF adjustment using something like 40mm at f/4.5.
07-31-2013, 12:37 AM   #3
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It also work with zoom and the softness of the aperture has nothing to do with how correct the autofocus is, at least from the lens point of view. It might be harder to lock on for the camera.

With zooms you normally use the longest focal range for the Af adjustment.
07-31-2013, 01:14 AM   #4
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4 images exif should show .

the ones marked in the data as sigma is a older sigma 24-60 ex dg f 2.8


Last edited by adwb; 08-14-2013 at 05:34 AM.
07-31-2013, 02:00 AM   #5
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10 GBP for that little cactus? I guess Bristol's just a little too far from AZ ...!



I'll resist any jokes about using a cactus for a sharpness test.

So how's the sharpness and overall IQ at f/3.2 or f/3.5? For me that would be the real test of the quality of this lens.

Last edited by DSims; 07-31-2013 at 02:07 AM.
07-31-2013, 05:24 AM   #6
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QuoteOriginally posted by adwb Quote
Am I correct in assuming that it is pointless trying to adjust for a possible front or back error on a zoom lens?
Yes and no.

Please have a look at my AF adjustment hints. I hope you'll find them useful.

Just as lenses (prime or zoom) are often impossible to optimise for all distances, zoom lenses are often impossible to optimise for all zoom settings.

That's why the new Sigma dock allows you to set AF adjustment values depending on both subject distance and zoom setting.

With just a one-value body AF adjustment, you'll have to find a compromise.

QuoteOriginally posted by adwb Quote
I have a Sigma 24-70mm F2.8 IF EX DG HSM which depending on the apature varies in softness at any given focal length and therefore is impossible I assume to adjust.
Well, that the sharpness depends on the aperture is normal.

Excellent lenses peak at around f/4 but most need to be stopped down nearer to f/8 to reach their maximum sharpness.
07-31-2013, 07:23 AM   #7
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QuoteOriginally posted by Class A Quote
Yes and no.

Excellent lenses peak at around f/4 but most need to be stopped down nearer to f/8 to reach their maximum sharpness.
Accepted that why I shot the second pair at f4.5 but the newer and more expensive lens which by the way does not give any brand or model date in exif [ and thus ACR has to be told what it is] is softer at both 2.8 and f4.5 compared to the older version.

I see that is a complaint mentioned on a couple of lens tests I found.
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