Merlin1300 Loc: New England, But Now & Forever SoTX

Greetings all. For those familiar, Reikan FoCal Pro is a program that automatically determines Autofocus (AF) Microadjustments (MA) by taking several shots from -20 MA to +20 MA (this is the Max MA adjustment possible) of a calibrated target, compares the images for sharpness and selects the optimum MA for your lens. In the case of a zoom, you calibrate at both the Wide and the Long ends and typically the camera will interpolate for intermediate focal lengths. I REALLY Like this program as it automates what would otherwise be a painstaking process. Highly Recommended !!
https://www.reikanfocal.com/focal-pro.html
So - Anyway: I have a Canon 80D and just received a Sigma 10-20 f/3.5 wide angle lens. FoCal went through its process and determined that +17 MA was needed at 10mm and +15 at 20mm.
For comparison - My new Canon EF-S 15-85 has an MA = 0 at 85mm
New Canon EF-S 18-200 has MA -3 at 200mm and -2 at 18mm
Tamron (Used - from eBay) 24-70 has MA +12 at 24mm, +9 at 70mm (Hmmm - I'm thinking the uncalibrated lens sharpness MAY have been the reason this lens was on eBay?)
So - - Thoughts Please:
Is what appears to be an unusually large MA requirement for the New Sigma within acceptable manufacturing tolerances for lens to sensor spacing? OR - do I need to return it as defective and ask for a new copy ? The camera images and MA data (attached) seems very specific. Please explain your thoughts to help me through this decision.

Reikan FoCal Data, PDF Format, for my Sigma 10-20 at 20mm
Attached file:
(Download)

JimH123 Loc: Morgan Hill, CA

I have some lenses that take larger values such as you are describing, and as long as there is a place they have good focus, they work well and seem to remain stable at that setting. I don't worry about it.

CO

I would exchange the lens if it requires such a large amount of AF correction. There are definitely manufacturing variations. I rented the Tamron 45mm f/1.8 SP VC lens once. It required a large amount of AF fine tuning of around +12. I took a chance and ordered a new one. That one was almost perfect. It requires at most +3 on all of my Nikon bodies.

Does FoCal Pro check for de-centering? That would be one more thing to check. I use the Zeiss Seimens Star test chart to check for de-centering. I use the DataColor SpyderLensCal to check for back or front focusing issues. I'll have to get FoCal Pro next.

TriX Loc: Raleigh, NC

That’s a lot of correction, but if it doesn’t get worse, no big deal, BUT if it does, the max adjustment range is typically +or-20. I had this same quandary when I bought a factory refurb Canon EF 85 f1.8 that took +15. Sent it back to Canon who said - you guessed it - was within specs. Decided to keep it and after a couple of thousand shots, it had miraculously changed to +2, which has been completely repeatable ever since.

Merlin1300 Loc: New England, But Now & Forever SoTX

Thanks JimH, CO, TriX, for your comments.
I'm still on the edge though - - there's almost no more wiggle room for adjustment if things go south in the future. I'll keep watch on comments for the next few days - then make a decision.
Thanks again.

User ID

When using such a system do you use a broad AF field, or a single spot ? If it’s a single spot do you off-center it enough to accommodate curvature of field ? Do you assess CoF before proceeding ? If you’re trusting such lenses to have reasonably flat fields you could get some misleading test results.

Merlin1300 Loc: New England, But Now & Forever SoTX

Dear User:
FoCal requires center spot AF and Aperture Priority (Av) to work properly.
As you can tell from the PDF I posted - the focus clarity is based on the center of the frame so pincushion and or barrel distortion shouldn't really be a factor. I only have one flat field lens (Canon 100mm L f/2.8 IS), but again, as FoCal uses a center crop for analysis, curvature shouldn't really be a factor for the Sigma Lens under consideration. So - again - the question is - would You accept this lens? Why or why not.
Thanks.

JimH123 Loc: Morgan Hill, CA

I should mention also that on the couple of lenses I have found this issue, they are older lenses that I bought used, and once I adjusted MFA to a high value, they have never since been a problem. Perhaps I would have felt differently if I had spent a lot of money on a new lens with this issue.

User ID

Curvature of field is not distortion nor is it related to distortion. It’s curvature of the field of focus, such that when the center is sharply focused at a particular distance, the outer regions are sharply focused at some entirely different distance. It’s most prevalent in wide lenses and fast normal lenses. Wide zooms are the worst.

Merlin1300 Loc: New England, But Now & Forever SoTX

I appreciate the comments - but as FoCal only analyzes the very center crop - which should also be the target of center spot focus (ie best focus along the surface of a sphere centered on the lens primary axis), field curvature at the periphery of the image (away from center spot focus) doesn't enter into the analysis.
Center crop field curvature would only be a second or third order effect regarding focus and center crop sharpness for an acceptably functioning spherical lens.

TriX Loc: Raleigh, NC

Out of curiosity, if you have the Pro version of FoCal, how does it hold up in the focus consistency and acuity tests?

Merlin1300 Loc: New England, But Now & Forever SoTX

Hi - TriX:
I ran FoCal Pro on the new Sigma lens twice. Both times the 20mm MA was at +15; the 10mm was 16 the first time and 17 the second time. I thought the center crop image at 16 was a tad better - so I did not change the 20mm MA (FoCal gives you the option of keeping the old value or updating to the new value).

TriX Loc: Raleigh, NC

Did you do the focus consistency or acuity tests? (Are you running the pro version that has these available?)

Merlin1300 Loc: New England, But Now & Forever SoTX

I did not specifically run the consistency / acuity tests. I do have the "Pro" version.
Just as an update - although there may be mitigating circumstances - I am a bit concerned regarding a new lens that is so close to the adjustable threshold. Should the lens deteriorate by just a little bit over the next 5 - 10 years, I'd have one that could not be adjusted back to acceptable sharpness. This lens is going back via return shipping on Monday.
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Reikan FoCal Pro 2.11