Front Focus Back Focus hocus pocus

[Abitconfused]

In a rare moment of overindulgence, (who is he trying to kid) I bought a Sigma 20mm for my D810. I like the results as the lens is very sharp but I miss the freedom of my wide angle zoom, also a Sigma, which produces great results too but not as pinpoint sharp as the 20mm. But the 20mm is fast at f/1.4. Anyway, I observed some front focusing so I went to the D810's menu and did a +10 lobatomy on it. Bingo, problem solved or solved enough for me. Am I a heretic? I didn' use any instrument or tripod or scientific analysis. The focus adjustments are from -20 to +20 so it's not like there are an infinite number of choices. Anyway, I suggest having a feel for the image is the gooder of two weevils.

[chauncey]

Huh?

[xpatUSA]

I've never really understood the front/back focus issue. I suppose certain types of photography demand absolutely precise focal plane placement but I've yet to encounter those types, it seems. I don't use AF much, anyway.

But we do hear horror stories aplenty of how someone might send a camera or a lens for service or repair and now none of them will work with each other, er, if you see what I mean . .

[William W]

Your experience concurs with other reports where the Sigma 20/1.4 Art requires micro adjustment both for Canon and Nikon bodies, whereas, (by contrast), the EF 24/1.4L MkII and the 24mm f/1.4G AF-S, do not, or require very little adjustment.

The (previous) Sigma 20mm F/1.8 EX DG was (is) well known for front focusing on many Canon DLSR/SLR bodies: for those without Micro Focus Adjustment, a common practice (at least amongst my cohort) was to physically adjust the camera position by eye to attain the correct plane of sharp focus, but this was really only necessary for the closer Subject Distances because there is usually an adequate DoF at ‘normal’ Portrait SD, even if used at F/1.8. The 20/1.8 was (is) quite popular amongst Wedding Photographers, and also for its slight ‘macro’ ability. The newer 20/1.4 also seems to hold in good standing – probably that standing is bettered because most ‘serious’ DSLR’s have a micro adjustment to assist/correct the focus issues especially noting that the Sigma 20/1.4 is less capital outlay than comparable Canon and Nikon lenses.

Not sure what you mean ‘heretic’ but I assume the comment refers to NOT using any technical scientific procedure to make the micro-adjustment – if that assumption I correct, I donlt think that is heracy, but I do think that soem analysis of the sample photos would be useful to nail down whether (or not) ‘+10’ is the best answer – that would be quite easy to do, just select +5 and then also +15 and compare by eye the results . . . you might find +15 is actually better – also remember that there might be a different bets solution at differing Subject Distances – but as already implied the most deleterious effect is at the shorter SD’s.

20mm is a very useful Focal Length for many Photographers – for what main purpose(s) did you buy it?

WW

[William W]

I've never really understood the front/back focus issue. I suppose certain types of photography demand absolutely precise focal plane placement but I've yet to encounter those types, it seems.

These are two examples where a W&P Photographer typically uses a W/A Lens at a close Subject Distance and at a large aperture. Other examples would be the Ring Shot and a Detailed Cake Shot. In all these cases accurate focus is required to nail the major (part of) an element in a frame where there is a limited DoF. (Both these were made with a 35/1.4 on a 5D Series Camera).

But as I mentioned once we get to longer Subject Distances the accuracy of the Plane of Sharp Focus becomes (much) less relevant.

Two reasons why the Sigma 20/1.8 (and now the 20/1.4) is popular with W&P Photographers is firstly the price, and secondly, for those who run a Dual Format Kit, then using a fast 20mm FL Prime Lens is seen by many as a more flexible option than using a fast 24mm Prime Lens.

For example (I use Canon) and the three fast Primes 20; 50 and 135 provide equivalent FoV of: 20; 32; 50; 80; 135 and 216 all at fast aperture. The alternate option of 24; 50 and 135 provide: 24; 38; 50; 80; 135 and 216 all at fast aperture.

The major feature is – there is a dramatic DIFFERENCE between the AMOUNT of FLEXIBILITY when one is using a 20 vs. 24 lens; or when using a 32 vs. 38 lens.

WW

All Images © AJ Group Pty Ltd Aust 1996~2016 WMW 1965~1996

[Stagecoach]

.so I went to the D810's menu and did a +10 lobatomy on it. Bingo, problem solved or solved enough for me. Am I a heretic?

Absolutely not !

If you feel somethings not right, investigate it. I recently AF fine tuned one of my long lens/+TX to my two bodies after suspecting it was front focusing having viewed many images from it.

I did go a bit more technical than you have done so far;

Masking tape with cross on it stuck to a wall.
Camera on tripod with remote release at FL and rough distance I wanted it 'best'.
Ran through a number of tests setting AF fine tune between -20 to +20 (at steps of 5), each shot de-focusing in opposite directions.
Sat at computer and studied each set.
Conclusion, +15 gave the sharpest cross on the masking tape in every test

Not sure about the Sigma with your 810 but with Nikon lenses the camera recognizes the individual lens and applies the AF bias to only that lens when it is fitted.

[William W]

. . . Not sure about the Sigma with your 810 but with Nikon lenses the camera recognizes the individual lens and applies the AF bias to only that lens when it is fitted.

? ? ?

Later this morning after I replied, I too was thinking about exactly that question.

WW

[george013]

The lens corrections are stored in memory under the lensname. This is only possible when the lens has a for the camera readable "cpu". I think you can attach the corrections manual too for the older lenses.

A simple trick to look for front or back focus issues is to compare the focus results between AF through the lens and LV manual.
Don't forget that the AF-sensor has a dimension and it's not always sure where in that dimension the focus is set. So if possible use a plane parallel to the sensor.

George

[Abitconfused]

Your experience concurs with other reports where the Sigma 20/1.4 Art requires micro adjustment both for Canon and Nikon bodies, whereas, (by contrast), the EF 24/1.4L MkII and the 24mm f/1.4G AF-S, do not, or require very little adjustment.

The (previous) Sigma 20mm F/1.8 EX DG was (is) well known for front focusing on many Canon DLSR/SLR bodies: for those without Micro Focus Adjustment, a common practice (at least amongst my cohort) was to physically adjust the camera position by eye to attain the correct plane of sharp focus, but this was really only necessary for the closer Subject Distances because there is usually an adequate DoF at ‘normal’ Portrait SD, even if used at F/1.8. The 20/1.8 was (is) quite popular amongst Wedding Photographers, and also for its slight ‘macro’ ability. The newer 20/1.4 also seems to hold in good standing – probably that standing is bettered because most ‘serious’ DSLR’s have a micro adjustment to assist/correct the focus issues especially noting that the Sigma 20/1.4 is less capital outlay than comparable Canon and Nikon lenses.

Not sure what you mean ‘heretic’ but I assume the comment refers to NOT using any technical scientific procedure to make the micro-adjustment – if that assumption I correct, I donlt think that is heracy, but I do think that soem analysis of the sample photos would be useful to nail down whether (or not) ‘+10’ is the best answer – that would be quite easy to do, just select +5 and then also +15 and compare by eye the results . . . you might find +15 is actually better – also remember that there might be a different bets solution at differing Subject Distances – but as already implied the most deleterious effect is at the shorter SD’s.

20mm is a very useful Focal Length for many Photographers – for what main purpose(s) did you buy it?

WW

I do some wedding, event and architecture but I like wide angle photography and the lens was there.

[Abitconfused]

Huh?

What?

[xpatUSA]

These are two examples where a W&P Photographer typically uses a W/A Lens at a close Subject Distance and at a large aperture. Other examples would be the Ring Shot and a Detailed Cake Shot. In all these cases accurate focus is required to nail the major (part of) an element in a frame where there is a limited DoF. (Both these were made with a 35/1.4 on a 5D Series Camera).

But as I mentioned once we get to longer Subject Distances the accuracy of the Plane of Sharp Focus becomes (much) less relevant.

T<>

Thanks for the examples, Bill,

Now I understand my lack of bad experiences with front/back focusing. For my still-life close-up work I always focus manually and try to get the DOF to "straddle" the item of interest. Plus, I'm using the Panasonic DMC-G1 much more often for close-ups these days and the 10X magnification 'focus by wire' is a pure joy to use.

And as to greater distances, I mostly shoot natural subjects, trees, woodland, streets, usually manual focus (retired, all the time in the world, etc). So, like, there's always something in focus.

Not that manual focus with my DSLRs actually gets me round any front/back issue - especially if I listen to the 'in-focus' beep from the Sigma AF sensor system.

[Codebreaker]

FWIW: I had a Samyang 8mm for my Canon 7d. It's a manual lens but would not focus beyond 1.5m . I tested the lens by laying a tape measure on the floor and then manually focusing as best as possible at various length marks along the tape. Also did the test at different apertures. In front of the 1.5m mark was ok but nothing beyond that could be achieved.

I returned the lens and the second model proved to be perfect, or as near as I can detect.

[Black Pearl]

I left front/back focus problems behind when I went mirrorless

[John 2]

I left front/back focus problems behind when I went mirrorless

Not to mention mirror slap when it come to the long end.

I don't have my Nikon D7100 any more but I found this useful when it came to calibrating the auto focus when I did:

https://www.youtube.com/watch?v=EqhH2ELuCaE