front/back focusing - to whom is it important?

[xpatUSA]

They're getting their knickers in a twist yet again over here on my "home" forum:

http://www.dpreview.com/forums/post/55918617

Folks are sending their cameras off to Sigma for AF re-adjustment on multiple occasions, agonizing over this lens vs. that, etc. etc.

Also consider that the forum (Sigma) has no 135 format "full frame" cameras in it, thus making "razor-thin DOF" almost a non-factor except perhaps for the f/1.4 crowd.

Enter your resident Philistine (me) who really couldn't care less if the focus is an inch or two off in a few yards or more away. Or a foot or more in a hundred yards away, e.g., it doesn't matter to me which leaves are in perfect focus in an oblique shot of a row of trees (hyperfocal matters notwithstanding).

Having said that, please allow me to explain my personal view.

- I don't go normally below f/5.6 (seeking best sharpness, i.e. lack of abberation).
- I normally prefer quite a bit of DOF in my shots, near or far, and will use up to f/16 to get it if necessary - recovering with LR deconvolution sharpness in post.
- None of my cameras or lenses have sufficient resolution even to worry about it.

If focus accuracy is important for the occasional shot, e.g. close-up of a coin surface or a watch dial, I'll focus-bracket with several separate shots, bearing in mind that any amount of front or back focus for such shots is more like a millimeter or fractions thereof.

So I'm trying say that, to me, the back/front focus issue is unimportant.

However, I would be interested to read comments from those that do find it important and, if so, how they fix it (apart from CDAF, live magnified view and manual focus, that is).

[William W]

I keep my eyes closed, that way I can't see it.

Can we discuss Protection Filters now, please?

[Shadowman]

I too suffered from the ailment, then I just became more attuned to my camera's focusing system and moved on.

[William W]

I keep my eyes closed, that way I can't see it.

Can we discuss Protection Filters now, please?

That's a very silly comment to a very reasonable question.

***

I have one little bugger that tries to cause me grief in that area, funny thing is it is an old Sigma W/A Prime Lens that was given to me by a Bloke who had no need for it. It Back Focuses. Seems to be reasonably consistent. Doesn't worry me much I occasionally use it on an APS-C for street stuff I use it at F/8 mainly for hip-shots.

WW

[xpatUSA]

That's a very silly comment to a very reasonable question.

No problem, Bill, I got your drift. And I'm sure that front/back focusing (back/front focussing in Oz) and causes thereof have been beaten to death here - and everywhere else, for that matter. A fascinating subject, not the least of which is it's obfuscation by linear measure - demonstrating yet again we photographers' natural aversion to angular measure of any kind

[bugster0121]

For me personally getting the focal point of the image i.e. The eyes(s) the sharpest in the capture is the most important thing. Viewers eyes are drawn to this. If the focal point in the image is large i.e clouds then as long as the focus point is close then i dont worry to much. In portraiture it matters greatly at least for me.

[DanK]

My opinion is that it usually doesn't matter but can.

I tested 5 lenses, if I recall, on a body I recently sold, a 50D. 4 were essentially spot on. The last one, a zoom, was a little bit off in one direction at the short end and a roughly comparable amount in the other direction at the long end. I adjusted it for the long end, but afterwards, I realized that I almost never shot that lens wide open anyway.

I think it is worth testing lenses, not because a very small adjustment matters much, but because there really are bad batches. Check out this posting from lensrentals. That said, I have had my 5D3 for a year and a half and just haven't gotten around to testing a single lens on it, because I have never noticed a problem. Someday, I will open a beer and start on them.

[jcuknz]

There are a couple of photos of a swallow at its nest over in the nature forum which everybody is going on over and in both the head of the bird is soft. I suspect not a lens error but the camera was set up and left, but a wrong focus point had been choosen .. so assuming you get the lens right are you going to focus correctly?
Side point ... does the camera have a sufficiently small focus area to accurately focus or is it being used with the hit-or-miss multiple areas system.

[xpatUSA]

Side point ... does the camera have a sufficiently small focus area to accurately focus or is it being used with the hit-or-miss multiple areas system.

But a good point, nevertheless. In the OP I should indeed have mentioned that I use spot metering (in manual) to estimate the best exposure required. Then I aim the spot at the plane of interest (half press) and twiddle the focus ring until the camera beeps (not too different to using AF really). Then I re-compose and push the button. I'm retired, so there's plenty of time for all that . .

[george013]

But a good point, nevertheless. In the OP I should indeed have mentioned that I use spot metering (in manual) to estimate the best exposure required. Then I aim the spot at the plane of interest (half press) and twiddle the focus ring until the camera beeps (not too different to using AF really). Then I re-compose and push the button. I'm retired, so there's plenty of time for all that . .

Recomposing is changing your focus by definition. With long distances you won't see it, but with short distances you will. With open diafragma off course.

George

[xpatUSA]

Recomposing is changing your focus by definition.

George, I should have made my method more clear, because you have misunderstood.

I always shoot with everything in manual, including the lens focus switch. So, by that definition, once I have adjusted the focus to my liking, I can move the camera around (which is what I meant by recomposing) and my focus does not change at all. On the other hand, should I move my tripod to somewhere else, then I get to start over again.

If you use a different definition for "recomposing" please let us know what that is.

[george013]

George, I should have made my method more clear, because you have misunderstood.

I always shoot with everything in manual, including the lens focus switch. So, by that definition, once I have adjusted the focus to my liking, I can move the camera around (which is what I meant by recomposing) and my focus does not change at all.

If you use a different definition for "recomposing" please let us know what that is.

You don't focus on a distance, but on a plane parallel to your sensor. If you recompose, rotating the camera, that plane is moving to. The distance to your subject might stay the same, but the plane has changed.
On short distances that's very noticible. You will get front focusing.

George

[xpatUSA]

Now I understand. I believe you mean rotation of the camera, for example horizontally commonly called "panning", but any axis will do. If I understand you correctly, my spot focusing places the subject initially at the center of the frame. Then recomposing to place the subject elsewhere, by rotating the camera, rotates the plane of focus in the scene and the camera will apparently "front-focus".

The odd thing is, I've never realized that until you just pointed it out, and I thank you most sincerely.

[george013]

Now I understand. I believe you mean rotation of the camera, for example horizontally commonly called "panning", but any axis will do. If I understand you correctly, my spot focusing places the subject initially at the center of the frame. Then recomposing to place the subject elsewhere, by rotating the camera, rotates the plane of focus in the scene and the camera will apparently "front-focus".

The odd thing is, I've never realized that until you just pointed it out, and I thank you most sincerely.

If you keep in mind that you focus on planes parallel to the sensor, it can be of a help to find alternative points to focus on, when needed.

George

[William W]

. . .I'm sure that front/back focusing (back/front focussing in Oz) . . .

That's funny.

[rpcrowe]

I keep my eyes closed, that way I can't see it.

Can we discuss Protection Filters now, please?

Or full frame vs crop format... I don't think that has been covered very much

[xpatUSA]

xpatUSA wrote: Re: front/back focusing - to whom is it important?

Bill wrote: I keep my eyes closed, that way I can't see it.

Can we discuss Protection Filters now, please?

Or full frame vs crop format... I don't think that has been covered very much

Yes, it's amazing how some people buy those slanted ruler thingies just to test for such unimportant matters.

I suppose for hairy subjects it really doesn't matter which end of the animal is in focus

My apologies for wasting people's time with such a frivolous post

[William W]

. . .Yes, it's amazing how some people buy those slanted ruler thingies just to test for such unimportant matters.

Hey Ted,

On a semi serious note, I never bought one of those slanted ruler thingies: I figured any rule in my workshop stuck at the bottom and top with Gaffer's tape on my work bench & wall and at somewhere about 45 degrees would do the job adequately.

Also - the brick wall on the front of my house is used for Barrel/Pincushion tests.

Then stick 5 sheets of newspaper on the same wall (Gaffer's Tape again) - set them out in a quincunx, that's my standard IQ test block.

(BTW 'quincunx' is a word to store away for huge points when playing Scrabble)

WW

[xpatUSA]

Hey Ted,

On a semi serious note, I never bought one of those slanted ruler thingies: I figured any rule in my workshop stuck at the bottom and top with Gaffer's tape on my work bench & wall and at somewhere about 45 degrees would do the job adequately.

Me neither, Bill (or, US: me either).

When I got interested in the subject, I took my nicest ruler (a W&G ) and did much the same but, being me, I agonized about the angle. Surely, I thought, the smaller the angle - the more "resolution" would obtain. Somehow 45 deg. seemed a bit coarse, if you will. Anyhow, a few shots at an angle of 15 deg. soon showed the error of my ways. At that angle, the degree of focus spread itself over the entire length of the ruler and it was virtually impossible to estimate where the center of focus was; whereas, at 45 deg. or even more, the center of focus was quite evident.

Gaffer's tape, aka bodge tape (RAF) or duct tape (US) is certainly the handyman's friend:

Wobbly tent?



Cut thumb?





Wife looks a little bemused

[DanK]

You don't focus on a distance, but on a plane parallel to your sensor. If you recompose, rotating the camera, that plane is moving to. The distance to your subject might stay the same, but the plane has changed.
On short distances that's very noticible. You will get front focusing

This should only matter if you rotate quite a bit and have a very shallow DOF. The ratio of original to final distance to the point of focus, looking straight ahead without rotation, is just the cosine of the angle of rotation, right? So a 10 degree rotation, for example, would make the distance to the focal point .985 times the original distance.