DOF, Front Focus, Back Focus - HELP!

Started Aug 24, 2005 | Discussions
edhannon
edhannon Senior Member • Posts: 1,761
DOF, Front Focus, Back Focus - HELP!

All these threads on DOF, Front & Back Focus have me very confused!

Let me put in my own words what I thought I understood before all this. Then maybe someone with a lot more experience than me (like Godfrey) can get me straight.

I thought DOF depended on three things: the subjective opinion of the viewer of what is sharp, the size of enlargement, and the geometry of the "circles of confusion". By the last one I mean the size of the circle projected on the film/sensor by a point in the subject.

I don't think that the first two have nothing to do with the lens/camera so I will ignore them in this.

I thought that the the size of the circle is only a function of the lens focal length, the aperture selected and how far the subject is from the center focus plane.

I can't see how the lens or camera allignment can effect how far front or back of the center focus point that a subject appears in focus.

What I can see is how the center focus point can be different from what I think it is. In manual focus I do not allways manage to focus where I think I am focusing. In auto focus I seem to focus where I don't want to more than in manual focus!

So are not front/back DOF issues really mis-focusing issues?

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volosong Veteran Member • Posts: 4,318
Re: DOF, Front Focus, Back Focus - HELP!

To help unerstand front or back focus, imagine a large letter "H" on a sheet of paper. The "H" will be printed in black ink (laserjet toner, etc.) on white sheet of paper, somewhere around 8.5 x 11 inches in size.

Now, take that large "H" shape and construct it so that the center, horizontal, cross bar of the "H" is solid and that the vertical, stands, of the "H" are a series of small, horzontal lines . . . each very thin, but extend horizontal for about two inches.

Place the paper on the floor and set up your camera/lens on a tripod so that the angle from the lens to the target (paper) is 45 degrees. Simple enough to set up.

Then, spot focus on the horizontal, cross bar, part of the "H" pattern and take your exposure. It is very important that the camera be in spot focus mode. Exposure setting are really not important, since focus is being tested. (By using spot focus, the camera/lens is only focusing on the center bar and does not try to focus on anything outside its "circle". Anything outside will focus as the lens design allows. Since we are measuring distance, only two dimensions are in play, not three.)

Take your shot, preferably in mirror lock-up, IR remote, cable remote mode. The idea here is to minimize any shaking that could cause blur or out-of-focus issues. Examine the resultant image on a computer . . . or print it out. On a perfectly focused image, the horizontal cross bar will be in perfect focus. As you view straight out to the sides, looking at the vertically stacked lines on the vertical part of the "H", they will be in focus also and the distance where the vertically stacked line become blured is equidistant on either side of the central bar. You can easily see what is in focus and what is blured.

Depth of field affects the results. With a narrow DOF, the horizontal part of the "H" will be in focus and only part of the vertically stacked bars on the side.

If more vertical stacked bars are in focus on the lower part of the "H", there is a front-focus "problem". If more vertically stacked bars are in focus on the upper part of the "H" (farther away from the "film" focal plane), then there is a back focus "problem".

Hopes this clarifies a bit. Both of my "pro quality" Sigmas have a front focus "problem". (70-200/2.8 EX and 100-300/4 EX) The issue is being worked by Sigma UK and Sigma Japan and our forum member, Richard Day. Richard is on top of this and expects Sigma to resolve the issue by the end of August (as of the last e-mail I received from Richard.)

If you want to run test yourself on your lenes, search some of Richard's posts and find the link to a PDF file available on the web with a calibration/text chart and extensive directions as well as samples.

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  • Steven

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Jonas B Forum Pro • Posts: 14,596
Re: DOF, Front Focus, Back Focus - HELP!

It's too much at one time, too much English for me to write and too much details to really be dissecated here.
So I go directly to the last sentence:

So are not front/back DOF issues really mis-focusing issues?

Yes, of course. The only other reason would be some sort of odd lens construction giving more DOF in front of the focus plane instead of having it the usual way.

So, I agree with you and the conclusion you made. I also think that the post I saw somewhere where somebody (my middle name is Senilix) was talking about the placement of the DOF was nothing but a way to express in what way he/she/the camera misfocused.

if I got you right,

Jonas

Jonas B Forum Pro • Posts: 14,596
Re: DOF, Front Focus, Back Focus - HELP!

Ah, Steven replied while I was contemplating my own reply. I can add the link for the Tim's focus test chart, http://md.co.za/d70/chart.html as I guess that's the he was thinkin' of.

Jonas

volosong Veteran Member • Posts: 4,318
Re: DOF, Front Focus, Back Focus - HELP!

Jonas B wrote:

http://md.co.za/d70/chart.html as I guess that's the he was
thinkin' of.

That's the one, Jonas! I must say that even though I have the front-focus "problem" with my two telezooms, in real world applications, I have not noticed any degredation of images, other than operator error (which happens al too often).

Most of my use with the 70-200/2.8 has been at weddings and the 100-300/4 have been at local league tennis matches and high school baseball games. At tennis and baseball, I've had a lot more problems with autofocus issues than with front-focus problems. I solved that problem by going to manual focus mode, keeping the shutter speed high to freeze action, and upping the ISO thereby letting me use a smaller aperture resulting in a decent DOF (so focus isn't as critical).

The wedding images came out pretty good. In fact, of the three weddings for family members this summer where I attended as a guest, in two of those weddings the bride and groom "begged" me for my images because the "professional" photograher did such a terrible job.
Good thing I used RAW.

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edhannon
OP edhannon Senior Member • Posts: 1,761
Re: DOF, Front Focus, Back Focus - HELP!

Just to make sure we are all saying the same thing, let me say it simpler:

Short of a lens whose correction was so bad that it could not be used even when "in focus", there is nothing that a lens or sensor can do to change the ratio between the distance from front DOF to true focus plane vs back DOF to true focus plane. That ratio is determined purely by the laws of geometry.

If the ratio appears off, it can only mean that the plane of focus is not where you think it is.

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Jonas B Forum Pro • Posts: 14,596
Re: DOF, Front Focus, Back Focus - HELP!

Now you are talking. And yes, I'm still with you.

There was perhaps something in my first post that was unclear or strange but to me it seems as you got it right, from the start and still so.

Jonas

popopow Regular Member • Posts: 342
Re: DOF, Front Focus, Back Focus - HELP!

Hi,
I have also the sigma 70-200 2.8.
Never noticed any front focus problem until last sunday with this shot :

As far as i remember i focused on the kid, but the sand in front of him is clearly more in focus.

So i tried the test chart :

.

It seems also that the lens really have front focus.
Does my lens have really has a problem ?
--
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popopow Regular Member • Posts: 342
Re: DOF, Front Focus, Back Focus - HELP!

In fact it seems that the problem also occurs with the pentax FA 50 1.7

Maybe be i should return my camera ?

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edhannon
OP edhannon Senior Member • Posts: 1,761
Ratio of Front to Back DOF changes with focal length!

I was curious as to what was the correct ratio of front DOF to back DOF. Found a site that said it varied based on focal length.

That did not seem right to me. But I found this DOF calculator that gives front to back %.

http://www.dofmaster.com/dofjs.html

Sure enough front to back ratio is 50% to 50% only at the higher focal lengths (at 50mm it is 47% in front to 53% in back, at 28mm it is 40% front and 60% back)

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Jonas B Forum Pro • Posts: 14,596
Re: Ratio of Front to Back DOF changes with focal length!

Ah, ok, then I didn't get you right from the beginning. But it is very nice anyway when we all can agree in the end...

regards,

Jonas

RJPhipps Forum Member • Posts: 94
1/3-2/3 Front/Back DOF 'rule' is flawed-

Looking at various DOF calculations using the DOFMaster website http://www.dofmaster.com/dofjs.html I observe the following trend for a 50mm lens at f2.8

Distance %DOF In Front
1ft 50%
10ft 45%
20ft 40%
40ft 29%
60% 19%
90ft 4%

Similar trends can be observerved for other focal lengths and apertures, though the rate of 'decay' differs.

Therefore is seems clear that the traditional 1/3 in front 2/3 behind 'rule' for Depth of field is a very badly flawed approximation. At some specific distance (for any given lens and aperture), this holds true, and DOF is always greater behind than in front, but at close distances the ratio approaches 50%, and at long distances the ratio approaches zero!

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rossphotos Junior Member • Posts: 28
Don't return it...

I had a 'front focus' issue with my first *istDs and every lens I own, some AF, and some MF (based on the AF indicator). It was a little worse than what you show for your FA 50/1.7 but not by much. B&H graciously allowed me to exchange the body but the replacement was still off, and comparable to what you show for your 50/1.7.

I rigorously tested both bodies using both manual and auto focus and came to the conclusion that (1) both bodies had perfectly aligned focusing screens and sensors since I could nail the focus manually just about every time. (2) The AF sensor locked focus over a range of distances which is weighted just slilghtly closer than the actual focal plane. When autofocusing a lens, focus tended to lock on the near end of that range, resulting in the 10-20mm of 'front' focus that I and you observed.

So I believe that the tolerances of the AF system in this camera is just not very good for critical DOF use, period. After doing lots of reading here and on other forums, all the dSLRs have some issues like this. We are stuck with the AF systems of $200-400 film based SLRs in $1000 cameras unfortunately. Only the top pro models (Canon 1D and Nikon D1/2) seem to have stricter QC for AF accuracy, but I am sure that you will not read much about such issues on the internet with these cameras if they do exist since folks using those have better things to do with their time then gripe about it in public like us!

rke21 Regular Member • Posts: 468
Re: DOF, Front Focus, Back Focus - HELP!

DOF is really an approximate boundary. That's why it's called the circle of confusion. Where does the dof end and where does the bokeh begin? It's not really known because it varies per se. I might see it in one place and you'll see it somewhere else.

The problem is we need a way to compute the dof. Since it's not known exactly where the dof ends, someone came up with a formula. And basically, that place 1/3 of the dof in front and 2/3 of the dof in the back. These are defined dof. It's just a mathematical model. what you or anyone else actually see will be different. But it allows us compute a dof based on aperture and distance.

As it turns out, a passive autofocus system will put you somewhere in the dof. How in the world does it figure out where to focus so that it's 1/3 and 2/3? It can't do it acurately. That's why you get the front focusing or back focusing. It can be annoying when you're dof is narrow. The best part of having a Pentax DA lens is you can manually focus the lens while using autofocus. That's why I like the DA lens. I had to mention that in case you're wondering why anyone would use manual focus in autofocus mode. It's great.

edhannon wrote:

All these threads on DOF, Front & Back Focus have me very confused!

Let me put in my own words what I thought I understood before all
this. Then maybe someone with a lot more experience than me (like
Godfrey) can get me straight.

I thought DOF depended on three things: the subjective opinion of
the viewer of what is sharp, the size of enlargement, and the
geometry of the "circles of confusion". By the last one I mean the
size of the circle projected on the film/sensor by a point in the
subject.

I don't think that the first two have nothing to do with the
lens/camera so I will ignore them in this.

I thought that the the size of the circle is only a function of the
lens focal length, the aperture selected and how far the subject is
from the center focus plane.

I can't see how the lens or camera allignment can effect how far
front or back of the center focus point that a subject appears in
focus.

What I can see is how the center focus point can be different from
what I think it is. In manual focus I do not allways manage to
focus where I think I am focusing. In auto focus I seem to focus
where I don't want to more than in manual focus!

So are not front/back DOF issues really mis-focusing issues?

Roland Mabo Forum Pro • Posts: 12,462
Re: DOF, Front Focus, Back Focus - HELP!

I believe front/back focus has always existed with AF bodies, but the problem is more noticeable with digital because we're viewing the images at 100% resoultion at a full screen - so it's easier to spot than on a smaller, printed, image. In the film days, with the small prints, we was happy shooters because we didn't know better...

Take care

  • R

popopow Regular Member • Posts: 342
Re: Don't return it...

Yep you are mabe right.

After foucsing at the foucs line of the test pattern, I tried to focus a little behind and didn't heard the camera and lens trying to focus...

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edhannon
OP edhannon Senior Member • Posts: 1,761
I think I have it now

After all the responses, I think I have an understanding of what is going on:

1. The threads about back/front focus are really talking about where the AF decides the plane on which it will focus is located. I got confused with terminology when some talked about how much was in focus in back of the focus plane vs how much in front.

2. DoF is not an absolute concept. It varies depending on a lot of variables: the viewer, amount of enlargement (on screen or on print), etc.

3. I learned that how much is in focus in back vs front is not constant but varies depending on lens focal length and distance. Thus the location of the plane of focus is not easily judged by looking at the front and back edges of the DoF.

4. The only physical constant in all this is that, for a given focus setting on a lens (distance from sensor), there is only one plane parallel to the sensor that is really in focus.

5. AF is even less deterministic than I thought. Only the Shadow knows where it will lock. Another reason for me to avoid AF as much as I can.

Ed

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racehorse in the desert Veteran Member • Posts: 4,103
Good discussion

Very good discussion. So, as has always been the case, stop down a bit more to increase DOF, unless you want bokeh for creative purposes. I have long wondered about the true accuracy of passive AF systems, and the points made here have pointed out the limitations of those systems.
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brian thomson Regular Member • Posts: 227
Re: DOF, Front Focus, Back Focus - HELP!

This has been an enlightening discussion so far, I was never that clear on the front/back focus problem. I certainly have never had the AF system get it totally wrong, like in the example on pg 10 of that test chart PDF. I do most of my shooting with the DA 16-45mm lens.

Here's a point that just occurred to me: when a lens is in A -mode, the camera body is always focusing with the lens open at its widest. Then, when the picture is taken, the lens is allowed to stop down to the selected aperture. Does this mean that:

a) we are (at least partly) talking about a change in a lens' focus point at different apertures?

b) shouldn't that mean there should be no problem with the lens at its widest for the shot - since the aperture doesn't change between focusing and the shot?

I have a Sigma 70-200mm f2.8 EX too, and while I haven't been looking for this problem, I will now I have read of it. curious to hear what Sigma UK come up with as a solution. Are we talking about replacing the lens? I bought mine literally days before they introduced the DG model, so that would be good... 8-)

Thanks,

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brian thomson
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edhannon
OP edhannon Senior Member • Posts: 1,761
Re: DOF, Front Focus, Back Focus - HELP!

brian thomson wrote:

stop down to the selected aperture. Does this mean that:
a) we are (at least partly) talking about a change in a lens' focus
point at different apertures?

The focus point does not change with aperture. What changes is how far in front and in back of the focus point a point on the subject produces a circle less than or equal to the circle of confusion. That is how far front and back appear to be in focus.

Ed

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