Depth of field question

[FWG]

My 5D Mk3 seems to have a DoF which is biased from the focus point towards the camera. In other words the depth of field is not equal about the point of focus. It is particularly noticable on my Canon 85mm f1.8 lens when taking "close up" photos at about 1m from the camera at wide appatures. To a lesser extent it also seems present on my 24 - 70mm f2.8 and of course the effect is less at longer focus distances and smaller appatures. Has anyone else experienced this and can it be adjusted (or is that normal)?

[Waddizzle]

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@RobertTheFat wrote:

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@FWG wrote:

Excellent - are there any instructions (Canon or otherwise) that can help me [use AFMA to correct my focusing issue]? I do not want to make it worse!

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Look it up in your instruction manual. AFMA isn't nearly as complicated as some make it out to be. All you really have to do is take a series of pictures, using different AFMA settings, of a scene that has objects at different distances and zero in on the setting in which the object actually in focus most closely matches what the camera thought was in focus. Your 5D3 allows you to record AFMA values at both ends of a lens's zoom range for greater accuracy.

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Psst.  EF 85mm f/1.8 prime.  I disagree.  While AFMA adjustments are not complicated, once you have done it a couple of times. Iit is all too easy to take bad test shots the first time you do it.  

The hardest part is figuring whether or not you need a positive or a negative adjustment value. I said that it was “back focusing”, while someone else said it was “front focusing”.  Which is it?

[RobertTheFat]

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@Waddizzle wrote:

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@RobertTheFat wrote:

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@FWG wrote:

Excellent - are there any instructions (Canon or otherwise) that can help me [use AFMA to correct my focusing issue]? I do not want to make it worse!

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Look it up in your instruction manual. AFMA isn't nearly as complicated as some make it out to be. All you really have to do is take a series of pictures, using different AFMA settings, of a scene that has objects at different distances and zero in on the setting in which the object actually in focus most closely matches what the camera thought was in focus. Your 5D3 allows you to record AFMA values at both ends of a lens's zoom range for greater accuracy.

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Psst.  EF 85mm f/1.8 prime.  I disagree.  While AFMA adjustments are not complicated, once you have done it a couple of times. Iit is all too easy to take bad test shots the first time you do it.  

The hardest part is figuring whether or not you need a positive or a negative adjustment value. I said that it was “back focusing”, while someone else said it was “front focusing”.  Which is it?

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If a +1 adjustment gives you better focus than zero adjustment, it needs positive adjustment. If +2 gives you better focus than +1, it needs more positive adjustment. Etc. Same with negative adjustment. And it doesn't matter if you get it wrong the first time; you just try again.

And you did notice the OP's reference to a 24-70 f/2.8, right?

[Waddizzle]

"And you did notice the OP's reference to a 24-70 f/2.8, right?"

Yup, I noticed. 

"It is particularly noticable on my Canon 85mm f1.8 lens when taking "close up" photos at about 1m from the camera at wide appatures."

His complaint is primarily with the 85mm prime, though, not the zoom.

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Is it back or front focusing?  I like your answer.  You proved my point.  It is not quite as simple and straight forward as it might seem.  You have to pay close attention to what you're doing.  The devil is in the details.  Plug and play, doesn't work well.

The OP says the [camera/lens] combo causes the DOF to be between the camera and the subject [plane of focus].  It is back focusing.  The camera is focusing behind the actual DOF.  You would enter a negative value to compensate.  A negative value decreases the distance to the plane of focus, which brings the plane of focus closer to the camera.  A positive increase the distance to the plane of focus, which moves it away from the camera.

[FWG · Accepted solution]

Hi Waddizzle

As you were so helpful re my question just to let you know the outcome. I followed all the tests you suggested and have concluded that I am not a very good photographer as I could find nothing wrong with my camera or lens settings. It is however sometimes good to know what the problem isn't as I can now concentrate on my technique knowing that there is a perfect shot to be had!

Thanks again and Happy 2018

[ebiggs1]

Most lenses do not need AFMA.  The easiest way to do it is with a ruler.  Lay it on the floor about 7 feet (not a fixed value as FL will change this) away.  Use center focus point and shoot towards the middle of the ruler.  It is easy to see the DOF.  Use a wide aperture and longer FL if it is a zoom lens.

[Waddizzle]

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@ebiggs1 wrote:

Most lenses do not need AFMA.  The easiest way to do it is with a ruler.  Lay it on the floor about 7 feet (not a fixed value as FL will change this) away.  Use center focus point and shoot towards the middle of the ruler.  It is easy to see the DOF.  Use a wide aperture and longer FL if it is a zoom lens.

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A ruler works, but it is bit short.  I think a yardstick or a tape measure works better, especially with longer focal lengths.

[ebiggs1]

"A ruler works, but it is bit short."

Hmmm, I didn't suggest to use a 6in or 12in 'ruler'.  But ti does depends on several other factors on what type ruler you use.

[Waddizzle]

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@FWG wrote:

Hi Waddizzle

As you were so helpful re my question just to let you know the outcome. I followed all the tests you suggested and have concluded that I am not a very good photographer as I could find nothing wrong with my camera or lens settings. It is however sometimes good to know what the problem isn't as I can now concentrate on my technique knowing that there is a perfect shot to be had!

Thanks again and Happy 2018

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You may be better than you think.  It takes a good photographer to recognize their flaws.  Besides, many consumer grade lenses do not seem to focus consistently enough to bother with using AFMA, not unless they are WAY off the mark.  A lens like that would need repair, anyway.

[RobertTheFat]

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@Waddizzle wrote:

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@FWG wrote:

Hi Waddizzle

As you were so helpful re my question just to let you know the outcome. I followed all the tests you suggested and have concluded that I am not a very good photographer as I could find nothing wrong with my camera or lens settings. It is however sometimes good to know what the problem isn't as I can now concentrate on my technique knowing that there is a perfect shot to be had!

Thanks again and Happy 2018

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You may be better than you think.  It takes a good photographer to recognize their flaws.  Besides, many consumer grade lenses do not seem to focus consistently enough to bother with using AFMA, not unless they are WAY off the mark.  A lens like that would need repair, anyway.

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OTOH, it's the consumer-grade lenses that are most likely to need AFMA. I've used five "L" lenses, and none of them needed it. I've always thought it a bit ironic that Canon's expensive cameras, which are less likely to need to use it, have AFMA, while Rebels, which often use cheaper, less accurate lenses, don't.

The lens that forced me to learn about AFMA was my first 17-55mm f/2.8 IS. (I later bought a second one for my wife that didn't have the problem.) It provoked one of the greatest errata of my photographic life. I had just bought my first 7D and the 17-55 to go with it and was eager to put them to use. So the next day I went into an important shoot (an award ceremony at MIT) without testing them first, only to discover that all the pictures I took with the 7D were unusably OOF. Fortunately, I was using two cameras (as I usually did at events), and the pictures from the other one were OK. But it was a very close call and an important lesson learned.

[ScottyP]

At first I thought you were complaining of a curved plane of focus, where the center focus point is right but the DOF curves making the edges OOF.  

Now i I think you are saying the camera front-focuses. Test for that using a ruler leaned on a pencil stabbed into a box. If you are focusing on the front of the box, near the edge and the ruler, you should see the zone in focus on the ruler should show 1/3 in front of the box and 2/3 behind the box. You can do autofocus microadjustment on each lens to address the problem, if a problem exists.