topic Re: Depth of field question in EF & RF Lenses

Depth of field question

FWG — Sun, 31 Dec 2017 16:21:20 GMT

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)?

Re: Depth of field question

ebiggs1 — Sun, 31 Dec 2017 16:27:27 GMT

" In other words the depth of field is not equal about the point of focus. "

That is correct, it is not equal. Unless the focal length is the same all lenses will be different.

Re: Depth of field question

FWG — Sun, 31 Dec 2017 16:34:37 GMT

Sorry - I did not understand the second part of your reply. My problem is that if I am taking a photo of an object using the camera focusing system everything in front of the point I focus on (within the DoF limits of course) is in focus but vitrually nothing behind the point of focus is "in focus". I can overcome this bias by using manual focus which is fine if the object is not moving but that is not always the case!

Re: Depth of field question

ebiggs1 — Sun, 31 Dec 2017 16:40:14 GMT

I think you are refering to the hypoer focus distance. The hyperfocal distance is the closest distance at which a lens can focus while keeping objects that are at infinity acceptably sharp. When the lens is focused at this distance, all objects which are at a distance from 1/2 of the hyper focus distance clear out to infinity and will be acceptably sharp.

Re: Depth of field question

FWG — Sun, 31 Dec 2017 16:49:10 GMT

If I am focusing on something only 1m from the camera with a wide aperture I know that there will be a point at which the image is blurred both in front of and behind the focus point. The DoF would not be more than a few inches in fact. The problem is that all the portion of the image which is in focus is in front of the registered point of focus. Pretty much everything immediately behind the point of focus is not sharp.

Re: Depth of field question

Waddizzle — Sun, 31 Dec 2017 16:57:43 GMT

@FWG wrote:
If I am focusing on something only 1m from the camera with a wide aperture I know that there will be a point at which the image is blurred both in front of and behind the focus point. The DoF would not be more than a few inches in fact. The problem is that all the portion of the image which is in focus is in front of the registered point of focus. Pretty much everything immediately behind the point of focus is not sharp.

It sounds ike your camera/lens combination could be back focusing. If the lens consistently back focuses wide open on subjects at different distances [25x and 50x the focal length], then backfocusing is the problem. Typically, you would want 2/3 of the depth of field to be behind the focus plane.

You can correct for it on a per lens basis using AFMA, auto focus micro adjustment, in the camera menus.

Re: Depth of field question

FWG — Sun, 31 Dec 2017 16:59:13 GMT

Excellent - are there any instructions (Canon or otherwise) that can help me do this? I do not want to make iut worse!

Re: Depth of field question

ScottyP — Sun, 31 Dec 2017 17:02:35 GMT

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.

Re: Depth of field question

FWG — Sun, 31 Dec 2017 17:05:25 GMT

Thanks - I will have a go at adjusting it.

Re: Depth of field question

Waddizzle — Sun, 31 Dec 2017 17:07:43 GMT

@FWG wrote:
Excellent - are there any instructions (Canon or otherwise) that can help me do this? I do not want to make iut worse!

Yes, do a web search on YouTube for " Dot Tune Method ". You will also need a tripod and a focus target. You can stretch a tape measure across the floor to use as a focus target, too. This adjustment works best with a broadband light source, like sunlight. Artificial lighting is not broadband, and may throw off the results.

Making the adjustment is not as routine as it might seem. The first thing you to do would be test your technique at taking measurements. You should be able to take several test shots, and they should all be consistently off by nearly the same amount. The lens itself will have some variation in how consistently it focuses, but this is why you take many test shots, and come up with an average.

When you perform the actual adjustments, once again you want to take several measurements, and come up with an average correction value to enter into the camera.

Re: Depth of field question

FWG — Sun, 31 Dec 2017 17:11:00 GMT

Thanks for your help and patience!

Re: Depth of field question

RobertTheFat — Sun, 31 Dec 2017 22:04:30 GMT

@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!

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.

Re: Depth of field question

Waddizzle — Sun, 31 Dec 2017 23:10:25 GMT

@RobertTheFat wrote:
@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!
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.

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?

Re: Depth of field question

RobertTheFat — Mon, 01 Jan 2018 00:59:15 GMT

@Waddizzle wrote:
@RobertTheFat wrote:
@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!
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.
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?

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?

Re: Depth of field question

Waddizzle — Mon, 01 Jan 2018 01:48:45 GMT

"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.

Re: Depth of field question

FWG — Mon, 01 Jan 2018 11:12:04 GMT

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

Re: Depth of field question

ebiggs1 — Mon, 01 Jan 2018 14:58:41 GMT

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.

Re: Depth of field question

Waddizzle — Mon, 01 Jan 2018 15:26:28 GMT

@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.

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

Re: Depth of field question

Waddizzle — Mon, 01 Jan 2018 15:28:46 GMT

@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

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.

Re: Depth of field question

ebiggs1 — Mon, 01 Jan 2018 16:03:38 GMT

"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.