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)?

[ebiggs1]

" 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.

[FWG]

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!

[ebiggs1]

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.

[FWG]

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.

[Waddizzle]

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@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.

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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.

[FWG]

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

[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. 

[FWG]

Thanks  - I will have a go at adjusting it.

[Waddizzle]

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@FWG wrote:

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

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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.