Really confused about DOF and how a camera's "focal plane" works???

Every since I got a Sigma 17-70 that front-focused really bad, I test all my equipment and am paranoid that my gear has focus issues.

Look at the this picture here. Doesn't it look like while the face of the squirrel is in focus, what's behind is not but a good portion of the front of the tree is also in focus. https://photography-on-the.net/forum/showthre​ad.php?t=481147

If I put my center focus on a single object and am using something like. f1.8. Will what's in front and in back be in equal focus or is what's clear in front more than back?

I'm confused...

It's not possible to tell much from that squirrel except that the critter itself is in focus.

For the most part, you should really expect about 50/50 DOF before and after the subject. There is a myth that there is a 1/3 front 2/3 back ratio, but that really occurs more when you are closer to the infinity focus range of the lens...like when you are shooting wider and more stopped down.

For shots that are tele and pretty open like that squirrel shot you should be expecting closer to a 50/50 set.

Thanks for the reply. It was also my understanding that DOF should be roughly 50/50 but it seems that a lot of my shots have front focusing and it was really confusing me.

cowboylife wrote in post #5262527
Look at the this picture here. Doesn't it look like while the face of the squirrel is in focus, what's behind is not but a good portion of the front of the tree is also in focus. https://photography-on-the.net/forum/showthre​ad.php?t=481147

From the picture it's hard to tell how far in front the tree is. To me it almost looks level with the squirrel, in which case it's supposed to be in focus.

blackcap wrote in post #5264084
From the picture it's hard to tell how far in front the tree is. To me it almost looks level with the squirrel, in which case it's supposed to be in focus.

It looks like that to me as well, almost as though the squirrel is standing on the edge of a cliff. Look how the claws are curled over the edge.

However, this was shot at 300mm and f/2.8. I don't know how far away the subject was from the camera, but if we assume 10' (120") the DOF was only 0.39" in total. That's just +/-0.2" from the focus point. Not much margin for error there. Even at 20' (240") the total DOF is only 1.66" - +/-0.83". See attached DOF calculations. You can play with figures for yourself here - http://www.dofmaster.c​om/dofjs.html

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cowboylife wrote in post #5262527
If I put my center focus on a single object and am using something like. f1.8. Will what's in front and in back be in equal focus or is what's clear in front more than back?

Canon's autofocus system is only specified to get the edge being focused on to be sharp when the image is printed at a 6"x9" size and viewed from a distance of 1 foot. Essentially, that means that the edge being focused on will be somewhere within the conventional depth of field. It might be at the near limit, the far limit, or anywhere in between.

In the case of high-precision AF sensors, the permitted circle of confusion is 1/3rd that of the standard AF sensors. In that case the edge being focused on should not be right on the edge of the conventional DoF. But it still isn't necessarily in the middle.

It appears to me that you have a perfect text book demonstration of DoF. Wide open long lense giving a very narrow DoF. Tdodd's calc sheet is right on. The front of the log is the same distance and would show the same focus. Past that, I am not sure what your question is about "focal plane"

Great shot of the squirrel

JeffreyG wrote in post #5262588
There is a myth that there is a 1/3 front 2/3 back ratio, but that really occurs more when you are closer to the infinity focus range of the lens...like when you are shooting wider and more stopped down.

For shots that are tele and pretty open like that squirrel shot you should be expecting closer to a 50/50 set.

Hi Jeffrey,

I agree that generalisation about a 1/3 front 2/3 back ratio are wrong, but so too is your statement that it "really occurs more when you are closer to the infinity focus range of the lens". In fact, it occurs only when the focus distance is about 1/3 of the hyperfocal distance. In other words, for moderately close shots. As one approaches the hyperfocal distance, the front ratio decreases to 0% (1/2 the hyperfocal distance) and the back ratio increases to 100% (infinity). Conversely, as one gets closer to the lens, the front & back ratios approach 50%.

Cheers

Bad generalization about about DOF.

While 50:50 applies to a 50mm f/4 lens on APS-C camera 2' from the subject, and at distances like 20' it is closer to 1/3 - 2/3 split, if you focus at 90', the DOF extends from 45' to almost 2600' which is nowhere close to 1/3 - 2/3 !!

cowboylife wrote in post #5262527
Every since I got a Sigma 17-70 that front-focused really bad, I test all my equipment and am paranoid that my gear has focus issues.

Look at the this picture here. Doesn't it look like while the face of the squirrel is in focus, what's behind is not but a good portion of the front of the tree is also in focus. https://photography-on-the.net/forum/showthre​ad.php?t=481147

If I put my center focus on a single object and am using something like. f1.8. Will what's in front and in back be in equal focus or is what's clear in front more than back?

I'm confused...

Aside from the technical discussion below, I agree that this shot is exactly what it should be: the front of the log is very nearly in line with the plane of focus (the face of the squirrel) and so will be sharper than things in front of or in back of the squirrel.

I'm not clear as to what about this shot bothers you -- it looks great to me!

The image linked by the OP is not his own and I think he simply was mistaken in thinking that it would make a good illustration of front focusing.