85 1.2L DOF question

[Kristofgss]

Is there anybody who could help me out with this? i was using the 85 1.2L wide open and noticed that when the face of my subject was in focus, her feet were out of focus and the grass in front of them was in focus. Doest his mean that the focus plane behaves like a cricle (anything with distance X from the camera is in focus) or is the focus plane supposed to be flat? (Sorry for the basic paintshop drawing, but I thought it would help explain the question)

 

[rfdesigner]

Is there anybody who could help me out with this? i was using the 85 1.2L wide open and noticed that when the face of my subject was in focus, her feet were out of focus and the grass in front of them was in focus. Doest his mean that the focus plane behaves like a cricle (anything with distance X from the camera is in focus) or is the focus plane supposed to be flat? (Sorry for the basic paintshop drawing, but I thought it would help explain the question)

1. Focal plane in theory "should" be flat, like a plate glass window, but in practice it isn't quite, nothing's perfect.

2. If your camera is exactly parallel to the floor then your image plane will be perfectly vertical vs the floor, if however your camera is pointing down a little then the image plane will tilt back and feet will be OOF, noticable on wafer thin DoFs like the 85 f1.2, not so on kit lenses.

3. If you want to point your camera down a fraction but get a vertical image plane, get a tilt shift lens.

For most portraiture work this is not important as the edges and corners are not on the focal plane anyway. If you calculate it accurately you'll almost certainly find the image plane is pretty flat and vertical over most of it's extent.

 

[Kristofgss]

I hadn't thought of that, but yes, my camera was tilted slightly downwards and I only noticed it when looking at the grass because it's easy to see where that is the sharpest.

I had been using a TS-E 24 to take the other images that day, which indeed, did not have that effect.

Thanks for the tips, i'll go try out the 85 again this evening while keeping it parallell to the floor. That will no doubt resolve it.

 

[Loswr]

This also highlights a related issue – focus/recompose with a very shallow DoF can result in your intended point of focus falling outside the focal plane.

Regarding things not being perfect, while the focal plane of a lens should be flat (it's callec a plane for a reason), some lenses exhibit noticeable field curvature – the original 24-70/2.8L is one example.