Field curvature?

[jbl]

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I've got a weird issue I'm seeing that I'm trying to make sense of. I have an MM1, an M240 and an M246. The 246 is the new one to the bunch and I've noticed I'm missing focus when I focus and recompose in a way I never have see before (on the MM1, M9 or M240). I've been trying to make sense of why. I'm seeing this primarily at f/1.4 and larger because I'm guessing DOF covers it.

 

As an example, take any of the Noctiluxes wide open.

 

Focus seems spot on down the center of the frame even wide open. If I focus and recompose to place the subject in the upper right corner, focus is slightly off, but here's what I don't get. Recompose should place the subject slightly behind the plane of focus but the only way I'm able to correct for this is to focus on the rangefinder spot and then when decomposing turn the focus wheel on the lens to bring the plane of focus *closer* to the camera, in other words backwards of what I would expect.

 

There are a ton of variables, I know. I've played with making shutter speed faster to account for camera movement and that doesn't seem to help (1/125 is my standard, I've tried at 1/250). The rangefinder being off does not seem to be what's going on since focus is spot on at the center of the frame.

 

The only thing I can think of is curvature of the plane of focus. On something like a Noctilux wide open, how does the plane bend? Closer to or away from the camera?

 

Along these lines, the only only explanation I can seem to come up with as to why I'm seeing this only on the 246 is that maybe the resolution finally pushed me over a limit?

 

Appreciate any advice anyone can offer.

 

-jbl

[pop]

Turning the camera after focusing will move the plane of focus behind your subject, i.e. the subject will be closer than it was before. However, this will only be the case if you turn the camera exactly around the optical centre of the lens. If you move the camera by twisting your body, the geometry becomes a bit less predictable.

 

Anyway, there is but one explanation if it occurs with your M246 only and not with the M240 or the MM; the adjustment of at least one of those cameras must be off.

[alex7075]

Tired of unpredictable sharpness areas, I ran a test to know the field curvature of my Noctilux f/1.0, recently calibrated by Leica.

For clarity purposes, I will post some images taken at f/2.8, where the phenomenon is better visible.

The shape of the field curvature don't change at other apertures and/or other distances. Only the "thickness" of the shape decrease or increase depending on the diaphragm aperture (depth of field), and this determines the amount of the correction which must be applied, whilst the kind of correction remains the same (front or back-focusing).

Camera is full frame (MM1). Sharpening 150 in Capture One Pro 9.

 

1. Here we have the scene. The two lateral clothespins are at the same distance from the sensor. Focus is on the central clothespin (I slightly front-focused to compensate the focus shift at f/2.8):

 

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You can see the moustache shape of the field curvature, which corresponds to the MTF graph. At 7mm from the center, the curvature starts.

 

 

2. Center crop:

 

 

3. Left crop (right is the same):

 

 

4. Now, I focused on the left clothespin and recomposed (always slightly front-focusing to compensate focus shift):

 

 

As you can see, focus is in front of the lateral clothespins and behind the central clothespin. Result: nothing is sharp.

 

5. Center crop:

 

 

6. Left crop:

 

 

So, the focus-and-recompose method doesn't work until f/8.0, where the depth of field catches the three clothespins.

At bigger apertures, one must choose between sharp central area and sharp lateral area through intentional back-focus, progressively along a curve defined by the field curvature of the Noctilux.