What's "back focusing Problems."

[Scott Root]

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I keep hearing about "back focusing problems." What is it?

[woodda]

How do you know you have back or front focus problem

 

Please see posts above

[Ben Marks]

"Back focus" is when a camera's lens focuses behind the indicated focus point. This may lead to unsharp pictures, depending on your subject and light conditions. I think of this as primarily a rangefinder problem where the focusing mechanism and the lens have a mechanical linkage that may be off by a few millimeters. However, it can occur with SLRs too if, for instance, the autofocus is off or if the camera's focusing screen is not the same distance from the focal point as the film plane/sensor plane. I noticed this with my 75/2 when I focussed on my son's eyes with the lens wide open and instead the hair between is eyes and ears was in critical focus. Note that this is different from "focus-shift" which is the tendency of certain lenses to change their plane of critical focus when stopped down to working aperture (see Sean Reid's excellent demonstration of this on a C/V 40/1.4 Nockton on his website reidreviews.com).

 

You can test for this in many ways. For me, the simplest was to lay out a tape measure on a table and set up my M8 on a tripod pointing down at the tape measure at a 45-degree angle. I focussed on a particular mark on the tape measure and then reviewed the result at high magnification to see what was actually in focus. My test showed that for several of my lenses with very shallow depth of focus (90/2 AA), the plane of critical focus was about 2 cm behind the plane of focus as indicated by the RF. A quick search of the forum turned up an easy fix for this; however the same would not have been true for my M6 (would have had to check for an aerial image with a loup at the film-gate) and not sure I have the tools for RF adjustment.

 

Hope this helps.

 

Ben Marks