Compensating for Focus Shift?

Question

Some lenses, because of a design that includes uncorrected spherical aberration, shift their plane of focus backwards as the aperture is narrowed. This is most apparent at close focusing distances and mid-to-wide apertures, but is not a defect and these lenses can have wonderful rendering.

But what techniques, approaches, or methods have people learned to compensate for the shift? Or do people even bother?

I ask because I feel the urge to load some film into my rangefinder and take my Zeiss ZM C-Sonnar 1,5/50 out for a spin, and this lens absolutely shows these characteristics but I need to work with it 'blind'. However, there are also some excellent lenses for digital cameras that exhibit focus shift, so I'm interested in that experience as well.

Answer 1

With a digital SLR there are several advantages your film rangefinder doesn't share. Combining Live View with the depth of field preview button (or any other setting that stops the lens down while viewing the scene via Live View) allows for precise manual focusing, usually with the subject magnified 5x or 10x on the camera's rear LCD. Another advantage is being able to instantly review the image and compensating for a follow up shot when the camera and subject are both static. The combined result of these advantages is that learning to shoot "blind" with such a lens is a dying art.

Before the advent of digital cameras in the hands of most every shooter, there were several ways to deal with the issue of focus shift.

• Don't worry about it. Expectations in terms of resolution and print sizes were often much lower before the advent of the megapixel race. Going back and examining some (but not all) works considered masterpieces from previous eras reveals a lack of tack sharp resolution that is surprising to our eyes.
• Experience/trial and error. Although the feedback from each attempt took longer than the near instantaneous results of the digital age, many owners of such lenses learned how to use them this way. Often entries for each exposure were made in log notebooks so that when the results were viewed later they could be interpreted in light of the various amounts of compensation tried at various apertures and focus distances.
• Systematic charting. The results from shooting at test charts to document the amount of shift using various combinations of apertures/subject distances were compiled into a chart that could be used to predict the amount of shift. Some lenses were supplied with such testing information when they were new, or the manufacturers otherwise made it available to the owners of their lenses.