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Field Curvature Focusing Tips
As discussed in MTF Measurement vs Field Curvature, MTF charts can show substantial differences with focus changes of as little as 50 microns at the sensor (1/20 of a millimeter).
In the field, being aware of the shape of the field curvature can help decide the choice of focus: depending on the scene, one might choose a slightly weaker result at center in order to produce a stronger result towards the edges or over the frame as a whole. By also stopping down, notably superior results are possible.
General tips
For a planar subjects:
- For focusing on a planar (flat) subject (e.g., a wall or a distant landscape), peak center focus and mid zones can be balanced in some cases by skewing focus appropriately (closer or farther at center). The problem being a hard infinity stop may prevent focusing any farther; this depends on small variations in the camera body flange distance.
- Since nobody can reliably control the focus with the necessary accuracy,
the use of focus bracketing and some stopping down are the best way increase the odds in one’s favor.
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Diglloyd Making Sharp Images articulates years of best practices and how-to, painstakingly learned over a decade of camera and lens evaluation.
Save yourself those years of trial and error by jump-starting your photographic technical execution when making the image. The best lens or camera is handicapped if the photographer fails to master perfect shot discipline. High-resolution digital cameras are unforgiving of errors, at least if one wants the best possible results.
- Eases into photographic challenges with an introductory section.
- Covers aspects of digital sensor technology that relate to getting the best image quality.
- Technique section discusses every aspect of making a sharp image handheld or on a tripod.
- Depth of field and how to bypass depth of field limitations via focus stacking.
- Optical aberrations: what they are, what they look like, and what to do about them.
- MTF, field curvature, focus shift: insight into the limitations of lab tests and why imaging performance is far more complex than it appears.
- Optical aberrations: what they are, what they look like, and what to do about them.
- How to test a lens for a “bad sample”.
Intrigued? See Focusing Zeiss DSLR Lenses For Peak Performance, PART ONE: The Challenges, or (one topic of many) field curvature.
