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Table of Contents   Page 1: My Approach to Testing a New Lens Page 2: Know Your Goals Page 3: General Issues Page 4: Choosing a Test Scene for Evaluating Sharpness Page 5: Focus Targets Page 6: Focusing Accuracy When Testing Page 7: Testing and Adjusting Autofocus Accuracy Page 8: Eliminating Camera Movement when Testing Page 9: Ruling Out Camera/Sensor Alignment Page 10: Checking for a Good Sample Page 11: Comparing Lenses Page 12: Testing for Focus Shift and Secondary Color Page 13: Testing for Field Curvature Page 14: Testing for Lateral Chromatic Aberration Page 15: Testing for Longitudinal Chromatic Aberration (LOCA) Page 16: Testing for Bokeh Page 17: Testing for Autofocus Accuracy Page 18: Testing for distortion Page 19: Testing for Flare Page 20: Downloadable High-Res Ruler Target

How to Test A Lens

With pentagonal diaphragm

Testing for Bokeh

Please see the Bokeh Introduction page for key information on the factors involved in bokeh.

Bokeh is a complex area, and full of subjective interpretation, in large part because evaluating bokeh is very much subject to the choice of subject, distance, aperture, etc.

The same lens might be described by two people as having good or bad bokeh. Occasionally, the choice of subject matter might suggest that a lens has unattractive blur characteristics but sometimes that’s just the way it is (not the lens at all).

Considerations when evaluating bokeh:

• Aperture — Behavior can change markedly by stopping down even one stop e.g., from f/1.4 to f/2. The Zeiss 50mm f/1.4 Planar changes behavior this way as do many f/1.4 lenses; wide open, it has a “lively” bokeh, a “whorl” effect that some find disagreeable (my personal take is that it can add interest).
• Effective aperture — The amount of optical vignetting can influence the perceived bokeh: the effective aperture is much smaller away from center, and thus the lens is effectively stopped down away from center, which yields less blur. The net effect can be distracting.
• Distance — The relative distance between in-focus and out-of-focus subject matter exerts great influence on the perceived blur qualities.
• Front or rear — most lens designs choose pleasing blur qualities behind the point of focus; blur close to the camera can be not so pleasing. Symmetric optical designs tend to perform better.
• Field curvature — a curved field has a profound influence on bokeh if the curvature throws portions of the subject into better focus, or more blur. The effect can be invisible, or very noticeable.
• Color correction — color bokeh can be very distracting with some subjects (magenta/green) shapes. Color bokeh can yield a two-tone magenta foreground and green background effect, a departure from reality that intrudes in subtle and not so subtle ways.
• Lateral color fringing — another type of color error which can result in ugly red/cyan fringes on high contrast edges, which not only looks harsh, but causes a sort of eyestrain like an out-of-registration print page.
• Aberrations — shoot night scenes to determine the extent of lens aberrations. At wide apertures, some some lenses show outlandish rendition of point sources which can overwhelm all other considerations.
• Diffraction star — for night shooting, a diffraction star can be an attractive quality. Stop down 1-3 stops for this; some lenses produce beautiful diffraction stars stopped down just one stop whereas others disappoint and/or require more stopping down.
• Your preferences — designs with aspherical elements and a high degree of correction sometimes can be thought of as “clinical”. Symmetric designs with uncorrected spherical aberration are often regarded as more pleasing, a more “classical” look.

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