Fig 1. Tamron 90mm f/2.8 Di Macro 1:1 SP AF (172E) lens (photo from the Tamron web site)
Fig 1. Tamron 90mm f/2.8 Di Macro 1:1 SP AF (172E) lens (photo from the Tamron web site)
This macro lens was mounted on a Canon EOS 600D / Rebel T3i. The lens was lent by a friend for this analysis.
As per the Focus Analyser procedure, the test target was a carbon-soot-coated straight-edge razor blade inclined (with respect to the optical axis) by 6.5° and tilted (with respect to the camera frame) by ~5°, elevated about 5mm above an out-of-focus white paper background. The target was illuminated obliquely by a single LED lamp. At maximum zoom and closest focus distance, the field of view was measured as 22.1 mm (on a sensor that is said to be 22.3 mm wide).
The camera was operated remotely (using Canon remote capture software) with 'mirror lock-up' enabled. Raw files were processed with dcraw ('-D -4 -t 0') to produce PGM files used by Focus Analyser. The camera's white balance was set to Auto.
At 1:1, the lens would have an effective f-number (working f-number) of f/2.8 * (1 + 1) = f/5.6.
The numeric aperture (NA) ≅ 1 / 2feffective = 1 / (2 * 5.6) = 0.089, which corresponds to an Raleigh Criterion resolution in green light of 0.61 * 0.55 / 0.089 = 3.8 μm.
At the maximum zoom / closest focus distance, there was about 10 cm of working distance between the end of the lens and the object in focus. The lens extends to about twice the length in figure 1 when at 1:1.
Below are Focus Analyser reports for selected aperture settings, ranging from f/2.8 to f/22. Recall that the target is an incline edge and that the curves show the degree of edge spread along the target (the less edge spread, the sharper the focus). In all cases, the lens is focused on a point near the middle of the target; the only change is the f/stop. The depth of field increases (indicated by a broadening of the curves) as the aperture decreases. At f/22, the entire length of the inclined target is nearly equally focused, but with less sharpness than at lower f/stops.
Ideally, the lens would focus all three colours identically; the curves would overlap. Offsets in the curves indicate chromatic aberration (colour fringes).
The lens performs best, in terms of resolution and aberration, in the range of f/4.5 to f/8. Diffraction begins to reduce resolution at about f/11. At f/22, the entire FOV is in equal focus but with about half the resolution, trading off sharpness for depth of field.
At the wide open end (f/2.8), the green channel comes to best-focus about 140 microns closer than the red and blue channels (longitudinal aberration); the green channel has slightly better resolution until about f/5.6.
Figure 2 shows two photos of an integrated circuit, lit by a ring LED light, taken at 1/800 s, f/4.5, with the macro lens at 1:1 and with a camera pixel pitch of 4.28 μm/px. The teeth in the comb-like feature in the red circle are spaced by approximately 11 microns. The images were lightly sharpened using Photoshop.
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Fig 2. 100%, cropped, 1:1 photos of an integrated circuit, lit by a ring LED light. The teeth of comb in the red circle are spaced ~11 microns. The camera pixel pitch is about 4.28 microns/pixel. f/4.5, 1/800 s.