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Lens: Kern Macro-Switar 26mm f/1.1

Vintage: 60's?

Lens Mount: C-mount.

Preferred Mounting: Reversed.

Filter Thread: About 38.5 mm

Needed Adapters: I found that the lens threads are between 38 and 39 mm. A 38 fits, but is loose and isn't secure. I added a little tape to the threads and it snugged up nicely. I use a 38-52mm step-up ring connected to a standard 52mm reversing ring.

Street Price: $1300 - $4000 in good used condition. Prices vary a lot online, shop around.

Controls: Aperture setting, aperture preset, focus setting.

Aperture: 9 blades.

Basic Function: Requires a bellows to set focus and magnification

Extension Magnification Working Distance
adapters 2.68 23 mm
25 mm 3.73 21 mm
5 cm 4.37 19 mm
9 cm 5.87 18 mm
13 cm 7.40 17 mm
19 cm 9.55 16 mm
Est. Focal Length: 26.9 mm

Appearance:

kern 26mm top view kern 26mm side view

aperture graphResolution vs. Aperture:

The lens is sharpest at an aperture of about f/4.7 (halfway between f/4 and f/5.6). The most resolving aperture is about f/3.3 (halfway between f/2.8 and f/4. f/4 is almost as sharp as f/4.7 and resolves almost as much as f/3.3. I used f/4 for testing.

corner sharpness graphCorner Sharpness vs. Aperture:

At a magnification of m=4.4, this lens shows good field flatness across the range of apertures. At lower magnification this lens shows quite a lot of field curvature (If the center is in focus, the sides are fuzzy - if the sides are in focus, the center is fuzzy). This will make any individual image look pretty bad (as shown by the peripheral fuzziness at m=2.7 and m=3.3 in the next graph), but stacked images will look considerably better.

resolution graphSharpness and Resolution vs. Magnification:

This lens shows pretty good sharpness and resolution acriss the magnification range, but the peripheral fuzziness at low magnification makes this lens better used on the bellows (5 cm or more of extension).

performance:sharpness graphPerformance: Sharpness vs. Magnification:

I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.

The Kern 26mm shows sharpness performance that varies from good at the lowest magnification to fair at the highest.

performance:resolution graphPerformance: Resolution vs. Magnification:

I have 4 lines that represent levels of performance from outstanding (top) to fair (bottom). This shows where this lens fits into the hierarchy that I have created.

The Kern 26mm shows resolution performance that varies from very good at the lowest magnification to fair at the highest.

resolving power graphResolving Power vs. Magnification:

This graph represents the smallest details that are able to be resolved by this lens at various magnificaitons. If the number doesn't get smaller as the magnification rises, there is little benefit to going up in magnificaiton with this lens. This situation is also called empty magnification.

Decent resolution although dwindling above 6:1. If you want to squezze a little more resolution out above 6:1, you could go to f/3.3.

lateral CA graphChromatic Aberration:

This lens shows mild color fringing in the center (about 0.10 - 0.60 pixel) and moderate/severe on the periphery (0.090 - 0.330%). Anything below 0.04% is not considered significant. Above 0.08% is considered moderately visible.

There is moderate axial CA on out-of focus details (see image below).

Longitudinal CA:

axial CA

Image Contrast: Image contrast is OK, less than most bellows and enlarging lenses.

Flare: This lens shows mild flare during testing. Contrast improves significantly with a black piece of velvet placed in front of the object being photographed to precent reflections from the base.

Distortion: This lens shows no significant distortion during testing.

Image Samples:

About 4.4:1 magnification, f/4, focus stack of 25 images, resized:

sample image

crop imagePixel level crop from the image above:

There is good pixel level detail, but the lowish contrast can be seen in this picture.

About 7.5:1 magnification, f/4, stack of 31 images, resized:

sample image

crop imagePixel level crop from the image above:

There is good pixel level detail.

Conclusion:

This C-mount video lens can be used on a bellows, but needs to be reversed due to its retrofocus design. If it is mounted normally, working distance goes to 0 at a magnification of about 2:1 and is minimal below that (mounted directly to the camera). This lens is quite expensive and has a large aperture, but its performance doesn't get particularly useful until you get at least to f/3.3 (most resolution) or f/4.7 (most sharp). The lens is also somewhat limited by fairly severe lateral CA and relatively low contrast. It does have pretty good working distance for the magnification due to its retrofocus design.

This lens is best used on a bellows with at least 5 cm of extension. If less extension is used, there is quite a lot of field curvature that makes imaging more challenging.

I think I would choose a short focal length enlarging lens or a similar bellows lens over this lens for general macro imaging in the range of 2:1 to 10:1 as they are cheaper and will tend to perform better than this lens. If you do have one of these lenses already, it will do a decent job, just not spectacular.