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Zeiss Batis 40mm f/4 CF: Bokeh Wide Open at f/2 Shows Nonagonal Shape of Lens Diaphragm

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Marat S writes:

Does your sample have aperture slightly closed when set to f2? It causes nonagonal shapes of OOF point light sources especially noticeable when focusing up-close and seems to be common among early Batis 40mm samples.
I know one person who contacted Zeiss about this issue as soon as first images surfaced after Photokina, but they haven't told him whether it's normal. I really hope it's not.

Eyal O writes:

I’m glad to see that you have the Zeiss Batis 40mm f/2 CF in hand and are actively testing it. I’ve been scouring the web for info/images and am really looking forward to your thoughts/review. One thing that has been noticed over on Fred Miranda’s forum is the tendency for the camera to render background lights in a stop-sign pattern when used for close focus at f/2. It is unclear if this is:
1. An issue with very early copies
2. An issue only when using the close focus
3. An issue at all distances

DIGLLOYD: Similar to the Hasselblad XCD 90mm f/2, the Zeiss Batis 40mm f/2 CF wide open at f/2 does not show circular bokeh on out of focus highlights. This appears to be as designed. I have an inquiry into Zeiss.

Further insight below—the apparent diameter of the diaphragm can be seen to change with focus distance.

* See Hasselblad XCD 90mm f/2 Bokeh: Out-of-Focus Lights at Night.

Zeiss Batis 40mm f/2 specular highlight bokeh at f/2
f2 @ 1/800 sec, ISO 100; 2018-10-22 12:02:36
Sony A7R III + Zeiss Batis 40mm f/2 CF

[low-res image for bot]

Infinity focus versus close focus

Toggle the image below to compare. As the Zeiss Batis 40mm f/2 CF is focused closer, the apparent size of the diaphragm opening contracts. Hence at everything but infinity, the background out-of-focus bokeh wide open at f/2 will show the nonagonal shape of the lens diaphragm.

I determined that there is a two stop difference in exposure for focus at MOD vs focus at INF. I used an even gray material and then manually focused at INF then manually focused at MOD. This is good and bad: it is good in that it looks like Zeiss is not using the trick (very common) of shortening the focal length in order to maintain aperture. The bad is that the lens becomes f/4 at MOD. This is consistent with the size of the lens diaphragm seen below, well stopped down when the shooting aperture is f/2.

This is kind of crazy: near MOD, the lens diaphragm is stopped well down even as the shooting aperture on the camera is f/2! That explains the light loss, but it does not explain why it is stopped down in the first place.

MOD = Minimum Object Distance e.g., very close focus
INF = infinity focus

Zeiss Batis 40mm f/2: toggle to see infinity focus vs close focus

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