Strange behavior of Sigma 40mm F/1.4 Art lens.

[Vroobel]

The Sigma 40mm and 2600MC got another chance. 🙂

A few days ago I received a custom made 24.5mm long (!) Canon EF / M42 adaptor. That means it's 2mm shorter than the ZWO filter drawer. Hopefully will test it tonight at F/1.4, I'm very curious about the result. 

 

 

 

Edited June 8 by Vroobel

[desmcm]

Hi

I've also dabbled in trying to use an asi2600mm/mc camera with the sigma 1.4 40mm art lens.

I believe I have a good copy of the lens as judged by its performance when using a canon 800d.

To summarise my experience when used with the ZWO EOS filter drawer, I could never find a backspacing where the performance of the lens with the asi2600mc/mm + IDAS NBZ filter approached that when using a canon camera with no filter. Best compromise I found was to use a 1.0mm spacer to extend the backspacing.

I've had email exchanges with others trying to use the sigma 105 art with dedicated astro cams with similar outcomes.

Some examples of what I have managed so far (don't get too excited!) can be found here:

https://www.astrobin.com/zlxguk/ with the 2600mc 

https://www.astrobin.com/dpjuqf/B/ with the canon 800da

Cheers, Des

 

[Elp]

2 hours ago, desmcm said:

Best compromise I found was to use a 1.0mm spacer to extend the backspacing

How did it work without a filter? I've found using these Eos adaptors (either the filter drawer one or the simpler one) when you use a filter the focus plane will move away from infinity without adjustment of the camera backspacing. And fast lenses you usually need to stop down to f2.8 bare minimum.

Edited yesterday at 11:16 by Elp

[desmcm]

6 hours ago, Elp said:

How did it work without a filter? I've found using these Eos adaptors (either the filter drawer one or the simpler one) when you use a filter the focus plane will move away from infinity without adjustment of the camera backspacing. And fast lenses you usually need to stop down to f2.8 bare minimum.

Think I only tried it with a filter either IDAS NBZ or L. Agree that stopping down is essential. May have another try later in the year.

cheers, Des

[Elp]

I just did a day test with one of my Sony AF lenses, as it's AF I can't focus it manually. The workaround I'm trying, is to unscrew the camera by a set distance controlled with thin spacer rings. The rough test (bearing in mind it's hard to judge far focus during the day) has shown, when the f stop is lower the spacing reduces. I could be wrong and need to test on stars but at F10 the spacing was around 0.9mm, at F4 around 0.6mm. If I tried keeping the spacing at one single gap distance whilst changing the F stop of the lens the image would be out of focus.