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A medium format lens on a Canon full frame

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Long time ago I bought a Pentax medium format lens. It's a Pentax SMC 67 165 mm f/2.8. The reason I bought this was to have as low vignetting as possible. Later I bought the Sigma APO 150 mm f/2,8. Very good but a bit high vignetting.

After this the Pentax have collected a lot of dust. But why not give it one more chance?

Here you can see some graphs over the vignetting where I compare the Sigma with the Pentax:

Later I shall also do some test how sharp it is, now with a live view camera it's much easier to get it in perfect focus, I think I failed with this earlier.

Any other out there using medium format lenses (for astro)?


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I was very curious to test my medium format Pentax 67 lens on an astronomy object. I couldn't wait until I got a clear sky and performed this test at the balcony without polar align. Very light polluted sky.

Here is the simple analyze I did:

I must say that it perform better than expected. It's a bit exiting with this medium format lens and I will later spend a night on some astronomy object at a place with darkness.


Edited by Astrofriend

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Pentax 67 lenses are the way to go for widefield astro and mono camera because if the long back focus and the sharp optics, especially since we only use the centers, I have a selection that I use with a full frame astro CCD:


50mm f4:



300 mm f4




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Last night we got a a clear sky, at least I thought so. My friend invited me to follow him out to a dark place outside the city. My plan was to test my medium format lens on a object that I also have taken photos of with my more modern APO lens. We started about 7 pm. We have winter now and it's dark already at this time. As usually we got clouds drifting in over us. Still we could doing some astrophotographing but not very high quality of the images.

Anyway I have the photos here, both my old Pentax 67 medium format 165mm f/2.8 lens and the Sigma APO 150mm f/2.8 lens:

The stars bleading over, the red and blue doesn't focus at the same place as the green wavelength as espected. The advantage of the old lens is that I can use it at f/2.8 compare to the APO which I set the aperture to f/4.0. I use these settings most to get the vignetting under control. But of course even the sharpness got better.

What I miss most is the focus motor that I have in my Canon lenses. Now when get used to control the camera (Canon 6D) over the smartphone I have found it very convinient.

One more thing I tested this evening was the new angled finder I bought to the Star Adventurer mount. What a difference, now I can place the polar star exact where I want it. Almost impossible to see throught the polar scope normally when living at +/-60 degree latitude. Even the red light lamp I have worked well. But need another holder for it.

I will post more about this later on my homepage.


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I did a new page where I did a crop out of the extrem corner from both the Pentax photo and the Sigma photo of M45. Very interesting to see that the Pentax isn't very bad even if it have colored rings around the bright stars. Or maybe it's only what I feel because I'm not used to have any good astro photos from where I live in city. Thee photos are from a full frame sensor with almost no crop at the edges.

Here are the photos I compare:


Now just waiting for a clear sky.



Edited by Astrofriend

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