Project:
Sigma APO 150 mm f/2.8
vs
Pentax SMC 67 165 mm f/2.8

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

Content:

1. Camera lens vignetting
2. Histogram and vignetting
3. Analyze of Sigma APO 150 mm f/2.8 vignetting
4. Analyze of Pentax SMC 67 165 mm f/2.8 vignetting
5. Analyze of Pentax 67 at f/2.8 and at f/4.0
6. Pentax 67 165 mm lens mounted on a Canon full frame camera
7. Pentax 67 and Sigma APO star test at center
8. Pentax 67 and Sigma APO star test at corner

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!

7. Pentax 67 and Sigma APO star test at center

At last I got the opportunity to do some astrophotographing, it's not easy when living in a big city. A friend asked if I wanted to follow him to a dark place outside the city and doing some astrophotographing.

Now I got the chance to take astrophotos with my Pentax medium format lens and compare with earlier photos with the Sigma APO lens. The object this night is the M45 nebula.

This is a crop of the M45 central region. The upper one is the Pentax lens photo. Haven't done very much more then flat calibrate it. The lower one is from the Sigma APO lens. Both has been scaled by a factor by 4 to let us see the details. Some ugly jpg arte facts can be seen.

Pentax 67 lens:

The strong bleeding from the old Pentax lens, the red and blue channel is not that ugly after color channel gain is set correctly. Actually it helps the oversaturated star to give some color information. The light spreads over more pixels.

I didn't have any problem to focus at infinity, not much margin though.

Sigma APO lens:

On the Sigma lens when using the aperture stop down to f/4 a lot of rays can be seen around brighter stars. A common problem when the aperture is not fully open. What is to prefer, colored ring as I get from the old Pentax lens or rays as I get from the Sigma lens when stopping down the aperture?

At that time I took the Sigma photo I had problem to achieve correct tracking at 120 seconds exposures with my Star Adventurer mount.

Conclusion:

Which one is best? Nowadays I'm strongly prefer a lens that have a focus motor, the Sigma and all newer Canon lenses have that. But when looking at these photos the Pentax doesn't look too bad, it even looks to be sharper. We had a lot of clouds rolling in over us when I took the Pentax photo so it's not so easy to compare them.

They are taken with different settings, the Pentax photo is a mix of short and long exposures with different ISO settings. The Sigma photo is only taken with 120 seconds photos.

You can find more details here:
../../my-astronomy-photo/open-clusters/m45-open-cluster.html

I have to do a much better test in the future with more control of the environment and at the same setup. But not sure if I will do that, there is so few times per year that I have the opportunity to do astrophotography. When I doing that I want to spend that time with equipment that I have more control over, like the Sigma lens with the focus motor.