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  #1  
Old 25-10-2018, 08:40 AM
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Astrofriend (Lars)
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A medium format lens on a Canon full frame

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:
http://www.astrofriend.eu/astronomy/...vs-pentax.html

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?

/Lars

Last edited by Astrofriend; 26-10-2018 at 07:49 AM.
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  #2  
Old 25-10-2018, 08:57 AM
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Retrograde (Pete)
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Hi Lars,

Very interesting. I also have the Pentax 6x7 165mm f2.8 lens. I have used it on a Pentax K-5 (APSC) DSLR. I found that I needed to stop it down to f5.6 - f6 to get rid of chromatic aberration (violet halos around brights stars).

My two best pics with this lens so far are probably these:
https://www.astrobin.com/278971/?nc=user
https://www.astrobin.com/264019/B/?nc=user

I would be interested to see your results.
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  #3  
Old 26-10-2018, 07:15 AM
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Astrofriend (Lars)
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Hi Pete,
Good, then I'm not alone with this lens :-)

I know this lens will not perform very well when it come to sharpness. But It had been fun to do some test and see how it perform in other aspects. I did that many years ago but didn't have full control of the details then.

Tonight my plan was to do some Moon shots. But direct I get in trouble, I can't past focus at infinity. It looks that I'm almost on spot at the infinity stop. It looks very complicated to adjust the infinity stop at this lens. Another solution is to shorten the adapter, but don't have the machines to do that.

Anyway I shall spend a night on some object to have more test photos even if the focus is not perfect, maybe the lens shrinks out in the cold :-)

By the way, I updated my test page:
http://www.astrofriend.eu/astronomy/...vs-pentax.html

There is another problem with a lens like this. Now when I have start using the smartphone app to control my camera and lens. I can adjust the focus without touching the lens or camera, no vibration! Tonight when I try to focus manuell on Moon with the Pentax lens it cause a lot of vibration and make the focus process difficult.

Ps.
Your astrophotos at astrobin looks very good!

/Lars

Last edited by Astrofriend; 26-10-2018 at 07:50 AM.
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  #4  
Old 30-10-2018, 08:54 PM
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Astrofriend (Lars)
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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 on the balcony without polar align. Very light polluted sky.

Here is the simple analyze I did:
http://www.astrofriend.eu/astronomy/...ax67-lens.html

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.

/Lars

Last edited by Astrofriend; 30-10-2018 at 09:08 PM.
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  #5  
Old 01-11-2018, 10:04 PM
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Looks good Lars, interesting work
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  #6  
Old 02-11-2018, 01:00 PM
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Lars,

Did you try this lens with external aperture (instead of internal iris)?


Prompted by Tair 11A optical design (iris at the front end of the system.. - and this is btw excellent lens - very sharp in centre but some coma present at APS corners and no CA)), I found out that telelenses (Canon FD 200 F2.8, Canon 300mm etc.) perform much better when f number is stopped down by external diafragm.
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  #7  
Old 03-11-2018, 10:00 AM
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Pentax 6x 7 lenses are great.

I have mainly used them for a (KAF16803 sensor 36mm square) CCD imaging so I don't see chromatic aberration much and haven't tried it on a digital camera.

The ones that are best are:

1. 75mm F4.5
2. 165 f2.8 (mine is good wide open)
3. 300mm F4 EDIF also the 645 300mm F4 EDIF

http://www.pbase.com/gregbradley/image/167800780/large 300mm

http://www.pbase.com/gregbradley/image/167787138/large 300mm

There are example images on my website for the other lenses.

These lenses apart from the EDIF versions are quite cheap as well. The 75mm cost me under $100, the 165 under $200.

Greg
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  #8  
Old 04-11-2018, 05:18 AM
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Hi,
I answere you all here.
I have not try to use this lens or any other with external aperture in astronomy. But I have done that with old CRT projectors with positive results. I have done these late test at full aperture, earlier test at f/4 I see lot of "star rays" around brigth stars because of the aperture. With an external aperture this rays will be reduced.

My Sigma APO has better sharpness and also have motorfocus built in. Now when I have been used with the motorfocus and remote control from the smartphone I miss that when using the Pentax 67 lens.

I hope in future to have a sensor in full frame + size, then these Pentax medium format lenses will shine. But already now can I see the very low vignetting on a full frame sensor as a positive thing. And as you say, monochrome images and even better narrow band reduce the chromatic problem.

But for the moment I see this it as an interesting experiment. I will do more test later with this lens.

The Pentax 67 ED 300 lens I have read a lot positive thing about. Maybe I found a 300/400 mm ED f/4 lens in future.

Greg, nice photos you had with your 300 mm lens!

Lars

Last edited by Astrofriend; 04-11-2018 at 10:07 AM.
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  #9  
Old 04-11-2018, 10:42 AM
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Merlin66 (Ken)
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Adding a front minus UV filter should suppress the "blue bloat".
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  #10  
Old 04-11-2018, 10:50 AM
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Hi Ken,
Do you think the camera sensor is sensitive to uv? The amera is not modified and still has all its internal filters. I can add a 48mm uv filter inside the adapter.

Thanks for ideas!

Lars
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  #11  
Old 04-11-2018, 10:56 AM
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Merlin66 (Ken)
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Lars,
I was really responding to Pete's comment.
The usual inbuilt filters in DSLR should normally be sufficient to suppress UV bloat.
I have to add a UV-IR filter to my fully FULL (both filters removed) modded 1000D if I do any "general" star field imaging with various Zuiko lenses.
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  #12  
Old 05-11-2018, 11:40 AM
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Quote:
Originally Posted by Merlin66 View Post
Lars,
I was really responding to Pete's comment.
The usual inbuilt filters in DSLR should normally be sufficient to suppress UV bloat.
I have to add a UV-IR filter to my fully FULL (both filters removed) modded 1000D if I do any "general" star field imaging with various Zuiko lenses.
Hi Ken - my main imaging DSLR is astro-modded rather than full spectrum so it cuts off most of the UV anyway. I did run a 'test' with the front UV filter still attached (I forgot to remove it) and ended up with weird double images of most of the bright stars 😆.

Here is another image I took of the Southern Cross with the aperture stopped stopped down to f-4. I did a fair bit of work in post to remove the worst of the violet halos but you can still see them (this one was with the un-modded DSLR).
https://www.astrobin.com/full/187395/0/

I now use a 3D-printed aperture mask which runs the lens at f-6 and doesn't give me spikes around bright stars.
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  #13  
Old 05-11-2018, 03:55 PM
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Merlin66 (Ken)
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Peter,
I'm about to trial a Zuiko 135mm telelens with the FULL modded Canon, but I'll add a UV-IR filter to suppress the NIR (previous images with a 85mm Canon prime gave good star images but also showed the small NIR bloat on fainter IR stars.)
(Which UV filter were you using? I have an ol' Lumicon -UV filter which I've used in the past with reasonable results)
Stay tuned......
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  #14  
Old 08-11-2018, 11:46 AM
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Astrofriend (Lars)
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Hi Ken and Pete,
Is this Zuiko 135 mm lens special good for astrophoto? I read about it at internet, but didn't find any about to use it for astrophoto. Looks nice anyway.

Pete your photo of Southern Cross is awesome.

By the way, I never put any protection filter on the lens when doing astrophtography nowadays. It just cause me problems, dew, reflexions etc. But later if I take away the filters in my camera I of course have to implement some filter to limit the wavelength spectrum.

Has anyone tried to divide a color photo in two steps to get the best resolution when not APO lens. One focus at blue and green and another for the red?

/Lars
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  #15  
Old 08-11-2018, 02:21 PM
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Lars,
I don't really do much AP....gave that up years ago to focus on Spectroscopy (and solar imaging) , however I do use the Zuiko lenses very successfully in spectrographs and the occasional lo-res spectro wide field image.
Although, I'm sure, not as good as some of the expensive lenses available today, the Zuiko 135 (and the Zuiko 200 f4) should give acceptable results.

Trying to use RGB to "build up" an improved image - I think you'll find is problematic. Start simple and see how things go.

(I've been hampered by clouds, rain and wind so unable to demonstrate any images)
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  #16  
Old 10-11-2018, 10:11 AM
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Astrofriend (Lars)
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Hi,
To take at least two photos of the object with different focus is in some way simular to what I do when doing macro photography and focus stacking.

http://www.astrofriend.eu/photograph...-stacking.html

It should work also with a lens that have a curvature to the field. Never tried that, and it is a waste of time to have to take several photos when not using all the pixels. More like, interesting things to examine.

/Lars
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  #17  
Old 06-12-2018, 11:30 PM
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Hi,
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:
http://www.astrofriend.eu/astronomy/...n-cluster.html

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 I need another holder for it.

I will post more about this later on my homepage.

/Lars
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  #18  
Old 11-12-2018, 10:31 PM
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Hi,

I made two new pages where I compare the stars from the Pentax lens and the Sigma lens. It's from center and the extreme corner. It's interesting to study these photos, in some cases the old Pentax non APO looks sharper then the more modern Sigma APO lens. And the Pentax is at full aperture (f/2.8) and the Sigma is stopped down to f/4.

Have a look at page 7 and 8:
http://www.astrofriend.eu/astronomy/...vs-pentax.html

What I know the Sigma APO 150 mm f/2.8 is very good, even for astrophotography when stopped down one step.

/Lars
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  #19  
Old 17-12-2018, 05:33 PM
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Quote:
Originally Posted by Astrofriend View Post
Hi,

I made two new pages where I compare the stars from the Pentax lens and the Sigma lens. It's from center and the extreme corner. It's interesting to study these photos, in some cases the old Pentax non APO looks sharper then the more modern Sigma APO lens. And the Pentax is at full aperture (f/2.8) and the Sigma is stopped down to f/4.

Have a look at page 7 and 8:
http://www.astrofriend.eu/astronomy/...vs-pentax.html

What I know the Sigma APO 150 mm f/2.8 is very good, even for astrophotography when stopped down one step.

/Lars
You can't compare one lens at F2.8 and the other at F4. All lens are better stopped down so the Sigma is made to look better. Even at F4 its showing a lot of coma. I prefer the Pentax. Its also showing star colour whereas the Sigma has all stars as white which of course they aren't.

The Pentax does show some chromatic aberration with those magenta rings around the brighter stars so you can either stop that down to F4 to help there or use the chromatic aberration slider in Lightroom to get rid of it.

Greg.
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  #20  
Old 17-12-2018, 09:51 PM
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Hi Greg,
I can't use the Sigma at f/2.8 with a full frame camera and get acceptable quality in the corners, most because of very strong vignetting.

Then it's more interessting to compare the two lenses where they perform at it's best.

The Sigma deliver ovesaturated stars at the same exposure because it's sharper and then concentrate the photons of a star to fewer pixels. I can take shorter exposures to bring out the color, but then I get problem with the weak details, more have to do HDR imaging as I did with the Pentax lens photo, one of them.

But this time I'm only looking at the vignetting and the shape of the stars at the edges. I will take more photos with the Sigma lens to find where it perform at its best. I had a small drift because of bad polar alignment in this case. I have overcome that problem now with a better view through the polar scope.

http://www.astrofriend.eu/astronomy/...iewfinder.html


My biggest problem as usual, there are clouds at the sky!

ps.
I had a look at your homepage, wonderful photos!

/Lars
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