Old lens revisited

[Mickoid]

I took these a few weeks back with the same lens I used with film over 25 years ago. I went back to the same location too, in rural Victoria, to use it under the same dark skies. It's a Pentax Takumar 300mm f4 prime lens but digital sensors really brought out the flaws of this old lens that film had disguised. It has chronic CA and coma wide open. These were shot at f8 to reduce the aberrations and although better at f8, f11 would have improved it further. At f8, IC2118 was captured using a modded Canon 550d with 3min unguided subs at 3200iso on a Heq5pro mount. I shot about 40 subs but heavy dew had settled on the lens for half of them so I only used 21. That's just over 60 mins on this dim sucker. The blue glow is from the stars Rigel to the right and Cursa from below.

M45 was shot earlier in the night at a lower altitude, so the lens dew shield was able to do its job. Same set up was used but at 1600iso and only 11 x 3min unguided subs were shot.

[CoolhandJo]

Seems that the clarity of that lense is nice. The images display a real soft but crisp quality about them!

[Mickoid]

Thanks for the comment Paul. Yes, resolution is still good with many of these old lenses ( this one could be at least 40 years old ) but their coatings deteriorate with age and they probably weren't as good when new as those on modern lenses today.

[Ryderscope]

A good fun project Michael and some nice images in the bag as well :thumbsup:

[bojan]

Try with external diaphragm in front of the opening.

I found out those optical system (telephoto 200~400mm) respond better in terms of lateral CA, compared to internal iris, see here:
http://www.iceinspace.com.au/forum/showthread.php?t=51866&highlight=external+diaphragm
and here:
http://www.iceinspace.com.au/forum/showthread.php?t=138076&highlight=EOS+conversion&page=2

[strongmanmike]

Some cool results there Michael. Using old lenses for digital photography can be a lot of fun but yes many old lenses produced bloated stars because of the CA, which as you point out tended to be somewhat hidden with film. These results suggest that persevering with some longer exposures would be worth it :thumbsup:

I dabbled in using a Pentax lens, that I had used in the mid 80's, using a small CCD (https://pbase.com/strongmanmike2002/pentax_200mm_lens_images), back in 2006. The Pentax 200mm F2.5 was a cool lens for photography but not up to speed for the sensitive CCD era. I haven't gone back down that route again since but have always wanted to have another go with some Pentax 6X7 lenses (ala Greg Bradley), the later models of which were pretty sharp :)

Mike

[Mickoid]

Thanks for the advice Bojan, with the weather predicted to improve here nearer the weekend, I'll try out that suggestion from home because it won't require dark skies to conduct the experiment.


Thanks Mike, the Pentax 300mm is a fairly sharp lens so I may just try Bojan's method of creating an external aperture to lessen the affects of CA. I agree longer exposures would have made these better images but dew defeated my attempts on the Witch Head this night from capturing more. Also, 3mins was the best I could do unguided at 300mm, I can't be bothered auto guiding when I have to drag all the gear to a dark sky site for one night. By the way, how would you attach the Pentax 6X7 lenses to a ccd camera? Are adapters available? Those lenses would create a nice big image circle enabling most of the coma at their edges to be cropped out by the sensor. ;)

[Mickoid]

Thanks Rodney.