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Porst 55mm f1.2 COLOR REFLEX MC AUTO
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PostPosted: Mon Sep 16, 2019 8:46 pm    Post subject: Porst 55mm f1.2 COLOR REFLEX MC AUTO Reply with quote

Hi!
I got this lens finally, the version with smallest iris at f22.
The condition was not the best: some fungus on a few internal glasses, dust, cranky focusing.
I decided to open the objective, quite easy, 'till the point of putting it back together Smile
I got some help from other people documenting the thing, like in this thread (german):
https://www.digicamclub.de/showthread.php?t=10107&page=36
or the pdf guide available here:
https://www.pentaxforums.com/forums/54-pentax-lens-articles/144553-assembling-porst-color-reflex-mc-auto-1-2-55mm.html
Cleaned all with isopropanol, greased the double helix thread (a mess to put back together with correct focusing!!)
The lens is ok now. Just to get the idea:







In general, especially with broad dailight available, the image is somewhat milky, as well known, but this can easily be corrected in postprocessing (i.e. I used the local contrast tool in Rawtherapee, with suffucuently high radius).
Another issue is fringing across strong intensity gradients. This is a characteristic of the lens and we'll have to live with it.
It causes a particular coloring along white/dark edges:



In front of the focus (lower part of the image below) red leaks from white areas into dark areas, leaving a blue coloring on white.
The opposite happens over the focal plane, coloring red the whites and blue the blacks.
This coloring effect can also be used to our advantage because it can help precisely focusing.

I tried improving the milky and low contrast in broad light.
First I created rear masks/flare baffles, as suggested here:
https://www.street-photo.fr/materiel/27/35
With various shapes (round and 2/3rd rectangles) and sizes.
No real improvement from supposed reflections at f1.2
I noticed that with smaller baffles openings every time the contrast was improving I could also notice an improvement in DOF and a strong decrease in exposure.
This means that I was effectively decreasing the aperture instead of blocking unwanted light leaks --> Experiment stopped.

What helped a bit was the use of a hood in front of the lens. This is the image difference (center of the image, 100% crop) on an evening with quite a lot of diffused light:





The intensity histograms show that the dark level is 41 without the hood, and 26 with the hood. Not rocket science but it's something.
Still, I think the lens is working better in darker environments.


Some random snapshots below at f1.2
I like the bokeh, and how the subject stands out.











PostPosted: Mon Sep 16, 2019 8:51 pm    Post subject: Re: Porst 55mm f1.2 COLOR REFLEX MC AUTO Reply with quote

leodp wrote:
I got this lens finally, the version with smallest iris at f22.


Congrats!

I always appreciate lenses a lot after cleaning and repairing them.


PostPosted: Tue Sep 17, 2019 6:42 am    Post subject: Reply with quote

hi, interesting thread and very nice bokeh shots!
i had both versions of this lens. the 0,6m mfd f16 and the 0,5m mfd f22 version. the f22 version produced much more glow and ca's. the f16 lens is a much better lens, more sharpness wide open, less glow, better contrast. or just a bad copy of the f22 version?


PostPosted: Tue Sep 17, 2019 6:48 am    Post subject: Reply with quote

planet.groove wrote:
hi, interesting thread and very nice bokeh shots!
i had both versions of this lens. the 0,6m mfd f16 and the 0,5m mfd f22 version. the f22 version produced much more glow and ca's. the f16 lens is a much better lens, more sharpness wide open, less glow, better contrast. or just a bad copy of the f22 version?


hi,
I have read on other posts that the f16 version has a better image quality than the f22.

Note:
My lens has 8 diaphragm blades, Pentax K mount
Images above acquired with Pentax K-70


PostPosted: Fri Sep 20, 2019 12:55 pm    Post subject: Reply with quote

I had to modify the lens mounting a bit.
The problem being that the camera (Pentax K-70) was not correctly perceiving the lens, so the expo for apertures different than f1.2 was consistently wrong, even if I was in manual mode M, with fixed ISO setting and allowing the use of the aperture ring in the camera config menu.
Exposition time is measured by Pentax cameras with manual lenses when the green button is pressed. That worked intermittently.

By scratching the black-anodized lens flange in correspondence of the camera digital pin (opposite to the red dot in the lens flange) the pin is effectively grounded and the camera "knows" that it has a manual lens with aperture set by the user.