Kowa Prominar 8.5mm vs. M.Z 12, 17, 9-18mm for Astro

Discussion in 'Astrophotography' started by tradesmith45, Jul 26, 2015.

  1. tradesmith45

    tradesmith45 Mu-43 All-Pro

    Dec 13, 2012
    This is the 1st in what will be a few threads on MW issues. Spent a few hours at a close dark site shooting 4 lenses & 2 cameras of the Southern MW. Unfortunately had poor seeing due to haze - NWS reported 10 mi. visibility at a near by airport. So these are the worst MW captures I've ever done. In some of the captures, you can even see inversion layers containing more haze. But for this testing, I think it won't matter. Just don't expect any wow in these images - especially these purposely under exposed ones.

    First up was a test of my new Kowa 8.5mm f2.8 lens. My hope was that it would be as good as the M.Z 12mm & 17mm that I have been relying on. The short version of the results is the Prominar is certainly acceptable. I plan to use this lens for future aurora photography among other things.

    So for testing I used the E-M5 but difficulty focusing the short lens due to the low rez EVF gave me pause. I especially wanted to see if there was much coma in the Prominar. So I purposely under exposed by 1 EV so only the brightest stars would show thinking this would be a good test of coma. Since the lens apertures range from f1.8 to f4 & I wanted to give approximately equal exposure to each capture, I varied the ISO from 800 for the f2, 1600 for f2.8 & 3200 for f4. All lenses were shot wide open except the 17 which was at f2. Here are the captures w/ no lens adjustments or NR (RAW development was done w/ DXO w/ everything turned off). I adjusted black point & exposures by eye to be about equal.

    20018717001_8dc63f75dd_b.jpg Kowa 8.5mm by tradesmith45, on Flickr
    19392506623_4372fbf4b3_b.jpg M.Z 9-18 @9mm by tradesmith45, on Flickr
    20005699852_1525d8cdb9_b.jpg M.Z 12mm by tradesmith45, on Flickr
    19825464540_e426585ed0_b.jpg M.Z 17mm by tradesmith45, on Flickr

    Here's a 65% crop of the upper right corner of the Kowa (on the left) & 12mm (right) for comparison.
    19987190656_2dfa643e58_b.jpg Kowa 8.5mm - M.Z 12mm by tradesmith45, on Flickr

    Here's what I'm seeing on my iMac Retina. Prominar has about the same or less coma, a bit more CA, and about as much vignetting as the M.Z 12mm. It has less CA & coma than the 9-18 at 9mm. Not bad for a fast UWA.

    These dark low pixel jpgs won't allow you to see much difference. But that's really the story, the Prominar is a solid match for the fast WA Oly primes for star photography (and from other's sample images, everything else too.) And it will take filters - yeah!

    Kowa put 17 elements into this lens in their commitment to low geometric distortion. So far, I've failed to be able to stack images from the 12mm due to either distortion or decentering. I now have plenty of captures to try again w/ both new stacking software & all 3 primes but that will be the subject of a new thread some time later.
    Last edited: Jul 26, 2015
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  2. T N Args

    T N Args Agent Photocateur

    Dec 3, 2013
    Adelaide, Australia
    call me Arg
    Thanks for this report. I am wondering whether perhaps there is more automatic correction going on with the native lenses, and whether the Kowa lens should really be worked on for lens corrections in postprocessing in order to really make a like-for-like comparison.
  3. tradesmith45

    tradesmith45 Mu-43 All-Pro

    Dec 13, 2012
    Good question Arg. According to DXO, they start RAW development using the unadjusted .ORF files and then apply their own corrections - if you turn them on. Which I did not. I can verify there has been no de-vignetting applied in the DXO tiff output compared to the developed RAW files in Aperture. Also, when you get the .ORF into DXO & turn off the distortion correction - big change in the preview image. So its looking like its doing what I'm expecting it to do - nothing.

    BTW, I suspect there is still all the demozacing, minimal NR & sharpening being applied even if the lens corrections are turned off.
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  4. tradesmith45

    tradesmith45 Mu-43 All-Pro

    Dec 13, 2012
    Forgot something important. The shutter times for all these were adjusted to give the same star movement in the frame using the Rule of 250. So SS=15" for 17mm, 20" for 12mm & 30" for 9 & 8.5mm.

    And just so you can see a modestly processed image from the Prominar when seeing is not so good, here an MW image @ ISO 6400 for 30 sec. @ f2.8 looking higher into the sky.
    19989081546_d18fcc9e5f_o.jpg Kowa 8.5mm+DxO Prime. by tradesmith45, on Flickr

    As you can see, there is a bit of coma at the edges but CA is easily removed.
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