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12-15-2014, 11:34 AM   #1
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100mm WR Macro Astro Photography

Hi

I have just bought the 100mm WR Macro Pentax and decided it would be cool to take some night sky photographs. I have done this before with my old K mount 50mm and thought it would just be as simple as setting the focus on the lens to infinite. However it seems that the lens isn't actually at infinite as the stars are still slightly out of focus. I was wondering if anyone has any advice as the lens performs spectacularly except in this function.

Kind Regards,

Chris

12-15-2014, 11:38 AM   #2
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this lens should focus to infinity ( or even beyond that - for AF use) so chances are that you either have it focused past the infinity point, or this particular piece is faulty.
12-15-2014, 12:35 PM   #3
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As far as I know, all DA lenses focus past infinity, mostly for reasons due to variation of focal length with temperature. You will need to do some experimentation to achieve critical infinity focus. You cannot expect to hit it by racking the lens to its end stop.

Jack
12-15-2014, 01:00 PM   #4
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QuoteOriginally posted by jbinpg Quote
As far as I know, all DA lenses focus past infinity, mostly for reasons due to variation of focal length with temperature. You will need to do some experimentation to achieve critical infinity focus. You cannot expect to hit it by racking the lens to its end stop.

Jack
This is the answer.

As for the 100mm WR for astrophotography purposes, at 100mm focal length and only f2.8 max speed, you might have trouble taking night sky shots without getting star trails.

12-15-2014, 01:45 PM - 1 Like   #5
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I suggest you figure out where infinity focus is in daylight, and remember the mark. That way, you don't have to look for focus on the stars, you just set it up.
The other thing is that a 50mm lens at f2.8 will have a wider DoF than a 100mm lens at f2.8. This means the 100mm must be focused more precisely.
I can't imagine why the DFA 100mm would not be good for astrophotography, other than its focal length/aperture. I haven't taken star photos with it, but I have taken landscape photos, that were very sharp and detailed. Main problem is that 100mm means you will capture star trails very soon. The stars are constantly moving, but a wide angle takes longer to register a movement, than a telephoto lens. This is why most astrophotos are taken with ultra wide lenses. Telescopes and telephoto lenses can be used if you have an equatorial tripod that moves along, tracking the stars. On some Pentax cameras you can also use the O-GPS accessory, which has astrotracer function! This will let you take astro photos even with tele lenses.
So I suggest you play with the focus to get it just right! If you think the error is not the focus, please post some sample photos or tell us exactly what you are doing, and what you want to achieve. Then we can give better advice

Hope you enjoy the DFA 100mm, its a great lens!

Edit: Oh, btw. To avoid star trails, with 100mm lens, you should keep the shutter speed under 5 seconds. Also, use 2 sec timer to prevent blur from button pressing and mirror slap. And use Dark frame (slow shutter) NR for better noise performance.

Last edited by Na Horuk; 12-15-2014 at 01:54 PM.
12-15-2014, 03:50 PM   #6
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QuoteOriginally posted by 1991Chall Quote
I have just bought the 100mm WR Macro Pentax and decided it would be cool to take some night sky photographs.
QuoteOriginally posted by Na Horuk Quote
Hope you enjoy the DFA 100mm, its a great lens!
Well, it IS a great lens and I can confirm that the focus issue only is as stated above: It focuses past infinity. I use liveview at some 6-8 times magnification to nail focus when I use this lens in MF mode.

I am extremely fond of this lens, both for macro- and for landscape photography and everything in-between - but, alas, not beyond. The reason for this is not lack of sharpness but solely because it produces a lot of coloured fringing around the brighter stars (something it has in common with my Pentax-M 100mm f/2.8 lens). For this focal length range, some of the 135mm Pentax lenses appear to be a better choice.

Too bad, because as the examples below (cropped images) show, it would otherwise be a perfect match for my Pentax O-GPS1. Well, you can't win them all.........
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PENTAX K-3  Photo 
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PENTAX K-3  Photo 
12-15-2014, 04:00 PM   #7
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Ah thanks for the demo images, mine are very similar. I agree I have absolutely fallen in love with the lens, it is always on my camera.

It's good to know you have the same issues.

Chris
12-15-2014, 05:43 PM   #8
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I was curious as well, so I took this quick snap! 8 seconds, so the stars are trailing, not pinpoints. And ISO 3200, so there is some noise. I shot raw and did some PP. Not cropped.

QuoteOriginally posted by Stone G. Quote
The reason for this is not lack of sharpness but solely because it produces a lot of coloured fringing around the brighter stars (something it has in common with my Pentax-M 100mm f/2.8 lens).
Yeah, there is some fringing wide open. And bokeh/axial CA, as well. These are the only optical weaknesses of this lens, and they are relatively minor IMO. What I miss more is a focus limiter, and maybe an aperture ring


Last edited by Na Horuk; 05-23-2017 at 11:18 AM.
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