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10-01-2015, 01:21 PM   #1
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how do these newer DA zooms handle CA?

So we've seen the Sigma 18-35mm handles CA pretty well and can be used to shoot astro

How about the 20-40mm and 16-85mm... how do these fare? Yes it would need OGPS1 or K-3ii GPS (astroctracer mode) has anyone tried this?

Some good results have been posted on here from the wide sigma zooms 10-20 and 8-16 which like the 20-40 and 16-85 aren't particularly fast

10-01-2015, 01:58 PM   #2
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Interesting question. I wonder how the DA 20-40mm works for astrophotos, too.
10-01-2015, 02:31 PM   #3
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I will let you guys know about astrophotos in 2 weekends =p
10-02-2015, 01:09 AM   #4
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do you actually mean CA (chromatic aberration) or coma?

10-02-2015, 05:28 AM   #5
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Take a peak here (especially Heie's comments):

HD Pentax-DA 16-85mm F3.5-5.6 ED DC WR Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database

As for astro with the 16085, here are a few:





10-02-2015, 06:23 AM   #6
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There is no simple answer to how well "zooms" handle CA. Different zoom lenses are better or worse, and many zooms handle CA better at some focal lengths than at others. The aperture setting can also make a difference.
10-02-2015, 10:49 AM   #7
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QuoteOriginally posted by WPRESTO Quote
There is no simple answer to how well "zooms" handle CA. Different zoom lenses are better or worse, and many zooms handle CA better at some focal lengths than at others. The aperture setting can also make a difference.
My impression was though, the new HD 20-40 and 16-85 are significantly better than the 18-135 or 16-50 in certain situations.. in this case we'd be looking to shoot at the wider end of both lenses so 20-24mm perhaps and 16-24mm perhaps
10-02-2015, 12:14 PM   #8
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QuoteOriginally posted by Conqueror Quote
My impression was though, the new HD 20-40 and 16-85 are significantly better than the 18-135 or 16-50 in certain situations.. in this case we'd be looking to shoot at the wider end of both lenses so 20-24mm perhaps and 16-24mm perhaps
Yes these lenses are better at controlling CA. The 20-40 is especially good (it's IQ is generally outstanding). The 16-85 has a bit more edge CA at the long and short end of the zoom range, and a bit more at largest and smallest apertures than at intermediate settings.


Last edited by WPRESTO; 10-02-2015 at 12:23 PM.
10-02-2015, 03:53 PM   #9
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QuoteOriginally posted by WPRESTO Quote
The 20-40 is especially good (it's IQ is generally outstanding).
Yes this is what I've read/seen

Haven't seen any astro or night sky specifically from the 20-40 though...
10-18-2015, 02:49 AM   #10
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still interested in this...

the 20-40 from what I've seen ticks a lot of boxes... but not seen much astro/night from it though... the sigma 18-35 will have an advantage here (due to low coma).. but how much?

I'm curious to know how the 20-40 @ 20mm and f3.2 with OGPS1 might fare for instance
12-20-2015, 04:04 AM   #11
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QuoteOriginally posted by JinDesu Quote
I will let you guys know about astrophotos in 2 weekends =p
Any chance of follow up
12-22-2015, 12:28 PM   #12
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QuoteOriginally posted by Conqueror Quote
Any chance of follow up
It unfortunately didn't pan out, haha, was going to do photos of a meteor shower and stars in PA but the night ended up snowing and cloudy all over. I froze my butt off camping in below freezing weather
12-22-2015, 10:19 PM   #13
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Nothing posted here would encourage folks to think of zooms as the right choice for quality astrophotography. The 16-85 is too slow, and not adequate wide open (but lovely in normal shooting stopped down based on the various indications). Possibly the 20-40 can marginally contend, but it isn't very wide or fast.

If you look at flickr and study the sharpness results of the various contenders, you'll see that the faster, wide primes showing high resolution and low coma at wide apertures are your best bets. Look to Lenstip.com for those tests. The better, more economical solutions are the Samyangs - the newer 14mm f/2.8 (FF) and the 16mm f/2 (crop) are particularly strong. For astro, Pentax does not have wide OEM lenses that compete.
12-23-2015, 03:51 AM   #14
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QuoteOriginally posted by ScooterMaxi Jim Quote
Nothing posted here would encourage folks to think of zooms as the right choice for quality astrophotography. The 16-85 is too slow, and not adequate wide open (but lovely in normal shooting stopped down based on the various indications). Possibly the 20-40 can marginally contend, but it isn't very wide or fast.

If you look at flickr and study the sharpness results of the various contenders, you'll see that the faster, wide primes showing high resolution and low coma at wide apertures are your best bets. Look to Lenstip.com for those tests. The better, more economical solutions are the Samyangs - the newer 14mm f/2.8 (FF) and the 16mm f/2 (crop) are particularly strong. For astro, Pentax does not have wide OEM lenses that compete.
I agree however... with the use of OGPS1 or K-3ii it might be possible to get decent results on the 20-40

There's also the sigma 18-35mm f1.8

I've got the 16mm Samyang f2... but desperately need to get round to calibrating the infinity
12-23-2015, 10:32 AM   #15
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QuoteOriginally posted by Conqueror Quote

I've got the 16mm Samyang f2... but desperately need to get round to calibrating the infinity
Make sure that lens doesn't have any focus shift toward greater distances as you stop down. For instance, on the 8mm lens infinity comes at 2 meters when the lens is at f/3.5, but at f/16 infinity is at infinity. No one would want to use that lens at f/3.5, and the ideal sharpness is reached at around f/8 where infinity is around 3-4 meters. A lot of folks who have recalibrated mistakenly at wide open end up complaining that the lens has lost some sharpness(!).

The 14mm is off a bit too, but I don't see any focus shift in that lens. No idea about the 16mm... all you really lose is a bit of close focus real estate if you don't re-calibrate. If you don't, just make a mental note of where infinity mark should be when focusing by judging distance. If you do choose to re-calibrate an UWA lens, you'll be most accurate by taking an intermediate distance such 2 meters, measure from the object to the focal plane mark at the back of the camera (not the lens mount and not the front of the lens), use LV magnified and focus peaking to nail focus, and make sure you don't see any shift upon stopping down. I'd still allow for just a little play beyond the infinity mark. Lenses are generally calibrated to focus just beyond infinity, allowing for filters and various environmental variables (or just plain slop).
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