DA* 16-50 weird aberrations

**SunValley** · 04-18-2016, 06:12 PM

Greetings!

I tested my relatively newly acquired DA* 16-50 on a starfield and found the results to be a bit strange. I've enhanced the contrast on the image below for clarity. This was taken at 16 mm f/2.8. It is a 120 s shot with astrotracer running. There is a lot of vignetting in that lens and a lot of coma? astigmatism? in the corners but that was expected from all the reviews in this forum and others. What puzzles me is the fact that these problems are worsts in the top corners compared with the bottom ones. Is this normal? (Click on the image to get a full res view)

Wheatfield · 04-18-2016, 06:15 PM

Originally posted by SunValley

Greetings!

I tested my relatively newly acquired DA* 16-50 on a starfield and found the results to be a bit strange. I've enhanced the contrast on the image below for clarity. This was taken at 16 mm f/2.8. It is a 120 s shot with astrotracer running. There is a lot of vignetting in that lens and a lot of coma? astigmatism? in the corners but that was expected from all the reviews in this forum and others. What puzzles me is the fact that these problems are worsts in the top corners compared with the bottom ones. Is this normal? (Click on the image to get a full res view)

The lens might be very slightly decentered.

mee · 04-18-2016, 06:16 PM - **1 Like**

The 16-50 is a Chromatic Abberation nightmare at f/2.8 ...actually there is a lot of vignetting and a bit of softness too at f/2.8 on that lens in general..

it is really an f/3.5 or f/4 lens for serious sharpness, reduced vignetting, and minimized CA.

I've had 2 copies of the lens and, in this regard, they performed similarly.. f/2.8 was weak and the performance jumped largely by f/4. From then on its rather gorgeous..

Ole · 04-18-2016, 06:33 PM

You shot with the astrotracer running, so the sensor moved during exposure and the corners will therefore be uneven.

You should test it without the astrotracer.

It might be an idea to buy a manual focus prime lens like 20mm F2.8 for this kind of shooting and avoid using the zoom.

Apart from that, you succeeded well with the astrotracer shot! It can be a challenge to calibrate it and make it work correctly!

Adam · 04-18-2016, 06:39 PM

I think these kinds of results are to be expected with wide lenses and the astrotracer. The shot looks great overall, though!

DSims · 04-18-2016, 06:56 PM

Originally posted by mee

The 16-50 is a Chromatic Abberation nightmare at f/2.8 ...actually there is a lot of vignetting and a bit of softness too at f/2.8 on that lens in general..

it is really an f/3.5 or f/4 lens for serious sharpness, reduced vignetting, and minimized CA.

I've had 2 copies of the lens and, in this regard, they performed similarly.. f/2.8 was weak and the performance jumped largely by f/4. From then on its rather gorgeous..

I concur - I like my DA*16-50 very much, but I seldom shoot it wider than f/3.5. Furthermore, it's at its weakest between 16 and 20mm. It's a very good lens, and I'm happy with it since I only expect it to perform like a zoom.

In fact I don't know of any brand of zoom in this standard focal-length range that has spectacular results near wide-open.

This is why lenses like the Samyang 14mm prime are popular among astro shooters. I've always wondered if the Samyang 16/2 is just as good (it's got a nice fast aperture), but I don't know.

**SunValley** · 04-18-2016, 08:08 PM - **1 Like**

Thanks everyone!

Indeed the lens is not at its best at 16/2.8 but this was a test to see its limits really. Good old M28/2.8 and A50/1.4 give fantastic results with the astrotracer but I like the idea of a AW lens for this kind of situation when dew and/or frost end up covering the equipment.

Originally posted by Ole

You shot with the astrotracer running, so the sensor moved during exposure and the corners will therefore be uneven.

I didn't think of that but I guess you're right, during those two minutes the sensor must have moved quite a bit, thanks!

Here's a shot from earlier that night still at 16/2.8. Two consecutive shots of 180 s iso 400, one with astrotracer on and the other off for the foreground. Blended in PSE 14 so everything looks in place.I cropped a bit the sides to avoid the softer parts of the image.

Ole · 04-21-2016, 07:34 PM

That's a great image, well done, very nice! I envy you - I have still to get a decent shot of the milky way!

**SunValley** · 04-22-2016, 04:43 AM

Originally posted by Ole

That's a great image, well done, very nice! I envy you - I have still to get a decent shot of the milky way!

Thanks! With legendary clear Arizona sky it's me who is jealous!

04-18-2016, 06:12 PM	#1
SunValley Site Supporter Join Date: Jun 2013 Location: Montreal, Canada Photos: Gallery \| Albums Posts: 574	*DA 16-50 weird aberrations** Greetings! I tested my relatively newly acquired DA* 16-50 on a starfield and found the results to be a bit strange. I've enhanced the contrast on the image below for clarity. This was taken at 16 mm f/2.8. It is a 120 s shot with astrotracer running. There is a lot of vignetting in that lens and a lot of coma? astigmatism? in the corners but that was expected from all the reviews in this forum and others. What puzzles me is the fact that these problems are worsts in the top corners compared with the bottom ones. Is this normal? (Click on the image to get a full res view)

04-18-2016, 06:39 PM	#5
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,608	I think these kinds of results are to be expected with wide lenses and the astrotracer. The shot looks great overall, though! Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

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04-18-2016, 06:15 PM	#2
Wheatfield Moderator Join Date: Apr 2008 Location: The wheatfields of Canada Posts: 15,987	Originally posted by SunValley Greetings! I tested my relatively newly acquired DA* 16-50 on a starfield and found the results to be a bit strange. I've enhanced the contrast on the image below for clarity. This was taken at 16 mm f/2.8. It is a 120 s shot with astrotracer running. There is a lot of vignetting in that lens and a lot of coma? astigmatism? in the corners but that was expected from all the reviews in this forum and others. What puzzles me is the fact that these problems are worsts in the top corners compared with the bottom ones. Is this normal? (Click on the image to get a full res view) The lens might be very slightly decentered.

04-18-2016, 06:16 PM - 1 Like	#3
mee Veteran Member Join Date: May 2010 Posts: 7,403	The 16-50 is a Chromatic Abberation nightmare at f/2.8 ...actually there is a lot of vignetting and a bit of softness too at f/2.8 on that lens in general.. it is really an f/3.5 or f/4 lens for serious sharpness, reduced vignetting, and minimized CA. I've had 2 copies of the lens and, in this regard, they performed similarly.. f/2.8 was weak and the performance jumped largely by f/4. From then on its rather gorgeous..

04-18-2016, 06:33 PM	#4
Ole Administrator Join Date: Sep 2006 Location: Copenhagen, Denmark Photos: Gallery \| Albums Posts: 4,409	You shot with the astrotracer running, so the sensor moved during exposure and the corners will therefore be uneven. You should test it without the astrotracer. It might be an idea to buy a manual focus prime lens like 20mm F2.8 for this kind of shooting and avoid using the zoom. Apart from that, you succeeded well with the astrotracer shot! It can be a challenge to calibrate it and make it work correctly!

04-18-2016, 06:56 PM	#6
DSims Veteran Member Join Date: Sep 2009 Location: Southern California Photos: Gallery \| Albums Posts: 3,236	Originally posted by mee The 16-50 is a Chromatic Abberation nightmare at f/2.8 ...actually there is a lot of vignetting and a bit of softness too at f/2.8 on that lens in general.. it is really an f/3.5 or f/4 lens for serious sharpness, reduced vignetting, and minimized CA. I've had 2 copies of the lens and, in this regard, they performed similarly.. f/2.8 was weak and the performance jumped largely by f/4. From then on its rather gorgeous.. I concur - I like my DA*16-50 very much, but I seldom shoot it wider than f/3.5. Furthermore, it's at its weakest between 16 and 20mm. It's a very good lens, and I'm happy with it since I only expect it to perform like a zoom. In fact I don't know of any brand of zoom in this standard focal-length range that has spectacular results near wide-open. This is why lenses like the Samyang 14mm prime are popular among astro shooters. I've always wondered if the Samyang 16/2 is just as good (it's got a nice fast aperture), but I don't know.

04-18-2016, 08:08 PM - 1 Like	#7
SunValley Site Supporter Join Date: Jun 2013 Location: Montreal, Canada Photos: Gallery \| Albums Posts: 574 Original Poster	Thanks everyone! Indeed the lens is not at its best at 16/2.8 but this was a test to see its limits really. Good old M28/2.8 and A50/1.4 give fantastic results with the astrotracer but I like the idea of a AW lens for this kind of situation when dew and/or frost end up covering the equipment. Originally posted by Ole You shot with the astrotracer running, so the sensor moved during exposure and the corners will therefore be uneven. I didn't think of that but I guess you're right, during those two minutes the sensor must have moved quite a bit, thanks! Here's a shot from earlier that night still at 16/2.8. Two consecutive shots of 180 s iso 400, one with astrotracer on and the other off for the foreground. Blended in PSE 14 so everything looks in place.I cropped a bit the sides to avoid the softer parts of the image.

04-21-2016, 07:34 PM	#8
Ole Administrator Join Date: Sep 2006 Location: Copenhagen, Denmark Photos: Gallery \| Albums Posts: 4,409	That's a great image, well done, very nice! I envy you - I have still to get a decent shot of the milky way!

04-22-2016, 04:43 AM	#9
SunValley Site Supporter Join Date: Jun 2013 Location: Montreal, Canada Photos: Gallery \| Albums Posts: 574 Original Poster	Originally posted by Ole That's a great image, well done, very nice! I envy you - I have still to get a decent shot of the milky way! Thanks! With legendary clear Arizona sky it's me who is jealous!