[interested_observer]

I'm hoping to get out to do some night/astro shooting this month (here in the next week), so I was going back, looking over some past shots, to see what I should do differently. I came across an interesting condition that I sort of stumbled over previously but was just too busy to really take any time to look at.

So the topic is vignetting on the 15-30. The problem becomes apparent when you stitch, especially stitch wide with multiple frames. I have 2 examples below (sky and foreground), each shot at 30mm f2.8. Each of the individual images were corrected for vignetting in Lightroom 5.5 - but as you can easily see - there is a paneling effect across the resulting stitched image.

It's interesting to note that at 15mm f2.8 (no example included), there essentially is no vignetting that I have encountered in other similar stitched images. Delving deeper here, I went and took a look at the Tamron 15-30/f2.8 lens review.

• Tamron 15-30mm f/2.8 Di VC USD SP Review

If you click on the vignetting chart, you will see that at 15mm f2.8 there is substantially less vignetting than at 30mm at f2.8. The vignetting substantially dissipates at f4.

• I didn't really expect or anticipate this, especially since I overlapped every image by 50%. I was expecting the stitching utility to essentially strip off the edges and only use the centers of each image. I stitched the examples with Microsoft ICE, but have decided to switch to PTGui, which affords greater control and provides for the user to mask off designated areas. That should help a great deal.

• The other aspect is stopping down when at the long end of the lens - 30mm to f4. That should reduce the amount of vignetting to less than the amount encountered at 15mm f2.8 which was not apparent in other stitched results. Stopping down to f4 will reduce the amount of light collected by a stop. However, due to the physical aperture size difference (between 15mm and 30mm), the lens will still collect a stop's worth of additional light beyond what 15mm collects. So, loose a bit of light but also loose essentially all the vignetting, which seems like a fair trade.

Also, this location - the Lost Dutchman State Park, the light pollution in the last year has just become substantially worse. It use to be pretty reasonable, but now it's just a large PIA.

Attached Images

   

[mroeder75]

I am not sure what PIA means -- Pretty Incredible Astronomicalphoto? Thanks for posting.

[interested_observer]

Pretty Incredible Astronomicalphoto - I gotta remember that one!!! Actually it's short for - Pain in the A**.....

[UncleVanya]

Very interesting - thanks for posting this.

[BigMackCam]

So the topic is vignetting on the 15-30. The problem becomes apparent when you stitch, especially stitch wide with multiple frames. I have 2 examples below (sky and foreground), each shot at 30mm f2.8. Each of the individual images were corrected for vignetting in Lightroom 5.5 - but as you can easily see - there is a paneling effect across the resulting stitched image.

I assume you used an official lens profile rather than manually correcting the vignetting yourself? If so, it looks like whoever developed the lens correction profile didn't test it for accuracy

The other aspect is stopping down when at the long end of the lens - 30mm to f4. That should reduce the amount of vignetting to less than the amount encountered at 15mm f2.8 which was not apparent in other stitched results. Stopping down to f4 will reduce the amount of light collected by a stop. However, due to the physical aperture size difference (between 15mm and 30mm), the lens will still collect a stop's worth of additional light beyond what 15mm collects.

So far as my limited understanding goes, f/4 at any focal length will let in one stop less of light than f/2.8 at any focal length. Or have I misunderstood your point?

[photoptimist]

So far as my limited understanding goes, f/4 at any focal length will let in one stop less of light than f/2.8 at any focal length. Or have I misunderstood your point?

That rule about apertures collecting the same amount of light regardless of focal length is true for diffuse or area-illuminated subjects (sky glow, walls, etc.).

But point sources like stars are different. A 30 mm lens at f/4 (7.5 mm absolute aperture) collects one stop more of starlight than a 15 mm lens at f/2.8 (5.4 mm absolute aperture).

For diffuse or area-illuminated subjects, the larger absolute aperture of a longer focal length lens also collects more light but then the magnification effect of the lens just spreads that light over more pixels cancelling the effect. For point sources on a dark background the larger absolute aperture of a longer focal length lens collects more light but the magnification does not cancel the light gathering effect because the star is a point.



P.S. I don't think this panel effect is vignetting. It's too linear and of constant width from top-to-bottom. This looks more like a resampling artifact.

[BigMackCam]

That rule about apertures collecting the same amount of light regardless of focal length is true for diffuse or area-illuminated subjects (sky glow, walls, etc.).

But point sources like stars are different. A 30 mm lens at f/4 (7.5 mm absolute aperture) collects one stop more of starlight than a 15 mm lens at f/2.8 (5.4 mm absolute aperture).

For diffuse or area-illuminated subjects, the larger absolute aperture of a longer focal length lens also collects more light but then the magnification effect of the lens just spreads that light over more pixels cancelling the effect. For point sources on a dark background the larger absolute aperture of a longer focal length lens collects more light but the magnification does not cancel the light gathering effect because the star is a point.

Very interesting! Thanks for the clarification. So... dumb question... does this mean at 30mm f/4, sky-glow would be captured darker than at 15mm f/2.8, but stars (as point light sources) would be captured brighter?

P.S. I don't think this panel effect is vignetting. It's too linear and of constant width from top-to-bottom. This looks more like a resampling artifact.

I did wonder if it might be something other than vignetting. You'd expect to see rounded shading in the corners of each panel for that (right?), and it's not evident here... just vertical lines.

[PJ1]

Vignetting or not, you have a fabulous photo location.

[photoptimist]

Very interesting! Thanks for the clarification. So... dumb question... does this mean at 30mm f/4, sky-glow would be captured darker than at 15mm f/2.8, but stars (as point light sources) would be captured brighter?

Yes, exactly! The 30mm f/4 has double the starlight but half the sky-glow of the 15mm f/2.8.

It's this effect that explains why larger telescopes can resolve fainter stars.

(Note: a similar phenomenon happens with thin-line subjects such as meteors but it's not as strong. Meteor traces will be 1/2 stop brighter with the 30mm @ f/4 compared to the 15 mm @ f/2.8. But 30 mm's FoV is half the width so it will see about 1/4 as many meteors for a given exposure time.)

I did wonder if it might be something other than vignetting. You'd expect to see rounded shading in the corners of each panel for that (right?), and it's not evident here... just vertical lines.

Yes, I'd have expected to rounded shading, too.

[BigMackCam]

Yes, exactly! The 30mm f/4 has double the starlight but half the sky-glow of the 15mm f/2.8.

It's this effect that explains why larger telescopes can resolve fainter stars.

(Note: a similar phenomenon happens with thin-line subjects such as meteors but it's not as strong. Meteor traces will be 1/2 stop brighter with the 30mm @ f/4 compared to the 15 mm @ f/2.8. But 30 mm's FoV is half the width so it will see about 1/4 as many meteors for a given exposure time.)

Fascinating! Thank you for the lesson... That's one of the most interesting things I've learned in months! I have more detailed reading to do, I think

[mattt]

I agree with BigMackCam on that - very interesting

[interested_observer]

I assume you used an official lens profile rather than manually correcting the vignetting yourself? If so, it looks like whoever developed the lens correction profile didn't test it for accuracy

So far as my limited understanding goes, f/4 at any focal length will let in one stop less of light than f/2.8 at any focal length. Or have I misunderstood your point?

photoptimist was much quicker on the draw, and provided a much better explanation!!!

That rule about apertures collecting the same amount of light regardless of focal length is true for diffuse or area-illuminated subjects (sky glow, walls, etc.).

But point sources like stars are different. A 30 mm lens at f/4 (7.5 mm absolute aperture) collects one stop more of starlight than a 15 mm lens at f/2.8 (5.4 mm absolute aperture).

For diffuse or area-illuminated subjects, the larger absolute aperture of a longer focal length lens also collects more light but then the magnification effect of the lens just spreads that light over more pixels cancelling the effect. For point sources on a dark background the larger absolute aperture of a longer focal length lens collects more light but the magnification does not cancel the light gathering effect because the star is a point.

P.S. I don't think this panel effect is vignetting. It's too linear and of constant width from top-to-bottom. This looks more like a resampling artifact.

Morning - it's 4am here, well now 5. I got up to get a drink of water, and had a nagging suspicion that perhaps another stitcher would do a better job - and perhaps what was appearing was an artifact of the stitching process, re-sampling error. Sure enough, PTGui has a much better result, no resampling problems, better color transmission through the process, among others. See the new example below.

Also, thanks!!! - an excellent description of the aperture effect with point sources.

Very interesting! Thanks for the clarification. So... dumb question... does this mean at 30mm f/4, sky-glow would be captured darker than at 15mm f/2.8, but stars (as point light sources) would be captured brighter?

I did wonder if it might be something other than vignetting. You'd expect to see rounded shading in the corners of each panel for that (right?), and it's not evident here... just vertical lines.

• Yes - I would think that the stars would be brighter.

Yes, again you would think that the corners would be darker, however due to how the light falls up at the very top of the resulting image the sky is substantially darker, due to the lack of stars (Milky Way being lower in the frame), so there is a natural fall off of light. Having said that, re-running the stitching in PTGui, shows a substantial improvement in the stitching, lack of artifacts (resampling errors, etc.). If I remember correctly, PTGui in the past (several years ago) had a major update in terms of how it handled astro imaging just due to producing similar results. I have been on the edge for at least 5 to 7 years on what stitcher to buy, PTGui or Autopano. This is pushing me off the mark to move away from ICE and make a decision.

Vignetting or not, you have a fabulous photo location.

It's a very nice and picturesque location. It has a lot of appeal, sunsets are spectacular here and the scenery alone just makes the location. The state park occupies the the absolute perfect vantage point. As time passes, the population growth from Phoenix just continues to grow, putting pressure on the outlying areas. The light pollution here is just getting worse by the year.

I'm going back to bed..... Thanks, for solving the re-sampling artifact faster than what I was doing in my sleep.
_________________________

Just one more thought. The Sigma 18-35/f1.8 lens does cover the full frame sensor starting around 28mm all the way to 35mm. At 35mm f1.8, there is at least another stop of light to be recovered. I had tested that lens on the K1 before getting the 15-30 and run in to the same "patch work quilt" problem - re-sampling error with ICE. So, now with this, I have another excellent lens to use.

[John Hales]

photoptimist was much quicker on the draw, and provided a much better explanation!!!


Morning - it's 4am here, well now 5. I got up to get a drink of water, and had a nagging suspicion that perhaps another stitcher would do a better job - and perhaps what was appearing was an artifact of the stitching process, re-sampling error. Sure enough, PTGui has a much better result, no resampling problems, better color transmission through the process, among others. See the new example below.

Also, thanks!!! - an excellent description of the aperture effect with point sources.

• Yes - I would think that the stars would be brighter.

Yes, again you would think that the corners would be darker, however due to how the light falls up at the very top of the resulting image the sky is substantially darker, due to the lack of stars (Milky Way being lower in the frame), so there is a natural fall off of light. Having said that, re-running the stitching in PTGui, shows a substantial improvement in the stitching, lack of artifacts (resampling errors, etc.). If I remember correctly, PTGui in the past (several years ago) had a major update in terms of how it handled astro imaging just due to producing similar results. I have been on the edge for at least 5 to 7 years on what stitcher to buy, PTGui or Autopano. This is pushing me off the mark to move away from ICE and make a decision.


It's a very nice and picturesque location. It has a lot of appeal, sunsets are spectacular here and the scenery alone just makes the location. The state park occupies the the absolute perfect vantage point. As time passes, the population growth from Phoenix just continues to grow, putting pressure on the outlying areas. The light pollution here is just getting worse by the year.

I'm going back to bed..... Thanks, for solving the re-sampling artifact faster than what I was doing in my sleep.
_________________________

Just one more thought. The Sigma 18-35/f1.8 lens does cover the full frame sensor starting around 28mm all the way to 35mm. At 35mm f1.8, there is at least another stop of light to be recovered. I had tested that lens on the K1 before getting the 15-30 and run in to the same "patch work quilt" problem - re-sampling error with ICE. So, now with this, I have another excellent lens to use.

Just one more thought. The Sigma 18-35/f1.8 lens does cover the full frame sensor starting around 28mm all the way to 35mm. At 35mm f1.8, there is at least another stop of light to be recovered. I had tested that lens on the K1 before getting the 15-30 and run in to the same "patch work quilt" problem - re-sampling error with ICE. So, now with this, I have another excellent lens to use.

Interested -- have you tried any of the Rokinon - Samyang lens for your Astrophotography?

[interested_observer]

Interested -- have you tried any of the Rokinon - Samyang lens for your Astrophotography?

Morning John - Yes, I had. There was about 6 months between getting the K1 and the 15-30. I tried the full frame lenses I had, which were all excellent, but each had some drawback. Thinking things through, I really wanted a zoom that was more general purpose. I did think about the Samyang/Rokinon line a lot though, but really wanted something sealed, with AF. Size/weight was important, but was more of a secondary consideration - and well with acquiring the 15-30 which is a giant as well as being a heavy weight, I guess you could say any size/weight considerations went out the window.

In late April early May, my wife and I took a trip up to Alaska. I was out on deck of the cruise ship shooting the glacier we were in front of doing a 360 degree spin and a stream of water came poring off the deck above - drenching my tripod, camera and lens. My wife was mortified, as well as everyone around me. I pulled out a little hand towel, dried it off and kept shooting. So, the weather sealing paid for itself several times over.

I figured that 15mm was as wide as I really needed (although I have a 8-16 on my K5IIs). Having the option to go to 30mm (where I gain a stop of light) is a bit of a bonus.