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08-22-2017, 10:32 AM   #1
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What lens is decentered if decentered at all?

Hello,

I'd like to ask if I have decentered lenses? I present photos of 2 lenses (4 corners for every lens). What is your opinion?


Last edited by Medex; 02-23-2018 at 06:35 AM.
08-22-2017, 10:43 AM   #2
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I typically worry about de-cenerting on landscapes where panos may be in order. For this type of image with the centre in focus and the edges out of focus, decentering rarely makes a difference.
08-22-2017, 10:54 AM   #3
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Hard to say with those variations in the position of the subject and at such a close shooting distance. Before we jump to conclusions I think you should try the test again as described below:

How to Check Your Lens for Decentering - Articles and Tips | PentaxForums.com

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08-22-2017, 11:01 AM   #4
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QuoteOriginally posted by Adam Quote
Hard to say with those variations in the position of the subject and at such a close shooting distance. Before we jump to conclusions I think you should try the test again as described below:

How to Check Your Lens for Decentering - Articles and Tips | PentaxForums.com
Yeap, I have center image too, and I made test according to recommendations. These lenses are at 15 and 14 mm focal distance.
So, if you say, that it is hard to make decision, maybe it is not worth to worry?

08-22-2017, 11:50 AM   #5
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Depends on the shooter and what you are shooting.... the soft edges on the 18-135 at 135mm rarely bother me because I don't shoot images with it that are expected to be sharp edge t edge. It's edge to edge sharp for landscape at 20-45mm, which is exactly what I want. And it gives me medium telephoto with excellent centre sharpness as a bonus. Some people seem to believe they need sharp edge to edge for everything, then they buy 1.4 lenses and shoot images that look just like mine, out of focus because of narrow DoF instead oft of focus because of soft edges, but in the overall scheme of things soft edges are soft edges, be they from a sharp lens shot with narrow DoF, or a cheaper lens that's always soft on the edges.

Sharp edges is pretty much a solution waiting for a problem, the one exception being pano shooters, or shooters who stitch their images together.
08-22-2017, 12:15 PM   #6
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QuoteOriginally posted by normhead Quote
Depends on the shooter and what you are shooting.... the soft edges on the 18-135 at 135mm rarely bother me because I don't shoot images with it that are expected to be sharp edge t edge. It's edge to edge sharp for landscape at 20-45mm, which is exactly what I want. And it gives me medium telephoto with excellent centre sharpness as a bonus. Some people seem to believe they need sharp edge to edge for everything, then they buy 1.4 lenses and shoot images that look just like mine, out of focus because of narrow DoF instead oft of focus because of soft edges, but in the overall scheme of things soft edges are soft edges, be they from a sharp lens shot with narrow DoF, or a cheaper lens that's always soft on the edges.

Sharp edges is pretty much a solution waiting for a problem, the one exception being pano shooters, or shooters who stitch their images together.
Thank you Normhead, I am planning to shoot stars. Therefore I used for test some tiny reflections of the halogen lamps - to simulate stars. The first lens is Pentax 15-30, second - Samyang 14. I have a plan to buy Pentax, so I would like to know opinion about possible decentering or astigmatism because this lens is not cheap compared to Samyang. In my opinion, the left corner in the second image is softer with some astigmatism maybe. Pentax has astigmatism too on the left egde, but smaller.
08-22-2017, 12:18 PM   #7
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And unfortunately, stars are probably one of the things you want sharp edge to edge for... I'll add that to my list next time.
08-22-2017, 12:57 PM   #8
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The 14mm isn't very good on FF so I'd avoid it. Would be best to do some test shots with some stars, IMO!


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08-22-2017, 01:25 PM   #9
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Those photos seem to show negligible decentering, if any at all. I wouldn't worry much with those results. I doubt it would be noticeable out in the field. The second one has more than the first

Oh and astrophotos are never pin sharp in the edges, unless its a combination of stitching or cropping. Astrophotos don't need to have pin sharp stars anyway. Getting the focus just right and getting good light conditions is important. And the lens should have low low coma for astrophoto
08-22-2017, 02:47 PM   #10
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QuoteOriginally posted by Na Horuk Quote
Those photos seem to show negligible decentering, if any at all. I wouldn't worry much with those results. I doubt it would be noticeable out in the field. The second one has more than the first

Oh and astrophotos are never pin sharp in the edges, unless its a combination of stitching or cropping. Astrophotos don't need to have pin sharp stars anyway. Getting the focus just right and getting good light conditions is important. And the lens should have low low coma for astrophoto
Thank you for your answer. I did test shots of stars. Tomorrow will show examples. At f2.8 pentax shows triangular stars in the upper edge. A little defocused they look like drops. F4 is better a little. Samyang is a little softer at f2.8 but dont show such triangular stars.
I will post pictures tomorrow.
08-22-2017, 06:46 PM - 1 Like   #11
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QuoteOriginally posted by Medex Quote
I am planning to shoot stars.
I've no comment on your decentering question but would like to point you to some info that might help working out a suitable lens for shooting stars. The following links discuss shooting the Milky Way so whether that's your intent or not the info will still assist in determining the best choice of wide angle lens for star shots.

This links talks about focal lengths, managing trailing stars and talks a small amount on abberations: How to Pick a Lens for Milky Way Photography – Lonely Speck (You also mentioned: "At f2.8 pentax shows triangular stars in the upper edge." This could be related to aperture, exposure time [based on focal length] or abberations, so the answer to your question could be one of these as opposed to decentering).

This link discusses their recommendations for lens choices and includes info on the Samyang and D-FA* 15-30: Lonely Speck’s Ultimate List of Best Astrophotography Lenses – Lonely Speck

If your goal is primarily for star images this link explains lens abberations related to star images, in detail with examples, and has a downloadable test chart to test your lens options: A Practical Guide to Lens Aberrations and the Lonely Speck Aberration Test – Lonely Speck

This link is to Royce Bair's blog, and he's a fountain of knowledge on Milky Way Nightscape imagery. The link has a lot of detail on Coma abberations and you will see he's a big fan of the Rokinon (Samyang): Into The Night Photography: Overcoming Coma Aberration - Part 2 Also, there's examples of him using the Tamron 15-30 and Rokinon lenses on his Flickr page (including nightscape panos) if you want to check out his work: Milky Way over Devil's Garden | Milky Way over ?The Two Spin? | Flickr

Hopefully the links can aid in answering one or two of your questions if not my apologies for being off topic.

Tas
08-23-2017, 01:59 AM   #12
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QuoteOriginally posted by Adam Quote
The 14mm isn't very good on FF so I'd avoid it. Would be best to do some test shots with some stars, IMO!
did test with stars. At f2,8 Pentax 15-30 @15mm has triangle stars in upper part of frame, even at f4,5 and f5,0 they don't look round, I say they look like drops. Is it the sign of astigmatism?

Last edited by Medex; 02-23-2018 at 06:35 AM.
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