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Archive 2016 · Does megapixels influence when testing for lens decentering?
  
 
nampramos
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p.1 #1 · p.1 #1 · Does megapixels influence when testing for lens decentering?


I picked up a used Loxia 21 today, which will come in handy next year in March when traveling to Japan. I've been wanting one of these since they were announced but just couldn't pay the full retail price.

Today I got lucky and picked up a MINT one for a very good price.

Since it was already dark when I got it, I couldn't really do much of a testing. Just took a few shots on the tripod from the balcony when I got back home, which you can find on this DropBox link. First two are corrected in LR (f/2.8 and f/5.6), the next two are SOOC f/2.8 and f/5.6).

Does anyone see any red alerts from these ones, even though they aren't the best examples?

I will do proper testing tomorrow morning and was wondering if shooting only with the A7s (12MP) could not show decentering problems that maybe the A7rII (42MP) would?

What would be the best way to test it on the A7s?

Thanks for the help!

Edited on Oct 13, 2016 at 05:03 AM · View previous versions



Oct 12, 2016 at 07:37 PM
snapsy
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p.1 #2 · p.1 #2 · Does megapixels influence when testing for lens decentering?


MP definitely has an impact on whether you'll notice decentering issues. When I bought my 5DSr I noticed decentering on several lenses that was never visible/obvious with my 12MP/22MP Canon bodies.


Oct 12, 2016 at 09:59 PM
Fred Miranda
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p.1 #3 · p.1 #3 · Does megapixels influence when testing for lens decentering?


Based on my own practical tests, the higher the sensor's MP, the more evident the ill-effects of decentering or tilt element(s) will be.


Oct 12, 2016 at 10:44 PM
DavidBM
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p.1 #4 · p.1 #4 · Does megapixels influence when testing for lens decentering?


Fred Miranda wrote:
Based on my own practical tests, the higher the sensor's MP, the more evident the ill-effects of decentering or tilt element(s) will be.


What Fred says is true viewed at the same pixel ratio (i.e. When both are viewed at 1:1 or 1:2 etc)
But not of course when viewed at the same absolute magnification (i.e. When the viewed image is understood as a multiple of the area of the sensor)

What this boils down to is that a higher MP sensor won't make the decentering look worse than a lower MP sensor on prints of the same size, for example.

But it does mean that decentering might get in the way of using the MP to make a larger print than you could with a lower MP sensor.




Oct 12, 2016 at 11:31 PM
 

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charles.K
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p.1 #5 · p.1 #5 · Does megapixels influence when testing for lens decentering?


Nuno, the A7s and A7rII are very different. Lenses I used on the A7rII behaved differently on the A7s in a positive way. The RAW files from the A7s felt in some similar to those of the Nikon D700 also 12 MP. I doubt you will pick up decentering on the A7s, whereas as suggested the A7rII will be brutal.

Edited on Oct 13, 2016 at 10:02 PM · View previous versions



Oct 12, 2016 at 11:35 PM
nampramos
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p.1 #6 · p.1 #6 · Does megapixels influence when testing for lens decentering?


Thank you! I'll borrow an rII from a friend then, just to be sure.


Oct 13, 2016 at 05:02 AM
justruss
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p.1 #7 · p.1 #7 · Does megapixels influence when testing for lens decentering?


Short answer: Yes, higher resolution makes it easier to spot (in the way DavidBM describes).

Longer answer/thought: An instructive way (via simplification) to think about it is that decentering results in parts of the image being less sharp than other parts of the image-- whereas in a theoretically centered lens both parts should be just as sharp. How visible such a difference is comes down to a matter of thresholds.

Here's a way to think about it: consider sharpness/resolving power on a scale of 1-100 (again simplification), with 100 being the highest. Let's say a 12mp sensor + specific lens never gets the sharpness above 50 anywhere in the image. That's the best you can get no matter what. You have a moderately decentered lens, so maybe the unsharp side of the image registers a 47, and the sharp side a 50-- not very visible. But consider a 42mp sensor + the same lens, and it scores an 80 on our scale in the sharpest part of the image. The decentered part of the image then scores, say, a 53 (because of the way resolving power works, even the bad side gets somewhat sharper-- but the lens is a greater limiting factor), but the sharpest part of the lens scores an 80. The difference is MUCH more perceptible. The main difference is that the sharp part of the image is a lot sharper (resolves more), whereas the unsharp/decentered part of the image is primarily limited by the lens defect and hasn't changed all that much between systems.

Again, keep in mind this is a major simplification. And keep in mind the limits to where this will show up (re: DavidBM). But, if everything is a little blurry to begin with (low resolution sensor), the difference between the sharp and not-sharp sides of a decentered lens may not be all that noticeable. But if everything is extremely crisp and resolved, the sharp side is much sharper than the side limited by the decentering of the lens-- and the difference between the two is totally visible.

Here's another analogy: If you have a towel that is 50% saturated with water in the driest part, and 55% saturated with water in the wet part-- can you feel the difference by hand? How about if you have a towel that is only 20% saturated with water in the driest part-- and 85% saturated in the wettest part? It's a bit like that. Increasing system resolution (in this case, coming from a higher mp sensor) makes the baseline of the towel drier-- and decentering makes it wetter (or limits how dry the wet part can be).




Oct 13, 2016 at 06:23 AM
nampramos
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p.1 #8 · p.1 #8 · Does megapixels influence when testing for lens decentering?


Thanks guys!

I shot the Loxia 21 today with a borrowed A7rII.

Now only if I knew how to do a proper 100% crop in Lightroom (instead of a printscreen), I could share my findings from today here.

I uploaded several shots to a DropBox folder and would be happy if someone with experience could have a look at it.

Each folder contains 5 shots, all in this order:
- Target in the middle
- Target on the bottom left
- Target on the top left
- Target on the top right
- Target on the bottom right

There are two different targets (building towers). Each target was shot at f/2.8, f/5.6 and f/11.
So there will be a total of 6 folders (3 folders per target), with 5 photos in each folder.
Distance to target 1 is almost 0.7 miles and distance to target 2 is almost 0.6 miles.

https://www.dropbox.com/sh/09s6o2lye1lzedo/AADO2GGYtlfLzgvlcrD1-Q1Fa?dl=0

Complicated? Too much?


Thanks!



Oct 14, 2016 at 04:30 PM







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