[frank]

Stimulated by [url="http://forum.photozone.de/index.php?/topic/830-do-it-yourself-centering-sanity-check"]Klaus' thread on testing lens-centering[/url], I have been trying to test if my PC-E 24mm f3.5 has the problem of decentering. During the test I found the following problem, which I find confusing: for some reason I rotated the lens by 90 degree to see if the image quality changes. Of course, the shift and tilt knobs were both setted at zero reading (and I confirmed it before and after rotation). But I found that, after the lens was rotated, the image in the live view screen had a shift in the direction in the "vertical" direction-the direction parallel to the short side of the censor (the 24mm side, D700). When the image was magnified to maximum magnification ratio the amount of shift was quite obvious. So I tried to calculate the amount of shift on the censor, surprisingly this amount was somewhat random:



Test 1: short side shift = 0.27mm, long side shift = 0.07mm

Test 2: short side shift = 0.58mm, long side shift = 0.19mm

Test 3: short side shift = 0.38mm, long side shift = 0.07mm



Is this normal for a shift/tilt lens? I can imagine that as you rotate the lens the lens could take a tiny amount of shift or tilt if you did not lock them firmly (and according to the Nikon manual they really can't be locked firmly), but I still wonder a shift like 0.5mm (about 2% of 24mm-the length of the short side of the censor) is too large. It may also be that the tilt/shift scales are not very precise so that althoug they read zero but the true amount of shift/tilt is not zero (indeed I already noticed that the distance scale of the lens suffers large errors).



To make sure that I did my calculation correctly, let me tell you how I calculated the amount of shift on the image censor: I use an image editor to measure the shift in pixels, then multiply it by 24mm (or 36mm for the long side) and divided by 2832 (or 4256 for the long side; the image resolution of D700 is 4256x2832).



Any highlights?



Regards,

Frank



Edit: In test 2 I put the camera in a portrait orientation. It seems that the shift after rotation always predominantly occurs in the short side direction. I don't know if I need do more careful tests. The LV mode is really power-consuming.

[mst]

I guess this is indeed because of some tolerance or lack of precision when locking tilt in the centered position. It's not something I'd be too worried about.



-- Markus

[frank]

[quote name='mst' timestamp='1326729995' post='14909']

I guess this is indeed because of some tolerance or lack of precision when locking tilt in the centered position. It's not something I'd be too worried about.



-- Markus

[/quote]



Thank you, Markus. Now I feel easy about it. So it should be normal for a TS lens.



Regards,

Frank

[frank]

I did some ugly test for the centering problem. The images are 100% croped. I tried my best to make the censor to be parallel to the laptop screen (as indicated by the frame of some images), and to put the "center point" at the center of the LV frame. Markus and Klaus, can you tell if there is obvious decentering problem with the lens?



Thanks,

Frank

[frank]

Now the lens is rotated by 90 degree. In this situation it seems there is indeed some obvious decentering problem. Am I right <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/unsure.gif' class='bbc_emoticon' alt=':unsure:' /> ?

[frank]

I just noticed that my test images appear on the front page of the forum. If this is not good, please tell me then I will delete them, put them in another place and link them to this thread.



Frank

[Klaus]

[quote name='Frank' timestamp='1326806168' post='14954']

Now the lens is rotated by 90 degree. In this situation it seems there is indeed some obvious decentering problem. Am I right <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/unsure.gif' class='bbc_emoticon' alt=':unsure:' /> ?



[/quote]



This one looks suspicious, yes.

[Klaus]

[quote name='Frank' timestamp='1326805239' post='14953']

I did some ugly test for the centering problem. The images are 100% croped. I tried my best to make the censor to be parallel to the laptop screen (as indicated by the frame of some images), and to put the "center point" at the center of the LV frame. Markus and Klaus, can you tell if there is obvious decentering problem with the lens?



Thanks,

Frank













[/quote]



These look fine to me.

[frank]

Thank you, Klaus. Then it seems the rotation does cause some problem. I wonder if I should send the lens to Nikon for a professional test.



By the way, I would like to check with you on two things:



1. The images are sharpened in the Camera. That is, I shoot them in Raw, then use NX2 to save them in jpeg without farther sharpening or de-sharpening. The visual effect of decentering is not affected by the sharpening, right?



2. As I checked the iamges I found that I cannot bring the black part and the white part into the dynamic range simutaneously, i.e., either the black was underexposed or the white was overexposed. So I used some exposure compensation during the shot and as a reault the exposure of the above iamges is sometimes different. I have tried to use the NX2 to bring out the highlight details (but the black details cannot always be recovered). Could the exposure affect the visual effect? Should I lower the contrast of the display before shot?



Thanks,

Frank

[Klaus]

[quote name='Frank' timestamp='1326809336' post='14960']

1. The images are sharpened in the Camera. That is, I shoot them in Raw, then use NX2 to save them in jpeg without farther sharpening or de-sharpening. The visual effect of decentering is not affected by the sharpening, right?



2. As I checked the iamges I found that I cannot bring the black part and the white part into the dynamic range simutaneously, i.e., either the black was underexposed or the white was overexposed. So I used some exposure compensation during the shot and as a reault the exposure of the above iamges is sometimes different. I have tried to use the NX2 to bring out the highlight details (but the black details cannot always be recovered). Could the exposure affect the visual effect? Should I lower the contrast of the display before shot?



Thanks,

Frank

[/quote]





1. a softer edge will remain comparatively softer regardless of the sharpening. Please note that this tests is for the center only. If there's an issue here it will be amplified in one (or more) border zone.



2. this is a bit surprising. The exposure should remain roughly constant. Have you used spot metering ? You shouldn't really try to reach 100% white. A light gray tone for the white field is fine.



I'd say that the decentering is not extreme in your sample. Nikon may still respond that this is within factory limits. Remember that no lens is 100% centered anyway. If you have an issue in the field it should be taken care of (I didn't follow the whole thread so maybe you've mentioned this already).