De-centred lenses?

[LucisPictor]

Often we tend to forget that lenses cannot only be scratched or suffer from fungus infections, they also can have de-centred internal glass elements.

This is a problem that you won't easily see in normal usage, but if it comes to quality crucial pictures it might be disturbing.

There is an easy way to check if your lens is de-centred (and hope it is not, because there is not much to do against it. )

Shoot a frame with a distinct subject in the center that is more than 20m away so that it only covers a small part of the frame, e.g. a tower or a house or something. Use the lens at f8 in order to get maximum sharpness.
Focus as precisely as you can.
Then place this subject in the four corners and shoot again.

You will get five pictures that enable you to compare the sharpness of the subject in each of the cases.

If the subject is in all four corners equally sharp, your lens is well centred. Congratulations.

If you have a slightly de-centred lens, you will realize a slight difference in sharpness - not compared to the centre image, but between the pics with the subject on the left and those with it on the right side.

If you can detect a considerable difference between the left and the right side, your lens is heavily de-centred (which is rare).

I tested three of my lenses (all excellent ones) today at lunch time.

My Nikkor-S.C 1.2/55 is perfectly centred! Thank God, because rumour has it that this lens is kind of prone to this problem since it is susceptible to blows from a side, e.g. when badly secured during transport.

My Auto Revuenon 1.4/55 (Tomioka) also is well centred and again it surprised me with its great sharpness!

To my big surprise my SMC Pentax-M 1.7/50, which I always rank as the second best 50mm lens I've got, is slightly de-centred. I have never realised that! Fortunately, it is not too bad, but it is visible in a test like this. The right side of the frame is sharper than the left side.

As soon as I find more time, I'm going to test some more lenses, esp. those I use regularly.

[bob955i]

The rear element of the Flek 4/20 for one has an eccentric adjustment in that you can slacken the retaining ring and the element can then be moved sideways.

[montecarlo]

Hello,

Here, in an old test, results a 2% decentering (is it much ?) for the Pentax 1.7/50 http://www.pbase.com/steephill/image/38540082
Here are other two:
http://www.pbase.com/steephill/image/38540080
and
http://www.pbase.com/steephill/image/38540078

[LucisPictor]

Thanks for the links. I don't think 2% is a lot, but surely not every M 1.7/50 will be de-centred like that (or at all). It is rather an issue of each copy and not of a whole lens series, I guess.
There surely is a certain variance in quality. Perhaps with Leica and Zeiss lenses a much smaller one than with other lenses.

But as I have written, I haven't realised that my Pentax lens is de-centred. And I still like it a lot!

[bob955i]

Chances are Carsten that you might not see the results of de-centred elements on a crop DSLR as the sensor wouldn't see the resulting soft corner/s either.

Unless it was really bad....

[patrickh]

and at wide apertures it can easily be disguised as normal OOF


patrickh

[LucisPictor]

@bob995i: Yes, you will more easily see at at a full frame body, but you can see it at a crop-DSLR. My Pentax 1.7/50 produces fine pictures, you would never think it was de-centred, but with this test I recognized a slight difference.


@patrickh: Of course. That's why I suggest to use f8.

[Orio]

Carsten, considering how small is the portion of frame used by a 1.6x crop camera, I'd say that if you can find a visible difference within it, then the decentering issue could be much worse than you think.

[LucisPictor]

Yes, might be. But then, effectively only for full frame cams, not for me with my 350D.

I still have to test it again, since it could also be an effect of an inconstant thickness of the adapter...

[Orio]

[LucisPictor]

[LucisPictor]

Today I got another PK-EOS-adapter (as it seems a better one), since with the new adapter, my M 1.7/50 does not show any de-centred images.

It seems that my old adapter causes the problem. Thank God!