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Wideangle lens - strange behaviour

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Gromit44:
I've got a Nikon 14-24mm f/2.8 lens. I did some long exposure architectural shots recently using a tripod, wireless remote and mirror lockup - some were at 16mm and some at 14mm.

All of the 16mm shots were sharp both in the centre and at the edges .  The 14mm shots were sharp in the centre but they were all extremely soft  and/or distorted at the left and right edges (not the top and bottom). Has anyone had a similar experience with this lens?   

Hayo Baan:
Hi Simon,

You already sent me some sample shots so I could have a look at the problem, but you can also upload them here so others can see them as well (use the "Attachments and other options" beneath the post to add your images, please make sure they aren't larger than 200kb).

The difference is in sharpness between your 14mm and 16mm images is quite extreme. While I have noticed softness in my 14mm images when viewed at 100% (12MP D3/16MP D4, so probably worse on a D800E), I can't recall it being so dramatic (plus the screenshots you sent were only at 50%). What does influence softness a great deal is whether or not you remove the distortion (e.g., in NX2); distortion removal makes softness MUCH worse (which makes sense as that process will stretch/collapse pixels, and has to do so more extremely at the wider lens settings).

From our private conversation I gather you had auto distortion removal turned-on in the camera? As you use NX2 for your post processing, this setting is inherited by the software. So you are effectively applying distortion removal. This is very likely the cause of your issue.

To verify this, turn off auto distortion control in NX2 and look at the results. The auto distortion control setting can be found under Develop, Camera & Lens Corrections, Auto Distortion

Hope this solves the mystery  :D

Gromit44:
Hi Hayo,

Yes, Nikon's in-camera 'Auto Distortion Control' was indeed switched on - but it was on for both the 16mm shots and the 14mm shots, so I don't understand why it only affected the 14mm shots. I just tried switching it off in Capture NX2 (Camera & Lens Correction section) but doing this does not improve sharpness in the original NEF files.

How do I post the images here? (I don't have them online anywhere, only on my PC)

Gromit44:

--- Quote from: Gromit44 on 08/12/2012, 23:36:18 ---I just tried switching it off in Capture NX2 (Camera & Lens Correction section) but doing this does not improve sharpness in the original NEF files.

--- End quote ---

All it does it reduce the amount of sideways 'stretch' at the edges (i.e. there's more barrel distortion with it on than off) - softness remains the same.

Hayo Baan:

--- Quote from: Gromit44 on 08/12/2012, 23:36:18 ---How do I post the images here? (I don't have them online anywhere, only on my PC)

--- End quote ---
If you click “attachments and other options” you can browse to the file(s) and upload them.


--- Quote from: Gromit44 on 08/12/2012, 23:36:18 ---Yes, Nikon's in-camera 'Auto Distortion Control' was indeed switched on - but it was on for both the 16mm shots and the 14mm shots, so I don't understand why it only affected the 14mm shots. I just tried switching it off in Capture NX2 (Camera & Lens Correction section) but doing this does not improve sharpness in the original NEF files.

--- End quote ---

The compensation needed on 14mm would have been more than on 16mm so the softness would have been more of a problem.

I've attached two 100% (!) crops of two of my images at 14mm (taken with f/16 and f/8 respectively). As you can see they are not perfectly sharp, but pretty good nonetheless. Note: these are crops from processed images, so sharpening etc. has been applied to these images.

(In order to see the crops at 100%, download the files to your computer and view them there)

The only two explanations I can now come up with are:

* Your lens is a lemon and you should get it fixed
* The relative distance to the point of focus and the outer areas of your image are such that they are outside the Depth of Field Zone.
Whether one or two can be tested by photographing something that is more at the same distance (e.g., a wall) to all of your image.  Note here though that the plane of focus is not perfectly flat and that you may see unsharpness in areas due to that.
Also use live view and focus in the corners, that way you can tell directly whether or not it is focus that is causing the unsharpness in the corners.

Hope this makes sense to you ;)

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