Wideangle lens - strange behaviour

[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 

[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]

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.

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]

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

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

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.

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

[Gromit44]

Here are the (NEF) image comparisions - 16mm vs 14mm. The first three are at 50% and the fourth is at 100%.

[Hayo Baan]

The softness in your samples looks indeed much worse than in mine. However, I also notice you likely have moved and, more importantly, you were using a different AF method. Why was that, and where was the focus point?

Anyway, like I said, you really need to perform a “controlled” test (as I described earlier) to analyse the cause further and determine whether or not it is the lens, or something else.

[Gromit44]

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

I've downloaded your shots and they certainly don't show the same issue - although your subjects aren't the same 'U' shape as mine.

I don't think the lens is a lemon. I've got some D3x shots taken with it inside a smallish room and the same problem is not present at the edges of the frame.

I just checked the lens manual and DOF at 14mm and f8 is from 0.54m to infinity when focussed at 1m. If focussed at infinity it's 1.26m to infinity. The closest part of the building in my shots is approx 55 metres away from the camera so I think we can rule out a DOF problem.

[Gromit44]

The softness in your samples looks indeed much worse than in mine. However, I also notice you likely have moved and, more importantly, you were using a different AF method. Why was that, and where was the focus point?

Anyway, like I said, you really need to perform a “controlled” test (as I described earlier) to analyse the cause further and determine whether or not it is the lens, or something else.

I used viewfinder focussing for the 16mm shots (focus point is shown in the 1st pair of images above). When I got home I tested the lens with both VF and Live View focussing  - the latter proved to be sharper so when I went back to do the 14mm shots, I used LV.

The 16mm and 14mm shots are taken from slightly different positions because there's a metal signpost stuck in the ground immediately behind the 16mm position - to get round it I moved 3ft sideways and 3ft backwards for the 14mm shots.

One possibility I thought of was frost beginning to form on the left & right edges of the front lens element - it was much colder on the 14mm day (-3C plus wind chill). This is unlikely though so I will do some controlled tests as you suggest.

[Gromit44]

I used viewfinder focussing for the 16mm shots (focus point is shown in the 1st pair of images above). When I got home I tested the lens with both VF and Live View focussing  - the latter proved to be sharper so when I went back to do the 14mm shots, I used LV.

PS. The focus point for the LV shots was the same (i.e. the two columns either side of the door in the centre of frame) - I zoomed to 100% magnification on the LCD and used the AF-ON button until the LV focus indicator rectangle turned green.