Distortion/quality when using a full frame lens on a crop sensor camera

Question

I am looking for a sharp, affordable wide-angle prime for my Nikon D7000, primarily for landscape shots.

I am really interested in the Samyang (aka Rokinon, Bower) 14mm f/2.8 lens due to its excellent resolution, but have a few questions/concerns:

1) While this lens has a lot of vignetting and lack of resolution in the corners, does the fact that I am using it on a crop-sensor camera mitigate those issues at all?

2) Will the barrel distortion in the center be exaggerated by using with a crop-sensor camera (since I assume it will now pretty much fill the image)?

3) Will lens distortion correction software such as Photoshop or PTLens recognize the lens was used on a crop-sensor camera and correct the image accordingly?

Answer 1

You would actually get at-par to somewhat better image quality using this lens on your cropped sensor, than using a lens designed for crop sensors, in some aspects:

• You would get rid of most of the vignette, as the DX cropped sensor is effectively stopped down 1.23 stops compared to full-frame. (ref. Wikipedia). Thus, from the review link you provided, the vignetting would go from greater than 3eV with the lens fully open on full-frame, to around 1.85eV worst case fully open on your D7000.
• The effective barrel distortion would actually reduce, not increase or be more visible: Try covering the edges of the distortion image from the same review, leaving an effective 1.52x cropped area open, and you will see this in effect.

On the flip side:

• The lens will no longer be a 14mm ultrawide, on the DX sensor it becomes a more pedestrian somewhat-wide, more or less 21.3mm. You might do better getting a 14 or even 16mm wide designed for the DX frame. That would be cheaper as well as sharper: see next point.
• Again quoting that Wikipedia page, "...to maintain the same absolute amount of information in an image ... the lens for a smaller sensor requires a greater resolving power": In other words, finer details would not be as finely defined on the DX sensor, as on the sensors it was designed for.

That last merits explanation: All lens designers are forced to make some design compromises when targeting a given sensor size, and since full-frames have a greater image gathering area, they allow greater leeway in resolving power required of the optics. Smaller sensors require optics of greater "clarity", to oversimplify a bit.

I hope this helped.

Answer 2

The crop will eliminate anything that happens outside of the sensor area. At worst you'll lose the worst of the vignetting, at best you'll lose all of it.

Because the worst of the barrel distortion is right in the middle of the sensor, you're going to be getting all of it. (Edit: Anindo Ghosh correctly points out in his answer that the edges are actually the more distorted area with barrel distortion. The same is true for pincushion distortion, too.)

You can correct for both problems in post, and any decent processing software will understand that the entire image is taken from the center of cropped cameras.

Answer 3

1. Yes, most of the vignetting and corner fuzziness will be gone.

2. The distorsion in the part of the image circle that you use doesn't get worse because you are using a smaller part of the image circle.

3. Yes, for Photoshop at least I know that it takes the camera into consideration, not only the lens characteristics.