Is That Lens Sharp In The Corners? No? Are You Sure?

Fujifilm X-T1, 23mm f/1.4 lens @ 1.4; camera/lens on tripod and square to test subjects (click to enlarge)

Often I hear photographers and reviewers remarking that a certain lens is not sharp in the corners. But is it really not sharp?  Or maybe what you are seeing is the result of a curved field lens being tested against a flat subject.  I wrote about curved field lenses versus flat field lenses with some illustrations on the difference here..  If you are interested, please follow the link and read it.

Here is a practical example of what I mean.  The image above was made using the highly touted Fujifilm 23mm f/1.4 XF lens.  The lens is very sharp, even wide open and contains an aspherical element.  I used it to demonstrate that, looking at the image above, one may wrongly conclude that the lens is not sharp in the corners.

For this test, I taped 5 pieces of old newspaper to a vertical surface (checked with a level), then set my camera on a sturdy tripod about 5 ft away.  I carefully measured the distance corner to corner and ensured the camera was exactly level, square and plumb to the the test targets.  I then set the camera for a two second delay, defocused and auto focused for each aperture used.  I made several exposures and used the best ones as examples (they were pretty much identical, in reality).

The image above was made at f/1.4 with the camera's single center sensor focused on the center target, normally the best and most accurate focus sensor.  I made a series of exposures at f/1.4, f/2, f/2.8/ f/4, f/5.6 and f/8.  Every image I made at every aperture was properly focused.

At f/1.4 the center of this top image is very sharp but the corners are out of focus.  Does that mean this lens isn't sharp in the corners?  Not necessarily.  It could be unsharp or it could just means that this lens is a curved field lens, which means that it doesn't focus at the same point across a flat field, such as these test targets.  Normally, a macro lens will be sharp from corner to corner on a flat surface because a macro lens is often used to photograph flat object, such as prints, painting, stamps and such.  It is designed to focus sharply across a flat field.

Areas in red showing where crops were made

I have included crops of the center focus point and the corner to show you what I mean.  The center is sharp, but the corner is not.  The above image shows read rectangles showing A) the center where I focused the lens and made the center crop and B) the top left corner showing smearing.

Area A from center of targets; actual point of focus; f/1.4

The image above is the a 100% crop of the center of the image.  No issue here with sharpness at all!

Area B from extreme upper left corner of targets; f/1.4

However, this image shows the top left corner and it is far from being sharp.  The farther you look toward the top left, the worse the image looks.

However, when I reversed the procedure by focusing on the top left portion of the image, the center went soft, showing me that the lens doesn't focus equally across the frame on a flat plane.  This is a typical behavior of a curved-field lens.

Top left corner now at f/5.6

The way to get sharpness in the corners of a curved field lens on a flat surface is to stop down the aperture to increase the depth of field.  In this test, the softness remained until I reached f/4, which as almost as sharp as the center.  At f/5.6 the corner matched the center for sharpness.  For my particular copy of this lens, at f/5.6 at this distance, the depth of field was deep enough to overcome the curved field design.

The lesson to be learned is to know and understand your gear and don't necessarily take what another says as the absolute truth as the reviewer or photographer may not exactly have a full understanding of their equipment.

Thanks for looking. Enjoy! 

Dennis A. Mook 

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