A practical field test for camera lenses


My primary criteria for testing cameras and lenses it to see how they perform in actual shooting situations.  As a commercial stock photographer, I am required to produce images of at least 50mb in size at 300dpi.  These images are expected to have no distortion when viewed at 100%.  While there are many detailed camera and lens reviews on the internet, I have found that there is simply no substitution to putting a camera or lens through tests that more accurately simulate real life situations.  One such test I devised I call the "Brick Wall Test".


The Brick Wall Test looks for three specific aberrations in a lens: rectilinear distortion, edge sharpness, and vignetting.  


Brick Wall Test Setup

The camera is set up approximately 10' from the wall and centered with a level making certain that the camera back is perfectly parallel to the wall.  If the camera back is not parallel, the edges of the frame will be slightly out of focus when the camera is focused in the middle. 

The wall should look like this through your viewfinder.  All lines should be parallel indicating that the camera back is parallel to the wall.  Ideally, your finished photo should look like this also.
Next I run a series of exposures in one stop intervals starting with the most open f/stop of the lens and going down to around f/8 or f/11, which  would be the practical range for most lenses I use.  When testing a zoom lens, I run the same test over again on various focal lengths. 
Testing for corner sharpness
One of the most prevalent aberrations with lenses for digital cameras, particularly prevalent on full-frame digital cameras, is corner softness.  This is most apparent with wide angle lenses.  It is almost impossible to correct in post-processing so we want to minimize this fault.  As a lens stops down, the corner sharpness tends to increase.  So I want to know the maximum aperture a lens can be used without any corner softness.  Stopping down to f/5.6 or f/8 will usually do the trick, but it does vary from lens to lens.

The blue circle in the image above shows where you want to look for softening of the corner edges.  The center is tack sharp but the sharpness falls off at it approaches the corners.
In situations with a center subject and an out of focus background this lens aberration does not matter.  When shooting flat buildings, as often happens in travel photography in cities. corner sharpness is important.
Rectilinear Distortion
Rectilinear distortion means that lines that are normally straight show up curved on the image.  The two most common aberrations are barrel distortion where the edges of the image frame bow outward at the center, and pin cushion distortion where the edges of the image frame bow inward at the center.  Both of these are easily corrected in post-processing software.  Keep in mind, however, that the correction does involve cropping into the image.  This cropping  results in diminishing the effective wide angle lens focal length in addition to losing some of the image size -- neither of which are good things.
This image illustrates pincushion distortion where the edges bend inwards
Some lenses produce a combination of barrel and pin cushion distortion at the same time.  Most commonly the edge of a frame will go up in the middle (barrel) and down on the edges (pin cushion).  This is almost impossible to correct easily with conventional software.

This image illustrates several distortions.  The darkened corners show vignetting. The middle of the top and bottom edge curve outwards showing barrel distortion, while the ends of the top and bottom curve inwards in a pin cushion distortion.  An image such as this would not be fun to correct!
Vignetting means that the image darkens towards the corners.  This aberration changes with lens opening.  It is easily corrected with post-processing software with no detrimental side effects.  On many new digital cameras lens vignetting is fixed within the camera through firmware that recognizes the lens being used and makes necessary corrections.