John Paul Caponigro's "Testing for the Ideal Lens Aperture"

March 27, 2011

John Paul Caponigro's "Testing for the Ideal Lens Aperture"
… optimum image sharpness is typically achieved with neither with the lens wide open or stopped all the way down...

A question that many photographers ask is, “what aperture on my lens delivers the sharpest images?”  With over 60 different Canon lenses for EOS cameras, not to mention those from other lens makers, there’s no one simple answer.  The best way to find out – test your lenses yourself!  It’s easy to do, and will provide you with a valuable insight into getting the best possible images.

It’s a fact that SLR lenses perform differently at different lens openings.  In nearly all instances, the widest possible aperture will not generate the maximum sharpness and contrast that the lens is capable of.  (Keep in mind that some high-end super-telephoto lenses are designed to provide outstanding performance wide-open, and you may see little improvement in stopping down.)  With most lenses, as the lens aperture is closed down several stops from wide-open, typically there’s an improvement in overall sharpness, contrast, and evenness of illumination from center to corner.

A false assumption is widely made that stopping down to the smallest possible lens aperture, such as f/22 or f/32, increases image sharpness even more. In fact, a phenomenon known as Diffraction begins to progressively reduce actual image sharpness and contrast once the lens aperture is stopped-down to a certain point (where this occurs varies from lens to lens – a good reason to test your lenses, as described below).  As the lens opening gets progressively smaller, images now actually become softer and have less overall detail and contrast, as you near a lens’s smallest possible aperture setting.  So, optimum image sharpness is typically achieved with neither with the lens wide open or stopped all the way down. You may hear that most lenses deliver optimum sharpness at 2-3 stops lower than wide open. You may hear that most lenses deliver optimum sharpness at f11 or f16. Which is it? Individual results vary. Find out which f-stop delivers the optimum sharpness with your lens/camera system by testing it.

Important considerations when selecting an aperture for actual pictures:

Light in the scene and final exposure settings needed
Even if your tests indicate that your lens’s best sharpness comes at a smaller aperture, in dim lighting conditions, it may be necessary to shoot wide-open, or with the aperture only stopped-down slightly, to get adequate exposure and/or fast enough shutter speeds for adequate sharpness.  Conversely, shooting a back-lit scenic with the sun in the frame, it may be necessary to stop the lens aperture all the way down to control exposure, even if this isn’t where your lens is at its optical best.  Knowing at which aperture your lens is sharpest is always valuable, but doesn’t relieve you of the need to think about all the conditions facing  you as you shoot.

Depth of Field. 
Smaller f-stops produce greater depth of field, allowing objects at a wide range of distances to all be in focus at the same time (Fig. 4).  Conversely, a wide lens aperture is useful to limit sharp focus to a single plane in the image (Fig. 1).  Regardless of which lens aperture is actually your sharpest, your need for a lot or a limited depth-of-field may dictate shooting at a lens opening other than your sharpest.

Vignetting
Wider f-stops may cause the intensity of light reaching the image plane to fall off toward the edges of the picture.  This can be especially noticeable in scenic images with broad areas of sky, or other shots with plain, continuous light-colored backgrounds.  It’s often beneficial to use middle or small apertures (f/11, f/16, etc.) if this is a concern.

Optical Aberrations.
While lens designers strive for the best possible image quality at all lens apertures, it’s a fact that all lenses have some residual image aberrations that are impossible to completely eliminate.  These often are most visible at the lens’s widest lens aperture (Fig. 1), and sometimes can be minimized by just stopping down one f-stop, if conditions permit.

Testing your lens for its optimum shooting aperture:
Take different pictures at each f-stop on your lens, from its widest to its smallest.  For simplicity, use full-stop apertures; there’s little need to set in-between apertures.  Be sure to take test shots of the same subject with high frequency detail and crisp contours. Make sure camera motion does not affect your results by using a tripod and/or high shutter speeds. Make sure subject motion does not affect your results by photographing a stable subject and/or using high shutter speeds (outdoors, remember that wind-blown subjects can be a problem). Compare the results. If you’ve shot RAW images for this test, when evaluating results make sure digital files are processed identically, turn off software sharpening and noise reduction, and view the same area of each image/file at 100% magnification.  Whether you shoot RAW or JPEG images, be sure your camera settings for image quality (sharpening, noise reduction, etc.) are where you’d typically have them set for actual picture-taking.  Test each lens you use and make a record of your results. Test zoom lenses at various zoom ratios, at a minimum fully zoomed in and out.

Once you know the aperture that produces the sharpest image with your lens/camera system, you may wish to favor it. Look closely at the examples shown here.  The best results with this particular lens occurred at f/11 (Fig. 2).  But as mentioned before, this doesn’t mean that from now on, every picture has to be taken at f/11 with this lens!  Don’t be slavishly faithful to this “best” shooting aperture;   shutter speed and depth of field may be equal or more important factors in specific situations.

Remember, aperture is only one of many variables that contribute to image sharpness.

Here is a short list.

•    Subject motion
•    Camera motion
•    Lens resolution capability
•    Sensor or film resolution capability
•    Sensor or film flatness
•    Aperture with fewest lens aberrations
•    Aperture with greatest depth of field
•    Noise or grain
•    Image contrast
•    Sharpening routines (digital or analog)
•    Resolving capability of output device
•    Resolving capability of substrate

 

John Paul Caponigro is an internationally respected fine artist, a member of the Photoshop Hall of Fame, author of Adobe Photoshop Masterclass, the DVD series R/Evolution, and a renowned Canon Explorer of Light. Learn more about him and his work at The Canon Digital Learning Center, or at www.johnpaulcaponigro.com

The CDLC contributors are compensated spokespersons and actual users of the Canon products that they promote.

All images are copyright JP Caponigro

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