Sharpest Macro Images Depend on Aperture

Written by Alan Dewey

Sharpest Macro Images Depend on Aperture

Here is a short article which illustrates that very small apertures may result in images with lower quality.  As we have learned elsewhere, small apertures are indicated by a high f/number; a large aperture has a small f/number.  An aperture of f/22 is smaller than an aperture of f/16; the higher the number, the smaller the aperture.

We choose the aperture setting of our lens based upon our desired depth-of-field.  The range of aperture settings we may use is determined by the sensor sensitivity (ISO number) and available light.  For a shallow depth of field, we use a low f/number (large aperture) and for much greater depth-of-field, we use a high f/number (small aperture). 

Unfortunately, the desire to obtain the most depth of field possible has a deterimental side effect; that side effect is called "diffraction".

Is small aperture diffraction noticeable?

This is the question that Palm Beach County diver Tim Krutz asked during a recent underwater photography workshop by Reef Photo & Video at the Blue Heron Bridge.  Tim took advantage of the workshop photo-dive's two-hour time limit and shot the same subject at two vastly different apertures.  Other than reducing flash output for the larger aperture, he attempted to keep everything identical between the shots.  Keeping in mind that he was shooting a living creature and was not using a tripod, the photos are remarkably similar.  His camera was a Nikon D300 with Nikkor 105mm macro lens in a Sea & Sea housing with flat port.  The Nikon D300 uses a 12.3 megapixel sensor.

His goal was to experience first hand, and to demonstrate, if possible, that smaller aperture is not always better.  When we are first introduced to lens theory we learn that smaller apertures (higher f numbers) means more depth of field.  However, there is a phenonemon known as 'diffraction' that occurs when light passes the iris blades (the aperture adjustment.)  When the aperture gets to a certain size, this effect becomes noticeable.  There are numerous articles to be found (short list provided below) that explain the theory of this effect and provide examples for illustration; however here we have a real life example of the effect on underwater photography.

Here are Tim's two shots.  These images, when scaled down to only 900 pixels wide, below, do not show a dramatic difference in quality, but it is there.  However, when viewing (or printing) these images larger, the effect becomes not only noticeable, but rather distracting.

Tim took this photo at f/36.  At first glance, this photograph appears quite sharp, well focused, and an example of a image we would all be proud to show our friends.

Tim took this photo at f/22.  You may notice that there is slightly less depth of field in this image, compared to the image above; this is as expected when shooting at f/22 compared to f/36.  What you may not have expected, though, is that the image is actually clearly sharper everywhere within the depth of field.

Let's take a closer look.  The following shots are "1:1 crops".  Now we can see how much difference in image quality there is by looking at only a small section of the image.

(1:1 crop means is that there was no scaling of the image here.  We cropped the full size image to 450 x 450 pixels (the image should consume 450 x 450 pixels on your monitor.)

f/36, cropped to 450px wide, image not scaled. f/22 cropped to 450px wide, image not scaled.
Note how sharp the focus is in the f/22 image (on the right).
f/36 cropped to 450px wide, image not scaled. f/22 cropped to 450px wide, image not scaled.

So if f/22 produces a sharper image than f/36, why does the aperture adjustment go all the way to f/36?  Though in our example here, the f/22 shot would likely be preferred when making a large print to hang on a wall as art, a person desiring to document a creature and its environment may choose to accept the decreased sharpness in order to obtain the greater depth of field.  Also, when shooting above water during daylight, there may be instances when a very small aperture is needed to attain the desired shutter speed.

Alex Mustard Diffraction Test

We have seen, above, that aperture diffraction is a now a real concern for underwater photographers.  As digital cameras evolve with higher and higher resolution sensors, the phenonemon transitions from an curiosity to a significant factor in image quality.  When Alex Mustard tested and reviewed the Nikon D7100 in December 2013, he chose to illustrate that aperture diffraction clearly is a concern with large prints when using a 24 megapixel sensor. 

Here is a composite image at 1:1 crop at aperture settings ranging from f/8 to f/45.  For more information, read the full article at Wetpixel.com.


1:1 crop (100% crop) by Alex Mustard, reprinted with permission. 
Read Alex Mustard's full article at Wetpixel.com
Nikon D7100, Nikon 105 mm. Subal ND7100 housing. 2 x Inon Z240 strobes.

Additional Reading: