Lens resolution

We photographers are often pretty pleased with many of our images. We enjoy the subject, color, composition and detail of our photographs. But since we are our own best critics, we often wonder if we are getting as much detail as possible. This question raises its head more often in macro or extreme close up photography than in other areas. Let’s look at some of the reasons why.

Several factors affect image quality. Quality of the optic itself, camera movement, subject movement, and lighting are several factors that can affect the resultant details of a photographic image. In the macro world where the subject is reproduced at magnifications of life-size or greater, each of these factors plays a major role.

As macro photographers we know that we must eliminate all motion from our subject and optical system. So we bolt everything down, using heavy stands, tripods, electronic releases, mirror lock-up and anything else we can rig to eliminate the possibility of image deterioration from vibrations. We also understand that perfect exposure using quality lighting from a bright, white source like a strobe or speedlight flash is best. And we do everything to prevent flare, glare or unwanted reflections from further deteriorating the image. We are there. Or, are we? What’s left?

The remaining factors have to do with the reproducing quality of the lens itself. Let’s call this lens resolution. This is a measure of the capacity of the lens to reproduce tiny details and may actually be measured by standardized resolution scales expressed as lines per millimeter, or now more frequently, line pairs per millimeter (LP/mm). Basically, resolution quantifies how close lines can be to each other and still be visibly resolved, or distinguished as separate. For example, a resolution of 10 lines per millimeter means 5 dark lines alternating with 5 light lines, or 5 line pairs per millimeter (5 LP/mm) may be visually distinguished as separate.

In the lab we measure lens resolution by photographing a standardized test target. This is known as the 1951 USAF Resolution Target. It is a 2 x 2 inch optical glass slide with test patterns deposited onto its surface. These targets are available from several sources including Edmund Optics. In use, this target is photographed with the subject lens at the desired magnification with all due precautions discussed above. The final image is visually inspected and, with the supplied chart, the resolving power is determined.

The USAF test target is shown below in the top image, along with an actual resolution test photo of a macro lens in my lab.

The test above demonstrates a resolution of 161 LP/mm or 322 lines per mm. This is exceptional quality resolution, even under ideal conditions. This test was performed with a Zukio 20mm F 2.8 macro lens mounted on a bellows (Magnification: 7.4 X on the sensor) and a Nikon D2Xs camera. Backlight  Illumination using a SB-800 speedlight.

So, we can see that many factors affect our images. It’s our job to reduce the potential for problems where we can and invest in the best glass we can afford. It does make a difference.

© Brian Loflin. All rights reserved.


Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s