Acutance Testing

Step By Step Instructions to

Determine a Lens’ Optimum Aperture

In addition to the camera and lens you wish to test, you will also need:

• Tripod
• Printer
• Software capable of displaying and zooming  two images simultaneously. Possibilities include:
  • Faststone Image Viewer – free and works well
  • Zoombrowser – provided free by Canon with some cameras
  • LightRoom from Adobe – superb, if a bit spendy
  • Aperture from Apple – reported to work well, (I haven’t tested it)  but also not cheap
  • If you have a favorite, especially a free product, please leave a comment on this blog: www.aheld.com!
• Optional but very helpful: remote release.

SETUP

1. Download and print off the Acutance Test Target PDF. (It is a full page of boxes containing x’s.)
2. Tape the target in landscape mode to the wall in a well-lit location.
3. Put the lens you want to test on your camera (don’t laugh;  it’s been done with the wrong lens…)
4. Set the camera to aperture priority mode.
5. Set your ISO to the ISO at which you will be doing most of your shooting.
6. Use RAW mode if you are familiar with shooting in RAW and your viewing software will open RAW files.
7. If testing a zoom lens, set it to the zoom level you wish to test. Leave it unchanged throughout the test.
8. Place your camera on a tripod and move it forward until target over-fills the frame on all sides. Square the camera to the sheet.
9. Set the focus to MANUAL and check it carefully.
10. Set the aperture to its largest setting (this is the SMALLEST number – a value like 1.4, 2.8, or 4.0.  The aperture on modern lenses is controlled from the camera. Older lenses designed for film cameras have the aperture control on a ring on the lens itself.  (And yep!, you CAN use those old lenses successfully, especially for macro photography of static objects where you don’t have to shoot fast.)

SHOOT  AND EVALUATE

1. Take a series of photos, starting with the largest aperture, and working stop-by-stop down to the smallest – usually f/16, f/22. (If using a flash, allow the flash adequate time between frames to fully recycle, especially at the smaller f-stops, where it will be using more power.)
2. Load the images into your computer and open your viewing software.
3. View the image taken at f/8. Beside it, load the first image taken – probably f/2.8. Display both at 1:1 magnification. (This zooms in so one pixel of the image file shows as one pixel on your monitor.) Adjust your view of both images so you are looking at the same corner of the image in both windows.
4. Compare the sharpness of the images. The f/8 image should be noticeably sharper.
5. Close the largest f-stop image (probably f/2.8) and in its place open the next smaller one (probably f/4). Compare again.
6. Keep comparing and changing images until there isn’t a detectable difference between the f/8 “standard” image and the second image. Note the f-stop at which it was taken. This is the beginning of your sweet spot. Now start working UP from f/8 doing the same comparison with each image. At f/11 you probably can’t tell any difference. At f/16 you will probably see the image begin to degrade. By f/32 you almost certainly will.

EXAMPLES

I recently ran this test on my favorite macro lens – a micro-NIKKOR 55mm purchased in the late 1970’s. Click on the image below to see a 1:1 display clipped from the upper right corner of the chart. It shows the noticeable decrease in acutance between f/8 and f/32, even on a lens designed specifically to work well at small apertures.

At first glance this may not seem like an earth-shaking change – perhaps an acceptable trade-off for the increased depth of field. However, this slight fuzziness may destroy the very detail that makes macro photographs exciting to view. Click on the image below to enlarge it and see the difference between f/11 and f/22 on the same lens when used to photograph a slice of asparagus. Look carefully at the fiber-ends along the lower edges of the cut in the upper left third of the images. Note how the crisp detail from the tiny strands disappears at f/22.

As you can see from the 1:1 clipping, there is a distinct decrease in edge sharpness and contrast as the f/stop number doubles.

RESULTS!

Once you complete your tests, you know the range of f-stops at which this lens is sharpest.  And you’ve seen for yourself how sharpness decreases as you move beyond the sweet spot to either larger or smaller f-stops. Whenever you shoot anything (be it macro, landscape, or portraits) where you need to get a pin-sharp image, be sure to use this lens within that sweet spot. (This is another good argument for being aware of the settings your camera is picking when you shoot in an auto mode.)

Useful links with more information

• Wikipedia on F-STOPS
• Wikipedia on ACUTANCE
• Cambridge in Color tutorial on SHARPNESS (resolution + acutance)
• FastStone.org: Free image viewing software

Happy shooting! May all your macros be as crisp as fresh carrots and as sharp as broken glass!