the last word

Photography meets digital computer technology. Photography wins -- most of the time.

  • site home
  • blog home
  • galleries
  • contact
  • underwater
  • the bleeding edge
You are here: Home / Lens screening testing / Theory of the test

Theory of the test

You can do the test perfectly well without understanding why it works, but knowing what’s behind it may make the whole procedure make more sense to you.

The basic idea is to focus with the target in the center of the image, then recompose so that the target is in the four corners of the image and optionally, at the middle of each edge, then compare sharpness.

But you have to be far enough away from the target. Here’s why.

Above is a crude diagram looking down with one target on-axis and one at the far right edge of the image. Because we’re assuming a rectilinear lens, in which straight lines in the image map to straight lines on the sensor, both targets are in focus. But what happens if we dispense with the second target, and skew the lens so that the first one is at the same place in the image?

Now the target is out of focus. Specifically, it’s too close. This isn’t a problem at extreme distances. Infinity is infinity no matter where in the frame it is. Depth of field (DOF) can cover up this focusing error. In order to put some numbers on it, I wrote a computer program to compute the diameter of the circle of confusion (CoC) for this kind of misfocus at the very corner of the sensor for lenses between 18 and 100 mm on a full frame (FF, 24×36 mm)) sensor at f/4 with a target distance of 20 meters.

We’d like the CoC to be quite a bit smaller than the pixel pitch. FF cameras of high resolution have pixel pitches on the order of 4.5 micrometers (um). Halving the CoC half that should be good enough. You can see from the above graph that we’re in good shape at 20 meters for lenses up to about 35 mm, but that beyond that, we need to be further away.

I created a set of curves that show the distance required for a 2.25 um CoC diameter for a full frame, 4.5 um camera for focal lengths between 12 mm and 100 mm.

 

This show you how far you need to be from the target. You can be farther than that without causing problems as long as your target is big enough on the sensor and there’s not so much air between the camera and the target that atmospheric effects are important. Stay under 200 meters, and you should be fine unless you’re shooting over heat sources. 

I’ve prepared versions of the above chart for MFT, APS-C, full frame, and 33×44 mm sensors, for three ranges of focal lengths. You can find them here.

March 2023
S M T W T F S
 1234
567891011
12131415161718
19202122232425
262728293031  
« Jan    

Articles

  • About
    • Patents and papers about color
    • Who am I?
  • Good 35-70 MF lens
  • How to…
    • Backing up photographic images
    • How to change email providers
  • Lens screening testing
    • Equipment and Software
    • Examples
      • Bad and OK 200-600 at 600
      • Excellent 180-400 zoom
      • Fair 14-30mm zoom
      • Good 100-200 mm MF zoom
      • Good 100-400 zoom
      • Good 100mm lens on P1 P45+
      • Good 120mm MF lens
      • Good 18mm FF lens
      • Good 24-105 mm FF lens
      • Good 24-70 FF zoom
      • Good 35 mm FF lens
      • Good 60 mm lens on IQ3-100
      • Good 63 mm MF lens
      • Good 65 mm FF lens
      • Good 85 mm FF lens
      • Good and bad 25mm FF lenses
      • Good zoom at 24 mm
      • Marginal 18mm lens
      • Marginal 35mm FF lens
      • Mildly problematic 55 mm FF lens
      • OK 16-35mm zoom
      • OK 60mm lens on P1 P45+
      • OK Sony 600mm f/4
      • Pretty good 16-35 FF zoom
      • Pretty good 90mm FF lens
      • Problematic 400 mm FF lens
      • Tilted 20 mm f/1.8 FF lens
      • Tilted 30 mm MF lens
      • Tilted 50 mm FF lens
      • Two 15mm FF lenses
    • Found a problem – now what?
    • Goals for this test
    • Minimum target distances
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Printable Siemens Star targets
    • Target size on sensor
      • MFT
      • APS-C
      • Full frame
      • Small medium format
    • Test instructions — postproduction
    • Test instructions — reading the images
    • Test instructions – capture
    • Theory of the test
    • What’s wrong with conventional lens screening?
  • Previsualization heresy
  • Privacy Policy
  • Recommended photographic web sites
  • Using in-camera histograms for ETTR
    • Acknowledgments
    • Why ETTR?
    • Normal in-camera histograms
    • Image processing for in-camera histograms
    • Making the in-camera histogram closely represent the raw histogram
    • Shortcuts to UniWB
    • Preparing for monitor-based UniWB
    • A one-step UniWB procedure
    • The math behind the one-step method
    • Iteration using Newton’s Method

Category List

Recent Comments

  • JimK on Fujifilm GFX 100S pixel shift, visuals
  • Sarmed Mirza on Fujifilm GFX 100S pixel shift, visuals
  • lancej on Two ways to improve the Q2 handling
  • JimK on Sony 135 STF on GFX-50R, sharpness
  • K on Sony 135 STF on GFX-50R, sharpness
  • Mal Paso on Christmas tree light bokeh with the XCD 38V on the X2D
  • Sebastian on More on tilted adapters
  • JimK on On microlens size in the GFX 100 and GFX 50R/S
  • Kyle Krug on On microlens size in the GFX 100 and GFX 50R/S
  • JimK on Hasselblad X2D electronic shutter scan time

Archives

Copyright © 2023 · Daily Dish Pro On Genesis Framework · WordPress · Log in

Unless otherwise noted, all images copyright Jim Kasson.