Comparing Lenses:
Equipment, Image Capture and Analysis
Equipment
Chrome on glass SFRplus Imatest target
Chrome multi-target on the LED light box with a 2-axis spirit level
Spirit level used to ensure the camera is level and parallel with the lightbox
Camera attached to a LPL 7700 series darkroom enlarger with a copy camera attachment (Cat: 40320)
Acquiring Target Images
Nikon Cameras used for image capture have Live View which allows the use of the fine focussing star on the chrome target. With manual focus only lenses live view and camera focus indication in the view finder was used and the sharpest images used for analysis. With Nikon autofocus lenses such as the AF-S VR Micro-Nikkor 105mm f/2.8G IF-ID camera autofocus provided in most cases the best focus. In a trial conducted on the D800 live view often proved to be a lot less accurate possibly due to the magnification available in Live View Mode. With chrome-on-glass targets focussing fully open is quite tricky. With the D500 focus accuracy was very similar between AF and LV modes with AF marginally better. Because focussing is critical to the data obtained with the chrome targets a number of images at each aperture were assessed to obtain the sharpest image possible for analysis. Even with this care there will inevitably be some measurement error in all of the measurements shown on this site.
Image Analysis
The Imatest software automatically analyzes the SFRplus chrome-on-glass image to find the upper and lower bar and the vertical edges of the 35 squares and calculates the geometry of the image. Imatest software is available in a "light" version (Imatest Studio) for individual photographers for 300 USD which I consider a bargain. The DIY light box used for chrome target illumination has a light uniformity of about 1/3 stop using the largest imaging target. As a result lenses with no light fall-off at edges may show a fall-off of 0.3 EV. If more accurate measurement of light fall-off is desired commercial light boxes can be purchased (at high cost!) with a 95% light uniformity. From August 2021 I have been using an Imatest LED light panel with better than 90% uniformity (QC value was 94%) which has recently come onto the market at a reasonable price.
The above image shows an analysis screen from the Imatest software. The slanting edges are used to determine the resolution and also chromatic aberration. This method for using stanting edges for quick MTF calculation is described by ISO12233. Detailed data from the analysis of images generated by Imatest can be displayed in a number of formats one of which is shown below.
A number of different output options are available using Imatest, in this example a 3D plot of MTF50 Cycles per pixel data
Mean resolution (lp/ph), chromatic aberration and light fall-off data is obtained for the centre of the image, part-way to the corners and at the corners of the image using Imatest and graphed using Excel.
The target geometry also allows Imatest to calculate distortion
