In this exercise, photographs were taken of a Koren 2003 target. All photos were taken with a Nikon D2x in NEF (RAW) mode. A Nikon 70-200 f/2.8 VR lens was set at f/8 and with an equivalent focal length of 200mm. Each image was passed through PhotoShop to create a JPEG file at maximum quality settings for import into the ImageJ freeware Java-based analysis package, described below. The aim of this test is to learn the resolution fall-off characteristics of the Kenko tele-converters.
Here is the Koren 2003 target. The yellow line represents the band upon which the analysis curves are based. This band is a sine-wave variant which spans between full black and full white. Subtleties in the imaging resolution capabilities are best revealed in this band (as opposed to the square wave band).
Data & Analysis
The ImageJ software provides for a means to make image-based, pixel densitometer measurements. No absolute distance scale has been maintained (i.e., no attempt was made to obtain absolute MTF and LPI measurements). The results shown here provide only a relative comparison.
The X-axis is normalized scale. Each photo was taken from the same distance at an equivalent focal length. This provides an image that spanned ~3/4 the distance across what was seen in the viewfinder. The Y-axis (grey values) gives a measure of grey-value; with 0 indicating absolute black and 255 indicating absolute white. Illumination was provided by an SB-800 used in TTL mode and bounced off a white ceiling.
In the plots below, the grey values cover 1 to 144, a span that indicates the illumination used did not yield perfect white, but only a light shade of grey. This does not detract from the usefulness of the results.
Ideally, in these graphs, a better lens will retain a strong shoulder-to-shoulder character moving from left to right (i.e., from lower to higher resolution). The more rectangular this character is, the better the resolution capability.
The top chart shows the results for the 70-200 f/2.8 VR lens without any converter in line. Blue lines have been drawn to indicate the resolution fall-off cone. These same blue lines were copied and pasted into the subsequent graphs in order to form a comparison of resolution degredation as converters are put in line.
The middle graph shows the results when a 1.4X Kenko Pro 300 converter is put in-line with a 70-200 f/2.8 VR lens. The zoom ring was backed-down to a focal length reading of 143mm, which provides a 200mm equivalent. Resolution degredation is evident on the black-side of the grey value axis (bottom part, shaded in yellow). It is not especially severe, but is indicative of a slight softening in sharpness, as compared with the example without converters in line (top graph).
The bottom graph shows the results when a 2X Kenko Pro 300 converter is put in-line with a 70-200 f/2.8 VR lens. Here, the zoom ring was set to read 100mm, providing, again, a 200mm equivalent. Further resolution degredation is evident on both the black and white sides of the grey value axis (shaded in pink). In this case, actual resolution failure is being measured on the image (as indicated by a randomly fluctuating mid-grey value at the right end of the X-axis). This is indicative of considerable softening that occurs when using the 2X converter.
Concluding, the 1.4X converter provides a tinge of softening, but of a level that, in my experience, is reasonably acceptable. On the other hand, the 2X converter adds considerable softening and loss of resolution. Again, in my experience, I have found such a characteristic only tolerable in the most contrasty of lighting situations. On days with marginal lighting, or under overcast skies, the 2X converter generally gives unfavorable results.
Last updated: October 25, 2005