RIT Alphanumeric Resolution Chart

Several months ago Popular Photography tested new Portra 800
and said resolution improved from "step 21 versus 19" in the RIT
Alphanumeric Resolution Target, based on 4x6 prints. Whazzit?
A picture of the RIT chart is in this PDF file:

http://www.image-engineering.de/de/downloads/Test%20Targets%20O-I-R.pdf

[thanks to Bill C of photo.net for the following]

The description says, "Alphanumeric configuration with frequency
range 1-18 cycles/mm in 25 groups." With this style of target
(equal width black and white bars) a frequency of 1 cycle per mm
means that one white bar and one black bar together fit in a space
1 mm wide. So they are just saying that these are the coarsest
bars in their target, and the finest bars can fit 18 pairs of
black/white bars into a 1 mm wide space.

If you take the 18 to 1 scale ratio and raise it to the 1/25 power,
you get about 1.12; ie, each step going 12 percent higher than
the previous. This is, coincidentally (?) the same rate as the
USAF 1951 res chart, which is typically stated as resolution
doubling every 6 steps, or 2^(1/6)= 1.1225 on your calculator.

Resolving an additional two steps (#19 vs #21) would imply an
improvement of 1.12 x 1.12 = 1.26, or about 26% more resolution.
Roughly 26% anyway (could be anywhere 10% and 40%).

Hi

It's just a resolution target. To get actual values you need to know the
reduction ratio. The progression is the 6th root of 2 so every 6th target is
twice as small as the first. So 3 steps will be about 50% increase in
resolution or in other words from 50 l/mm to 75 l/mm. It could also be 100 to
150 l/mm. Six steps would be from say 100l/mm to 200 l/mm.

Larry