MTF50 in Cam Scan of USAF... How?

[ColSebastianMoran]

Looking for help. What the right way to assess MTF 50 in a camera-scan of the USAF 1951 glass resolution target?

Background:
- I have been testing lenses for camera scanning at 1x to a 3.8µ sensor (Equivalent to 50MPx FF, I'm using a 24MPx APS Sony A6000 body).
- I know how to assess ultimate resolution; are the line pairs distinguished?
- But how to determine which set of lines show MTF50

Here's my approach so far:
- Shoot the test target at 1x, I set the camera at +1.something EV
- Load into LightRoom
- Adjust WB on the clear area of the test target (it's a bright gray)
- Adjust Exposure to bring the white area to 92 on the 0-100 LR scale
- With good lenses, this leave the big black areas at <10, same scale.
- So, full contrast is 90-10 = 80 LR points.
- Find the set of lines with 40 LR points difference between the center line and the adjacent white areas.

Generally, the discrimination is pretty good. One line set is clearly closest to 40 point difference; adjacent line sets are more and less than 40 points difference.

Here's an example, 1x to a 3.8µ sensor with a very good lens, the 70 Sigma Macro ART. I read the extinction resolution as G6-E3 = 80lp/mm (maybe G6-E4). This is among best lenses I've tested.

The MTF 50 for this same file I read at G4-E6 with 88 and 45 LR points; This is 29 lp/mm, just under 40% of the extinction resolution.

My concern: The IMATEST results on coinimaging.com generally have MTF50 at about half the extinction resolution.

Am I mis-using the LR brightness points? Any suggestions to making a reliable and comparable assessment of MTF50 for macro lenses at 1x?

For those interested:
- Here's a link to the full RAW file
- Here's a link to the XMP for my WB and Exposure treatment of the file.

 

[SCHWARZZEIT]

Richard,

the most common method to determine SFR or MTF of an imaging system is through slanted edge targets. There are a couple of transmissive test charts with slanted edge features such as the ISA Standard Film Format Targets that are available in 35mm, 6x6cm and 4x5” size at $1025 each or $2500 as a full set, or the Imatest SFR+ target that is available in various sizes and substrates for analysis in Imatest. But you can also use your USAF 1951 glass target at a slight tilt when you make the test shot. You can then have the test shot analyzed in a software such as MTF Mapper which is free. MTF Mapper automatically finds rectangular shapes, analyzes the edges and gives you SFR / MTF information on these edges as well as some data on chromatic aberrations. The edges of the larger lines of your USAF 1951 target which are black rectangles should be picked up by MTF Mapper. The very clear edges of a chrome-on-glass resolution target are great, but the very high contrast is not ideal according to this Imatest article.

I expect to get a low contrast chart later this month and will test if there is a significant difference vs. high contrast MTF results when properly exposed.

-Dominique

 

[ColSebastianMoran]

Hey, a big thank you. Will investigate.