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How to read MTF charts
#1
Can I ask Klaus to explain me how to read the resolution charts? In other terms, when I look at the Leica Summilux 25mm f/1.4 MTF and read 2658 LW/PH at f/2, what that means in terms of L/mm? Should I divide that number by the height of the sensor which is 13,5mm or should I divide it by 25,4 assuming that the value refers to an height of 1" ? Considering that the circle of confusion for a good eye is about 4-5 L/mm at 25 cm distance, this converts into 60-70 L/mm on the 4/3 sensor which should be considered as a sort of decence threshold for a good lens, is that right ?



Now, when I look at an MTF chart of a full frame lens such as the Nikkor AF-S 24-70mm f/2.8G ED (FX) at 24mm f/4, I read 3988 LW/PH. If I convert this number using the picture height of a FF sensor I will divide it by 24mm and will get 166 L/mm which is almost twice the resolution of the Leica (I don't think this is correct). On the other hand if I divide it by the unit (1") I will still get an impressive 157 L/mm (still unrealistically higher than the Leica). Where is the error in my assumptions?



Thank you

Carlo Borra
#2
[quote name='pesitalia' timestamp='1290072721' post='4233']

Where is the error in my assumptions?

[/quote]



Somewhere in the Leica part <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Wink' />



LW/PH is line widths per picture height, where PH is in pixels. So, to convert to L/mm, you need to divide by the physical height of the sensor.



For the Leica that results in 2658/13.5 ≈ 197 L/mm



Nikon, as you calculated: 166 L/mm.



So, the calculated resolution value is actually higher with the Leica lens.



However: you simply cannot compare resolutions that way. There are many parameters that can influence the MTF results, including the anti-alias filter of the camera as well as the RAW converter used. So, unless you have a really static and identical test setup (and with two different cameras obviously you don't) you cannot compare the measured and published results here directly (regardless of the unit used).



-- Markus
Editor
opticallimits.com

#3
If you are interested in reading MFT in general, there is an excellent report about just that at Zeiss Website. Go to http://www.zeiss.de/cln and have a look at the archive, CLN No 30 and 31. They are available in german and english.



Christian
#4
[quote name='TheChris' timestamp='1290090340' post='4252']

If you are interested in reading MFT in general, there is an excellent report about just that at Zeiss Website. Go to http://www.zeiss.de/cln and have a look at the archive, CLN No 30 and 31. They are available in german and english.



Christian

[/quote]



Thank you. For the moment I'm happy just to be able to convert LW/PH into L/mm.

Carlo
  


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