What does MTF mean?

Could you guys explain to me how to read the MTF graphs that are on the lens

PDFs on the Leica website? I am trying to compare my 75 Cron with the 50 Lux -

Thanks

Discussion of MTF testing:

 

 

http://www.photodo.com/topic_108.html

 

 

Limitations of MTF testing:

 

 

http://contaxg.com/archive/articles/beyond_the_mtf.htm

Nothing.

 

Get out and shoot photos.

A brief, good explanation can be found in "MTF chart: How to read" at

 

http://www.usa.canon.com/consumer/controller?act=GlossaryAct&fcategoryid=216&alpha=MNO

"Subject: What does MTF mean? "

 

In other words, WTF is MTF?

MTF = Modulation Tranfer Function

Good luck...anybody can read the charts, the art is in understanding the nuances. See Irwin Putts' analyses...I liken a newbie trying to understand the charts to a 10 year old swimmer deciding to swim the English Channel because he/she just got their swimming certificate from the Red Cross. Somewhat of a gap there.

I think I can add a little. A transfer function of anything is a statement of how what comes out of that thing compares to what went into it. So anything that has an input and an output has a transfer function. A lens has an input: the light entering it. And it has an output, which is the light exiting from the rear element. The MTF graph compares the contrast of the image as it exits the lens, compared to when it entered it.

 

The test target being photographed has black bars separated by white spaces. If we assume that the black bars on the chart are as black as they can be, and the white spaces are pure white, then we can treat the target as having 100% contrast. If the image exiting the lens was still pure black and white, then we could claim 100% contrast for the lens as well.

 

But that won't happen. Not in a real lens. There has to be some reflection, some absorption, some imperfections--so that a little white gets mixed into the black and vice versa. So now the black bars are really a dark gray, and the white ones are a light gray. Now the contrast is no longer 100%. If the lens is really good, maybe it will be 99% or 98%. Still some very high, almost perfect number. When the going gets rough, the contrast could drop to 50%, 20%, or less. If it gets too low, the white and black bars will look about the same, and you won't be able to tell them apart. When things get that bad, we say that this lens can no longer resolve the image at that spacing of the target lines (line frequency).

 

MTF graphs I've seen show how the contrast varies across the width of the image, from center to corner. Since lenses are sharpest in the center, the graph line tends to droop lower as we go from left to right--center to corner.

 

So why is there a whole family of lines across the chart? They represent the performance at several different spacings of white and black lines. The wider the spacing, the easier it is to keep the black and white separate, so the better the lens looks on paper. The next set of graph lines ups the ante with target bars that are more closely spaced. This continues for maybe four different target spacings (line frequencies).

 

So the MTF graph shows us something about both the contrast and the resolving power of the lens. It does this by treating resolving power as as a by-product of good contrast--an idea that makes sense to me.

 

Where there are both solid and dotted lines, they show the difference between the performance with lines that are aligned radially center to edge, vs. tangentially (at right agles to the radial ones).

 

A perfect (unobtainable) lens would have all lines be straight across the top under all conditions (all contrast, all the time). Real lenses can't do that, but the better the lens, the less the graph lines will sag down into the soup at the bottom of the chart.

 

Or at least that's what I've been able to figure out so far, as of 3:38 CST, on Wednesday, August 16, 2006. Hope it helps.

Rob, you explained that really well.

You have excellent teaching skills Rob!

Thanks Guys and Rob you for an easy to understand description.

It\'s the complement to bokeh. If you can focus a $3000 lens, then you can claim a drum full of MTFs. If you are falling down drunk at a wedding and snap off a roll of pants cuffs, then you have good bokeh. Most of their marketing can be contained by BVDs.

I'd give Rob a Hero icon for that response alone!

Hi There

 

MTF = GIGO.

 

Worrying about the MTF of a lens is the modern equivalent of the medieval contention of the number of angels dancing on a pinhead.

 

Cheers