Someone asked me how to read an MTF chart on another forum, so here's my answer I posted there. FYI.

Regards, Art.

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Ok this one's going to take a while to explain, so bear with me...here goes:

MTF stands for Modulation Transfer Function for anyone that wants to know. A set of charts are made for each f-stop for each lens. For a zoom lens a set is also made for each focal length in addition. You have to read the set as a whole to get an idea of the contrast and resolution charactersitcs of a lens.

So far, so good, right?

Basically the MTF chart plots the resolution power of a lens from the center of the image frame (not the center of the lens) outwardly to the far corner of the image frame. It does so by measuring the ability of the lens to distinguish between parallel black lines as the spacing between the lines gets smaller and smaller - ususally from 5 lines/mm to 30 lines/mm - and as the lines move away from the center of the image frame to the corner.

There are two sets of parallel lines for every spacing - meridional (parallel to the line from the center of the image frame to the corner) and saggital (perpendicular to the meridional lines).

Still with me?

OK so the chart itself has two axis. The horizontal (X) axis is the distance from the center of the image frame to the corner. About 21mm in total. The vertical (Y) axis is the degree of accuracy to which the lens distinguishes the individual parallel lines from each other.

No notice that there are several sets of plots on one chart. First off the solid lines are usually the meridional lines and the dotted ones are the saggital lines. Some manufacturers have this the other way around. Next each group of plots represents each line per mm spacing between parallel lines.

"So what, Professor Art?", you ask...

Basically the 'higher' the plots on the y-axis across the entire x-axis, for all line spacings, for both M and S lines and all f-stops, the higher the resolution power of the lens at all parts of the image frame - the 'better' the lens. Probably more $$$ too.

What does this have to do with contrast?

Well, contrast is the ability to distinguish between light and dark. Therefore a lens that offers good resolution of very narrowly spaced lines across the entire image frame is said to be more 'contrasty'. Hence, this is why optical specialists laud the Zeiss and Leica lenses so much. Their MTF charts are almost always near the high end of resolution at all f-stops for all their lenses.

Now one thing I should mention is the that the MTF charts are only typical for a lens. The manufacturers take a (maybe) statisticaly significant sampling of their pre-production lenses and perform these measurements on those lenses, average it out and this is the set of charts you see published. Ideally individual lenses should have their own unique set of MTF charts.

Just like DNA.

Since the manufacturing process of the lenses have variations, even at six sigma quality levels (I would guess even Zeiss and Leica are not at that level - probably high three or low four sigma, Nikon and Canon maybe low to mid three), theoretically, you could buy a very e$pen$ive lens, with known good results, and get poor resolution results. What distinguishes a good manufacturer from a poor one is the capability to produce consistent quality lenses that have characteristics that match these published charts on a repeated basis.

There use to be truly independent reviews in photography magazines once where they would send a lens to an independent lab to perform their own MTF charts. With today's advertising dollar, these independent reports are no where to be found now.

So really, when selecting a lens (or I would dare to say, when selecting a manufacturer), these charts are really what you want to see first. It's the quality of the glass that you can afford that counts in the end.

Hope this helps.