Understanding MTF Charts

It's a million dollar question for all photographers - 'which lens would fit the best to my requirements?' Well, there are a lot of parameters that play the game when you try to pin down your best option - starting from the weight(mass) of the lens, convenience of carrying it, range of the focal length, whether handholding is possible, if not what type of a support is necessary, the aperture (fast or slow), one touch or two touch focusing, 'IS' or not, 'L' or not, the optics quality, so on and so forth.... oops! I forgot about the most important deciding factor : the price.

But which one should be the key player of the game? Here, except the optics quality of the lens all other parameters are subjective and depend on personal taste. I might find a lens light enough to be my all-time and all-purpose lens and Mr. X might have rejected the same one because of its weight. The costliest lens I can afford is Mr. Y's cheap lens. The only parameter that everybody would agree on is a quantitative measure of the optics quality. Those figures would remain same and equally acceptable for me, Mr. X - Mr. Y and you. That's the reason people rely more on sample images taken by the lens rather than a one page bold review. And the best proof would be a test shot of the calibration chart....

Once I had a great confusion - whether to buy the Canon EF 100-400mm f/4.5-5.6L IS USM or the Canon 400mm f/5.6 USM, though I didn't have enough money to buy either of them :) - just for a future planning. The reviews available in the net made the situation even worse... they were so evenly balanced in numbers. Oh! lucky me - just then I happened to stumble upon a site which is the most useful site (IMHO) to compare between any two Canon EF lenses (of similar focal lengths)... but that took more than an hour to do a complete comparative study - judging the image sharpness and contrast at the centre, far away from center, on a diagonal line, along the perpendicular direction to that etc... and all these for all the aperture settings. I thought if someone could summarize all the results for me in a single graph.... and at that very moment I learned MTF charts are nothing but that - what I just wished for. MTF - Modulation Transfer Function.

Now, let me share here how to understand the MTF charts. (a part has been taken from Canon Glossary )

MTF charts (short for Modulation Transfer Function) provide a graph analyzing a lens' ability to resolve sharp details in very fine sets of parallel lines, and a lens' contrast or ability to provide a sharp transfer between light and dark areas in sets of thicker parallel lines. Fine repeating line sets are created parallel to a diagonal line running from corner to corner of the 35mm frame, directly through the exact center of the image area. These are called sagittal lines, sometimes designated "S" on Canon's MTF charts. At a 90° angle to these, additional sets of repeating lines are drawn, called Meridional (or "M") line sets. Repeating extremely fine short parallel lines spaced at 30 lines per millimeter measure the lens' ability to record fine details, or its resolution.

Even more important in the eyes of many optical designers is the lens' contrast capability, which is measured with thicker sets of parallel repeating lines drawn at 10 lines per millimeter. At first glance, it would appear that any good lens would record lines running parallel to a diagonal drawn across the film with the same accuracy as lines drawn perpendicular to them. However, in real-world testing, this is often not the case. Especially in the Meridional direction, faithful reproduction of fine line sets becomes increasingly difficult as you move away from the center of the image toward one of the corners. And it's a fact that almost all lenses produce sharper results in general near the center of the frame than at the outer edges.

MTF charts display the lens' performance from center to corner. Running along the chart's horizontal axis, labeled 0 to over 20, is the distance from the dead center ("0") of a 35mm image along a diagonal line to the corner of the frame, which is about 21.5mm away. On the chart's vertical axis is a scale representing the degree of accuracy with which the fine and coarse line sets are reproduced, in both the sagittal (parallel to the diagonal of the film format) and meridonal directions. Solid lines on the MTF charts indicate the performance of sagittal lines (parallel to the diagonal of the film), dashed lines are for the perpendicular meridional test target lines.

MTF chart for the Canon 100-400mm f/4.5-5.6 L IS USM Lens

In theory, a perfect lens would produce nothing but straight horizontal lines across the very top of an MTF chart, indicating 100% accurate reproduction from the center of the picture (toward the left of the chart) to its outermost corners (at the right side of the chart). Of course, no such thing as a perfect lens exists from any SLR manufacturer, so MTF charts typically show lines that tend to curve downward as they move left to right (tracking the lens' performance from center to corner of the frame).

Canon's MTF charts give results at two apertures: wide-open (black lines), and stopped down to f/8 (blue lines), with the lens set to infinity focus. While MTF charts don't include many factors that can be important when selecting a lens (size, cost, handling, closest focusing distances, AF speed, linear distortion, evenness of illumination, and of course features like Image Stabilization which may produce superior real-world results), they can indicate to the knowledgeable reviewer some of the optical characteristics they can expect from a particular lens.

So to summarize -

1. The thick solid lines represent contrast of image
2. The thin solid lines represent sharpness/ fine details of image
3. The dashed lines represent bokeh characteristic etc. of image
4. The more parallel are the lines the image quality is degrade less from center to corner. and
5. The higher the position of the lines in the graphs the better is the image quality.

Now if you are a digital user with a smaller sensor (not a full frame) - don't worry about the fall off of the lines at the right side. Your sensor does not capture that part of the image. E.g. if you use a 20D body which has a CMOS sensor of size 22.5mm x 15.0mm, the curves only up to 13.5 (along the X-axis) matter to you - just ignore the behavior of the curves beyond that. That's one big advantage of the smaller sensor.... your image sensor captures only the central part of the image where the sharpness and contrast do not degrade much from center to corner.