MTF Charts

How do you read these charts?

EF 70-200mm f/2.8L USM

ChrisBlaze wrote in post #2222543
How do you read these charts?

EF 70-200mm f/2.8L USM

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Have a read here http://luminous-landscape.com …s/understanding​-mtf.shtml

ChrisBlaze wrote in post #2222543
How do you read these charts?

EF 70-200mm f/2.8L USM

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[GIFS ARE NOT RENDERED IN QUOTES]

MTF stands for "modulation transfer function". The key word there is "transfer". Essentially, it shows how well the detail in the subject transfers to the film. It's measured by how black the black bars are and how white the white bars are on a special test chart. (Actually, they don't use black and white bars, but rather a strips that varies smoothly between black and white according to a sine wave.)

The "modulation" is there to show that the effectiveness of the transfer varies with spatial detail. Some lenses do very well at low spatial density, which indicates lenses that are peceived to have good contrast. Other lenses do less well at low spatial density and better at higher spatial density, revealing lenses that have good resolution but not as good contrast. Of course, some lenses do well at both.

And some lenses do better with bars oriented through the center of the lens compared with bars oriented along the tangents of circles around the center. These differences might affect perceived bokeh.

All lenses transfer information more effectively at the optimal aperture, and so lenses are usually evaluated both wide open and at f/8.

In the charts provided by Canon, they show the theoretical modulation transfer at various points from the center of the frame to one of the outer corners (those are the numbers at the bottom, with 15 being the largest number of interest for those of us with 15x23mm sensors). The modulation transfer is measured in percent, meaning approximately that "x percent of the original image was transferred by the lens at this spatial density".

The thin lines are wide open and the thick lines are at f/8. The solid lines are bars aimed through the center and the dashed lines are bars along the tangents of circles around the center. The black lines are at high spatial density (resolution) and the blue lines are at low density (contrast). The thin lines are wide open and the thick lines are at f/8.

A lens with good center performance and soft corners will show curves that drop precipitously as you move to the right. I would say that corner performance is excellent if both densities are above about 60% in the coners. The lens you showed is quite good, especially at the 200mm end.

Canon's MTF charts are theoretical and are produced by analysis of the optical design, while other manufacturers show charts that are made by actual testing. So, Canon tells you what a design is capable of, while some others tell you what the produced lenses are capable of.

Most typical test-chart tests report transfers that are just discernable on a bar-chart test target such as the popular USAF target. This corresponds to a transfer of about 10%, which is the ultimate resolution of the lens. I prefer MTF results that tell me the maximum spatial density achievable at 10% and at 50% modulation transfer.

The only really useful way to use Canon's charts is to compare different lenses in the Canon line. Charts that show curves that are high and flat represent lenses with more potential than charts showing curves that are lower and that drop off more quickly.

Rick "who suggests a search on Norman Koren" Denney