# MTF Charts, Lenses, and Crop Sensors... a question.



## Wrathwilde (Mar 16, 2012)

Since MTF Charts are from the center of the lens to the outer edges and Canon usually shows their lenses out to about 22mm from center, am I right in assuming that a FF sensor (36mm across) would only be applicable to about the 18mm portion of the MTF chart, and a crop sensor, say the 7D's 22.3mm, would be applicable on the chart to the area just above 11mm?

I've included a mock up, shown in the attached image, where the red line would indicate the edge of the 7D's sensor, and the green line the outer edge of a full frame sensor. I'm just wondering if my understanding of this is correct, and that the 7D is using a much more uniform part of the lens than a FF sensor.


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## Tijn (Mar 16, 2012)

Yes and no. Your lines are in the wrong spot, but you can indeed "crop" the crop-sensor out of that full-frame sheet.

This chart is for a 35mm sensor. That's because the sensor is 36 x 24mm in size. The diagonal is sqrt(36²+24²) = around 43,26mm. The chart left side (0mm) means 0mm _from the center of the sensor_. Seen from the center of the sensor, the furthest away part of a full-frame sensor is half the diagonal away, or ~21.63mm. And that's exactly how far the chart goes.

Canon's crop sensor is 22,2 x 14,8mm in size. Diagonal 26,68mm. Half a diagonal 13,34mm. So you can set the fullframe line to the far right edge of the graph, and the crop sensor line at 13,34mm. To the left is the APS-C relevant performance.

Most or all lenses are sharpest in the center, and weakest at the farthest distances from the center (the corners of the image/sensor). Crop sensors crop away the edges, so they get a bigger benefit of the sharp center of EF lenses. Lenses that give soft corners for full-frame users may be sharp from corner to corner on a crop sensor. This is often referred to as the "sweet spot" effect.


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## Wrathwilde (Mar 17, 2012)

Thanks, didn't even cross my mind about the diagonal measurement.


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## denisccote (Mar 18, 2012)

What does the y-axis represent? Many Thanks


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## Mt Spokane Photography (Mar 18, 2012)

denisccote said:


> What does the y-axis represent? Many Thanks


 
It represents MTF (Modulation Transfer Function), which can be considered as another term for contrast. We often perceive contrast as sharpness, but there is a difference.

This link will help explain it.

http://www.cambridgeincolour.com/tutorials/lens-quality-mtf-resolution.htm


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