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

[Wrathwilde]

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.

 

[Tijn]

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.

 

[Wrathwilde]

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

 

[denisccote]

What does the y-axis represent? Many Thanks

 

[Mt Spokane Photography]

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