September 20, 2014, 06:18:36 PM

Author Topic: MTF Charts, Lenses, and Crop Sensors... a question.  (Read 2305 times)

Wrathwilde

  • Rebel T5i
  • ****
  • Posts: 141
    • View Profile
    • Anarchy Photography
MTF Charts, Lenses, and Crop Sensors... a question.
« on: March 16, 2012, 03:46:53 PM »
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.

canon rumors FORUM

MTF Charts, Lenses, and Crop Sensors... a question.
« on: March 16, 2012, 03:46:53 PM »

Tijn

  • Guest
Re: MTF Charts, Lenses, and Crop Sensors... a question.
« Reply #1 on: March 16, 2012, 06:17:43 PM »
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.
« Last Edit: March 16, 2012, 07:06:01 PM by Tijn »

Wrathwilde

  • Rebel T5i
  • ****
  • Posts: 141
    • View Profile
    • Anarchy Photography
Re: MTF Charts, Lenses, and Crop Sensors... a question.
« Reply #2 on: March 17, 2012, 01:14:06 AM »
Thanks, didn't even cross my mind about the diagonal measurement.

denisccote

  • Guest
Re: MTF Charts, Lenses, and Crop Sensors... a question.
« Reply #3 on: March 17, 2012, 11:09:01 PM »
What does the y-axis represent? Many Thanks

Mt Spokane Photography

  • Canon EF 600mm f/4L IS II
  • ********
  • Posts: 8683
    • View Profile
Re: MTF Charts, Lenses, and Crop Sensors... a question.
« Reply #4 on: March 18, 2012, 01:45:04 PM »
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

canon rumors FORUM

Re: MTF Charts, Lenses, and Crop Sensors... a question.
« Reply #4 on: March 18, 2012, 01:45:04 PM »