Canon 70D Focus - Printable Version +- Opticallimits (https://forum.opticallimits.com) +-- Forum: Forums (https://forum.opticallimits.com/forumdisplay.php?fid=4) +--- Forum: Just Talk (https://forum.opticallimits.com/forumdisplay.php?fid=17) +--- Thread: Canon 70D Focus (/showthread.php?tid=1766) Pages:
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Canon 70D Focus - Guest - 08-28-2013 Quote: I tried a quick example for blur size based on the bottom of the wikipedia link I gave. Magnification given as 1:10 (let's say for a 50mm lens, object is ~ 50cm in front), the other object (that is blurred) is at infinity. Aperture is 50mm (e.g. a 50mm f1.0 lens, or a 100mm f2.0 lens). This gives the following graph (if I didn't make any mistakes), with the x- axis being the aperture (1 = f/1, 11 = f/11), and the y axis is the circle of blur in that condition. [sharedmedia=gallery:albums:126] At f1.0 you have a circle of 5mm, at f2.0 a circle of 2.5mm, at f5.0 a circle of 1mm, at f10.0 a circle of 0.5mm Thus, at the 80% edge of the 70D, you should be able to AF with f5.6, but not with f2.0. In the center no problem. I think the reason that only the center point is sensitive for fast lenses with normal PD AF is that all these PD AF sensors sit in the bottom of the camera. Only a fraction of the light in the center is redirected down, and in that little space down there, it would be difficult to install the larger fast PD AF sensors also towards the edge of the image. I think it's simply a practical limitation why you don't have fast PD AF outside the center. |