beanpkk Offline Upload & Sell: Off
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Thanks for the comments. All I can do is give you my method and you can tell me where I went wrong:
Set the camera and lens up at MFD, and measure the width of the image at the subject. Then measure the distance from subject to sensor, and calculate the angle of view. Angle of view leads directly to focal length given a constant format, 35mm in this case. Then repeat the procedure farther away. Here are my numbers:
With the camera sensor about a foot from the subject (MFD +/-), and going by the viewfinder in a 5D, the width of the image at the target was 36mm. I calculate that to be an angle of view of about 6.4 degrees. At 10 feet, image width was 2'10" for an angle of 17.7 degrees. That calculates to about a 120mm focal length. I didn't try it at Infinity but I assume it would come out 100mm for that. At MFD, 6.4 degrees comes out to 300mm.
Here's the site that gave me the focal length: http://tawbaware.com/maxlyons/calc.htm
According to that site, a FL of 100mm should give a horizontal angle of view of about 20 degrees, right in line with my 17+ at less than infinity. To get an angle of 6.9 degrees requires a FL of 300mm. B&H lists the angle of view of the 100mm non-L at 24 degrees (diagonal) which lines up with my 20+/- degrees horizontal.
If you put the 100mm lens on a camera and just move the focus ring, you can see the image get dramatically bigger as you focus closer.
What am I doing wrong?
Thanks,
keith
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