gmf wrote:malbrand wrote:according to aberlcine fov calculator crop factor for fullframe is 2.3, and for aps-c is 1.5
Please correct me if I'm wrong, but there is no APS-C lens which has a "1.6-crop-factor-focal length" written on it - it's always the focal length according to full frame, so to see what focal length a 17-50 mm APS-C lens will have on the BMC you still have to multiply that with the factor 2.3 - same of course with full frame lenses.
Focal length is an optical measurement. It has nothing to do with the sensor size.
http://en.m.wikipedia.org/wiki/Focal_lengthA 50mm is a 50mm no matter what camera you put it on- a BMCC, Super35/ASP-C, Full Frame, Medium format, Large Format etc. still a 50mm.
What changes with the sensor/film size is the crop into the image circle.
So if you are used to shooting on Super35 film cameras with PL lenses you don't say "oh this is a 50mm on a Canon 5D so it must now be a 75mm on my Arri Alexa". It's still a 50mm.
So the crop factors are there to give you a guide as to what your lens crop will look like based on your point of reference. For many coming from the 5D that point of reference is Full Frame 35mm (2.3x) and for those coming from arri Alexa or a 550D it would be Super35/ASP-C (1.5x).
So your 17-50mm from a 550D will have the 550D image cropped by 1.5. Where as the 16-35mm from the 5D will have its image cropped by 2.3.
Both lenses at 17mm would look the same on the BMCC.