Half-Frame lens field of view and crop factor

35mm half-frames are not 18x24 (the exact half of 24x36, but in general a little more than 17x24 because you need at least 1mm to separe two adjascent frames. It depends on the camera.
POLKa

I put together this detail to explain crop factor.

wiltw said:

And since 24x36 is actually a crop of all the larger formats, the 'crop factor' is proven to be a mental crutch for FL 'equivalence' to change from format to format for those unaccustomed to working in different formats. (I shot in 4 formats but never previously used any 'crop factor' -- except initially when going from 135 to digital APS-C...using the same set of lenses for both, which had never happened before.)

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That's why I think we should not use the crop factor any more. We had all the different formats before and didn't need the crop factor. One thing I don't like about the crop factor because it makes the 35mm format as the one all other formats are measured by. I often joke that I only use full frame because I can't figure out the crop factor.

Chan Tran said:

I often joke that I only use full frame because I can't figure out the crop factor.

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I only used 135 film when I could not afford to routinely shoot anything larger !

Nothing has created more confusion in the world of photograph than "crop factor" (no, not even the zone system.) I used to frequent a mostly digital forum and almost every day I'd see one or the other strange misreading of what it was and how it worked. One person was asking some questions about using an 25mm APS-C lens on a Micro 4/3 camera and was convinced that he needed to do two calculations--APS-C to FF, then FF to M4/3 because a APS-C 25mm was different than a FF 25mm--so (25*1.5)/2. People on the forum tried to patiently explain that 25mm was an unalterably real world physical value that was independent of sensor size, but he kept arguing and arguing until I decided never to click on that thread again.

Today I just tell people to look in the viewfinder to figure it out, and learn that each sensor/film size has its own view of a focal length.