I know it's a dumb question, but I really got confused... It's about the "crop factor" again, so please bear it with me

Below are 2 screengrabs straight from the FS100; one is with the pancake 16mm lens, the other - with the Tokina 17mm F3.5.

Shouldn't they both be more similar as far as the angle of view is concerned? I mean, it's just 1mm focal length difference... Does it account for the difference seen, or do the 2 lens have different crop factors?

Please note - I did put "crop factor" in quotation marks :)

I understand the NEX is an APS-C lens, while the Tokina 17mm is a FF lens - but their focal lengths are supposed to be expressed in FF terms... So, again: can the 1 mm difference make such a big difference?

Based on a straight angle of view calculation the difference between a 16 and 17mm on a 1.5x crop imager should be 3-4 degrees (~70 vs ~74deg).

What you are demonstrating is the focal length stated on one or both of the lenses are approximations only.

but their focal lengths are supposed to be expressed in FF terms...

Focal lengths are ABSOLUTE. They aren't expressed in "terms".

The 16mm is almost 6% wider than the 17mm, BTW.

The photo on the right side looks like it has cropped in a lot more on the right side than on the left side.

From your screen grabs, the camera seem to have been panned to the left and tilted down a bit between the two shots, so it's harder to spot the differences.

Off topic, the tokina is a lot sharper!!! ;-)

Yeah - the camera wasn't at the same exact position, as those pics were not even taken on the same day.

Which doesn't change the fact the lenses' angle of view is considerably more different than one would expect basing on the 1mm difference in their respective focal lengths...

As to the sharpness - well, the Tokina is actually very soft when wide open (at F3.5).

Think about it. The shorter the focal length, the greater a difference per millimeter. What would be the difference between a 1mm and a 2mm lens that is twice the focal length? What would be the difference between a 399mm and 400mm lens? Anything below 24mm is really wide and has serious perspective distortion so it doesn't surprise me that two lenses with slightly different focal length from different manufacturers might have a different framing.

To echo some things above - there are no "terms" on focal length - just like there really is no "crop factor". 16mm is always 16mm.

I use to have a Canon 16-35L for my 5DII. I almost never ran it at 16mm because I didn't like the look. I would usually bump it in to about 17 or 18mm - so there should be a difference.

There will be a very noticeable difference on wider lenses, however the really key factor is the change in camera location. Try the same setup again and this time swap lenses without moving the camera. The pictures are pretty useless as an example unless the only thing different is the lens. In fact, 17mm shot looks like the camera may actually be closer than the 16mm shot, in addiction to be higher (and tilted down to compensate) and then panned left a hair.