Comparison of focal lengths

[Remo Fritzsche]

Hi Folks

We're gonna be shooting a short on an Angénieux Optimo Style 16-40mm with a x0.75 wide angle converter (PL mount). On the widest end, this would result in a focal length of 12mm.

For prep however, we're not able to rent this exact lens, which is why I want to frame the shot using my cheap Tokina 11-16mm (EFMount).

Two questions, just to be absolutely sure:

1. Are both focal lengths (12mm, 11-16mm) of both lenses based on 35mm back and therefore comparable?
2. Distance to our subjects is very tight and I want to make sure to have the exact same field of view. Is it correct that this is relative to the film plane (sensor) and not to one of the lens elements? In other words, that the physical length of the lens doesn't matter and the two lenses can be directly compared?

Thanks a lot for your insights.

Cheers
Remo

 

[ericyoung]

1) Film frame/Sensor size does not affect focal length. But it does affect the usable field of view ("crop" size). A lens like the Tokina 11-16mm is an EF-S lens and has an "image circle" designed for a smaller APS-C sensor (about 1.6x smaller than a full frame stills 35mm sensor - hence 1.6x crop factor) which is roughly equivalent to Super 35mm movie film, or I believe 4K resolution on a Scarlet-X. If used at higher resolution, or on a larger sensor camera, there may be vignetting from seeing the edge of the image circle. But that's OK, as the Angénieux Optimo Style 16-40mm is also designed as a Super 35mm film/sensor lens, so should be comparable to the Tokina.
http://reduser.net/forum/showthread.php?66210-Scarlet-X-Data-Sheet

2) Focal length is focal length. In the case of multi-element or thick lenses, it's Effective Focal Length (EFL). It's definition is the distance between the effective optical centre of the lens and the point of focus of parallel rays of light (usually the film plane/sensor). The physical length of the lens has no direct relationship with the focal length. So in theory a 12mm lens from every manufacturer will have exactly the same field of view. But note that how each manufacturer measures the EFL and marks it on the lens scale/zoom ring/electronically isn't always very accurate, especially with stills lenses. So expect some variance. Ie, it'll be an approximation only. If distance is very tight to the subject, then you may run into problems with the minimum focus distance.
https://en.wikipedia.org/wiki/Focal_length

 

[Remo Fritzsche]

Great, many thanks for your detailed answer!