"To me a standard lens for 35mm film would be a 32mm and for 35mm stills it would be a 35mm. For 16mm the lens would be about a 16mm. On film, I would call a 50mm a close up lens rather than a standard because it tends to give little distortion when used to frame a face in a close shot and a 27mm a wide lens as it gives distortion to a wide shot."
I've been trying to adapt my sense of the usefulness of my lenses to the 2.5k Blackmagic Cinema Camera. I'm used to shooting with a Canon 550D/T2i. It has a sensor that is approximately the same size as Super 35, which is the current standard of cinema. The 2.4k Blackmagic has a sensor that is significantly smaller, about 65% of the size. It is effectively like a Super 35 image is cropped on the sensor, and then blown up to fill the frame. This has quite significant implications.
Click the image below to view comparison of lenses on Canon 5D (full frame), T2i/550D (APS-C, Super 35) and Blackmagic Cinema Camera. The images are simulations made with Artemis Viewfinder.
With full frame 35mm sensor (5D), a 50mm lens is considered standard. The actual figure based on the diagonal of the sensor is 43mm. This is widely accepted as the proper way to calculate a 'standard' or 'normal' lens.
With a Super 35 (APS-C) sensor, a 31mm lens is standard, calculated with reference to a 50mm lens being standard on 5D. The actual figure based on the diagonal of the 550D sensor is 27mm.
Hitchcock liked 50mm, which is close to the double diagonal idea of viewing distance, photos vs. cinema. For cinema, since the viewing distance is usually double, the "standard lens" distance is doubled. Based on the diagonal of the sensor, the actual cinema version of the standard would be 54mm.
With a BMCC sensor a 22mm lens is standard, calculated with reference to the 5D.
The actual figure based on the diagonal of the sensor is 18mm. Doubling this for cinema, we get a 36mm lens.
On Super 35 (Canon 550D, APS-C) my standard lens - one that doesn't compress or expand visual depth compared to the human eye - is 28mm. It's a good all round lens, which is the mark of a standard lens. You can get a wide enough shot. You can take a head and shoulder shot of someone and they won't look distorted. Depth of field is ample and controllable. And it's wide enough that it's relatively steady. So it's pretty good at everything, and it presents a seemingly natural perspective.
On the Blackmagic, my standard lens size - which I got by calculating the diagonal of the sensor dimensions - is 18mm. I do have a lens to cover that: a Tokina 11-16mm f2.8. At 16mm it's just slightly wider than standard. With a standard lens you can expect a very natural looking perspective. People would appear approximately the same size in the viewfinder as they would to the naked eye. This is part of what is illustrated by the images comparing lenses on different sensor sizes, above.
However, the Tokina lens is not really a good lens for shooting faces because it's so wide - even though the field of view is cropped and it matches the point of view of Super 35 with a 28mm lens. On the Tokina there are necessarily facial distortion issues because it's so wide. Look at the 19mm face in the comparison photo below.
And there's little depth of field with the Tokina lens because it's so wide. For shots of people I could switch to a 50mm f1.4 which is more flattering than the 28mm, assuming there is space. Since the BMCC image is cropped and magnified, lenses appear more telephoto and you have to move back. With a 50mm on a Super35 sensor you might stand 6 feet away from someone. With the BMCC, you might need 9 feet of space to get the same shot. So even though I can switch to a lens more appropriate for faces, it can be awkward in a standard sized room.
In an ideal world, a full frame sensor would be the standard, 50mm would be a standard lens - wide enough, beautifully rendered faces - and then you can go anywhere from there. That's unrealistic, but it's illustrative of the problem. The BMCC sensor makes a standard lens so wide you can't shoot faces with it. Whether it seems significant or not, in a head to head comparison with longer lenses, the effect of distortion is obvious.
One other unexpected result of the smaller sensor is an increased need for stabilisation. With a more telephoto image, it is more difficult to keep the camera steady. While you can post-stabilise a shot, rolling shutter artefacts make it look like the image is projected on moving jello. While the rolling shutter exists on HDSLRs like the 550D/T2i, the smaller sensor makes the problem significantly worse. This makes image stabilised lenses very appealing. And it makes the 2.4k version of the Blackmagic Cinema Camera much better for cinema than run-and-gun documentary.
If there is one standard I want to stick to, it's Super 35. In July, Blackmagic promise to have a 4k version of the cinema camera with a Super 35 sensor and global shutter. I imagine I'll swap my 2.4k version for 4k version for the Super 35 sensor size and global shutter. Then my lenses will make a lot more sense again.
As the Blackmagic Pocket Camera was just released - with an effective sensor size of 2.48mm x 7.02mm - the standard lens figure, based on the diagonal of the sensor is 14mm.
In this video, Cinematographer Roger Deakins talks about lens choice on Cohen Brothers movies Barton Fink and Fargo. I do believe he is talking about choosing a standard lens size as a general look for a movie, which is why all the options discussed seem rather wide, from 16mm to 40mm. He would be shooting on a S35 sensor.