"Phil Hobbs" wrote in message
...
But why can't we use a bigger film then? Should we always compare 135
film against CMOS sensors of different size?


A bit of possibly useful discussion:

https://electrooptical.net/News/photographic-film/

One other factor to bear in mind: the depth of field varies with lens focal
length, not field of view of the subject. This means that if you take a
photo on 120 film and on 35 mm, with appropriate focal lengths of the two
lenses to give the same field of view of the subject in both cases, and use
the same aperture, the DOF will be less on the 120 photo than the 35 mm
photo. So if 80 mm gives a certain field of view on 120 and 50 mm gives the
same field of view (ie shows the same extent of the subject) on 50 mm, and
both lenses are at f 4 (and so both will use the same shutter speed for the
same speed of film), the 120 photo will have a shallower DOF. That is why it
is so difficult to get shallow DOF on a compact or phone camera, because the
lens is such a short focal length to suit the very small sensor, that almost
everything is in focus even at a wide aperture (and the lens might have more
artifacts and aberrations than the comparable lens that gives the same field
of view for a 35 mm camera). In all this, I'm talking about the field of
view of the *subject* - ie how much of the subject (wide/telephoto) is
included within the frame of film or the sensor.

This is why some drama TV programmes are shot on 16 mm or with a similar
size CMOS sensor, but with a 35 mm-format movie camera lens and an
intermediate ground-glass screen. This allows a shallower DOF to be achieved
for artistic reasons without having to open up the (16 mm format) lens to a
wider aperture which might show more lens flaws. The lens for 35 mm format
produces an image on the ground-glass screen that has a certain field of
view and depth of field which would be recorded on 35 mm film. The 16 mm
camera focuses that image (which is all at one plane) onto 16 mm film.

I wish I could find a URL that describes it, but I'm obviously not feeding
Google with the correct search keywords - a common problem I have.