When shooting miniature sets what's the best way of calculating what the 'appropriate' depth of field should be?
For example a full frame shot of a car on a 50mm spherical prime (35mm) at a scale of 1:25...???
When shooting scale miniatures, All linear measurements are scaled by the scaling factor. Linear measurements are things like lens height, distance to subject (from the nodal point of the lens, not necessarily the film plane), focus distance. The aperture number is also one of those linear measurements. It is the physical diameter of the aperture opening, as a function of focal length. Hence, a 50mm lens set at a f/4 is set to a aperture of 12.5mm. To shoot a 25:1 miniature with similar depth of field, you would scale the f/number by the scale of 1/25th, set the focus at 1/25 the distance of the full scale model. This would give you an aperture of 50/4 (50mm at f/4) *1/25 = 50/100, or f/100. Ideally.
So, you would be best off, as Mark suggested, to base the full size stop at wide open and find a lens to shoot the miniature that will stop down to a f/32 or f/45…OR get a bigger miniature, like 1:4 scale, or 1:6 scale.
Gear+Rose Motion Control
Here's how I think about it...
...If it were a full size car and I were shooting it from, say 10 feet away at an f4, with the driver at 12' away, I would probably have focus set to hold from 10' to 13' or something like that....or maybe I have focus on her which means I hold from 10'6" to 14ft.
If I were shooting a miniature car with a 50mm lens to match that shot, (and I will use 12:1 as an example so I don't have to find my calculator)
then with the lens would be around 10" from the nearest part of the car and about 12 inches to where her head would be...I would want to hold the same DOF relative to the size of the car, so I would want to hold from, say10 1/2 inches to 14 inches...Oh, boy...looks like we will either be melting our car or shooting at a very slow frame rate, because with a 50mm lens, to hold from 10 3/4 inches to 14 inches, I need a whopping f 45....gulp!
Now if my live action car were in a night shoot and I were shooting at an f2, I would only be holding from around 11ft 2 in to 12 ft 11 inches, and now, shooting my 12th scale miniature could work at only an f 32: that's half the lights and half the heat (or twice the frame rate)
I'm hoping this helps - by the way, I was using pCAM with Super 35 1.85:1 for my calculations.
The way I look at things, whatever would be in focus if it were a life-size thingie should be in focus at whatever scale you shoot...DOF scales, if you will.
Obviously, this is one factor pushing us to build things in as large a scale as we can afford to....the other being level of detail and texture.
LA based VFX guy