Depth of Field and Field of View Equivalents
06. Mar, 2010 
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Focal Lengths are Absolute
When it comes to lenses it is important to remember that focal lengths are absolute and it is the field of view that is relative to the format you are shooting. This means that a FF35 still shooter will think of 24mm as a very wide lens while someone who regularly shoots in the 2/3″ format will think of 24mm as a fairly long lens.
I have used an image shot on 645 medium format 120 film with an 80mm lens to demonstrate this. You can see that on medium format 80mm becomes a fairly wide lens. On 2/3″ format 80mm becomes telephoto.
Focal Lengths and Field of View Equivalents
Unfortunately there is no metric to describe the all important field of view so we have come to rely on field of equivalents or “crop factor” to describe whether a lens is wide, medium or long. If you are used to shooting in a certain format you have learned to associate a certain field of view with the focal lengths of your lenses. Say you are a FF35 stills shooter and you are trying out the RED camera for the first time. You throw the 25mm RPP on and you notice it looks longer then 25mm usually looks on your Canon 1D. What you can do is multiply the 25mm RPP by the S35 – FF35 crop factor (1.62) and you see that it has the same field of view (field of view equivalent) as a 40mm lens on your Canon 1D. Going backwards gives you the ‘crop factor’ if you want to put your Canon 25mm lens on the RED camera. Multiply your Canon 25mm lens by 1.62 and you see that the image circle will be cropped to the same field of view as if you shot on your 1D at 40mm. You have probably heard APS-C (very close in size to S35) cameras referred to as 1.6 crop cameras. This is where that term comes from. Some of the more common crop factors are listed below.
- 2/3″ to S16mm = 1.24
- 2/3″ to S35 = 2.47
- 2/3″ to FF35 = 3.56
- S35 to FF35 = 1.62
DEPTH OF FIELD ACROSS FORMATS
BULLET POINTS:
- depth of field becomes shallower as a lenses f-stop decreases.
- at the same f-stop, lenses with a longer focal length have shallower depth of field.
DEPTH OF FIELD
It is especially important when shooting on Scarlet and Epic to understand exactly what affects depth of field because of all of the sensor formats, windowed sensor crops (for high speed) and the multitude of lens mounts available to the DSMC system.Depth of field refers to the section of an image that appears acceptably sharp or in focus. While other factors can have an effect on depth of field once a final image is being viewed (see circle of confusion below), it is important to remember that the cinematographer/photographer ultimately only has control over the depth of field created by the lenses focal length and aperture. There are the two absolutes that determine depth of field:
- depth of field becomes shallower as a lenses f-stop decreases.
- at the same f-stop, lenses with a longer focal length have shallower depth of field.
This explains why we talk about the difficulty of getting shallow depth of field on cameras with smaller sensors like the 2/3″ Scarlet. 16mm is a very wide lens on S35 but becomes a medium lens (S35 equivalent of 40mm) on the 2/3″ Scarlet. Lenses that we use to get the extreme shallow depth of field S35 is known for, like an 85mm, become very long telephoto lenses (S35 equivalent of 210mm) on 2/3″ and only usable in specialty situations.
The depth of field created by the lens remains the same no matter what size sensor you put behind it. It is the need to use unusually wide lenses (thus deeper depth of field) due to the small sensor that makes it harder to get thin depth of field.
VIEWING SIZE, DISTANCE AND CIRCLE OF CONFUSION
The size an image is viewed at and the distance from the screen does effect the perceived depth of field. You can see this in the strip of images we used above. When the huge medium format image is shrunk down to that size everything appears to be in focus. When we effectively zoom in to the S35 format the background now appears slightly out of focus.
This effect is fairly small and only apparent in drastic cropping situations. The cinematographer/photographer really has no control over what size the final product will be viewed at so it is generally not taken into consideration when shooting.
