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Howdy….
Just a few things to understand about using Artemis (from the person that invented it, back in 2008) or any other directors viewfinder system.
Firstly: Artemis is an idealized digital directors viewfinder. It is 100% accurate, given the voracity of the data it is provided with. It describes the angle of view of a specific focal length of lens when used with a specific camera format. And there is the rub…
With all lenses there is the measured focal length, the indicated focal length and the functional focal length.
The measured focal length (in lay terms) is the physical distance from the rear exit pupil of the lens to the image plane, when the lens is focussed at infinity, measured in millimeters.
I have a lens I use to test and calibrate camera systems. It’s a Xeen 50mm that I have had meticulously mapped and measured. At infinity the measured focal length is 52.28mm. At close focus it is 52.36mm. The discrepancy between these two values is an indication of the degree of breathing in the lens, which is to say, the difference between the measured focal length at two different points of image focus. Nearly all lenses have some degree of breathing, including those where there is compensatory optics to reduce the effect.
No two seemingly identical lenses will never have exactly the same focal length either. For anyone selecting lenses for 3D rigs, this becomes immediately apparent. Finding two lenses that are a close match can be a major headache.
The indicated focal length is merely the value used by the manufacturer that best fits its characteristics. This value is based on a few factors, not least of which is convention. The film industry does not have many indicated 52mm lenses, though most 50mm lenses tend to be closet to 52mm than 50mm in real terms.
The governing factor of the indicated focal length value is what the lens “appears” to be, compared to other similar lenses. A lens measured to be 18mm, with a high degree of horizontal field curvature, might appear to be as wide a 16mm, on the basis of its angle of view. Thought this angle might be exaggerated by distortion at the edge of frame, it still “looks” wider. So the manufacture might indicate the lens as a 16mm on that basis, which makes the marketing people happier.
The functional focal length is the number we should, as discerning and critical craftspeople, should be concerned with. In Artemis there is the facility to add custom focal lengths. This has two purposes; one being the ability to add new focal lengths as and when they magically appear (thinking of you here Panavision), the other being to allow for refinement of focal length values.
A number of years back Jim McConkey outlined his meticulous approach to shot planning. This conversation precipitated the inclusion of custom focal lengths because Jim, knowing that his 50mm Primo was closer to a 53mm in practical terms, wanted to increase the accuracy of his planning using Artemis. Determining the functional focal length of any lens helps the operator, cinematographer or director plan more accurately.
Later Jim bought an Artemis Prime viewfinder which was used extensively on Marvelous Mrs Maisel, which removes all the guesswork of what the functional focal length ism by using the actual lens. This also displays all the distortion and optical characteristics of any lens, spherical or anamorphic, minus the added distortion introduced by purely optical viewfinder systems. You can see Jim using this device here: https://www.youtube.com/watch?v=JT_J8umefQk
Hope this was helpful.
