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358 THE PRINCIPLES OF CINEMATOGRAPHY
The T Stop' Method of Lens Calibration
With the introduction of so-called 'lens blooming', or coating applied to each surface of a lens system so as to reduce considerably the reflection and consequent loss of image definition, it quickly became apparent that the f / stop system of lens calibration was not sufficiently reliable.
As a ready example of what is meant by this, let us remember that the f/ stop system is merely a geometric relationship based upon the ratio between the focal length and the diameter of the entrance pupil of the lens it thereby does not take into account the absorption due to the number of elements in the lens system and always assumes a perfectly circular lens diaphragm.
Because of this it is possible to find one lens nominally rated at f/2 and passing considerably more light than a second lens of similar f / stop rating particularly if the first lens has been 'bloomed' and the second has not. Clearly, the only satisfactory method to adopt is one which calibrates all lenses in terms of the actual amount of light transmitted to the film.
Whilst this object is only too easy to define, it is very difficult to standardise as a general practice; not the least difficulty being that a purely geometric-ratio-system such as the f/ stop nomenclature can be rapidly applied in the mass production of lens iris rings and the engraving thereof whereas, with T/ stop calibration, each lens must be individually measured. That these difficulties have been overcome and the photometric transmission of lenses is now measured represents a very considerable advance in technique and should lead to greater uniformity in results obtained when various lenses are interchanged. A very detailed account of the whole problem and the advantages obtained is given in a Report of the Lens Calibration Sub-Committee of the Society of Motion Picture Engineers, published in the October, 1949 issue of their journal.
To be able to use a series of wide angle, normal and telephoto lenses when such a range is fitted to the turret of some cameras it is necessary to remember the following points if optimum results are to be obtained: (1) A lens of long focal length will have a smaller 'depth of focus' than will a lens of short focal length.
(2) As the size of the aperture in a given lens is decreased the f / number will increase and the 'depth of focus' will become greater.
(3) With any given lens, at a given aperture, the 'depth of focus' will increase as the distance between the camera and the subject is increased. (4) the 'depth of focus' is always greater beyond the distance to which the lens is focused than it is in front of that distance.