Auteur: Jean-Yvon
Date: 10-04-2003 10:52
Bien sûr, il faut faire avec ce qui existe, mais on peut regretter qu'en 15 ans peu de progrès aient été faits sur ce problème, alors qu'il existe des solutions. L'étroitesse du marché des posemètres est sans doute responsable de cette situation.
Autre reflexion de Richard Henry dans le même livre :
Measuring Exposure Values (EV’s) of different colors. One large source of difficulty in using a spot meter, and perhaps to a lesser extent with other meters, is meter response to different colors. This is a problem that apparently is overlooked by most authors. One would predict that meter response cannot be the same for all colors and blacks, grays, and whites simply because no light sensitive element has a uniform response throughout the spectrum. Neither the Soligor nor Minolta instruction manuals accompanying their spot meters even mention this subject although Soligor does state that correct exposure will be indicated if a meter reading is taken through the same filter to be used on the camera. Theoretically this could be valid only if the spectral response curve of the film being used is the same. as that of the light detector in the meter, a most unlikely situation. Smith (435) in his The Tiffen Practical Filter Manual states that meter readings taken through a filter may vary from the exposure derived from an unfiltered reading and application of an exposure factor for that particular filter. He says that this is due to variations in subject color, lighting conditions, and the meter’s sensitivity to various colors of light.’ My own experiments taking exposure readings with a red filter 25A and then using the same filter on the camera agree with Smith. This, as predicted, does not appear to be a reliable solution to the problem.
The instruction booklet accompanying the Pentax spot meter presents a table of correction factors for meter readings taken of objects of different colors. These corrections vary for the colors given from -1 to + 2 stops. My attempts to verify these correction factors met with only limited success but this may only mean that my “yellow” object was not the same as Pentax’s “yellow” object. There are differences in yellows, reds, blues, etc. This is an area deserving further investigation. From a practical standpoint, however, it means that when we meter a red barn, a yellow truck, etc., we may end up with film D’s off by 1 zone, or rarely as much as 2 zones.
The obvious answer to this important problem is to alter the exposure meter so its response closely matches the color spectrum sensitivity of film. One solution to the problem with color film was offered by Staes in which 3 photoreceptors were used, each individually filtered for responses similar to those of the 3 emulsion layers of a color film. Horowitz was able to achieve this to a considerable extent for black-and-white film in 1983 using only 1 photoreceptor for the Zone VI Studios, Newfane, VT modification of the Pentax 1° Spotmeter. This subject points out the seemingly impossible challenge of designing a perfect exposure meter. Even the best-designed meter represents a compromise. For a meter to be perfect the detector of the meter would have to match the response curve of a panchromatic film to daylight. Fit the film sensitivity curve exactly is predictably next to impossible, although Zone VI Studios, Inc. obviously have made strides in that direction.
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