From: "J. C. O'Connell" <[EMAIL PROTECTED]> Subject: RE: Exposure (WAS: Re: OK Survey time)
Weve been down this road before, unless your aiming your camera at a full screen 18% reflectance subject the meter will over or under expose the subject. the only way you could be accurate is if you manually compensated the meter reading based on the KNOWN reflectance of the subject and that is nearly always UNKNOWN. REPLY: Arrgh! This is not about autoexposure but metering which something entirely different. It is about being able to set metering within 1/3 of a stop. Known reflectance has nothing to do with it. What matter for a photographer is tonality on FILM. Ansel Adams didn't worry about known reflectance but the tonality of the various part of the scene on film. It is exactly the same principle with slidefim just the difference that developing for all practical purposes is a constant and not something that can be used to adjust the contrast range. Have you seen an Ansel Adams print. Do you really think that whats medium toned on the print was medium toned in real life? Maybe maybe not (most likely not) but who cares and does it matter? If I meter a medium toned piece of rock it is totally unimportant if the rock is medium toned or not in reality. If I want it medium toned I expose it as such, which is 0, it doesn't matter if the rock is really white or black - it will be medium toned on the film. If I want it + 1/3 I do that. If I want it as silhouttte I expose it at -2. I don't care if the green grass over there is really 0, -1/3, +1/3, -0.5 or whatever. What I do want is to render it on film at +1/3 (with Velvia) because thats nice in my opinion, and I get that with precision every damed time. I also want to asses the contrast range of the scene fast and with precision. I can do that as well. This will tell me how all parts of the scene falls in the tonality range if I expose for this or that. Say theres a patch of snow in the picture I don't want to burn out. I need to expose it at +1,7. How then will the tonality of the snow-free foreground turn out? Is the image possible at all? I can figure this out in seconds because I have that tool to nail it. If I expose snow at +1,7 it always get +1,7 on film; never +2 which ruins the shot. I can promptly figure out the contrast range between the forground and the sky. Do I need a graduated Nd filter? How many stops? These are the reasons why I want ot be able to nail exposure within 1/3 of a stop fast and efficiently. - Pro slide film are within 1/6s tolerance. That is a worst case scenario; they are practically speaking dead on. Amateur slide film are within 1/3s but in reality amateur film is exactly the same film as the pro version but is released earlier to the store so that the manufacturer cannot guarantee its history. Whatever differences this may make it is usually in subtle colour balance, not exposure. - Pro Labs are within 1/6 stops - Modern Pentax lenses have apertures calibrated within 1/10s accuracy. Canon claim 1/12s for the L lenses. - All slr camera made after approximately 1985 have 100% shutter accuracy. This have made the once common camera shutter tests redundant. Even on older, less precise cameras the errors are usually only significant in the real short speeds. - All Pentax cameras I've bought since the 90's have 100% correct calibrated meters right out of the box. In the old days, pre 1990, a how to photography books would tell you to calibrate you meter. They don't anymore as new camera bodies are usually dead on. This usually no real problem as this is usually a constant and any erors can be calibrated away. All the errors above don't really add up to anything significant unless faulty equipment is at play or an accident happens. They are too small to be of any concern and they are more likely to cancel each other out than to add to each other. I've shot several side by side test images where I compare various Pentax 645 lenses. I shoot at various apertures and varios zoom setting with various lenses. There are no exposure differences regardless on Velvia. On this film 1/3s is screaming obvious. Whats annoying with this futile discussion is that some in reality are claiming that those of us who do nail the exposure we want within 1/3s are lying bastards. It is somehow not surpising that these allegation comes from people who use very old equipment. That probably explains it all. I cannot nail exposure this accurat with my older cameras. Life is really too short for these futile discussions. Pål
