| David Heath |
April 19th, 2009 12:40 PM |
Quote:
Originally Posted by Tom Hardwick
(Post 1073964)
When you switch an ND (or two) into the light path you're adding extra elements to the lineup of 15 or so you already have. Extra elements always up the flare levels by tiny amounts and in some cases can alter critical focus at the film plane. The NDs are way out of focus though, so even if they collect dust and debris it's not a problem.
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Not really Tom. The "no ND" position is a clear bit of glass, not an open space, so you always have the same number of glass/air surfaces in the optical path.
It's pretty critical that all the filters (including clear glass) have not only exactly the same thickness, but the same coefficient of refraction. If they didn't the lens tracking would vary from filter to filter, and if "no ND" had no glass the tracking would be so different from the ND positions as to be unusable at wide angle fields of view.
Quote:
Originally Posted by Steve Renouf
.....if you move indoors where the illumination is, say, 1000 lux - using your same objects - the black is still reflecting no light, but the white object is now only reflecting 1,000 lux (it can't still be reflecting 10,000 lux because there is no longer 10,000 lux available) so the contrast is now only 1,000 to 1 and the colours are less saturated as a result.
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I disagree. Nothing will have 100% reflectance, and nothing will reflect no light at all - even the deepest black will reflect some light. If we say the figures are 95% for the nominal white, and 1% for the deep black, then *all else equal* the outdoor/indoor figures using your figures will be 9,500/100 lux outdoor and 950/10 lux indoors - exactly the same ratios. Same if you add a 1 stop ND - the former case goes to 4,750/50 - same ratio again.
The reason contrast range may seem to decrease with lower light levels is to do with directionality, and especially highlight/shadow ratios. Light a subject with a highly directional spotlight in an otherwise dark environment, and the contrast range of the subject may be just as high as in sunlight, even though the overall level is far lower. |
