Edited by James Steven Beverly, 04 March 2010 - 08:55 PM.
Posted 04 March 2010 - 08:54 PM
Posted 04 March 2010 - 10:15 PM
Posted 04 March 2010 - 10:28 PM
Posted 04 March 2010 - 10:30 PM
So don't think that there's some big change in dynamic range when you are only talking about rating 500T at 400 ASA. It's more just extra insurance against underexposing rather than consistently overexposing.
You'd have to rate the stock 2/3's of a stop slower to be more consistently overexposing it and getting a denser negative, i.e. rating 500T at 320 ASA.
You are getting the same dynamic range but you are biasing information more towards the shadows when overexposing, and considering film's tremendous overexposure latitude, it's not a bad idea to worry more about the dark end of the scale and give it more density.
A denser-than-normal (overexposed) negative, printed at higher printing light numbers to compensate, i.e to print "down", gives you deeper blacks in the print, which improves the appearance of color saturation and contrast, plus tightens up the grain structure since you've exposed more of the tiny, slower grains in the film.
A simple test would be to take an unexposed roll of film, develop it normally so basically you have a totally black image (clear on the negative), and then print it at different printer light numbers. You'll see that at higher printer light numbers, the blacks in the print are deeper.
So ideally you'd want to expose your negative so that it prints in the mid to high 30's on average (except the blue record) rather than the mid to high 20's (the scale goes from 0 to 50).
Posted 05 March 2010 - 01:28 AM
It's not a hard-edged science. It's not like cutting a piece of glass to fit a window frame. More like cutting a panel of wood to block a hole in the wall. The panel of wood will be bigger than the hole, and you can position it a little to the left or right, so long as the middle is over the hole.
If the film is rated at 500, the latitude is fixed within those perameters,
Film has a very wide useful exposure range, which means that with a low to average contrast scene you can overrate or underrate the film and get away with it. Like a big panel of wood and a small hole.
And if you are shooting a very contrasty scene, with abrightness range outside the film's abilities, don't forget that the film characteristic curve has gradual slopes on the toe and shoulder, and you can nudge the image up and down the curve to control which end of the range gets better treatment.
In essence, exposing according to the recommended EI will get your mid greys exposed in the middle of the range. If you have no really extreme highlights, but you want to see into the shadows as much as possible, then you'd rate your 500 EI film at 250 or even lower. Or, you'd rate it at 500 and over-expose by 1 or 2 stops. It is exactly the same thing.
If you are doing a TV commercial in a white-tiled bathroom, no shadows, you might rate at 800 or 1000. Doesn't matter what midgreys look like, you want as much range in the whites as possible.
Other non-sensitometric reasons for varying the EI rating would be to do with minimising grain or trying to get away with fewer lights.
Posted 08 March 2010 - 07:35 PM