Dynamic Range vs. Latitude
Posted 03 June 2011 - 11:51 PM
Posted 03 June 2011 - 11:58 PM
i heard recently that these two terms refer to different things, I was under the impression they were the same thing. so what's the difference/relationship between DR and latitude
Generally speaking, dynamic range refers to the entire exposure range captured whereas latitude refers to the ability to make corrections, particularly exposure mistakes. So latitude tends to be narrower than dynamic range. If you say "I overexposed her face by three stops by accident but was able to print it back to normal" you'd be referring to latitude, if you say "an 18% grey card goes pure white when six stops overexposed and goes pure black when six stops underexposed" you'd be talking more about dynamic range. Obviously the two concepts are interconnected -- a format with limited dynamic range (like slide film) is also going to have limited latitude for corrections.
Posted 04 June 2011 - 08:05 AM
Latitude is related to Dynamic Range, but it is also scene dependent. Latitude is the degree to which you can over or under expose a scene and be able to bring it back to a usable exposure value after recording. It is dependent upon dynamic range, which is going to set the overall boundary of by how much you can over and under expose, but it's limited by the scene too, and how bright and dark the scene itself goes.
Say a scene has a range of brightness of 5 stops (a typical Macbeth chart for instance), and let's use a camera that has a 12 stop dynamic range. If we place the scene in the middle of that camera's recordable range, we have 7 stops to play we can we could over or under expose by 3.5 stops and still recover the scene.
But if the scene was a real world scene of actor against a sunlit window and the range of brightness of 15 stops, you don't have any latitude at all - no matter how you expose that scene you're going to loose shadow or highlight information.
So yes, Latitude and Dynamic Range are related, but different. Often people who really should know better use the term Latitude when they mean Dynamic Range, and indeed that's quite common.
Posted 05 June 2011 - 12:19 PM
Posted 05 June 2011 - 12:31 PM
so latitude refers to an actual shot scene, whereas DR refers to a stock/sensors range of exposures it can render?
No, latitude refers to the correctable-back-to-normal range -- you could shoot a latitude test with the camera pointed at a MacBeth chart for example. It's just that it's a bit more of a real-world subjective issue -- like notions of "acceptable grain or noise" -- rather than a purely scientific measurement. One person might say "wow, I can underexpose by three stops and print it back to normal and it looks fine" while another person might say that the result wasn't acceptable.
I also wouldn't use the word "render" for DR because that sounds like "display" -- and display issues will affect how much DR can be seen or used. I'd say that DR is about a luminance range that can be captured by the sensor (and presumably recorded). But as Graeme says, the figure can be a bit debatable because often there are discernible steps of luminance in the noise floor of the sensor which some people count (either just to be complete or to get the maximum figure possible) while others don't. That may make the difference between saying that the camera captures 12.3-stops versus 12.6-stops, let's say.