I like stuff too, but this makes my head hurt. What exactly is Optical Density? In English please.
great! I kind of got that part already but putting word on it like that really helps.I like stuff too, but this makes my head hurt. What exactly is Optical Density? In English please.
OK You asked for it :)
The degree of darkening of film is called Density. Density is a measure of the light-stopping ability of the film. To determine the meaning of density, we must first get familiar with the terms transmission and opacity.
Transmission is how much of the light gets through the film, and opacity is how much of the light doesn't get through the film.
Thus, opacity is the reciprocal of transmission (O = 1/T). For example, if 100 lux of light is falling on one side of the film, but only 25 lux gets through, then the transmission is 0.25. The opacity would then be 4. Notice that, like all reciprocal relationships, transmission multiplied by opacity equals 1.
Now, back to density. Density is the logarithm to the base 10 of opacity (D = log10O). Since the only logs we will be talking about are to the base 10, we will omit the subscript from now on (log = log10).
The density of your film can be measured by an instrument called a densitometer.
If you use a densitometer, you do not need to figure transmission, opacity, or the log of the opacity, because the instrument will indicate the density on its scale (or readout)
So basically the optical density is how dark the film is :) the more light that hits a negative the darker it gets, a curve is a pictorial representation of that.
I like stuff too, but this makes my head hurt. What exactly is Optical Density? In English please.
OK You asked for it :)
So basically the optical density is how dark the film is :) the more light that hits a negative the darker it gets, a curve is a pictorial representation of that.
haha i am sorry to have inflicted this on folks - BUT I LOVE IT!A famous photographic scientist said "all the magic happens in the toe" (of negative film) that is where you put your shadow detail and has a bigger effect on overall quality than I can possibly state.
PhotoUtopia, I am busy reading that great kodak PDF, I will get back to you. But I have already understood that I was incorrect about the flat part of the curve being the latitude. I thought the toe and heel were too crazy to use but it sounds like that is not the case! back to reading...
IWhich volume was that? I ask because the info might not be current. The original 1930's method was evaluated with a Dlog 1.5 E units and in the 1958-63 period they changed it to DLog 1.3 which gave a stop of extra speed overnight (Tri x went from 200 to 400ASA)
But it at least was quite interesting. I did learn that the reason the old films has a different speed than the new ones was because of the way that the tests were designed was different. From 1943 to 1960, the ASA rating was based on both curves and user input. But in 1960, the method was changed as to better match the easier to calculate European technique.
I am just messing with you guys with the flower pics.
With you there, Satish: as I see it there's no substitute for "shoot some and see" as the best solution, especially when you're using films for purposes other than they were intended for. James has got some nice flower shots with interneg stocks...
But seriously, my vision of old school CN filmstock developers is them just shooting rolls at different EIs, looking at them and saying "ayup, that one there's gonna be what we rate it at" ;DWell, ya I think that likely is the case. I mean if nothing else they are likely rounding things to the nearest stop. But I think Delta 3200 is a case of what you are talking about because Ilford says 3200 and world+dog says 1600
I don't know if I fully buy satish's story, whenever I hear musicians starting to talk about music and explaining how things work, it gets pretty math-y sounding pretty fast. I think he is a 'denier'! :P
The sensitometry workbook is an example, as is a whole range of educational material about production organization, lighting, shooting and handling film and postproduction that I and many other teachers have been using in our film classes for a long long time.
this sensitometry jibber-jabber :DI hope you understand that part of me totally agrees with you!!
My thoughts:
https://www.youtube.com/watch?v=X8CrHrx2k5M (https://www.youtube.com/watch?v=X8CrHrx2k5M)
The one question I have is that it does not really explain what to do if the density difference is not within .75 and .85. That is the case in the example below where the density difference was only .66. Now I was just goofing around in pshop from a screencapture of a PDF, etc. So there is lots of places for the error to pop up, but clearly there could be situations where the "Gamma" (to now use a term that I learned from the PDF) is going to be too high or too low to give the desired density range of .75 to .85. Photo-Utopia, what do they do then? I am presuming they would say that the film was not developed correctly for the test?Sensitomtry is just a lab based value. The density slope indicates what quality image can be expected from the negative. It is often an important value that photographers fix depending on the type of enlarger they use. Kodak recommends aiming for a density range of 0.8 for condenser enlargers and 1.05 for diffusion types.
Any thoughts on how the curves change as film ages? Does the iso decrease in a linear fashion or maybe an inverse log?
(http://cdn.ipernity.com/144/96/39/33619639.e450142e.640.jpg?r2)
A curoius effect I've noted is that if I'm shooting in very cold weather (well below freezing, say) I don't usually notice the pain in my fingers until after I've made the last exposure....
That is a good question. The old "a stop per decade" always seems too easy and too linear. But now I think about it, given a stop is a geometric unit of measure (a doubling or halfing), I guess even with that example we are talking about its sensitivity changing in a geometric manner and not linear. Right?Actually it isn't linear since stops aren't from the start.
I love that pic btw
Incidentally, what is a "Katy Perry"....?It's another one of those cute fuzzy bunnies... I think ;)
Incidentally, what is a "Katy Perry"....?It's another one of those cute fuzzy bunnies... I think ;)
I agree! The attraction of FW is in large part the absence of snipers here. We can have a discussion about useful-if-esoteric stuff like sensitometric curves and box speeds and seamlessly integrate it with pictures of pretty flowers. How much better can it get??? (Edit: and bunnies too.)
I know, I know. You are a fine bunch and I'm proud that you choose our little corner of the web-u-verse to share this kind of thing. Huzzah.