tianxusky

Thanks very much, super super helpful! First time posting in this community and am delighted to received such a detailed and kind response. Thank you. With this background, I can now go on to research waveforms and digital cameras and their log systems without being even more confused! Since this post is about understanding film response to light, I looked into exactly why film responds logarithmically, but couldn't find an answer, I can only speculate, for anyone who might be interested, or offer better insight, I've written my thoughts, as I noticed there aren't many threads about this online: It might be to do with the fact that silver halides, once exposed to photons (unit of light) that reduce them to pure silver specks (which form the latent image), cannot 'record' another photon exposure, so with increasing number of photons, there is a decreasing probablilties of hitting another silver halide, so film response slows down, because there is less and less material to be exposed with increasing exposure. Or maybe its because when a photon hits a silver halide, it splits the silver and halide, ejecting an electron which needs to travel and find a silver ion (a silver atom with a missing electron) to attach to, before turning it into a pure silver atom, if it doesn't find one in time, it might return to form the silver halide it originally was a part of. Maybe with increasing exposure, increased number of ejected electron flying about, something happens to the probability of finding silver ions, because there are also equally more silver halide bits for the electrons to return to, spread out across the crystal lattice. Or maybe its to do with development. Developers turn the remaining unexposed silver halides into pure silver specifically in areas with already reduced silver (silver specks formed by exposure), in a way amplifying the latent image. But developers work faster in areas with more silver specks, slower in areas of less silver specks, this makes sense, areas with more silver specks have more exposure so need to be developed more. But are they developed in proportion to the original speck amount? As developer reduces silver halides, there will be increasingly more pure silver, maybe causing exponentially faster developing. But with areas of less silver, developers may take a long time to accumulate enough silver to increase development rate. This suggests that the development phase is not linear either. I might be really wrong about this, and would need a scientist to expalin this properly, but essentially, all this put together might somewhat explain why film doesn't just count photon by photon like a digital sensor does, and it's good enough for me :)

Wow thanks so much! I couldn't figure out how to convert the log graph, I drew one, is it right? Is the film response vs. light intensity graph (arithmetic scales) more or less the same as human percieved brightness vs. light intensity? And the digital sensor response would look like a straight line? (attached image) I understand why a digital sensor would respond linearly to light, it just counts how many photons are hitting it. I understand why human response isn't linear, pupil dilation in dark areas, cone and rod distribution in the eye, plus other stuff, so it's said to be logarithmic. But why doesn't film respond to light linearly? Beyond the inertia point and saturation point, shouldn't it be just like digital sensors, responding linearly to the photons that hit the silver halide crystals? Is it because once a molecule is exposed, it can't be exposed again, so there are less and less molecules available with increasing light photons, but surely that doesn't matter when you're exposing at fractions of a second? Is it something to do with the way developing works? Why don't we just present linearly-recorded images of the world, with light values as they really are, and let our eyes distribute the tones, like the way we see reality? Is it because we cannot reproduce light at the range of intensities of reality, our monitors only emit certain range of light values, so if we don't stretch our camera values, they'll all be produced in a narrow intensity range, and look very flat? If this is correct, why don't we stretch it out linearly in proportion, but impose an S-curve on it? When we use the waveform scope on Premiere Pro or Davinci, what exaclty do the IRE values/0-255 values show? I understand it is logarithmic, in 0 black - 100 white, 50 isn't actually 50% reflectance, so how should we understand it? Given different dynamic ranges of cameras and monitors, this all becomes so complicated, so I thought I should get a grasp of the basics first. Thanks, any insights would be appreciated, am struggling to find answers :D

Hey everyone, I've been doing extensive research into this area for personal interest as well as for digital video color grading, but am really struggling. My question is, how does film really respond to light intensity compared to digital sensors? I understand that human response to light intensity is very complicated but can be simplified as being non-linear, and often logarithmic. My question is what about film, and digital? How do they respond to light? When a film negative is exposed, how is the data recorded, is it logarithmic, how? How do digital sensors record light, is it linear? What I am seeing everywhere is H & D graphs, which are density vs log exposure graphs, showing S-curves. But these are log graphs, more so, they are log-log graphs, both density and log exposure are logs. Why do we measure density logarithmically in relation to opacity and light transmittance? Where can I find a normal, arithmetic film exposure vs light intensity graph that can clarify things? I know that human response to light graph (percieved brightness vs light intensity) looks like a r-shape. What about film and digital sensors? Also, I read that digital sensors try to output values of 0-255 according to the S-curve from a film H&D (density vs log exp) curve, why is this? I'm just confused about all of this! There are a lot of questions but would appreciate any answer to any of them! Imgur Imgur Imgur Thanks!