It’s been an embarrassingly long time since I’ve written a blog post, and so I logged in today and discovered a bunch of really good comments and questions have been left on my blog recently. This one is a good one and so I want to post my response as a standalone article.
1) No question, film has better dynamic range than digital. Admittedly the difference is increasingly becoming slimmer. My question is, when film is converted to digital for special effects purposes, does it not lose that dynamic range? I read that digital typically has 256 shades of grey (lol!) but film is infinite. When the film is captured by the digitising machine and all, doesn’t it lose that range, and maintain that loss through to when it is spewed back on to film and shipped to cinemas?
Color bit depth, dynamic range and gamma function are the three things to consider when looking at how film is scanned, and also to digital imaging sensors.
Color Bit Depth
Color bit depth in a digital file, whether is it a scan from negative or from a digital camera is determined by the number of bits of data that represent the values of R,G and B channels from back to white. If you imagine each channel separately as shades of grey from black to white, then a 8-bit per channel color bit depth would give you 256 increments or “steps”. However this is not a likely color bit depth you will come across in professional digital cinema acquisition or post production. The minimum bit depth considered adequate is 10-bits per channel, giving you 1024 increments per R, G, B channel. Above this is 12-bit, 16-bit per channel and 32 bit per channel.
As you can imagine the file sizes per frame ramp up very quickly for any given image resolution with this increase in color bit depth. Dynamic range has nothing to do with color bit depth, it is purely a factor of a camera imaging system sensitivity limits. It’s typically referred to in “stops”. It’s the exposure range between the darkest blacks and the brightest whites that a film emulsion (or sensor) is able to resolve and still capture incremental detail. Think about the difference in light levels between a shaded interior, and a exterior lit by full sunlight. There is a massive scale in play there, and typically digital imaging devices have been poor substitutes for silver halide when it comes to capturing a wide scale above or below any given exposure. This has changed and I would argue is no longer a limiting factor.
The last thing to keep in mind is how the image information is recorded and interpreted. Our eyes perceive finer increments in brightness on the darker end of the scale than the brighter end, however light levels in the real world are of course linear.
Imaging sensors are natively linear in sensitivity to light, and film emulsion has a more or less linear response with a curved “toe” at the bottom end and “shoulder” at the top end.
How exposure values are recorded when scanned from negative or read from a digital imaging sensor can be linear, but commonly will follow a roughly logarithmic scale, assigning more incremental steps at the low end than at the higher end. This is one very good way to minimize wasted data with a form of “natural” compression as a digital image that has been captured in 10-bits per channel on a logarithmic scale can visually be very similar in fidelity to a higher bit-depth linear encoded image, and at a much smaller file size.
These three factors all come into play when comparing the performance of celluloid and digital in image acquisition.