Film versus DSLR dynamic range

Started Jul 24, 2007 | Discussions thread
Lictor Regular Member • Posts: 346
Re: Film versus DSLR dynamic range

Alfredo Li Pira wrote:

A number of posts in recent threads state that film has more dynamic
range than DSLR sensors.

DR is not a problem with digital. Anyway, you don't want to use a media with huge DR, because you will get a very flat image as a result with no contrast whatsoever.

The advantage film still has over digital is elsewhere. Linear range is similar, but film wins outside of that range. It's due to the way digital and analogic react to saturation.

When you saturate a digital device, it clips. In 8 bits, anything above 255 is 255. That's the dreaded white hole in your blown highlights. Worse, not all channels clip at the same time - causing color shift when the highest channel clips while the other keep responding.

Another problem with digital is that CCD/CMOS are absolutely linear : twice the amount of light will cause the output of the sensor to double. Which means that shadows will get a very limited range of values to express the different shades, leading to posterization in the shadows. On the other hand, the human eye does not have a linear response.

Clipping is a problem much broader than imaging. You can actually hear that with low quality digital amplification, where the sound becomes awful when it's too loud.

On the other hand, analog does not clip, it compresses. This means that outside of the linear response zone, analog still responds, only with diminishing returns. If you double the light, you don't make the response twice as bright, but only a fraction of that. This means your blown highlights will fade to white, while your shades will fade to black.

Likewise, film response is not linear but logarithmic IIRC - much like the human eye. The available shades are split equally on the white-black spectrum.

Compression is what all radio station use : quiet sounds are made louder and loud sounds are made less loud. Compression is also what happens when you saturate an old tube amp : rather than ugly clipping, you get distorted sound with lots of harmonics. It's no more realistic than clipping, but it sure sounds better.

That was used a lot in B&W film photography. Rather than use bokeh to fade an ugly background, you could also flood it with flight while keeping the foreground in the shade and exposing for it. This worked very well for portrait. This does not work so well with digital...

That's why to be effective, digital needs a more DR than film. Because you will have to sacrifice some of that DR (the part that has a linear response) to simulate the non-linear portion of film (the shoulder you had for highlights) and get a smoother transition from highlights to pure white. Likewise in the shadows with the foot of the film response.

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