Digital BW, The Print

Steve Kale wrote:


> And better yet, more bit depth....unless you don't like dynamic range... ;-)

Wandering a bit off topic here, but it is kind of a myth that bit depth 
correlates to dynamic range. Dynamic range is physically related to the 
range of possible photosite well potentials, often called "well depth." 
This range can be sampled in as few or many bits as you want at a 
constant dynamic range.

This is why even an 8 bit ADC on an old MF digital back with gigantic 
24mu pixels buys you so much more dynamic range than any miniature 
format (35mm size or smaller) digital camera on the market today, even 
if you read out the latter at 12 bits. The latter cameras typically have 
photosites on the order of 6 to 12 mu, and that provides a firm limit to 
the potentials available in each pixel.

What I'm getting at is that if you want more dynamic range, you should 
obsess about getting a bigger sensor with bigger photosites, rather than 
one that has an ADC that reads out more bits. I'm sure Nikon or Canon is 
going to come up with a 16 bit RAW file in the next couple generations, 
and photographers are going to freak out over it as though it is the 
Second Coming of dynamic range. But it won't actually make any 
difference to dynamic range unless there are also significant 
differences to the sensor engineering compared to current sensors.

Couple examples: I use a couple of scientific cameras that read out 16 
bits, but they have more than a stop less dynamic range than Canon's 1Ds 
II which reads out at only 12. Similarly, comparing a jpeg to a raw on 
the latter camera convinces me that there isn't a great deal of 
difference in dynamic range between the two, if any; but there is a 
useful difference in how well that range is sampled. I keep hearing 
photographers say that raw has more dynamic range than an in-camera 
jpeg, but I have a very hard time measuring this difference with my cameras.

One of the most frustrating things about digital is the fixed bit depth 
and dynamic range of the sensors. Whereas you can switch to a high 
contrast film to sample subtle tonal differences in the real world over 
a larger range of densities, digital is not so flexible. I think this is 
the source of a lot of the 'growing pains' that former film 
photographers experience when they pick up digital for the first time.

--
Jeff Medkeff
Eagle River, Alaska