It is misleading in the extreme to assign a firm ISO number to an old colorblind or orthochromatic emulsion.  As I have addressed in previous sections, the old emulsions respond differently than the modern miracles we've grown up with.  Any emulsion produced without a gamut of panchromatic sensitizers and industrial cooking controls will exhibit a range of 'speeds' — determined by the temperature of the light, the color and reflectance of the subject and the inevitable variability of each handmade batch, and the developer used.  The old emulsions are best described qualitatively and with an assumed '~' sign. 

For better — and for worse — modern film emulsions have a lot of leeway for exposure and development.  The bright side of this is obvious: it’s pretty hard to make an exposure/development mistake serious enough to ruin an image.  The disadvantage is the loss of control previous generations of photographers took for granted.

The following sets of images cover three overlapping areas of artistic control/potential pitfalls (two sides of the same coin) — exposure, development and filter factors.

The histogram on a digital camera is the best way I know to understand the old emulsions. Unless you are deliberately choosing otherwise, you are aiming for a smooth, balanced bell curve that doesn't compress or clip either the shadows or the highlights.  Visualize a histogram when you look at the bottom row of the ColorChecker chart.  A nice 'curve' in our case is six, easily distinguishable density blocks.  The six plates below illustrate a number of overlapping issues that influence the exposure curve of a negative.

The six plates go from one exposure extreme to the other.  The first plate is very much underexposed and the last plate is very much overexposed.  There is quite a bit to learn here.  All the plates were photographed on a light table and output with no manipulation beyond inverting and cropping.