I agree with Ron that a simple adjustment by moving he middle slide in Levels may not result in a good adjustment curve. In fact, depending on the needed correction it might actually make output less linear.
Attached is a raw output graph that is not atypical. Making an adjustment curve by the use of Levels will make either the highlights or shadows less linear, depending on the direction of adjustment.
Sandy King
I purchased Mike Ware's pdf and did some quick testing about three weeks ago to investigate his premise that simply setting the gamma from 1.0 to 2.2 in a Photoshop levels layer would be sufficient to create a digital negative with roughly linear output.
I don't have time to comment fully right now, but here is my short take:
Dr. Ware is correct that gamma adjustment can be useful in increasing ink laydown in the midtones. This won't impact the white or black endpoints, so if you can't achieve sufficient density for paper white with your printer and inkset this provides an incomplete solution. I did not find changing the layer opacity to be helpful in adjusting the white point, but I do get sufficient density so I didn't need this tweak and forged ahead. While it was possible to create a printable negative adjusting just the gamma, the output was significantly nonlinear and a correction curve was always required. The gamma adjustment might be used to create a less severe correction curve, but using my materials and printing carbon transfers it could not eliminate the need for a curve. The two biggest technical challenges in printing digital negatives are 1) obtaining sufficient density for paper white and highlight separation and 2) achieving linear output. Dr. Ware's method did not provide solutions to these in my workflow.
These are just some initial thoughts. I may retest using palladium to see if my results are consistent. The pdf is articulate and concise and documents a simple but structured approach for creating digital negatives. It is worth a read and I’m sure will provoke some useful discussions.
As always, your mileage may vary. Here is what I tested:
Epson R800
Arista OHP
Canon Transparent Clear Film
single transfer carbon
X-Rite 810 densitometer (for measuring linearity of prints and tablets)