Disclaimer, 22 december 2015: Only recently I realized that the variations measured are in tens of stops. F5.6 7 on the meter means is F/5.6 + 2/3 stop or F/7.1, rather than close to F/5.7. This make the measured deviations and gradients considerably larger. With proper illumination these gradients can still be eliminated.
Making wet prints is at the heart of what I do here, but what would a photography blog be without photos of the prints I make?
The first thing that might come to mind is simply putting the print onto a flatbed scanner. A proper photo scanner will allow you to make high resolution scans of your prints and the included software can correct for glare and color shifts. As you might have guessed when you entered the methods section, I chose for a more elaborate and more sophisticated method to get more control over the lighting conditions and be more true to the real viewing experience.
The method I opted for consists of making digital photos of the prints under reproducible, consistent lighting conditions. For this the print is laid on the table and lit using a studio strobe from one side under a 45 degree angle. The strobe fires into a 150 cm diameter white umbrella to create a nice large diffuse light source. The camera is at right angles above the print.
The power is tuned to illuminate the photo at a consistent exposure value, for which I chose an aperture of f/8.0 and a shutter speed of 1/125th of a second at ISO 100. I chose these settings to allow for more than sufficient depth of field and to be close to the sharpness sweet spot of the lens. The shutter speed is close to the flash sync speed of my camera (1/160th). Firing at the sync speed allows me to reduce the influence of ambient light as much as possible, so that independent of the time of day or the weather, I will get consistent results. Above you will find an overview photo of the setup that I use. In test shots I add a black-grey-white card set to allow easy white balance corrections. The evenness of the lighting was checked using an incident light meter. Measuring at the four corners and at the center results in a consistent f/8.04 ± f/0.02.
Compared to using a flatbed scanner this setup has the advantage that all the controls are with me, instead of a black box algorithm in the scanner or the scanning software. Additionally, it allows me to reproduce photos that are still wet. Typically one would only view and judge prints when they have fully dried, as I will discuss in another post, but given that I will be talking about the printing process you might also see some prints that are still wet or treated in such a way that does not allow me to use a flatbed scanner.
When looking at the example image below, I noticed a few differences between the print and the reproduction. I will have to tweak the post processing steps by correcting for any exposure offset created by the camera. It seems that at f/8.0 at 1/125th of a second the reproduction is slightly over exposed (about half a stop) and could use a little extra contrast to come close to the print. I will append this article in due time to have a complete overview of the reproduction process.
I hope this explanation was clear and instructive. If you have any comments or think that the method can be simplified or improved, please drop me a line!