How to illuminate a flat object uniformly

Just recently I was working on the project that involved shooting 24x36 inch poster on high-contrast film. The uniformity of board illumination was absolutely crucial as film being of high contrast would immediately amplify even slightest differences in my target brightness. My light source were three CFL photo lamps - one placed well above the camera and two other on the sides. To add insult to the injury the lamps were of different output and age so just placing them on the equal distance from the chart did not solve the problem.

Of course I did not have any decent light meter, as my last Selenium based light meter lost its senses many years ago. Certainly I could use my digital camera as a spot light meter but that was a pretty clunky solution. I wanted to see the whole chart and be able to move lights around and see immediately the result.

At some point the light bulb went on (pun intended ) in my head - I realized that I can use "heat map" - the approach in which each level of brightness is expressed by different color. After some searching I found out that app "Camera MX" as one of its effects has the "heat map".

In no time I installed the app and eventually located the "heat map" filter among free options. Now it was easy - I pointed my cell phone toward the chart, turned on light and observed the map. Moving slider up and down I found the camera sensitivity level just good enough to highlight (literally) the differences in the brightness. It was easy now to put my lights into the positions where all board would come up one color, with different gray scale elements being of another color. It was much easier than I anticipated. Certainly that would not work had I have the strobes, but this time around the strobes were not used. And if you ask why I needed that chart on 35 mm film - the quick answer is to get high -contrast, high resolution target for DSLR scanning setup. With the target like that tuning up and checking the lens quality or scanning rig alignment became a very easy and simple task - but that is beyond the scope of this post.


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