The original thread with a question about using barn doors on powered strobes turned into a great thread with tons of information, tutorials, diagrams and examples. I've decided to continue any lessons and challenges here in this thread and I'll start things off with the last challenge from Scott (lightingman).
HI Robert:
I have a new challenge for you. It is based on one of the most common problems inexperienced photographers encounter as a result of not yet understanding the science of light. Here is the scenario. The photographer photographs a full length bride on black seamless paper. Rather than the background appearing as black, it looks decidedly gray in the photographs. This all stems from lack of independent control over each area within the scene.
The challenge:
Create a portrait (head shot) using black seamless and using the science of light, create a uniform background that ranges from 18% gray to maximum black.
Show me a series of images each with identical lighting on the face and hair but with the background ranging from 18% middle gray to max black in half stop increments.
The goal here is to teach how to maintain control and accurately predict what your background will look like even before you have made an exposure.
Although I did get some help I never really got the answers to my questions and still had to do some thinking to solve the problem. I knew how to lighten a dark background and isolate that from the subject and the lighting created for the subject but I wasn't exactly clear on what Scott wanted and I did not know how to know where the heck 18% gray was....BUT, I knew that if Scott was asking for this that he knew I had the means to do it and figure it out. Wilt helped a bit but honeslty between using EV numbers and percentages of reflected light, I really was not getting it.
OK, I knew that the way to get a dark color to appear lighter is to add light, and I knew that I could get enough light on the black seamless to have it appear gray, so I had to figure out where 18% gray was and use that as my lightest image in the series. The only thing I could come up with that made any sense was that I know when I look at a histogram I see 5 distinct regions which I believe represent a 5 stop dynamic range, going from shadow (left side) to highlight (right side). I also know that when I shoot a gray card (which should be 18% gray) it appears smack in the middle of the histogram. So that tells me that if 18% is right in the middle of a 5 stop range then it would be 2.5 stops down to black or 2.5 stops up to white. That also told me that there would be 6 images in the series ranging from 18% gray down to black in half stops. If my logic is correct then it also tells me that black is 2.5 stops less than the taking aperrture, which is metered for 18% gray.
Assuming all this is correct then here is what I did.
I set up for a portrait with my main light behind a diffusion panel, a reflector as fill, hair light with barn doors, and placed the background light directly behind the subject, perpendicular to the background and parallel to the ground to get the most even lighting without creating gradients.
Once I had my main, fill and hair light set up I metered the shot, pointing the meter at the main light from the subject's position and got a taking aperture of f/16. Based on that, I knew that I needed a reading of 2.5 stops less than that, or f/5.6 + .5 to achieve black and that if the background metered the same as the taking aperture then that would be my 18% gray background.
So that's where I started, by metering the background at the background, pointing the meter at the background light and setting the power on that strobe until I metered f/16. I took that shot and then adjusted the power until I metered f/11 + .5, f/11, f/8 + .5, f/8 and f/5.6 + .5 and took a shot at each setting. This gave me 6 shots, in half stops, ranging from 18% gray down to black.
I've attached the images I shot and a diagram of the setup. Nothing else changed from shot to shot other than the exposure for the black seamless. All other lighting, taking aperture, shutter speed and ISO remained the same throughout.
I believe I was successful but the only thing I can see is that the background is not 100% uniform. I did try a few different positions for the background light and got maximum coverage and even lighting with the light directly behind the subject. If there is a way to get a completely uniform background then I would love to know how it's done. I had thought about not using the hair light and trying to light the background more evenly with a second light but I figured I would post and start here before making changes that might not be necessary or correct.
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