One of the most frequently asked questions I get is how I use the "Flash/Ambient Hybrid" technique when photographing interiors. Although it sounds somewhat complicated on the surface, the concept is really quite simple. The execution, however, takes some practice but nothing that can't be achieved through proper planning and of course a little "good ol' fashioned know-how". Before I delve into the technique stuff, I think it is important to know the "why" behind the reasons I choose to utilize this technique.
Truth be told, the genesis of this approach was actually me trying to find a work around for the exaggerated color casts and unacceptable window pull (and bloom) I was getting from HDR at the time. Like the Prodigal Son, I had strayed from the rigors of becoming adept at using multiple strobes for the greener pastures and simplicity of HDR. After a few months of shooting for HDR I managed to refine my processing so that it appeared realistic, but it was still lacking. It was lacking the snap and the crispness that I got from using an off camera flash. Not only that, but I never found an acceptable substitute for managing color casts without having to spend way to much time in post. So it was back to strobes, at least for a little while, that is until the learning curve reared its ugly head again. While I found it easy to nuke a room and get a good window pull and consistent 5400K across the scene, the harsh shadows and lack of ambient feel again had me questioning if a purely strobist approach was for me.
After reading a few comments from Peter Lyons on the PFRE flickr group regarding using a flash lit frame as a layer in "color" mode, it hit me. "Why not shot one frame with strobes and one with just the available ambient light and then use the best parts from each one?" I know, pretty basic stuff but then I was faced with the challenge of implementing this into a consistent work flow. Ultimately I settled into breaking the shots into two parts. The first part was my Strobe lit scene (Iran uses anywhere from one to four strobes, the main being a 580 EXII and the rest Vivitar 285s all triggered with Cowboy Studio triggers). I discovered this frame didn't have to be perfect but it did have to meet a few criteria if it was going to solve my problems. First, the scene has to be evenly lit. It was okay if there wasn't a hint of ambient light in the frame as long as the shadows that needed to be filled in were. Secondly, the shutter speed has to be fast enough to negate the color casts of the other lights, typically lamps and fixtures. Since a fast shutter speed was required, I also used this as in opportunity to dial in my window pull. Finally, the strobe lit frame should not have any harsh shadows or hotspots that couldn't easily be fixed in post. The result is something similar to the lighting in the photo above. (click to see a larger version)
Once that frame was captured, I turned off my strobes and simply changed the settings on the camera to shoot a series of bracketed exposures. I found that working within the limitations of my particular camera's sensor (Iran uses a Canon Rebel XTi), I needed approximately 5 exposures 1.3 EV apart (Iran used to bracket manually using the histogram, but now uses the Promote Control by Promote Systems). Since my ISO and Aperture were already set to make the most of my strobes, I varied the shutter speed for each exposure. The criteria for this frame was minimal. All I had to do was capture the desired dynamic range needed to retain the sense and direction of the natural light. Above is the ambient version of the above photo. (click it to see a larger version)
From here, it is all post. After doing typical RAW edits on the flash frame and then rendering a HDR or Exposure Fusion image from the brackets, I bring the two parts into Photoshop. I made the "ambient" frame the background and drug the flash frame on top as an additional layer in "normal" mode. Using just the "opacity" slider I find a sweet spot between the two images. If I need a better window pull, I use the quick selection tool to lasso the windows, make a new layer via copy and again adjust opacity to taste. If the WB isn't consistent I make a duplicate copy of the flash frame, set it to "color" blend mode and use the opacity slider to force the color temp of the flash frame. Of course there are several other issues that can be addressed with these separate frames and I will borrow from each accordingly. The result of blending the two parts is the first image in this post marked "hybrid final".
As I continue to improve my strobist skills I find I have to rely less and less on workarounds such as this. Nevertheless, this approach provides great flexibility in post and less finesse needed with the lights on site. Oh ya, and a fairly decent end result!