> > Hello All: > > Assume: > > Set the white balance of an *istD to Flash. > > Make a properly exposed photo of a gray card ... by Flash. > > Make properly exposed photos of a gray card ... by ambient light only > .... in various ambient lighting situations (all-tungsten, > tungsten-daylight mix, various fluorescent-lit environments, etc.). > > Read and record the RGB values of all the gray card images using Mac's > Digital Color Meter utility or Photoshop. > > Question: > > Is there then a way to use the difference between the RGB values of the > Flash gray card exposure, and the RGB values of the various no-flash > ambient lighting gray card exposures to determine a suitable filter to > place over the flash to match the ambient light?
Simple answer: Yes (more or less). In fact some flashes come with a set of filters and diffusers, which quite often include a filter to balance the flash to tungsten lights. More complicated answer: Yes, but with some reservations. To truly match the flash to the ambient light, you need a filter that converts the whole power spectrum of the flash to the profile of the ambient lighting. That can't be done from just a three-value measurement (the R, G & B values). In practice this doesn't matter, as most lights (and most materials) have fairly smooth frequency spectra, so you can get close enough by just matching the curves at a few points. > Finally, assume no filter over the lens ... just match the flash output > to the ambient light, and color-correct the image globally in PS > after-the-fact, or in-camera by custom-white balance or by choosing an > optimal camera WB setting. If you want to correct after-the-fact, shoot RAW.
