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have three SB-900s so I do have flexibility and expansion. However, I found that when I tried to use all three Speedlights, I wasn't getting the results I wanted...even after reading all those excellent books. So, I figure I better get the hang of one light and learn to use it effectively before introducing others.
The technical shortcoming of digital sensors (and color films) is not being able to handle the contrast created by sunlight outdoors (where there is skylight fill) or indoors when direct flash is used off axis and there's no bounced "spill fill" off walls and ceilings back into the shadows. That's why when exposure is set for the highlght detail the midtones and shadows wind up darker than seen by eye.
What using two flashes in a key over even fill does is change the scene range where it hits to fit the fixed range of the sensor. Adding a third flash before understanding how to do that with two WILL confuse you.
Visualize this cause and effect with two flashes, then try it:
Start in a dark room with the lens set at f/5.6 and throw a white suit and black suit over a chair Put the Mr. Gnome on top of them if you'd like
1) With just the flash on camera over the lens raise manual power until detail is seen in the folds of the black suit.
That's not how you'd typically set lights but it will show the role fill needs to play in a two flash scenario. The resulting shot will be flat (no modeling) except for any highlight reflections off the higher parts bouncing back to the camera and underexposed (white shirt will be gray) because fill is set for the shadow detail, nothing else. But that's exactly what you want for fill; even shadowless light. The goal here with fill is to just lay down an even foundation to build the key lighting pattern on, not model the object with directional lighting.
2) Once fill is revealing the shadow detail at f/5.6 raise power of the off axis "key" light. The parts of the 3D objects the key light hits will get brighter. Keep raising the manual power level until you start to see clipping in the brightest highlights in the playback warning (i.e. the blown highlights black out in the playback). Since we want detail in those highlights back off the key flash power one-click (1/3 stop).
What you will do with those two simple steps is arrive at "optimal" exposure where both shadows and highlights are reproduced with detail as typically seen by eye in person. The lighting on Mr. Gnome will unremarkably normal because everything in the middle between solid white and black will be rendered as typically seen by eye. That happens automatically when you get the two ends of the scale exposed correctly because it's how the process is engineered to work.
What makes it easy to fit scene to sensor with two flashes it the ability to independently control the fill intensity and angle, keeping it as shadowless as possible from near camera axis to avoid unfilled areas, and being able to adjust it easily to the point your sensor, at whatever f/stop you select, is recording detail above the noise threshold (the rainbow speckling seen in dark shadows) of the sensor.
If you were to do the test outdoors at light where there is no bounced "spill" fill you wound not see the core shadow tone change at all as key light is added. Indoors in an average room when you turn on the key light and raise it some of it will be bouncing off walls and ceiling and will make the shadows lighter than they were with your camera fill alone. That's just a variable you need to be aware of when setting the lighting ratio.
One you try this and see how easy it is with two flashes with key overlapping even fill turn off the fill light, grab your reflector and try to get the same results. That will work also, but you may find the reflector fill shaded in some low area shadows because it's off axis.
Next try the CLS system with two flashes. The thing to grasp about CLS ratios is that only one of them will fit scene to sensor range exactly and you need to experiment to find it. The constant for the test is how you expose the highlights, just below clipping via the playback warning. The variable is the Master (fill): Slave (key) ratio you set on the flash.
With the same targets, aperture and camera/flash positions set the ratio master and slave are equal (1:1) then adjust FEC (flash exposure compensation) until highlights on the white shirt are one click (1/3 stop) below clipping. Next change the ratio so master is 2x greater than fill. CLS will reduce fill power and at the same time increase key power to keep the highlights the same. The net effect you will see is the same highlight exposure and pattern with darker shadows. But don't expect CLS to do this perfectly every time. You need to monitor the exposure via the playback and may need to adjust FEC if CLS doesn't not keep the highlight exposure the same as in the 1:1 shot. Run the table on the ratios making the key 3, 4, 5... 8 times brighter and see what the results are.
I did a similar test the first day I got my Canon flashes back in 2004:
I set up the white binder so the centered group A master fill over the camera on my bracket hit both sides and the group B slave key only hit the right side. I started at 1:1 and adjusted FEC to get the highlights below clipping (leaving headroom for overexposure in this test) then change the ratio without doing any FEC compensation to see how consistent the metering would keep the highlights as the shadows changed.
The ratios in the top row all look pretty similar because in all of them the group A fill overpowers the group B key light. At 1:1 setting both sides of the binder get 1 fill and the right side gets 1 fill + 1 key, a 2:1 reflected highlight:shadow ratio. In the bottom row key light is progressively increased and the fill progressively decreased automatically. The metering keeps the highlights consistent as it makes the shadows darker. From 1:1 to 1:8 (fill:key) the eyedropper reading on the highlighted side varied from 227 to 232, a 5 point variation. That's a 1/6th stop difference in exposure variation.
If you try a similar test and examine the files you'll see at some point the shadows get so dark the detail in the black suit is lost. That's the ratio which exceeds your sensors range. The one that fits the sensor perfectly will have detail in the shadow and highlights and like the manual test above everything in the middle will look very normal, as seen by eye.
This test shot for a portrait session is typical of the results when ratio is set for a full range of detail:
Note the right and left sides of the histogram (from Levels in the SOOC file). The just barely kiss the edges indicating that few if any highlights or shadows are being lost.
The ratio numerically? For that shot I set my lights manually both at 1/2 power with key at 5.5 ft (my arm span) and fill at 8ft over the camera on a bracket where I shot from. The difference in distance between key and fill make key 2x brighter resulting in a 2+1:1 highlight:shadow REFLECTED ratio.
To get the same results using my Canon ETTL ratios would set the ratio to 1:2. As above that also makes key 2x brighter than fill at the subject (incident) producing the same 2+1:1 = 3:1 reflected ratio the camera sensor records.
Want a "softer" look? Here's the same file with a small Levels correction with the middle slider....
What the middle slider does is shift the midtones without changing the highlights or shadows much. Once the full range of detail is recorded at capture there are many ways to manipulate the detail in Photoshop that will change the holistic perception of "hard" and "soft". The modifiers I used for that shot were very small, about 8" in diameter.