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With still life subjects, animals and full-body shots of people in actions I find one of the more effective ways to create the illusion of 3D is to place one light behind the object striking the back at 45° to create rim-light defining the overall shape, with the second light centered and raised above the camera so there is natural looking downward modeling on the front but none of the no harsh unfilled shadows while will occur if the frontal light is placed off to the side.
Simply put, shoot into the shadow side of the light to make it look round by curving the light around the the object from behind. Most of the time I use a master flash on a bracket with an off axis slave key light and park the off camera flash behind to define the shape and created foreground > background separation.
The shot above were taken with two speedlights, one about 45° behind and to the left as "key", the other over the camera on the bracket as "fill". The fill, while centered horizontally, isn't totally flat. The downward angle created by the bracket mimics the downward angle of skylight outdoors. Natural key and fill come from overhead. In general when using flash you also want to keep them at 30° - 60° degrees downward for a more "normal" look because out baseline for normal is seeing things at illuminated naturally that angle in daylight. That's not to say you can't put lights at other angles, but the result will not match the "normal" baseline. For example if the light hits horizontally it will be similar to the setting sun, which is ideal if all the other lighting clues in the background say its late in the day. But if the things in the background cast 45° shadows from the natural light and the flash is sending shadows sideways on the foreground the two lighting angles, by comparison, will seem unnatural. That's why eye / lens level flash tends to look "fake" when used outdoors.
The ideal strategy is three flashes: one for rim light directly behind in back to define the overall shape (accent)
one off axis in front (key) at a 30°-60° angle to create shape clues there
and one near the camera to control the tone of the shadows in front the camera sees (fill)..
Combined they create the clues the brain uses to imagine 3D in the 2D photo:
If only using two lights I change up the strategy a bit. Instead of putting the rim light directly behind I move it to the side behind which wraps its highlights around the front like the crescent highlighting on a new moon. Then I move my frontal light up off axis more than I would as fill in a three light scenario so it is directional, creating some directional modeling, but not so far off axis in will create harsh unfilled shadows. The rounder and flatter the object the more the frontal light can be moved off axis. For people the tall nose shadow cast sideways is unflattering, but that's not a problem for birds or most other animals.
I shot some hummers a few years ago using my Viivitar 285HV flashes triggering one via Radio (PW) and the other optically because I only had one set of radio triggers. I mounted the rim light flash above the feeder pointing down at about a 45° angle. The hanger bracket for the feeder is the lower right of the photo below...
The other flash triggered with the PW I placed on a stand so I could also move it around rather than keep it centered on my bracket as I would shooting people. I started shooting in the early evening with the setting sun coming from the left not hitting the subjects but providing background light. I controlled the tone of the background with camera shutter speed.
This is an ambient only shot:
Here I added the flash.
The off axis flash on the super clamp is coming from about the same direction as the natural light in the first shot.
When shooting into the shadow side of ambient light I take note of natural modeling on the shaded side. Skylight isn't just "flat" fill it has a dominant direction, always from above (even at sunrise and sunset) and from an angle where the sunlight hitting the atmosphere and clouds is brightest. The simplest way to train your eye to see it is take some ambient only shots first, then find the spot where it is modeling the subject in a way you like, then add your "key" (main modeling source) so it hits at the same angle. The produces a double key light effect like putting a small direct flash in the middle of a huge softbox.
One thing adding the flash does vs. natural light alone is create more contrast in the gradients due to the inverse-fall off effect. Skylight isn't just "fill" for the sunny side. When the sun is at the back of a subject is the "rim" light and the skylight hitting the shady side has both a downward/sideways "key" component in addition to the 360° wrapping "fill" .
The other thing flash does is add stronger, contrasting specular catchlights. Compare the catchlights in the eyes in the two shots. Also notice how the flash helps reveal the texture of the feathers? That's because feathers have gem-like facets that will mirror a direct collimated source like a small flash head more than the diffuse sunlight. So for birds smaller modifers / direct flash or shiny silver reflectors are actually better than softbox.
The bigger the source the bigger the catchlight in the eye will be. For example of you took that aluminum tray you cooked the Turkey in yesterday and bounced your flash backwards into it you'd have the perfect modifier for shooting hummers: big but specular creating.
Note how in the flash shot the catchlight is "dead" center? That's what happens with the frontal light is close to the lens. There is it most effective for fill in a three light scenario, but when using only two you'll want to move the frontal light to where the catchlights look more "natural"
In the shot above the frontal flash was moved lower and to the left of the camera moving the catchlight and creating the illusion the bird is looking down and revealing the colors on the front and the detail in the wings. It was still daylight but I cancelled it with a faster shutter speed. With just the flash and less ambient you can see how the specularity in the feathers is better revealed. That's also the result of USM (sharpening) in PP. The AA filter in the camera blurs tiny specular refections so USM is needed to bring them back to "normal" as perceived by eye. I do and overall sharpening, dupe and over-sharpen even more, then very selectively add it where I want texture.
The more you cancel the ambient light the better you will be able to freeze the acton of the wings with a short flash duration. That's why I was shooting late in the day. The way speedlights work the lower the power the shorter the flash duration. Using the flashes at low power very close will create the shortest burst and freeze action the most if that is your goal.
Freezing a hummer in mid-air is a POV reveals details not see by eye. It's a bit like what you'd see on a rod in a case in a museum. For someone how has seen a hummer in person buzzing with wings blurred that frozen POV helps them understand what they aren't seeing by eye. But consider a person seeing the photo who has never seen a hummer in person. For that viewer the shot with the blurred wings conveying the idea it is hovering like a bee over a flower would convey better what is is like to see one in person. To cover all the bases shoot it three ways as shown here: ambient freezing partially with shutter, ambient + flash with blurred wings slow them down, and frozen in mid-air so he shape of the wings can be seen.
Again there is no rule that says YOUR lighting goals need to be the same as mine. Most of the time I go for a natural seen by eye in the natural light look when adding flash. But if you understand and can execute that strategy it's easy to get a less natural look for dramatic / creative effect — just do anything else. For example if you wanted to make the hummer look like it was rising from the bowels of Hell you'd want to light directly from below with a red filter on your flash. If you wanted to make it look like an angels descending from heaven on gossamer wings you'd want to light from above from a higher that "normal" angle with a lot of lens flare for an otherworldly look.
Moving the frontal flash to the right would have created more 3D modeling with light, but put the front of the bird and wing more in unfilled shadows (a third flash for fill would be needed). I took some that way with the two lights but didn't like them as much because of the dark shadows.
What I noticed using only one radio trigger with an optically fired slave is that the latency between the first flash firing and the second seeing it's light and triggering caused a bit of blur in the wings in the mostly flash shots like the one above – a double exposure effect. It wasn't planned but I liked how it added just a bit of blur. If you want totally frozen action it would be best to fire the lights with radio triggers on all of them so them all fire at exactly the same time.