cgardner
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DYI is a good way to learn the cause and effect of how flash and diffusion work. Size of the modifier and the distance it is used from the subject are variables which affect how parallel the rays of light hitting the object and creating the shadows are: the more parallel the light rays from the source, the more distinct the shadows it creates will be.
What makes a shadow look hard or soft is a function the ratio of umbra (dark core of the shadow) to penumbra (fuzzy transition zone). The softer looking penumbra is created when single light source is so large relative to the object creating the shadow that the light hits it from many different directions; from the outside-in. Hence the rationale for using large key light modifiers. The larger the modifier, the more it wraps the light around the object making the penumbra larger and the darker core umbra smaller having the net effect of making the shadows lighter.
But there's another way to make light hit an object from different directions: use more than one light. That is the cause and effect relationship between a key light (i.e. the one which creates the highlights on the front of the subject facing the camera) and a secondary source for fill. The fill creates a second directional component which works to modify the shadows cast by the key light in a way similar to how a single large modifier will wrap the object to lighten the shadows.
Its possible to create very flattering lighting with direct flash if the key light is placed to model the shape of the face in a way that is perceived as natural and enough neutral fill is used to keep the shadows light and reveal detail. What the key and fill do when working together cooperatively is reduce the contrast of the scene to the point the camera can record detail over the entire range when the exposure is set for the highlight detail. The lighter the shadow become the softer the lighting will look perceptually, even if the sources are direct unmodified flash. I learned this in a very practical way: learning to shoot wedding receptions with two direct flashes back in the early 1970s.
Most thing in terms of starting with the key light to create the lighting pattern then filling the shadows, but the best way to grasp the cause and effect of how key and fill work together perceptually is to start the other way around with only the fill light on to start.
FIll placement is important because its role is to lift all the shadows the camera sees. Logically if the fill source creates a shadow on the face the shadows are not lifted there. That will not be a problem if the key light also hits that area, but what happens when a fill shadow crosses a shadow cast by the key light? A dark harsh void will be created. The #1 rookie mistake in lighting is putting key and fill on opposite sides of the face. The face will look evenly illuminated, but because the shadow cross in low areas there will be very dark and harsh looking shadows in low areas like the smile creases around the mouth, the corners of the mouth, and the base of the nostrils where small very dark crescent shaped shadows will appear. If you see any of those characteristics in a lighting pattern its the result of the fill creating shadows which overlap the shadows created by the key light. Logically the way to avoid the problem of shaded fill and crossed shadows is to start by placing the fill where it will create minimal shadows on the front of the face: directly over the camera. Yes that is a recipe for flat shadowless lighting but flat lighting is what is ideal for fill because the other component, the key light, has the role of creating the illusion of shape.
So humor me and place your unmodifed fill flash directly over the camera lens (a flash bracket is ideal for this) put it in M mode at f/8 and gradually raise the power of the fill. What you'll see in the images is the detail emerge from the base noise level of the camera sensor. Keep raising the fill until you see the desired amount of detail in the darkest areas of the face.
Once the fill is set to reveal the shadow detail put the key light, also unmodified, 45 degrees from the nose and a foot or so higher than eye level at its lowest power setting. In that configuration where you've already established the shadow level with the fill even small amounts of light from the key light overlapping it will start to form highlights and create the illusion of 3D. As you gradually raise the power of the key light the areas it hits will become progressively lighter, but the areas you first illuminated with the fill will remain the same. Eventually when the key light power is raised high enough you'll see the highlight areas start to clip and blackout in the camera warning. When that starts to happen back down the power of the key light until it disappears.
What you will wind up with in that little 15 minute experiment is a perfectly exposed image in which the camera is able to record the full range of tonal detail. It will teach you the cause and effect of how key overlapping even fill work together to reduce contrast to fit the range the sensor can record. Equally important it will give you a good baseline for evaluating the effectiveness of your DIY modification experiments. What you will discover is that when the key light is well placed to put light in the eyes and make the nose shadow fall down naturally along the base of the nose and over the top of the nostril, and sufficient foundational even fill is used to first raise the shadows and reveal their detail, its possible to achieve very flattering lighting with direct flash. What softer looking lighting? Start with more fill power (i.e. a lower light ratio).
From that direct flash key over neutral fill baseline as you add modifiers of increasing size to your key light you'll have a good basis for comparison. What you'll find, as I have, is that a point is reached where the size of the modifier placed on a hot shoe flash begins to make the use of the flash more cumbersome. Your DIY creation is making the light more diffuse than direct flash, but its cumbersome and impractical for field use and it will also block the optical sensor on the slave flash requiring the additional expense of radio triggers.
I learned flash using two direct flashes used as described above and experimented with various bounce and DIY modification techniques over the years. I've found, all things considered, a reflector-diffuser to be the best general solution for the type of location shooting I do with my Canon flashes. I don't try to make them into something they are not designed to be -- studio style flash heads - but have used what I know about the physics of light and cause and effect of making lighting flattering to find a good balance between results and practicality.
Because I used two flashes in a key / fill configuration the top of the diffusers extends over and covers the vertically oriented flash head. Instead of just blasting the lens focused light through a layer of diffusion to change its direction the design changes the reflective angle of the light: two different means to the same end. But what I found with experimentation is that the reflective angle approach diffuses the light more than a similarly size softbox would. So instead of moving around with a cumbersome SB on camera or stand I get similar diffusion from a reflector-diffusers which weighs a few once and folds flat for storage. By orienting the flash head vertically I can position the slave anywhere without blocking its sensor, which on Canon flashes is on the front over the red AF assist lamp (which has no role in the wireless control). The signaling is done via visible pre-flashes from the Master on the flash bracket and the identical diffuser on it spreads the light on a wider arc than the bare flash head zoomed to 24mm so I rarely have any signaling problems and haven't found the need to use radio triggers. On occasions where I find I need more diffusion over a wider area on the key light than the foam diffusers provide I use an umbrella on the key light (which is mounted on a generic light stand umbrella bracket). Its one of a million possible modification options, but the one I've found to be most convenient and compatible with the way the Canon flash system is designed to work. 
Chuck
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