I'm in the market for a light modifier, but the list of candidates has me dazzled... umbrellas, soft boxes, brollies, barn doors, snoots, and all sorts of exotic thingies. But as I understand it, the Strobist approach is based on simplicity and ease of use/portability, and some of these attachments are big and complicated... as well a expensive!
For example, I can buy a 45" white umbrella with a removeable black backing, which means I can shoot into it or through it, for $29, but a 46" Softliter II will cost more than twice that amount. And I've been warned that it's "complicated" to attach an SB800 to a Softliter.
Help me out, folks. I only have little 32" white umbrellas (with black backing) that came with my B&H lighting kit.
http://www.bhphotovideo.com/c/product/425220-REG/Impact_DFUMK_Digital_Flash_Umbrella_Mount.html
They're too tiny to light a group. What would you recommend? Keep in mind that I'm geometry challanged, and need to Keep It Simple.
I cannot speak for the Softlighter, but it does appear that its shaft should be right at the edge of the light. Studio lights are made that way, but (without some rube goldberg modifications), the SB-800 would be off center. The tilted and longer shaft of other mounts help to center the offset SB-800.
What is the diameter of your 32" umbrellas? Often the marketing number is the longer curved path over the top (fabric dimension if it could be laid out flat), but I am asking the straight across diameter? I dont know which way yours is marketed.
Page 121 of the SB-800 manual shows the beam angles for the various zoom angles. This is important to umbrella users. Specifically, it shows 60x78 degrees at the 24mm zoom, which seems right to fill an umbrella. No law says every inch of the umbrella has to be filled, maybe call it 75 degrees.
The SB-800 Guide Number is not large at 24 mm. With both metered in the same umbrella, a 24 mm SB-800 meters one stop less than an AlienBees B400, so my claim is that in this case at 24 mm, the SB-800 is about 80 effective watt-seconds. The link you posted saying the SB-800 is stronger than a B400 is dead wrong, she obviously doesnt understand GN is about the specific reflector used, and that the "stronger" number is only at the 105 mm zoom position. The B400 GN is for its 80 degree reflector. When the SB-800 is zoomed to 24 mm for 60x78 degrees, its GN drops to 98.
Back to the point, on the umbrella, suppose the straight across diameter is the 32 inches. Half of that is 16 inches. In trigonometry, call that the Opposite side. The effective shaft length (fabric to flash head) is the Adjacent side of the light beam angle. Maybe the effective shaft is 20 inches (in front of SB-800), just making up a number to have a number for the example. My 40/45 inch umbrella "effective" or usable shaft length of about 24 inches.
So, may be the umbrellas arc tan (16/20) is 39 degrees, and double width is 78 degrees. This is a good match at the 24 mm zoom setting, but you could hone it, either with zoom or shaft length. To judge this result closer, take some flash pictures of the wall BEHIND the umbrella, opposite the subject, to judge how much light spill you see back there.
What size of group? 8 people or 80 people? Umbrellas are inexpensive, no reason not to get what you want, and you probably do want larger, but I bet what you have would do 8 people. It becomes larger if you move it back a little, but it needs more power then.
For portraits, the idea is that moving an umbrella in close, as close as possible, stand pole at 2 feet instead of 4 feet, makes the fabric be larger in relation to the subject, for better smoother lighting, with more wrap around smoothing effect for portraits. The flash tube is the source for computing inverse square law, but the fabric is the source for computing width or angle of the beam. Substituting a larger umbrella appears the same size as a small one up closer, for this same better quality of light, so large is good for that reason. But of course, up close the illuminated area gets smaller... no room to spread out. A shoot though umbrella is great because it can be even closer. and larger... fabric at 2 feet instead of 4 feet on opposite side of pole. The general idea is large, and getting them up close is one way to appear large compared to subject, for the smoothing quality of light this large light produces.
Up close, you do see the umbrella spokes in the reflection in the eyes however (either way), which makes soft boxes popular for studio lights. My opinion is there isnt much difference other than the spokes and the spill. The key factor is "large", which may mean "close". I bought a 60 inch Photogenic Eclipse Plus white satin umbrella because it has white satin liner inside, over the spokes. It hides the spokes in the eyes, but I dont use it because it just doesnt give the same magic color image that the translucent white has (when used as reflected).
Anyway, instead moving the umbrella back, 12 feet instead of 6 feet, makes it smaller, and more "harsh" as a point source, but it illuminates a larger area for the group (needs considerable more power or more lens aperture). There is a large difference in the "quality" of light between 2 and 4 feet (due to relative size), but not much between 6 and 12 feet (because 6 is already small with respect to the subjects face).
The trick for groups is to have each flash wide, with the stands about inline with the edge of the group, and far enough out to be pointing in at a 45 degree angle towards center of the group, or very slightly past center (lights crossing, so to speak). The 45 degree angle is a magic number concerning the square root of 2, etc. The light at the center from TWO lights at 45 is about the same intensity as one light at the edge of the group... i.e., very even lighting across the group. The angle makes the shadows be worse however, so you pay close attention to the shadows. At minimum, each head has to have a clear view of both the camera and the lights.
That should cover the width of the group. As to height, point the shafts lower than the subjects eyes, if you need it to reach the floor. If centered on the heads, light going over the heads probably doesnt help anything.
