45" Smith Victor - 60" Photogenic Eclipse - 32" B&H Impact
Three popular white umbrellas, 32 inch B&H Impact, 45 inch Smith Victor, and 60 inch Photogenic Eclipse, of different sizes. Generally the rule of thumb is that the umbrella evenly illuminates an area of its own size when located at a distance about its own diameter, and will be adequately soft at that distance. A greater distance will of course be be a wider field, and will be less soft.
We hear that the larger umbrella requires a larger flash, which is not actually true, unless you plan to place it farther back. It should not be ignored that a 60 inch umbrella can be a bear to work around in a smaller room, but pretty much any two sizes of umbrella (assuming same type of reflective fabric, and same light) will meter just about the same if the fabric is at the same distance. Not exactly, as the larger umbrella has a longer shaft so the light can fill it, and that path distance back to the light is a factor. But the umbrella is just a fabric panel which reflects the light hitting it, same as the ceiling reflects our bounce (umbrella is easier to aim however). Same as the ceiling, the distance is the factor that requires the power, not the size of the reflecting surface.
The table results are with the same SB-800 (24mm zoom) at full manual power, metered at six feet with a Sekonic meter (ISO 200), carefully moving the light stand to place each umbrella interior fabric center at the same distance from light meter each time (using a string plumb bob and a measuring tape on the floor). All umbrellas are mounted at full shaft length.
|at 6 feet|
|at 6 feet|
3 foot off center
|at 6 feet|
6 feet off center
when off center
|32 inch||49 points|
|f/8 + 4/10 stop||f/5.6 + 8/10 stop||f/5.6 + 0/10 stop||57 inches|
|27.5 inch||42 points|
|f/8 + 6/10 stop||f/8 + 1/10 stop||f/5.6 + 1/10 stop||53 inches|
45 inch silver
|27 inch||37 points|
|f/8 + 9/10 stop||f/8 + 0/10 stop||f/5.6 + 3/10 stop||40 inches|
|22 inch||33 points|
|f/8 + 7/10 stop||f/8 + 1/10 stop||f/5.6 + 1/10 stop||51 inches|
Three feet off center would be a six foot wide subject, at six feet. Engineers think of field width as that width where it falls to half strength, in the usual units, which would be one stop here (photographers may not want the edges to be one stop down however). The 12 foot wide field would exceed that limit here, and the umbrella needs to be moved back farther for that width (back farther is weaker). The Alienbees silver umbrella is built to be curved more, to be a couple of inches "deeper", where the others are a bit more flat, less deep.
In the next pictures below, each umbrella is simply swapped out, at full shaft length, without moving the light stand or changing the flash power. Therefore, each smaller size is closer to this background (Savage #27 Thunder Gray), due to the shorter shaft lengths. The SB-800 flash (24mm zoom) is 1/4 power for all (and f/11 for all, ISO 200), and neither power nor exposure was changed for shaft length distance (umbrellas differed about 1/3 EV at each length, due to closer distance of the shorter shafts). Brighter naturally appears wider than it really is. Yard stick is about 8 inches from background. The end of the yard stick starts 12 inches from the center axis of the umbrellas (marked with a dab of tape). Camera is off center about four feet, mostly seeing the dark outside spill, but also peeking under the umbrella slightly.
60 inch Photogenic Eclipse
Fabric center (interior) is 46 inches from background.
45 inch Smith Victor
Fabric center is 42 inches from background.
45 inch Alienbees Silver (it is silvered inside, and has no black cover)
Fabric center is 41 inches from background. This silver is about 1/3 stop brighter than white, but this one is a deeper umbrella, and is not as wide.
32 inch Impact
Fabric center is 36 inches from background.
Black cover is a bit thin, does not fully block the light.
FWIW, the 32 inch umbrella fabric is at 36 inches and the flash tube is another 22 inch shaft length (less about 3 inches overhead for the flash head and mount) which is 55 inches path of the light.
The 60 inch umbrella fabric is at 46 inches and the flash tube is another 32 inches shaft length (less about 3 inches overhead for the flash head and mount) which is 75 inches path of the light.
75 inches / 55 inches is 1.36x longer path, and the inverse square law computes about 0.89 stop difference (1.414x longer would be 1.0 stop). And that is about right.
In the next pictures, in order to compare widths at a more constant brightness, the umbrellas are swapped out again, at full shaft length, but the stand is moved each time so each fabric center is kept at the same 72 inch distance from the background (for all). The SB-800 flash is full power (f/11 plus 3/10 stop, or f/13, ISO 200), and unchanged for any. These are fake flowers, and they are between 2 and 3 feet off center of the umbrellas.
60 inch Photogenic Eclipse, fabric center (interior) is at 72 inches.
Umbrella distances were measured with a plumb bob string from outside center apex, but the Eclipse is built different, with the white fabric under the ribs, so five inches was always added to it, to compensate for where the interior fabic was (so interior was at 72 inches). The dab of tape on background is the umbrella center axis.
45 inch Smith Victor, fabric center is at 72 inches
45 inch Alienbees Silver, fabric center is at 72 inches.
Silver is a bit brighter, but not as even for pictures of people. Notice the harder shadow above the yardstick, in both samples. I love the Alienbees lights, but I prefer the Smith Victor umbrellas.
32 inch Impact, fabric center is at 72 inches
32 inches seems small to me, but I cannot fault it.
40x32 inch Alienbees Large softbox panel is at 72 inches, with an Alienbees B400 light metered to f/13, ISO 200. This one is cropped a bit wider to include the edge of the softbox at left.
These results show no evidence that umbrellas have more frontal side spill than softboxes. Looks like the opposite, but excess spill certainly seems negligible to me. They are all big wide lights. (However shoot-through umbrellas do put 2/3 of their light out the back side, to scatter all over the room, which cannot be good).
Make no mistake, the umbrella is an extremly fine light source. The light from an umbrella is the same as, and is as good as the light from the softbox. Both work by the virtue of being large, like 40 inches in size. Size is what matters (and closeness, to preserve that size). The softbox can be more convenient to use. As compared to reflected umbrellas, the light stand is not in the way out in front - can be closer. So can shoot-through, but as compared to the shoot-through, softbox has a flat panel, and of course there is no rear spill. Also most softboxes are doubled baffled with less hot spot than shoot-trough umbrellas. The softbox is simply handy to use. But the light is the same (if same 40 inch size). The softbox cost is relatively high, and attaching speedlights is a real issue, and the disassembly and storage can be complications (all of which is solved by umbrellas). There are pros and cons regarding how handy, but never doubt that the umbrella is an extremely fine light source, and all that you need.