Monday, August 25, 2014
Day 82, Learning 82: HSS, IGBT and other Lighting Alphabet Soups
There's plenty of acronyms in the photography world. Generally, you don't need to be well-versed in them... until you're trying to buy a piece of equipment. Then it's the difference between having the ability you want and being frustrated in the field.
Nowhere in the photography world is this more evident than in the highly technical realm of lighting, so today I'm going to write about a couple of terms that are important when choosing a studio strobe or flash. First, a definition: by studio strobe I mean something other than a hot shoe flash. There are numerous varieties and a jaw-dropping range of prices. That's where deciphering alphabet soup becomes important.
Today: GN, Watts-Per-Second, HSS, IGBT, and T.1 versus T.5. Basically, every important consideration when it comes to assessing the light produced (not the fancy bells and whistles).
GN vs Watts-Per-Second
While hot shoe flashes general talk about power in terms of GN, watts-per-second is the lingo of choice among studio strobes. For folks making the transition, there's not a neat way of comparing flash power to strobe power. GN isn't calculated in a very uniform way among manufacturers, and the reflectors and other modifiers make the same muddle of watts-per-second. However, general rule of thumb is most hot shoe flashes produce between 50 and 65 watts-per-second. That means a 100Ws strobe produces about twice as much light-- or 1 stop more light-- than your average strobe. A small increase. 200Ws produce 2 stops; 400Ws produces 3 stops, or the equivalent of eight SB800s firing at once. 800Ws equals sixteen hot shoe flashes.
Let's put that in field-terms: with an unmodified 400Ws strobe you can shoot at f/4, 100 ISO, 200 Shutter speed. More or less. That'll be some hard light-- add a scrim or softbox and you're shooting at about f/5.6.
HSS & Hypersync
HSS stands for high speed sync. This is located somewhere above regular speed sync. Why is it important? Nutty things happen when you start speeding. Most cameras sync at either 1/200 or 1/250 of a second. The Pentax 645D, alas, syncs at 125. It works this way: flashes are faster than 250th of a second. Most hot shoe flashes range widely, from 320 to 1/40,000 of a second, depending largely on power: Full power equals slower flash. Lower the strength and you lower the flash duration. This is important because your shutter doesn't open all at once. In order to move as quickly as it does-- sometimes with shutter speeds of 1/4000th of a second, it opens two parts, forming a moving slot that crosses the sensor from top to bottom. If the flash duration is shorter than the shutter, then a portion of the sensor does not receive any light from the flash.
There are times when the object you're photographing is moving so quickly that a shorter duration than 1/200th is needed in order to freeze the action. For this to happen, one of two things must happen: either the flash needs to be evenly distributed over time so the faster shutter can complete its action, or the flash needs to be shorter in duration so that the flash freezes the action. In the latter case, you keep your shutter at 1/200th of a second and let the flash freeze the action.
For hot shoe flashes, the approach is to "lengthen" the flash duration by produces a series of rapid small flashes. In doing so, however, the power is also reduced, generally to about 1/4. If you're in bright light (i.e. outside) forget about it. Your flash won't make a dent in the ambient sunlight. If you're shooting indoors, this can be helpful if 1/4 power is what is needed: i.e., your ambient light is reduced by the high shutter speed and your aperture/ISO are sufficient for the flash.
Certain flash triggers provide another option for hot shoe flashes: HyperSync. This process improves the alignment of the shutter and flash so that the sensor is exposed while during the tail of the exposure, rather than at the start. This prevents a black band from appearing (due to the complete absence of light), but results in both less power and a gradation of light from top to bottom.
The other approach is favored by high end studio strobes: shorten the flash duration using IGBT.
A traditional studio strobe has a sharp spike and a long tail as the light dwindles. Half of the total amount of light it produces occurs in the tail. This isn't good for high shutter speeds which cut off before the tail is completed. To freeze action, you need the light cut off like a switch rather than the shutter. Enter IGBT technology. Used with longer shutter speeds in darkened environments, IGBT lights release a burst of the power that stops abruptly.
T.1 versus T.5
These aren't Terminator movies. The flash duration of studio strobes are typically discussed in terms of T.1 and T.5 amounts. T.5 levels refer to how long it takes for half of the light to be released, and are generally useless. A strobe with a fast T.5 (the first half of the light produced) can have a very long tail. T. 1 refers to the time it takes for 90% of the light to be released. This is the measure you should be following. Different strobes have different T.1 times. Alien Bees are great reliable lights, but slow. My NiceFoto nFlash 680WS strobe has a T.1 time of 1/1,400th at 1/4 power. Not the fastest of IGBT strobes (or the most powerful), but that is the equivalent of two SB800s firing at full power. Strobes with high watts-per-second can be very useful in reducing ambient light outdoors, too.
Bottom Line: What do I look for?
Everything is a balance of cost, ability, and reputation. In the realm of ability, you want a strobe high watts-per-second (400Ws would be the minimum) and IGBT technology that creates a short T.1 time. Hot shoe flashes all have IGBT technology, so flash duration isn't a factor; they also all have fairly low watts-per-second-- calculated as GN-- making them useful in enclosed areas but problematic outdoors or when photographing large groups. After those functional elements, I'd consider bells and whistles-- do you want built in power, trigger, or faster recycle time? Having a traditional hood mount, such as a Bowen, is helpful with accessories. Reputation is a harder call. For the risk adverse, stick with the established brands like Einstein, Alien Bee, Profoto, etc. However, whether they are more reliable than many of the better-known Chinese imports-- Mettle, NiceFoto, Neewer, etc.-- is yet to be seen. And the fact that it's not clear already says something positive about the import quality.