Back to Basics: How to Choose an Umbrella
Below are some of the pros and cons of each of the major types of umbrella. You'll find that choosing is easy, once you figure out how you will be most likely be using it.
Shoot-Thru or Reflective
The first choice you'll make is whether you will bounce off of an umbrella or diffuse light through it. This is also the biggest variable in the quality of light from an umbrella, so you'll want to make this decision first.
Shoot-thru's offer softer light, both because they are white (vs. silver) and because you can place them very close to your subject. This is simply because the shaft will not protrude into the frame in advance of the umbrella, because it is pointed in the other direction. So you'll get a larger apparent light source, and thus softer light.
Since the light source can be very close with a shoot-thru, it can also be very powerful. If you do outdoor headshots and compete with the sun's light level, this kind of thing matters. You'll want a shoot-through. Just come in as close as possible with that light source and keep it barely out of the frame.
Another advantage of a shoot-thru with a removable black backing is that you can partially gobo the light with the backing. This will remove some of the degree of coverage coming from your light source. This technique can keep light off of, say, the bottom of your frame, as seen here.
But given the same lighting distance, shoot-thru's are usually less efficient than their silvered reflective counterparts. And efficiency is important if you are lighting larger objects (such as full-length portraits rather than head-and-shoulder shots.)
To get even coverage over a larger area, you need to back that light up. And a silvered reflective umbrella will let you do that and keep a lot of light hitting your subject.
A good example is in this group shot, done with two, horizontally ganged 43" reflective umbrellas to make one large, efficient source for key lighting this group shot. It was done for one of the location shoots on the lighting DVDs inside of a dark, shiny room that just threw one problem at us after another.
The silvered umbrellas helped us keep as much of the flash's power as possible with a longer working distance. Additionally, the black-backed silver umbrellas also gobo'd themselves for us, as they were just out of the frame to camera left.
Rather than choose, I like to have this one both ways. I use shoot-through umbrellas with removable backings most of the time, but always keep a silvered umbrella or two with me. Fortunately, they are very cheap ($26 and $20, respectively) and the double-folds take up very little room in the light case. So it is easy to splurge for those two reasons.
Double-Fold or Regular
This one is easy. If space in your bag (or on your shoulder) is at a premium, go with a double-fold. If you value durability over marginal portability, get a standard umbrella.
Apples to apples, the prices are all in the same neighborhood, so this is really a physical size choice. The double-folds are wonderful little umbrellas, and at 43" they are great for general use. But you do have to be careful with them because they are constructed in a fairly lightweight manner to allow every thing to collapse upon itself.
That said, if you aren't using them in windy conditions they will hold up fine if you are careful. And you can make the tip of the shaft (where it gets clamped) stronger with a pencil stub.
Another factor to consider is whether you are using a normal stand or a 5-section compact model. With a normal stand, there is no reason to sacrifice strength and get a double-fold, as you are already committing to the linear space for the stand. May as well get the more sturdy umbrella, too.
Conversely, your choice of umbrella may dictate your choice of stand, so keep them both in mind when choosing.
Medium or Large
First of all, why not a small?
Because you can make any umbrella into a smaller light source by zooming in the flash head or choking up on the shaft (or both). And the smalls are only a few bucks cheaper than the mediums. If that. So go with at least a medium and you'll have more flexibility.
So your choice pretty much comes to something in the medium range (~43") or something in the large range (~60"). And speaking of inches, that measurement is the distance around the face of the umbrella, from tip to tip.
Why? Because it sounds bigger that way.
Second, a myth to dispel: You shouldn't use a huge umbrella with a small speedlight.
You absolutely can, but with the lower-powered flash you lose one of the biggest advantages of a huge umbrella, which is that you can back the light way up and still keep it soft.
With a speedlight, if you back that umbrella way up (say, to light a large object) you will lose effective power before you lose softness. But if you are shooting in a very low-light situation and your aperture is accordingly set (i.e., close to wide open) you can get away with it.
For just a few dollars, a speedlight in a 60" umbrella makes a gorgeous, soft, light source for close-up portraiture -- i.e., head shot, 3/4 shot. Just keep it in pretty close to the subject. And it doubles as a sweet main light mod if you add a more powerful monobloc to your bag later.
But for most shoots, a standard 43-45" umbrella is a great fit for a speedlight. It also gives you a nice power-to-effective-size ratio that works well with the small flashes.
For most people, I would suggest starting with a pair of ~43" umbrellas for your first flash. One silver reflective, and one a removable-back shoot-thru. Standard or double-fold is a pick 'em, and your choice depending on the variables above. They are cheap, and adding a second won't really bump up the shipping much, either. Go crazy for once in your life.
For your next umbrella (say, if you have two flashes) I would suggest a second black-backed shoot-thru. This gives you the option of ganging them together for a huge light source in close, and/or using them high and low for "clamshell" lighting, seen in the head shot at left.
If you want to create very soft indoor light on the cheap (at close range) consider a 60" shoot-thru as a value-oriented option. And especially if your lighting arsenal includes a monobloc. But it will not have as much versatility as will a medium-sized umbrella.
Here are some of my favorite choices, all of which I consider to be good values, in umbrellas:
:: Westcott 43-inch Shoot-Thru Double-Fold ::
:: Westcott 43-inch Reflective Double-Fold ::
:: Paul C. Buff 60-inch Shoot-Thru ::
:: Eclipse 45-inch shoot-Thru Standard ::