Zoom and Shoot Through It

Today I finally got around to testing my strobes and the power they put out through various light modifiers. Yes, a slow day indeed! I was able to get a better sense of f-stops, distances and such, and learned some interesting (I’m using that word loosely) and surprising things. And to my non-photographically-minded readers (clients, etc), my apologies for such a dry post. 🙂

So put on your nerd hat, and let’s dive in.

[click “continue reading” below for more…]

I tested my Norman 200B, a single Nikon SB-28, and a pair of SB-28s ganged together. I also tested a shoot-through umbrella, a regular silver umbrella, hard light with no modifier, and a Lumiquest Softbox III (for the single SB-28 only).

The first surprising thing I found is that my Norman doesn’t work properly! It has settings for 200 watts, 100 watts and 50 watts (i.e. full, half and quarter power). But the 100 and 50 settings both output the same amount of light. Not sure if that’s a defect…well it’s gotta be a defect, but I don’t know why. This flash unit could be 30 years old though, so I’m not alarmed.

Another thing I learned was that, at least in my particular circumstances, a 45” shoot-through umbrella was equally as efficient as a 45” silver umbrella (and sometimes better). It had been my understanding that shoot-through umbrellas ate up a lot of light, but they made up for it by allowing the user to position them closer to the subject. But for my particular circumstances, at the same distance, the two types of umbrella are very similar. Since I can position a shoot-through closer to the subject, and it tends to make a softer lightsource than a ‘hard’ silver umbrella, I’ll be choosing the shoot-through even more than I already do.

And the third surprising thing I found is that zooming a hotshoe strobe has very little apparent effect on lightsource softness (contrary to the prevailing wisdom), but can give you up to a half-stop more light output. This was most significant using the Lumiquest III, where the softness was least effected by zoom. So when in doubt, zoom!

First, some details on how I measured. I used a light meter for all this, with a tape measure on the floor to keep the distances accurate. I measured everything at 3ft, 5ft and 7ft. This was done indoors, in my living room, so there might be some bounce fill from the walls altering the data a little. But I don’t think it was significant.

When measuring the distance from the silver umbrella to the light meter, I measured from the rim of the umbrella. For the shoot-through umbrella, I measured from the tip of the umbrella. This was under the theory that those two points were the closest one could get to the umbrella without being inside it, or beside it as the case may be.

I always tested several pops with the light meter to make sure I was getting an accurate reading. My light meter reads in f-stops and tenths of an f-stop. So in the charts below, if you see “f/4.0-6”, it means f/4 and 6/10ths of a stop. If two successive pops were within 1/10th of each other, I recorded the lower of the two readings. If they were further out (on a misfire perhaps), I would test again until I had two readings that were within 1/10th of each other.

SHOOT-THROUGH VS. SILVER

The data doesn’t always line up as one would expect, but keep in mind these readings are from multiple pops and therefore aren’t random errors. The numbers are very close to each other on all the strobes when comparing shoot-through vs silver, with the exception of the single SB-28 zoomed to 85mm. here the shoot-through umbrella was actually more efficient. Conversely, the Norman showed a slightly higher efficiency using the silver umbrella—up to 1/2 a stop at some distances.

Note the three rows in each table are for 3 foot, 5 foot and 7 foot measurements.

Norman 200B:

shoot thru            silver
200w       100w       200w       100w
f/11-0     f/8.0-3    f/11-5     f/8.0-9
f/8.0-1    f/5.6-4    f/8.0-3    f/5.6-6
f/5.6-4    f/4.0-7    f/5.6-6    f/4.0-9

Single SB-28 (half power)

shoot thru            silver
35mm       85mm       35mm       85mm
f/4.0-6    f/5.6-2    f/4.0-7    f/4.0-8
f/2.8-7    f/4.0-2    f/2.8-7    f/2.8-7
f/2.8-0    f/2.8-5    f/2.0-9    f/2.8-1

Dual SB-28s (half power)

shoot thru    silver
85mm          85mm
f/5.6-8       f/5.6-9
f/5.6-0       f/4.0-9
f/4.0-4       f/4.0-2

Conclusion: Silver umbrellas are not the automatic choice when you need the most efficiency. They control unwanted spill in the studio better, since you’re only lighting half the room instead of all of it. But the benefits of a shoot-through umbrella will often be the deciding factor for me.

TO ZOOM OR NOT TO ZOOM

Now take a look at the data again, but this time compare the 85mm “telephoto” setting, vs the 35mm “wide angle” setting (the strobe will go wider, but this is what I tested). First, you’ll notice that I didn’t even bother testing the wide angle setting on the dual SB-28s. Why? Because I came to the conclusion that I would never set it to wide, ever again, unless it was being used as a ‘hard’ light! That, and having two strobes next to each other made the wide-vs-tele measurements a little confused anyway. Do two SB-28s set on telephoto equal a wide angle? Who knows? Who cares?

But look at that efficiency increase from wide to tele: for the shoot-through, it’s half a stop difference! Yet the “softness” of the light doesn’t seem to change very much at all (see images below). On the silver umbrella however, the change in light output is negligible. Why? No idea.

And then on the Lumiquest (see below), it’s the same thing. At least half a stop difference. And yet no change in softness.

Lumiquest III

35mm       85mm
f/8.0-0    f/8.0-6
f/4.0-7    f/5.6-3
f/4.0-0    f/4.0-5

Conclusion: when in doubt, zoom. At the very least, nothing will change. But you might gain half a stop of light.

Note in the gallery below, you can tell which were the tele and which were the wide settings when viewing the actual umbrella. But the effect on the subject was hardly noticeable. In fact, I’m not sure which image is which for all the self-portrait test shots I took. So I didn’t bother posting them.

Also, while I didn’t measure the 35mm setting using dual SB-28s with the light meter, I did take a shot of the umbrella at the wide-angle setting.

Edit 02/25/09:

I just got an Alien Bees AB800 today, and tested it. I was initially disappointed that the hard-light measurements were less than my ancient Norman 200B! But with an umbrella, it’s matched with it. The Norman drops 3-1/2 stops with the umbrella added, whereas the AB800 only drops 2-1/2 stops. I think this must be due to the different type of reflectors on the units. The Norman has a 5″ shiny reflector, and the AB800 comes with a 7″ ‘pearl’ finish. The AB800 has the added benefit that it will shoot all day, and has a modeling light. The Norman 200B is good for about 150 pops at full power, and then you’re done (or have to charge it for 45 minutes or so).

I must admit, I was expecting something with “800” on it to be massively more powerful than something with “200” on it.

While I won’t bother to add the images, I did shoot the umbrella to see the coverage of the AB800. Wow, what a difference! The speedlights essentially light the top half of the umbrella. The AB800 lights the whole umbrella very evenly, and thus is going to be a softer light. I didn’t shoot the Norman, so I don’t know how it compares.

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