Archive for August, 2010

Strobe Misadventures, Part 3: Strobe Flash Duration Answers

In part 1 I got some new monolight strobes and saw some strange results suggesting the flash duration was much longer than the 1/600th – 1/1,000th that was listed. In part 2 we did some crude testing of the strobe flash duration and verified that yes, it was indeed the flash duration and not something else that was off.

After the strobe testing I finally did some research and discovered the answer: yes the strobe flash duration was much shorter than it seemed it should have been, and the reason has to do with the way that manufacturers measure the flash duration.

Flash Duration – T.5 vs T.1

When a flash goes off, it gets up to 100% brightness really fast, then trails off mush more slowly. Because you can never say for certain exactly where the point is that you’re back to ambient light, flash durations are measured in terms of how long they are over a certain percentage of their brightness.

Most manufactures measure flash duration as T.5, or the duration that the flash is over 50% of its brightness — the amount of time from half brightness to full brightness and back to half brightness again. The problem with this is that the flash curve gets really long on the tail end, and even though it’s less than 50% of maximum brightness, that’s still plenty of light to expose.

The more accurate measurement in terms of establishing the stop motion capabilities of the flash is T.1 — the duration of time that the flash is 10% brightness or greater. This is about three times as long as the T.5 measurement.

Monolight Flash Duration, T.1 vs T.5 measurements.

Thus that 1/600th flash duration, as reported my the manufacturer, is suddenly only a 1/200th or so duration. Plenty slow enough for motion blur, especially if you’re using a longer focal length.

Furthermore, for most monolight strobes — like my lovely Flashpoint 1820a — when you turn the power down (as I was) you actually get a longer duration, not a shorter one like you might think.

The Monolight Strobe Solution

There are basically three solutions here. Firstly you can just get yet faster strobes. Be sure to check the T.1 rating on them so you know how fast they’ll really be. Of course faster strobes tend to be far more expensive. If you want really, really fastness, you’ll want to use flash heads (but that’s a whole series of stuff for another day).

If getting much more expensive strobes isn’t a great solution (which it is not for me) then solution #2 is just to put the camera on a tripod, which you should really be in the habit of doing anyway, even if that’s a habit that I’ve never developed.

Finally we have the WoodJr solution, which is to go to the max sync speed of my camera. I had been shooting with the default flash shutter speed, which for some reason my D200 thought should be 1/30th of a second. However, the maximum sync speed of the D200 is actually 1/250th of a second. While this isn’t blindingly fast, it’s plenty fast to avoid any hand-shake blur. In effect it’s just cutting off the tail end of the strobe (and I had to turn the strobes up / open the aperture to compensate) but I still got great, even, consistent lighting every shot.

Once I popped my camera to 1/250th I went back into the studio and had no more hand shake blur issues at all, and I’m back to loving my strobes to death.

1/250th at f10 on ISO 100 with 38mm focal length

1/250th at f9 on ISO 100 with 26mm focal length

Strobe Misadventures, Part 2

Having established in Strobe Misadventures Part 1 that something was amiss with my focus, I promptly set out to do some testing to find out what it was. The focus could be off on my camera. I could be blitheringly unable to use the auto-focus. The flash duration of the strobes could be insanely slower than reported. Or I could be so hyped up from years of over-imbibing caffeine that my hands shake violently in a way my shell-shocked brain refuses to acknowledge.

It seemed to me the easiest thing to cross of the list was the flash duration of the strobes. Admittedly, this also gave me the chance to play with them some more, which may have influenced the decision.

I grabbed some paper from the printer and jotted down a handful of crude tests then grabbed a friend to hold them for me and be blinded by a series of flashes. My concept here was simple: take a control shot with the camera on the tripod. Then, with the camera still mounted on the tripod, have the model move her hand around and different speeds, to verify that we’re getting no blur there. Then take the camera off the tripod and take some shots with spastic, over-exaggerated shaky hands.

This isn’t the most scientific experiment here: my definitions of speed include “fast, medium, slow.” But the point is a 1/600 – 1/1,000 of a second flash duration should easily stop anything but the speediest of hand motions.

Here are the test shots, all taken at 44mm, f11, ISO 100.

The control test. Focus is good here, which also suggests there's nothing wrong with the camera's auto-focus.

Here the model is moving her hand up and down furiously. Clearly that's a lot of motion blur.

Here the model is moving her hand at a moderate rate, as though lazily fanning herself. We *still* have motion blur! Oh noes!

And here in the spastic shaky cam, with the model standing still, we still do not have a sharp image.

Without question we are seeing that the flash duration is apparently much, much slower than thought. The room was very dark, so there was no chance of ambient light affecting things. No, we’re seeing something that I’d normally attribute to something like a 1/125th shutter speed.

But… that’s not possible, is it?

The shocking answers next time in part 3: WoodJr just looks it up on the internet like he should have done in the first place!

Strobe Misadventures, Part 1

It was with great excitement that I unpacked my first monolight strobes, a pair of Flashpoint 1820a 900 watt/second strobes. Up until just recently I had always made due with continuous lighting — the lights that also doubled as video lights for video projects.

But of course I had all the problems that everyone has with continuous studio lights. With 1,750 watts of lights (requiring 2 circuits) I was still struggling to get better than 1/60th of a second at 24mm and ISO 400 on full shots. I just needed so much light. And of course they were hot.

The temperature in the room was easily 10 degrees higher within minutes of turning on the lights — and another 10 degrees in front of the lights. My models didn’t just have sweating problems — after a while they simply burst into flame, and as you can imagine this made cleanup a chore.

But model combustion aside, the bigger issue was brightness and what it did to my shutter speed. You can pretty safely guess that any studio shot I take that I showed to my father, the conversation will go pretty much like this:

WoodJr: Hey, here’s some recent shots I took.

WoodSr: They’re out of focus.

WoodJr: No… no they’re okay.

WoodSr: They aren’t tack sharp. Did you use a tripod?

WoodJr: … No. I couldn’t. I was suspended above the model from cables… in a 300 mph wind… underwater. I couldn‘t use a tripod.

WoodSr: Well that’s why they aren’t sharp. Camera blur.

But no longer! With these strobes I no longer have to care about shutter speed! They fire at the speed of light, c, 186,000 miles per second. Well, more importantly the flash duration is — optimistically, but I’m of an optimistic mindset with these strobes — 1/1000th of a second. Unless I’m using a crazy long lens, which I’m not, the studio room isn’t that big, there is no way I’ll get camera blur ever again.

I can finally show shots to my father and have him exclaim “Dear boy! These are as sharp as your very clever mind!”

So I quickly badgered a friend into posing as I tested out the lights. The weather was against us with a nasty and humid 92 degrees — so much for no more sweat — but I’m happy to report the model never once burst into flame.

And I was delighted with the strobes. They were bright. Stopped down to 1/4 power I was still getting f8 at ISO 100. It was fantastic! I had flexibility I’ve never had before. The lights didn’t have to be just on the outside of the frame. I could move them across the room if I wanted. And as they weren’t within a few degrees of the surface of the sun, I could toss whatever kind of filters or material I wanted over them.

Then I got back to the computer, to look through the shots, and saw several shots like this one:

This shot is… not sharp. In fact, it looks very much like blur caused by camera movement. Suspiciously so. But at 1/1000th of a second, that shouldn’t be happening. Okay, optimism aside, the low end of the flash duration is 1/600th of a second. Still plenty fast to stop any incidental motion.

Was it possible that I was legitimately out of focus? Does the auto-focus of my camera require adjustment? Are my strobes somehow insanely slower than they should be?

The only way to find out is to run some tests!

Part 2 of Strobe Misadventures: the disturbing results of some crude testing!

Night River

I was out camping along the St. Croix river for a few blisteringly hot days hoping to relax, do some swimming, some reading, and of course take some photos. As always when I’m not backpacking, I brought my entire camera bag with me and as always when I’m camping, I went most of the trip without finding anything much to take pictures of (though this may have been aided by 90+ degree temperatures, horrible humidity, and swarms of mosquitoes).

Several times during the day I saw bald eagles flying around on the other side of the river, but when I popped by 200-400mm lens on I still couldn’t get in close enough for a good shot. Not that it would have mattered a lot if I did — with the birds constantly swooping over the river, I couldn’t keep them in frame for more than a fraction of a second.

Finally after a big storm the first night I was sitting around the fire around 2am when the fog started rolling in across the river, illuminated by the nearly full moon through the clouds. This, I finally thought, could make for a decent shot.

I tossed the camera up on the tripod and tested out a few exposures until settling on this. Shot at 24mm at f3.8 on bulb setting with a cable release, ISO 100, for about 1 minute (I was counting — and chatting with my campmate —  so the time may not be that exact).

I’m always happy to return from any trip with a shot that I’m pleased with, and always frustrated to return without any decent shots. I’m quite fond of this one — I like the muddy foggy look of it and thus I’m declaring the trip a success!

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Photo Bug is the home of Brian's very occasionally updated photography journal. Posts are sporadic, and I keep swearing that some day I'll dig through the archives and start filling things up. Some day...


 
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