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03-10-2017, 12:22 PM - 4 Likes   #1
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Flash Duration Measurements for K3D Pop-up Flash

A few years ago, I analyzed the flash duration at various power level settings for the Pentax AF-540FGZ flash. The reference is here: Flash Duration Measurements - AF-540FGZ - PentaxForums.com

A few days ago, as a result of discussion of flash duration for a member’s macro project (62mm: Flash and 1/180 - PentaxForums.com), I pointed out my ‘540 results. That generated a request from BrianR for information about the duration of the pop-up flash that many of our cameras have.

Thus inspired, I made some measurements, and report here some preliminary results of flash duration measurements for the flash on my K3D camera, at various power levels from full all the way down to 1/128 power (as low as can be set).

I operated the camera in manual mode, with the exposure set at 1/180 sec - the default for flash.

This time, I used a photodiode with very fast response time (a few nanoseconds - billionths of a second!) and my digital oscilloscope to measure the time-behavior of the light output of the flash.

The results are, ahem, illuminating.

I have attached here three plots showing the results.

The first plot shows the light curves overlaid for all the power levels. Each horizontal division on this plot is 100 microseconds.

At full power, the pop-up flash is fairly fast, but no speed demon. The effective duration is somewhat less than 800 microseconds, or about 1/1250 sec.

At half power, the flash duration is much shorter: about 300 microseconds, or shorter than 1/3000 second. This is fast enough to stop almost any motion that you might want to photograph (except maybe hummingbirds). In particular, I think this should be adequate for 62mm’s salamanders.

At even lower powers, the flash duration gets shorter and shorter. When the flash is operated at 1/128 power, the duration is around 20 microseconds, or around 1/50000 sec! There’s not much power here, but it certainly is short.

The second and third plots show the light curves on expanded times scales. The second shows the higher power levels (full, half, quarter, and eighth) and the third the lower power levels (eighth down to 1/128).

Here is a little album with images for the various power levels: K3D-Popup Flash Durations

Note that the time per division varies on these plots. The exact value is noted just above the “type/image” button at the center bottom of each frame. The units are microseconds (“us”).

Here’s a list that gives the approximate t0.1 times for the power levels. Note that the t0.1 times are greater than the effective flash duration because of the rise and fall of the light curves within the t0.1 interval. When I find and resurrect my analysis software, I will give a more detailed report.

Full - - - - 820 microsec
Half - - - - 290 microsec
Quarter - - - - 165 microsec
Eighth - - - - 95 microsec
Sixteenth - - - - 63 microsec
32nd - - - - 40 microsec
64th - - - - 28 microsec
128th - - - - 21 microsec

(Sorry for the lousy format - there does not appear to be any way to actually insert this as a table here at the forum!?)

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03-10-2017, 01:56 PM   #2
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Very interesting. Appreciate it.
03-10-2017, 02:38 PM   #3
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Super! That's so much for testing! As you said, it may or may not apply to other built in flashes, but I can't recall seeing a built-in flash tested anywhere so it's absolutely awesome to have this data point.

It's faster than I would have expected (though not really based on anything), and possibly borderline if it has to fire full power and you're working at a high enough magnification. Definitely something to be aware of.

The K3 flash has a guide number of 13 (ISO 100/m). For comparison, the AF540fgz has a guide number of 13.5 (ISO 100/m) at 85mm setting and 1/16th power (your measured t0.1 was 1/5400 seconds), or 13.5 (ISO 100/m) at 35mm and 1/8th power (your measured t0.1 was 1/3080 seconds).
03-11-2017, 06:40 AM   #4
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Very informative. I think you have done us all a great service!
Some years ago, there was much discussion about the relative importance of the t0.1 and t0.5 values to real exposure conditions to catch motion. I remember entering a discussion summarized here:

What’s Up With “Flash Duration”? : Photo Arts Monthly

A useful quote from that blog is here:
Once a photographer is on set, shooting, it’s important to remember strobes are at their quickest when power is at its lowest settings. If images are falling victim to motion blur (blurred details, trailing edges, or less-than snappy objects in motion) dial packs down. The difference will have to be made up with aperture, ISO, or light placement, and a close eye will have to be kept on ambient light, but the strobes will be working at their fastest. Need to freeze really speedy subjects? Consider buying, or renting a big pack that both has impressive duration numbers and power to spare, since you might be often relying on a very low power setting.

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