Debunking the Myths of Flash Power

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(unknown member) Junior Member • Posts: 42
Debunking the Myths of Flash Power
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I've been trying to research exactly how to discover exactly how many stops of light any particular flash may have.  After reading article after article, I've realized that there are deep misconceptions that have spread, even among very well known editorials.

As an engineer I had to figure it out myself for the sake of my own sanity, and hopefully, yours also.  So here we go:

First:  Guide numbers are totally useless because they can, and are manipulated by marketing.  This is endemic in all industries that like to display the specifications of their products big and bright all over the box they come in.  SO DON'T USE THEM!

Second:  The Power rating is not alway intrinsic with the ultimate luminosity of the light.

Third:  The only way that you can truly know how a particular flash works with your particular camera is to TEST IT.

... So how do you test it?

This is called Metrology (the study of weights and measures).

First:  In metrological calibration, which is to verify how a something compares to an established reference, you first need to establish this reference.  The one that I use (from the Beyond Photography YT channel), is (8888)

(8888) is:  f8;  800 ISO;  8 ft;  1/8 power

... Why 8's?  Because these are all reasonable midpoints on the system settings for both the camera and the flash, and usable distance.

Second:  Setting up the calibration test:

You will need a grey-card, a camera, and a flash.  Set up the grey-card exactly [8 ft] from the flash.

Next, set the flash to [1/8] power and point it directly at the card, and also make sure that its zoom setting is at its widest focal length (because this is how you are going to use it 99% of the time).  This also works best with Off-Camera Flash.

Now, set your camera to [f8], and [800 ISO].  Stand close enough to the grey-card so that you can fill up the frame as much as you can and take the picture.

What you will see with a standard camera flash are relatively narrow lines near the center of your histogram.  These may be partially or several stops of light from center depending on the model of flash you are using, but most of the top camera flashes are actually factory calibrated to something near this exposure (The exact center of the histogram is where [0 EV], or correct exposure is).

My Nissin Di700A is [-1 stop] from center.  To find the deviation, adjust the settings of your camera and retake the shot.  In my circumstance, I just changed my aperture to [f5.6].

If this interests anyone, I will make another post on exactly what the "Real Math" is in calculating the power.  But I think this is enough for now.

Thanks, and I hope this helps.

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