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I picked up my flash a few months ago and am extremely satisfied with it for the price, considering that it’s much cheaper than the Canon version and still pretty powerful and full of features.

Since buying it i had no idea how to actually work it though. I mean I could set it up to do off camera shots with a radio trigger, I could set it up on camera and use it in ETTL metering mode, so I didnt have to calculate much, essentially the “P” mode of DSLRs.

Recently, someone I met in Seoul through the photography group, Seoullighters, shared an interesting article called, Reversing the Inverse Square Law. First, it does a really quick run down of basics of photography like ISO, aperature and shutter speed and how they all interrelate.

I get aperature but this solidified my confusion; increason from ISO 100 to ISO 400 is going to make an image TWO stops brighter, not 4. We can counter this by either increasing Shutter speed two stops, from say 1/125th of a second to 1/500th of a second, 1/250th being the stop between the two. Or we can stop down the aperature two stops, from f8 to f4, with f5.6 being the stop between the two.

After the short review it got into flash and how their power output is rated; Guide Numbers. The reason for understanding this, for me, is so that I can confidently use flash with my film camera, with digital it’s forgiving unless in critical moments. Anyhow, Guide numbers are measured at ISO 100 and change depending on the focal length of the lens you are using. They also change depending on teh flash model you are using, check the manual.

Say I am shooting on a 28mm lens; my guide number will be 100 but if i switch over to my 85mm lens that number jumps over to 180. What does this number mean? How can I use it? First you need to calculate the distance to your subject. Some lenses have focus distance meters built in, if not guesstimate. So, we are using the 28mm lens and our subject is 5 feet away. We will take the Guide number and divide by the distance to the subject, 5 feet. 100/5= 20 so 20 is the aperature setting for that situation given that you set the flash to full power or 1/1 power.

What if you want a shallow depth of field? you need to open up the aperature to say 3.5. You have now just added 5 stops of light. Congratulations that image will be extremely blown out. dont worry, we can fix that we just need to reduce the flash power by 5 stops from 1/1, …1/4, 1,16, 1/32, 1/64, …to 1/128.

The other thing I enoticed is that shutter speed doesnt really matter for exposure as long as you are within the camera’s limit for flash sync. For me if I stay under 1/200th of a second I will get the same exposure as if I were shooting at 1/16th of a second. Not sure whats up with going slower that 1/16th. I guess at that shutter speed there is enough available light without the flash and any aditional light from the flash is causing the image to over expose.

I also noticed that taking away a stop of light helps fill in shadows without over exposing the subject, when the flash is not the primary light source. And when bouncing the flash off of a wall or ceiling to create softer shadows, you have to add an extra 3~4 stops of power.

With the Guide Number you can figure out teh settings for any of your lenses for any given distance to your subject. This helps you understand what is going on inside of the flash when you use ETTL while allowing you more control.

Finally, what exactly is the inverse square law? all it really means is that if you double the distance between a light source and a subject you make it 4 times less bright… it’s how light works, it’s how sound works, it’s how gravity works. We can use it to make our mental math easier. if you look at the chart I made you can see some paterns emerge when comparing settings for the same lens at different distances.

Flash Power Chart

For example at 28mm and 20′ we see we need 1/1 power at f5 if we half the distance to 10 we keepf5 but reduce the flash power by NOT HALF but by 1/4 and if we half the distance again to 5 feet keeping f5 we again quarter the flash power to 1/16, NOT hallf it.

Sometimes the best way to learn something is to try teaching it and then doing it. Seems my images are pretty on target!