A flash extender, in case you’re wondering, is a device designed to extend the range of a flash (duh), most commonly a hotshoe-mounted Speedlite style flash.  They are of particular use to bird and wildlife photographers, although they can be used by any photographer who works with telephoto lenses and needs some extra punch from a flash.

But why do you need the extender?  Consider the Nikon SB-900, the Speedlight with the longest zoom range: 200mm.  Now consider the lenses that wildlife photographers use: 400mm, 600mm, 800mm.  There’s a major disparity there.  At 200mm – of the 105mm that my Canon 580EX maxes out at – the flash is too diffuse & weak by the time it reaches a bird on a branch to do any good.  Enter the flash extender.

A flash extender is simply a fresnel lens mounted on support arms in front of the flash head.  The fresnel lens condenses the flash beam; a tighter beam suffers less fall-off and reaches farther.  There are several commercial models available, the best known probably being the Better Beamer.  It runs about $40, and I’ll be the first to admit that while I’ll drop $1k on a lens, I’m too cheap to drop a pair of Jacksons on a flash extender.  Come on, it’s a plastic fresnel lens mounted on a pair of support arms.

DIY Flash Extender

Turns out that fresnel lenses can be purchased at Barnes & Noble in the form of reading magnifiers.  Since I don’t have a Barnes & Noble within 45minutes, and my local Borders didn’t carry any, I settled for an 8×11 inch lens from Staples.  Cost $8.  I brought it home, pulled up some product photos of a Better Beamer mounted to a Canon 580EX, and took some measurements of different parts of the Speedlite and Beamer in the photo, then of the actual Speedlite.  Some mathematical magic gave me conversion ratios, which I used to calculate the dimensions I needed out of the 8×11 sheet.

Since a frensel lens works in increasing “ripples” from a central point, I plotted the center of the lens sheet using a Sharpie marker, measure out my dimensions, and cut it down to size using a Dremel tool & cutting wheel (gotta love those things!).  Managing not to lose fingers in the process, the result was a roughly  4×6 inch lens.  Since the cuts were somewhat ugly – not to mention sharp – I cut three open rectangles of black foam; one traced to fit tight around the edge of the fresnel lens, the other two wider to cover it on either side.  Some hot glue, and my lens was suddenly mounted in foam and looking better.

DIY Flash Extender Lens

Use the same mathematical magic as before, along with some hand held tests, to determine the best length of the fresnel from the flash head (about 6″).  I cut the support arms out of foamcore scraps I had lying around.  Nice and simple.  Now, when it comes to assembling the whole thing, I thought about just hot gluing it, but decided that I’d rather be able to break it down so that it would all lie flat in a camera bag pocket.  Slapped some Velcro on the end, which turned out to be sturdy enough to hold it together in use, although gale-force winds might tear it free.

Finally, a foam strap with Velcro to mount it to the flash head, and it was done.  The whole process took about three hours, much of that measuring and cutting of components.

The result?  A working DIY flash extender.  Out in the field it works as expected; I’m able to throw a flash beam onto a bird that I’m shooting at 400mm.  The beam is tight and balanced, and not only does it allow the flash to work, but it allows the flash to fire at less than full power, which is what it defaults to at that range without the beamer.

Black-capped Chickadee photographed with DIY flash extender

Of course it’s not as simple as all that to use – there is a learning curve.  The trick is figuring out how much flash compensation to dial in.  The amount varies depending on the ambient conditions, but it’s almost always a negative amount, since you’re looking for fill light, a catch light in the eye, instead of full-on primary lighting.  You also need to make sure that the flash head is aligned with the fresnel lens, otherwise the beam won’t be straight and centered, but off to one side.

There’s still a rang limit, as well.  Although the extender will reach to 400mm (my maximum focal length), it works best on subjects that fill a significant portion of the frame at that range.  If you try using it to light a bird that is very small in your frame, you’ll still see fall off, to the point where you may not notice the flash’s effect at all.

Bottom line, if you’re shooting birds or wildlife, this is something to really think about having in your bag.  Is it worth building your own?  That depends on how much $40 is worth to you, vs. $15 in supplies and a few hours of creative energy.  For me, part of the drive to make DIY gear is the fun I have in the process – I like designing and building useful tools out of unlikely stuff.  But if that’s not you, go for the commercial option.