Semi-automatic macro flash

Using Self-contained automatic flash for closeups

I came up with this technique when I used film on a pre-TTL (through-the-lens; a revolutionary metering method when it was new) camera.  It bridges a gap between all-manual flash and fully-automatic TTL flash.  It can still work, if you have an older flash (or newer ones with the right settings).  I have never heard of anyone else doing this.

Caveat/warning

Some older flashes used a much higher voltage than current flashes. Read the documentation for your camera and flash system, and learn the voltage of your old flash before you try this. You can fry your camera or flash and this is done at your own risk. For example, my Canon cameras use 6V for the flash, while my old Vivitar 283 uses about 120V and will fry the Canon if used on the hot shoe!

Background

One of the greatest aids to flash photography was the invention of through-the-lens (TTL) flash metering. With TTL flash, correct flash exposures may be made with little or no knowledge of flash theory. Since light intensity varies inversely with the square of the distance between the flash and the subject, measuring the precise flash to subject distance is the major consideration when shooting closeups, and manual flash units essentially limit you to the use of one f/stop for any given distance. TTL flash eliminates this worry, but while I would have loved TTL flash, I refused to part with my Minolta XD-11 until I moved to digital. I did not want to resort to manual flash to shoot closeups, so I used a self contained automatic flash, my Vivitar 283.

With a self contained automatic flash, a sensor on the flash unit reads the amount of light reflected from the subject and automatically shuts the flash off when sufficient light is detected. As long as you keep the flash closer than its maximum range, good exposures are virtually assured; the photographer need not worry about distance or guide numbers.

However, the instruction manuals also list a minimum distance for good exposure. The manufacturers expect most people to mount the flash on a hot shoe and make no provision for shooting accurate closeups. At ranges less than about 2 feet, parallax becomes the limiting factor: the hot shoe mounted flash fires at one place and the sensor reads another, neither of which correspond with the subject seen by the lens. The light entering the lens (at this time) is less than the light reaching the sensor, and the result is underexposure. The solution is simply to hold the flash off camera with a bracket, connect it to the camera with a PC cord and aim the sensor at the subject. Now you can get really close!

Increasing Depth of Field

Depending on the model, the flash may have one or several usable f-stops in the auto mode. The Vivitar 283 has 4 such f stops, selected by a color coded dial. For ISO 100, the "purple" mode selects f/11. This is the smallest f-stop that the 283 is designed to handle. But what happens when you need to stop down for even more depth of field? You can trick the flash into giving you more light by placing a neutral density (ND) filter over the light sensor (but not the flash head). I use the light control kit that I bought from Lepp & Associates years ago, and attach the flexible filters with velcro.

Minolta 200X control panel

Vivitar 283 control panel (pardon my dust)

With a neutral density (ND) filter in place, the sensor receives less light and the flash compensates by increasing the output; all you have to do is stop down the lens by the appropriate amount. For example, if you set your flash to the f/11 exposure mode and place a two stop ND filter over the sensor, you must close the aperture two stops to f/22 (other things being equal). You might worry that the flash won't have sufficient power to give you a couple more stops of light, but I have never found this to be a problem with short flash-to-subject distances. Exactly how many additional stops you get depends on the power of the flash you own. Experiment! I have found that I can get up to 5 additional stops from my Vivitar 283 at ranges less than about 1.5 feet.

Minolta 200X flash with 1-stop ND sheet over sensor

Light loss in lenses at close range

There is one complication. Fortunately, it is the only complication I’ve encountered with this method. Most macro lenses (in fact, most lenses) do not transmit the same amount of light at all focusing distances. For any given f-stop, lenses transmit less light at closer focusing distances than they do when focused to infinity. The closer a lens focuses, the less light is transmitted. My old 100mm macro loses two f-stops at the closest focusing distance (lifesize, or 1:1 in this case). It loses one f-stop at half-lifesize (1:2), and about a half f-stop at one-fifth lifesize. If you ignore this fact, you will under-expose your image (and proper exposure was super-critical when I shot slide film). This is why I said “other things being equal” earlier. One notable exception to this rule is internal focusing lenses; they do not lose any light when focused closer.

Some lenses have an exposure scale built-in. These lenses tell you how many stops of light have been lost at a given focusing distance. Not all lens manufacturers are this considerate. If your lens does not provide this scale, you will need to perform a simple test. To find out how much light your lens losses, you will need to mount your camera with the macro lens on a tripod and point it at a uniform surface (such as a wall). Start by focusing to infinity and take note of the exposure. Now manually focus your lens closer and take note of the exposure change. Write the information down for each 1/3 or ½ stop of light lost. I wrote the information on a sticker and placed in on a non-rotating part of the lens, in easy view.

Sticker with EV adjustments on my old macro lens

Getting the right exposure

Now all you need is some basic arithmetic. Suppose your flash will expose correctly at f/5.6 with ISO 100. If you place a four-stop (16x) ND filter over the sensor, and the focusing distance requires an additional stop of exposure, what is your final f-stop? You need to stop down four stops to f/22 for the ND filter, then open up one stop to f/16. Simple! Set your aperture to f/16 and shoot. As long as you remain within the range of the flash, the exposures should be good.

What is your final aperture if your flash is set to expose correctly at f/4, you use a two-stop (4x) ND filter, and your lens will lose a half-stop of light? Close down two stops for the ND filter to f/8, then open up a half to f/6.3. If you didn’t know it was f/6.3, just set the aperture halfway between f/8 and f/5.6. No problem.

The exposure technique also works for two or more self-contained automatic flashes, if you prefer multiple flash units. I experimented with two flashes at various angles and found that the exposures were still good. Be certain to set each flash to the same setting, unless you are experimenting.

Diffusion

Sometimes you may want harsher light, sometimes you may want softer light. You might want to try using a wide-angle diffuser to soften the light. Or if you don’t have one, experiment with varying thicknesses of tissue taped over the flash head. I used the translucent plastic from a lemonade mix container; I cut it and shaped it to fit over the flash head. Velcro secures it. Note that I reduced the output of the flash somewhat by using such thick plastic; the maximum flash distance is reduced and I can not use that particular diffuser for “normal” photos (meaning, at distances greater than a couple of feet). 

Another great idea is this custom diffuser from Brian Valentine (aka LordV): coke can diffuser.  Actually I find the quality of his light to be the most natural I've seen from a flash.

There are many other options, which is an entire subject unto itself.

Remember not to cover the flash sensor.

That’s a lot of equipment to hold!

If you hand-hold, you will also want a way to hold the flash(es). Many photographers have designed or built flash-holders/ macro flash kits. Your local store is likely to have a very generic unit. I think the majority of these are too bulky. Search the magazines for catalogs, or make one yourself. For one of the best ideas I’ve seen, I highly recommend that you read John Shaw’s Closeups in Nature. He is a great writer, a superb photographer, and the you’ll get a lot out of the book even if you never borrow his flash mount idea. For myself, I use a Wimberley macro flash holder, or I reverse the lens collar and mount a flash on that, depending on the lens I'm using at the time.  Really Right Stuff also has some interesting ideas.

Lens collar with QR foot, small clamp with small ballhead reversed.

In conclusion

If you have a camera capable of TTL flash metering, then by all means use it. TTL flash is the easiest method available. But if you do not have TTL flash available, then using a self-contained automatic flash is a very workable alternative. I found it more flexible than using a manual flash method. I came up with this technique while I was still in school and could not afford to “upgrade” my system.  I confess, I now use a TTL flash.