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Flashes emit a strong pulse of heat, and at the extreme macro scale this can and will move or disturb your specimen.

In the case of butterfly scales it can move them, rendering flash useless for stacking, and in the case of small flies (especially Doli flies) the fly reacts faster than the second flash (if using TTL).

Proving Subject Movement

too slight to be felt by us, or to disturb a bigger insect

With flash, a tiny blast of air shoots forward, for a short distance, from the front of the flash tube when it fires because of the heat generated during discharge causing the air next to it to expand rapidly. Boyle’s Law says that gas at a constant pressure will occupy a larger volume as the temperature increases. Joules of energy are released when a flash fires and some of this must heat adjacent air molecules, causing an outward puff of wind. This is too slight to be felt by us, or to disturb a bigger insect. However, small insects may be blown out of range of your lens, as may even smaller things like butterfly scales.

There's a great little test for this if you want to see it for yourself. Put a piece of black paper flat against the light of the flashgun and sprinkle some salt over it. Pepper works too as it is lighter. Pop the flash, even at quite low power, and watch the salt and pepper dance. Set it to repeat, say 16 flashes, and stuff will move right off the paper! Because the paper is black and masks the light, you can watch it happening.

Insoluble Problem At 50:1

at extreme macro levels of 50:1, there is an even bigger problem

Heat from a flash coming onto the subject is one thing, but at extreme macro levels of 50:1, there is an even bigger problem. When the subject absorbs light energy and heats itself up due to this, it excites nearby air molecules which push on the subject and move it. The same principle as a Crookes' Radiometer, there is no obvious immediate solution to this other than to invest in cold continuous lighting systems.

Insects & Flash

Insects can and do react to the pop of a flashbulb, which makes the use of TTL flash metering problematic, as the first pop may cause your insect to fly off before there is an opportunity to take the main shot with the second flash pop. That said, insects reacting to flash is the exception rather than the rule.

In terms of hurting an insect through the heat emitted by a flash, in fact this rarely happens. I have seen instances of this only when a bare flash is used and only if the flash is extremely close to an insect. The overwhelming majority of insect photography uses diffusion though, with which heat emission is not an issue. On cold mornings, insects can warm up after a number of flashes from close range, but there is no noticeable change in behaviour in broad daylight when using flash.

Bright Metallic Coloured Insects

Recently, fast reflex responses of skipper butterflies (Hesperiidae) to flash was reported and were found to be among the fastest ever recorded (17ms). Using a similar technique, even faster response times were found in Condylostylus flies (Diptera: Dolichopodidae), of 2-5ms. Skipper butterflies and dolychopodid flies both have a habit of perching openly on the upper surfaces of leaves in sunny areas of forests. This allows them to attract mates and repel competitors by territorial behaviour, but it also makes them obvious prey for birds. The Condylostylus flies and some of the very fast species of skipper butterflies also possess bright metallic colours. The explanation of the fast reaction times may that insects with bright metallic colours may use this ability to escape repeatedly from predators, and thereby instilling into predator memory the learned reflex of avoiding this particular prey.

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