Jürgen Niermann Wildlife photography

Double-polarization Flash Technology

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In macro photography, particularly when the magnification ratio exceeds 1:1, natural light is sometimes insufficient. However, direct illumination with a torch or flash – just like direct sunlight – often leads to distracting glare on shiny surfaces such as chitinous exoskeletons, reptile scales or damp amphibian skin. Such effects can be avoided by using a polarising filter both in front of the lens and in front of the light source.

This can be illustrated below using a photo taken with a flash against a mirror. What you see is the view through the camera’s viewfinder.

flash combination
There is a (circular) polarising filter in front of the lens and a polarising film (linear) on the flash. The planes of polarisation are the same, so the light transmitted by the film can pass through the filter unimpeded.
flash photo
When you take a photo with a flash, everything in the mirror is overexposed. The same effect occurs when attempting to take a photo through a pane of glass with vertical alignment of the flash .

polarising combination
Here, the planes of polarisation are at a 90° angle to one another. Only light whose direction of polarisation is altered by the illuminated object upon reflection can pass through the filter.
polarised flash photo
No glare in the mirror or on a glass pane.

frog


Phantasmal poison frog in a terrarium. The flash was directed more or less vertically at the glass pane.

However, using this technique results in a loss of three stops of light. It also always produces very high colour saturation. To achieve a natural look, it is often necessary to reduce the saturation when processing the images.

Unless you are working at a considerable distance, a clip-on flash must be ‘triggered’ via a radio remote control. Specialised macro flashes may not have sufficient power to compensate for the loss of light. For shorter working distances, the use of a diffuser at the end of the lens is particularly recommended.

Here are a few tips on DIY flash diffuser for macro photography.