Unwanted background light when using flash

[gpmatthews]

I'm suffering from a bad cold with aches & pains in my limbs, high temperatures, coughing, snuffling, shivers etc... but although I don't feel well enough to spend my usual hours at the microscope it hasn't stoppped me thinking and trying some simple experimental techniques.

Sometimes it is inconvenient having to use low levels of lighting for manual observations in order to minimise background non-flash light from the normal illuminator when taking flash pictures. I find this especially so when using short flash durations.

One solution is to place a polarising filter underneath the flash mirror/splitter and another crossed with the first at the camera. This way you can vary the light intensity for manual viewing without introducing unwanted light from the normal illuminator when taking the flash picture. The direct light from the illuminator is cut right down by the crossed polars.

The disadvantage is that there is a polarising filter in each of the manual and camera optical trains which could produce some unwanted polarisation effects and also will produce some light loss (I'm always looking for brighter LEDS...). The crossed polarisation image from the direct light could also be seen at the camera, but hopefully will not show through much and should not be a problem for objects that do not show up under crossed polars anyway. If you are unlucky, depending upon filter orientation, you may get attenuation of the reflected flash, because reflections tend to be polarised. It is worth setting up to maximise the reflected light intensity.

I have tried a couple of test shots, and the technique looks promising.

Of course you need your camera setup to be parfocal with the manually observed image, because you shouldn't see a lot on the camera viewfinder. This is OK for my GFL, but something I have yet to achieve for the Olympus FH.

[pollengirl]

What a great idea, might get some interesting ideas from the polarizer. Thank you. Any photos handy?

For a second there, I thought you were going to post a close up of your virus, lolol. Hope you are over it!

[gpmatthews]

Linda, it's difficult to post any examples, because the final photographs should be just as if the microscope was being used for flash only, without any other strong source of illumination. Normally, if the constant light source is similar in intensity to the flash, the strobe effect of the flash in apparently stopping motion is eliminated as it would be when using flash for an ordinary picture in full sunlight. My trick effectively hides the sunlight from the camera but not from the photographer. The chief disadvantage appears to be light loss.