New answers tagged night
Well they move way too fast for stacking photos. You may want to use a wide aperture lens with ISO 1600 or more and capture trails of the fireflies using bulb mode. Shoot it in such a way that the fireflies are bright while the yard is quite dark. Next, shoot with proper settings so that the yard is bright enough. Ignore the fireflies. Then in Photoshop, ...
You can't do this - the brightness of light and it's colour cannot be separated, so you cannot filter out colour whilst leaving the intensity. You say you cannot shoot B&W because of the girl & the glowing balloon - I assume you are trying to keep the colour of the balloon? You might have some success with a filter the same colour as the balloon ...
There is no way to desaturate an image optically (short of a deep infrared filter that only allows frequencies that the sensor colour filter array is invisible to). Assuming you are shooting digitally, why don't you just turn down the saturation setting on the camera? That doesn't require any post work and counts as doing things "in camera".
So, if I'm correct you want to make the moonlight slightly blue, the only way I can think to achieve this is by playing with White Balance. There is no way to "desaturate" with a filter and have the result not heavily colour cast.
Try using manual focus. There probably is not enough light for your camera's auto focus system to work properly.
Actually, the HS25EXR has a software limitation that decides the max shutter speed of the camera according to the ISO value. At ISO 100, you can get a shutter speed of 30s but as the ISO increases the shutter speed is automatically reduced. So far, I haven't found a way to override this but if you do, be sure to share it...
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