Long Exposure Calculations

We'll be heading out for some night photography next month. Playing with long exposures is one of the thing you can't get around. Especially since I don't have any f/0.2 lenses in my bag.

Most lenses are sharpest (the so-called sweet-spot) around 2 stops from their biggest aperture (mostly around the apertures of f/8 and f/11) , but the problem is that these apertures kill the available light hitting the sensor of your camera. You could turn up the ISO (to 102400 if you have a Nikon D3s), but that gives you noise, and lots of it with most consumer dSLR's.

The only proper thing to do is adjusting the shutterspeed. Leave the aperture on its sweet-spot (f/9 or f/11), and the ISO at ISO100 or ISO200 (for the lowest amount of noise). But if you do that, chances are that you need exposures much longer than the available maximum on your camera, which is 15 or 30 seconds (depending on your camera). Which leaves you with the bulb setting on your camera. But how do you calculate the time needed for an exposure?

Bulb means that the shutter stays open for as long as you press the shutter release button.

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Unpredictable Exposures

When shooting from a tripod and/or with Live View on your (d)SLR you may find strangely exposed photos. Some are darker than others.

This could be related to the light seeping in through the ocular on your camera. Normally you have your eye against the ocular, but when you use e.g. Live View on your camera, you watch the LCD. This leaves the ocular 'open', and unwanted light might enter the camera and screw up the exposure of the photos you're taking.

I created a small example were I shot several high-speed images (in Aperture priority mode) while I used a small flashlight to pass a beam of light over the back of the body (e.g. car lights passing by in low light circumstances). I made sure that I also 'touched' the ocular with the beam of light.

You can see that the exposure goes from normal to dark, and back to 'normal'.

I haven't found any real differences when you're shooting under normal circumstances, but when (a strong) light is shining in or on the ocular, you might be in trouble.