As a photographer, there are certain times when you might want to use a relatively slow shutter speed in order to increase exposure time. An example of such an occasion is when photographing a scene containing moving water and you want the movement of the water to be captured as a milky mist for dramatic effect.

One way to allow a longer exposure without over-exposing a photo is to use a neutral density (ND) filter. These attach onto the end of a lens and restrict the amount of light which can pass through them resulting in the shutter having to stay open for longer in order to maintain the correct exposure.  They are neutral because they shouldn’t affect the final image colour balance.

If you compared two images of the same still scene at the same time and with the same aperture; one taken with a ND filter and one without, as long as they were exposed correctly they should appear identical although the ND filtered exposure would have required the shutter to remain open for longer.

When it comes to film cameras, once light has entered the camera body you no longer have any control over how it’s recorded. Apart from varying the aperture and shutter speed (which we may want to keep at specific values), using a ND filter is the next most obvious way to control the amount of light reaching the film.

Once the light of a scene has entered a modern day digital SLR, every part of the resultant image is controlled hardware and software.  My question is why isn’t there an electronic ND filter built into my DSLR?

From my days of science class in school, I recall that digital sampling of an analgoue waveform effectively takes samples at discrete intervals.  When a photograph is being taken with a DSLR, the analogue light is digitised by the camera.  When carrying out this conversion process, why can’t it dampen down the sampled values by a specific amount i.e. effectively reduce the amount of light registered?

Maybe I’ve got physics wrong, but I’d like to know why there aren’t ND filters built into modern day DSLRs or maybe there are, but I’m just not aware of them?