I have found interested comments on 500px: Non-HDR-processed / Non-GND/ND-filtered / non-blended● Black Card Technique and group on Flickr. Could somebody explain how to and cons and pros of this technique, who uses it of course.
The Black Card Technique is a workaround for the problem of the limited dynamic range of digital cameras. The dynamic range of a camera describes the difference in light levels it can record.
The limits of dynamic range are often seen in landscape photography - you will often see photos where the sky is nicely exposed, but the ground is underexposed, or vice versa. This is because the camera can't record enough of a range.
The Black Card Technique attempts to get around this problem by essentially allowing two different exposure times in one shot. The card is used to block the brightest part of the image for the majority of the exposure, allowing the darkest part to be exposed properly without overexposing the bright part.
The technique is relatively simple. Simply set your camera up on a tripod, and set it for spot metering. Meter the ground and note the setting given - let's say you're photographing a sunset and the ground reading is 8 seconds. Now meter for the sky, which will give a faster shutter speed - let's say 2 seconds.
Set the camera to Manual and dial in an 8 second exposure, for the ground. Using either the viewfinder or live view, compose the scene and place the card over the lens so that the sky is covered. Fire the shutter and time 6 seconds, then remove the black card for the final 2 seconds.
The ground, which wasn't covered by the card, has now been correctly exposed for 8 seconds, while the sky was covered for 6 seconds and then exposed for 2 seconds, again correctly as per the meter reading. You should therefore have a shot with correct exposure in both the dark and light parts.
The pros of this technique are that it is cheap and relatively simple to achieve. It precludes the need for multiple Neutral Density filters, which are the 'proper' alternative. However, neutral density filters are less fussy to use, offer a range of gradations for a more natural transition from sky to ground (amongst other things), and offer less chance of introducing camera shake by nudging the lens with the card.