Obviously it was a thing for film cameras, but now that everything is digital, something like “just make the picture darker” seems extremely easy to do with software
quick edit, I know what ND filters are for and how to use them, no need to explain. it just seems to me that it could be engineered in a way that doesn’t require them, which is what I’m asking about
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There are two reasons for needing an ND filter with a digital camera. One good techinal limitation, and one that could indeed just largely be fixed in the digital realm.
First, the technical reason: Sometimes there just is way too much light (and our digital sensor doesn’t support a fast -enough minimum “shutter speed” to get the image we want). This, however, is increasingly rare with better electronic “shutters” that can support shutter speeds below 1/10,000 seconds. This used to be a major problem in the film era with cheaper cameras.
Second, a legacy reason. As the very bright scenes, or the desire for very long daytime exposures for artistic effect, are relatively rare, the camera manufacturers have not added the ultra-low ISO modes. (Where the camera would take multiple consecutive frames and instantly average them in post (average rather than sum).)
There is, however, a small technical component to this: Such stacking would need more battery power and could also cause problems with the camera overheating for very long exposures. As, now, the camera is reading the sensor hundreds of times. So, for best results in very bright light, a mild ND filter might still be optimal (when combined with the software approach to make it a heavy ND filter) as it will make each individual frame more reasonable in its duration for very bright schenes (reducing the risk for image artefacts arising from the time not actively exposing).
The complete lack of the second feature in most cameras is a shame, as the image quality of such software or hybrid ND filter would (in most situations,) be superior to a physical ND filter (which will not be perfectly neutral, but will always leave a small tint).
As a sidenote, the second approach is obviously possible manually. Just take a pile of images and average them in postprosessing. But, the manual process makes the time gaps between frames undesirably long (leading into bigger risk of motion artefacts), and obviously results in way too much a data being stored temporarily on the memory card.
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