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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It’s all to do with the amount of light reaching the sensor inside the camera and how much light the sensor can handle!
The sensor has a maximum and minimum amount of light it can deal with. More light that it’s maximum sensitivity will be recorded as 100%, the sensor doesn’t have the range to tell the difference between bright and really really bright. At the point it’s been recorded as 100%, there’s nothing you can do in software as if you try to make it darker the bright and really bright bits will be made darker by the same amount as they both got measured at 100%.
There’s are 5 ways you can change how much light hits your sensor. 3 of them are known as the exposure triangle:
Aperture – a expanding and contracting ring that physically blocks light, this also has the affect of changing the “depth of field” which is basically the depth of the area in focus, the smaller this is the more cinematic the look of the photo/video is.
Shutter speed – essentially how long the shutter is open and allowing light into the camera. This also affects the motion blur of the image, if the shutter is open longer you capture more “time” and if the thing your capture moves in this time you get a kind of ghosting effect called motion blur!
ISO/Gain – this is kind of the power of the sensor. You can turn the power of the sensor down which has the affect of making it less sensitive to light, so it can “see” brighter things and tell them apart. But this has the affect of increasing noise, like the white noise on old TVs. This is because there’s always random electronic interference in all things, as you turn the sensitivity down it’s harder to tell this interference apart from actual data.
So each of those three settings has a trade off, in an ideal world you want minimum noise, a depth of field that suits your look (normally narrow and cinematic) and a shutter speed that has a natural amount of motion blur (our eyes have motion blur too, so an image with none at all can look very un-natural and fake, too much and you can’t see what’s going on)
When you have those settings correct and the exposure/brightness isn’t what you want, you have two more options, more lights! And ND filters to just block x% of the light.
ND filters are often more relevant in film than photography as in photography you can use shutter speed much more and the motion blur is less relevant.
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