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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Even digital camera sensors have a range of light intensity that they capture best and are limited in the dynamics range that they can capture. That is the difference between the darkest and lightest it can capture at the same time. Using a ND filter allows the photographer to get the camera to better capture the detail of bright things.
Any given light sensor will have a range of possible responses, from 0-100%.
If you have regions where your sensors are giving 100% (because some are 115%, some 125% and some actually 100%) you lose the information of which bits are brightest. At that “just make the picture darker” will just turn them from all-white to all the same grey.
The ND filter gives it a -30% before it hits the sensors, and thereby means that what would have been all 100% becomes “this bit 85%, this bit 95%, this bit 70%”.
Cameras make the picture darker by capturing less light, which is either done by shorter capture times, or done by using higher f-stop (which makes backgrounds less blurry), or done by using a lower ISO value.
But if you want a super long exposure, even f22 and ISO 100 may not be enough to get the image dark enough.
For video, you have the 2x rule, in which the shutter speed (capture time) is 2x the frame rate, unless going for some stylized effect. In which case your 3 tools are resized to just 2 (f-stop and ISO), so an ND filter is needed even more.
There are couple of things you can play with. Exposure time – how long the sensor gathers light, ISO sensitivity – which is just preamplification before ADC times sensor intrinsic efficiency, the F# number – what sort of lens you use and how you adjust the iris.
And all of those things have other effects too, they don’t just make image lighter or darker. Cameras are built so that you don’t need filters for most normal pictures. But if you want very long exposures, or very bright subjects or combination of them, then ND filters can shift your working range to cover extreme situations.
There are three main settings in a camera, ISO, Shutter speed, and aperture.
ISO is how sensitive the digital sensor is. Higher numbers artificially boost the amount of light, but degrades image quality
Shutter speed is how long the camera lets light in for. Slower speed means more light, but more blurring from motion
Aperture is how large the shutter is overall. Wider aperture means more light, but effects how in focus near and distant objects are.
For video work, you want a certain amount of motion blur, or the video will look “off”. So a specific shutter speed is desired in many cases. You might also want everything behind the object to be “blurred” out, so you use a wide aperture. The problem is now you have too much light and the whole scene will be white, you can’t digitally darken a pure white scene, because it’s just one color and nothing else. You can’t lower ISO below a certain amount either, so instead, you put a darkened piece of glass over the lens to lower the light level.
Not all do. Some (particularly in phones) will use HDR to the same thing: several almost simultaneous photos then combined to widen the effective dynamic range.
ND filters allow you to finely control aperture and speed to get the result you are looking for.
Software now makes a reasonable attempt at blurring the background but it’s still obvious it was software created. Whereas using a wide aperture gives wonderful bokeh with beautiful gradations. Ditto for wanting to use a slow shutter speed to grab motion blur.
They are mainly for outdoor video shooting. Because the ISO is static and shutter speed is based on frame rate the only way to adjust exposure is apature without an ND. The change of apature also effects dept of field. So the ND filter gives videographers a second way to adjust for exposure.
Ooooooof I love this type of question. But, can I make it ELI5? That’s going to be harder.
So, do you know how you first wake up and EVERYTHING is so super wayyyyyyy to bright? Well an ND filter can drop that by removing a bunch of light so it doesn’t hurt your eyes- like a camera sensor.
Camera sensors have gotten REALLY good- REALLY REALLY good. In fact, with the introduction of the Canon 10D I’d say they finally started to surpass film in terms of sensitivity for a given viewing distance* (beyond ELI5)
All of that… ND filters ‘knock down’ the amount of light coming into the sensor, so the sensor can put it in a range that the software that makes the pretty photos on the screen look good can work with.
….
When it comes to HVS and variety of sensors and whatnot, there are hundreds of tricks that have been used / played/ that digital is finally realizing, including ‘over bright’ (+130%) illumination highlights to give things certain looks, that just can’t be captured on a screen.
My cameras’ shutter speed is maxed out at 1/4000 sec. If I want to use my lens full open at f1.7 aperture in bright sunny day. I would need an ND filter so the exposure isn’t too bright.
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