Why are nd filters still used with digital?

[CgRay]

I'm kind of new to the world of DSLRs, but it just occurred to me from what I know, that nd filters should really be a thing of the past. I mean nd filters are just there to reduce the amount of light right? This is such a basic thing to do when dealing with digital imaging. Other than holding onto the past, why wouldn't there be a way to set your ISO as low as 1? Or am I just totally missing something? I know some professional digital video cameras like the sony f65 have internal nd filters, but unless that's a software thing, then that also seems a bit silly. Can someone educate me if I'm not understanding.

Cheers,
Cg.

[TCampbell]

Many "effects" filters are no longer required as the effects can be applied with digital editing software... so while it's true that many filters are "a thing of the past", ND filters are _not_ actually "effects" filters.  What they do cannot be simulated by editing an image.

An "good" ND filter wont change the look of the image itself... everything should appear sharp and without any color cast.  What it does... is change your shooting CIRCUMSTANCES.

Suppose I'm shooting with an outdoor "fill" flash on a sunny day and my flash can't do "high speed sync" mode... so I'm limited to max flash sync shutters speeds of, say, 1/200th (this varies based on your specific camera model.)  The "Sunny 16" rule says that I the normal exposure for full mid-day sun using f/16 is to use the inverse of the ISO as the shutter speed.  So at a low base ISO of ISO 100, that'd be 1/100th.  But the side-effect of f/16 is a _very_ broad depth of field and suppose I want a deliberately blurred background.  I can increase the shutter to 1/200th and this will let me drop my f-stop to f/11... but that's still a LOT of depth of field and no nicely blurred background.  If I wanted to bring that down to, say, f/4... I'd have to find a way to dump 3 full stops worth of light.    I can't just drop the f-stop to f/4 and increase the shutter by 3 stops to 1/1600th because that's WELL beyond flash-sync speed and my flash (for purposes of this example) can't do high-speed sync.  

One really great way to dump 3 stops of light so I can drop my f-stop down without having to speed up my shutter is... a 3 stop ND filter (sometimes called an ND 0.9 -- every "0.1" worth of density blocks 1/3rd of a stop of light.)

Another _VERY_ common use of the ND filter is to get shutters speeds much slower than possible without the filter.

Suppose I want to take a photo of a waterfall and I want a slow shutter so that I can have that gorgeously blurred dreamy looking water.  At f/16 my shutter is still at 1/100th... I could go to f/22 and drop the shutter to 1/50th... but that's still pretty fast.   If I used 5 stops I could get the shutter down to 1/4 second.  A 10 stop ND (ND 3.0) would let me bring that down to 8 seconds... 

The final images in these examples would still have the same amount of light and there'd be no tint or color-cast on the images... what you're REALLY doing is changing your shooting "circumstances" which allows you to use different exposure settings than would otherwise be possible without the filter.

Hope that helps!

Regards,

Tim

[ScottyP]

I guess they might be able to make an ISO setting of "1" or something like that, which would be like a 6 stop ND or so. Not sure if that would produce some kind of electronic distortion, or would require sensor design modifications that would impair "normal" ISO performance, or anything. My camera has ISO 50 as an unlockable "extended ISO" so why not 25 or 12? I think there probably just isn't enough interest in that feature to justify it, especially as the ND filter is a pretty workable solution to that problem already.

[CgRay]

Hi Scotty, yeah the nd filter thing works totally fine. Except that they're not cheap and you have to buy them for every different lens you use and you have to put them on and take them off again. A software option seems like a much saner way to go in the future.

Cheers,
Cg.

[CgRay]

Hi Tim, thanks so much for your in depth reply. But you're still talking as though you're in the film world. I've been working as a digital commercial artist for 20 years and I also do a bit of related programming. So I'm coming at this from a bit of a different angle. If you take your example of waterfall, then all you really want to do is reduce the amount of light that is being recorded so that when you want plenty of motion blur and you have a very slow shutter speed, your image doesn't end up all blown out. I was more complaining that the camera manufacturers are still thinking in old film terms. ISO used to be governed by the chemicals on the film. We don't have that limitation any more so why are camera manufacturers still putting hard limits on DSLRs like the lowest ISO being 100 when all they'd have to do is only record a fraction of the light that was captured by the sensor to get that a lot lower. Which is pretty much what an nd filter does yeah? This would only need to be a software thing. Maybe magic lantern or something similar will sort this out in the future if the manufacturers don't. I guess the fact that lots of people still think in film terms is the reason that dslr cameras are still so tied to the past. Thanks again for taking the time, I'm looking forward to learning much from this forum and if there's generous people like you around, then I feel like I'm in the right place.

Cheers,
Cg.

[TCampbell]

The sensor is covered with "photo-sites" which you can think of as photon counters... While the sensor is digital, the photons are not.

Here's the rub... in photography, most camera makers don't talk much about "well depth", but in astro imaging CCDs "well depth" is a big deal (sometimes called "full well capacity") All digital sensors have a well depth. It's basically the maximum number of photons that a single photo- site can "count" before it over-flows and you can think of it as a measure of the camera's dynamic range.

That means you can't necessarily just let the light keep coming in and then divide the count by 100 to get ISO 1 because the photo-sites would "overflow" the "well depth".

You could do this if you had a sensor with a significantly higher well depth but those sensors are significantly more expensive. It's vastly more practical (from an economic perspective) to just use ND filters.

[CgRay]

Wow, this is getting good. Tim, from what I understand about how digital camera sensors capture light, they do it sequentially rather than in parallel like film would. And this is why I end up having to deal with rolling shutter artefacts in digital footage. So from my understanding, if a shutter is open for a long time, the sites in a sensor each take their turn to count photons over and over and over again til the shutter is closed. Or is it different for stills compared to footage? I'm guessing the well you speak of is like a buffer that each site has that it uses to hold onto data til it's time to send it to the next stage. I thought there'd be time to manipulate the data each time it is captured, rather than having to do that after the buffered stage. Or am I way off?

[TCampbell]

I don't know the nuances of "how" the CCD collects the light... only that it's basically counting a score for the amount of light recieved and that score is held digitally in the sensor while the exposure is being taken.  When the exposure is finished, the data is "read out".  

I believe all photo-sites are actually able to count up light values at the same time... not taking turns, but we'd have to rely on someone else for that info.  I only know that the well-depth basically limits how hight that count can go before it overflows (any light received after it hits the cap is simply lost since it cannot count any higher.)

My partner and I had been speculating if there might be a way to create a long exposure mode which causes the sensor to sample light, pause, sample more, pause, etc. but do this rapidly so that in a 30 second exposure, the well-depth wouldn't overflow, but you'd get motion blur similar to what an ND fitler can do.  I suppose it's possible... but the question is would it actually be as good and would it impact the price by more than the cost of an ND filter?   There is such a thing as an "electronic shutter" where no mechanical shutter actually opens or closes... it's really "open" all the time but it's just digitally activating and de-activating.  I seem to recall there was a technical reason why the mechanical curtain and leaf shutters are better than electronic shutters -- which is why high end cameras still have mechanical shutters, but low-end cameras use electronic shutters.  In "image stacking" (very common among astro-imagers) you can do image stacking by "averaging" frames (which is used to eliminate noise even when shooting at very high ISO because real stars will appear as faint points of light but in the same spot in every frame whereas noise will appear as tiny specs but with random positions in each frame (and you can use "dark frames" or "dithering" techniques to eliminate any re-occuring noisy/stuck/hot-pixels.))  You can also do additive stacking... lots of short frames but you're just adding the amount of light frame after frame to simulate having taken a very long exposure.  I suppose what you're looking for is something along those lines... lots of fractional short exposures, but stretched over time so that the small blur in each frame is added together until you get that soft dreamy look that the ND filter would have given you.

[hsbn]

To lower the ISO, you'll have to divide the signal or reduce the sensor sensitivity. Divide signal will cost you dynamic range, and reduce sensor sensitivity is kind of counter intuitive (we all want higher sensitive sensor, don't we?). So from practical stand point, it makes more sense for camera manufacture just to make you purchase a ND filter. You can buy the biggest size for your lens and use a step down ring for the rest. A good ND filter is still not that expensive comparing to your camera system.

[CgRay]

Hsbn, thanks for the step down tip.