ND filters for 640T
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vr-pilot
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I also wondered about having ND 8 and/or ND 16 filtering available in order to reduce video flickering due to oscillating prop shadows by using longer exposure times. This will "stretch out" the shadow phases. For longer exposure times according to the "180° shutter angle rule" it would also be technically possible to use lower or "negative" ISO values.
Maybe an "electronically induced ND filter" (like i.e. in the FX9 camera by Sony) will become a more common feature also for the smaller chipset cameras...
BTW the RGB camera of the Dual 640T is really great and powerful!
I have some very lightwighted ND filters for a Typhoon H CGO3+. Maybe there is a way to place it i.e. with a tiny ring of velcro...
If you read this thread you will be provided with an alternate viewpoint on ND filters. Why I NEVER use ND Filters with Drone Cameras. And if you read this thread you will also be presented with an alternative viewpoint on shooting at 24FPS. Why I NEVER Shoot Video at 24FPS you will then see why I said you probably don't need them.
The solution to your flickering issues as well as additional information on why an ND filter is not needed is also provided in this thread: Why I NEVER use ND Filters with Drone Cameras. The short answer to your flickering problem is either get/make a lens hood, or rotate/tilt the camera/drone until the flickering is gone. Attempting to lower your shutter speed to what is potentially a very low number which will add undesirable softness to images is not a good solution not to mention for video you would be limited by the video's framerate meaning you wouldn't be able to go below 1/24s for shutter speed anyway. Even if 1/24s was slow enough, it would be unlikely to produce reliable results since the blades rotational speed changes based on what the drone is doing at the time. It is much better to just prevent it from happening in the first place (lens hood, camera's relation to the sun), than try to fix it with a filter.
As far as "negative" ISO values goes, there is no such thing. Cameras have one or more ISO values baked into their sensor circuitry by the maker; for most cameras that is ISO100 unless shooting in a LOG profile which could push it up to something like ISO800. ISO is simply the amount of circuit gain applied to the incoming signal. It is possible to go below the native ISO if the software/camera lets you. For example, my cinema camera's native ISO when shooting in a LOG profile is ISO800; I can go below it to ISO640 or ISO400 but it is still not negative. Since ISO is a function of circuit gain, it is not possible to go below zero which would result in no signal at all.
I have in the past dropped to ISO50 with my regular camera to get more motion blur for something like a waterfall on a bright day when the ND filter was not strong enough. I do agree that it would be great to see more cameras come out with eND technology; no idea why Sony is still the only one with this. None of the latest cameras from other makers have it; possibly due to production costs, Sony patents, or something else that we are not privy to.
vr-pilot
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I get your point. With longer exposure times I was thinking of prolonging them for video shootings i.e. from 1/2000 s (or even shorter in bright sunlit conditions) to 1/50 for PAL or 1/60 for NTSC respectively. For sportive action we use 1/100 s @ 25 fps or even 1/200 s @ 50 fps for 25p-slomos (PAL, Europe).
ND filters (smoked glass) were primary invented for getting rid of excessive light when ISO , F-number and ET are already at their limits. But "some" drones (like the Dual 640 T) do not have variable aperture, just a fixed f-number, so here ND filters are the "only help". (For cameras with variable aperture the ND filters are "the only way" to get stronger Bokeh effects though, in a given light situation.)
I know that mathematically it would not exist, but there is a technique that is called "extended ISO" for going below (LO) or above (HI) the sensor's standard range which can be stretched even beyond the negative range. That is why I put "negative" in quotation marks in conjunction with ISO values. But technically (in terms of electronically) downgrading the sensitivity of a sensor would be no rocket science at all. Maybe the term "negative noise level" would be more appropriate in this context.
Sony's "variable light barrier" ND filter is optical. The first time I used it with an FX9 I was reminded of the B787 (Dreamliner) dimmable windows. They use a gel, Sony maybe something more "sophisticated".
Fixed aperture definitely makes things harder, if you cannot properly expose a scene when the shutter speed is at its max then an ND filter is the only remaining option. I used to shoot with the DJI P3 and it had a fixed aperture and I never needed an ND filter for that one either but I my approach is to simply increase the shutter speed.
I do use ND filters for portrait shoots and combine then with flash for fill lighting to get a shallower DOF which translates into stronger bokeh, but for drones and their wide angle lenses and typical subject to camera distance bokeh is practically non existent.
I am not sure how far down they could go though before there was some other negative impact. I have to believe that the imaging circuitry has a minimum threshold beyond which something else would go wrong (i.e. color reproduction) otherwise we would never need ND filters and could just turn down the circuit sensitivity until the desired exposure was reached.
Sony uses electrochromic glass for their eND technology. It has been around for quite some time, but they probably patented using it inside of cameras which is why no one else is doing it at the moment. You can buy external eND filters for certain lens mounts and you can even get a drop in eND filter, but for some reason you cannot buy a non-Sony camera with a true eND internal filter. Another reason could be because the max strength is 6 stops, vs my cameras which can provide up to 10 stops of ND internally. A few times I have needed all 10 stops when filming in direct sunlight with an F1.4 lens.
vr-pilot
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Thank you for all the details and explanations. I agree with your insights on i.e. "DOF for drones", "downgrading sensors" and the "eND".
The phenomenon "light" is quite a science (in this case much more than "only" rocket science) of which the underlying human knowledge is still "quite basic", IMO.
Reducing a sensor's sensitivity would mean worsen its quantum efficiency (ability to change photons into electrons). It was always of higher economical priority "to speed up the ISO" in order to make low noise images of fast moving objects with less motion blur under poor lighting conditions.
A reduced performance sensor would only help for situations were movements and/or depth of field shall be blurred intentionally under excessive light conditions. Instead of an electronic degradation of its high speed sensor i.e. Sony decided to optically build a photon barrier, which is another way of reducing the quantum efficiency of the system.
The phenomenon "light" is quite a science (in this case much more than "only" rocket science) of which the underlying human knowledge is still "quite basic", IMO.
Reducing a sensor's sensitivity would mean worsen its quantum efficiency (ability to change photons into electrons). It was always of higher economical priority "to speed up the ISO" in order to make low noise images of fast moving objects with less motion blur under poor lighting conditions.
A reduced performance sensor would only help for situations were movements and/or depth of field shall be blurred intentionally under excessive light conditions. Instead of an electronic degradation of its high speed sensor i.e. Sony decided to optically build a photon barrier, which is another way of reducing the quantum efficiency of the system.
