By technical editor Matt Allard:
Douglas Underdahl from Long Valley Equipment has come up with a prototype built-in ND filter system for RED cameras called the ND Slide.
It has two filter stages, each loaded with a different ND value, giving four settings:
1) no filter – no light reduction to the OLPF and sensor;
2) slide one engaged: ND .6 (two stops) light reduction:
3) slide two engaged: ND .9 (three stops) light reduction;
4) sildes one AND two engaged, giving a light reduction of ND 1.5, or 5 stops.
This from Douglas Underdahl:
Filters used will have both ND and IR reduction. Phil Holland seems to think that Formatt Firecrest filters are the best and we will test them along with others. Also, other values might be used, for example, 3 stops, 6 stops, and the combo would yield 9 total stops along with IR attenuation. Or more?
LENS MOUNTS – right now, I’m using this prototype with our Long Valley Equipment Posi Lock Nikon mount for Epic/Scarlet/Dragon/Weapon. This mount uses the Red One Nikon locking system and is very robust. The chassis is CNC machined from a single billet of 6061 aluminum. It offers manual iris control for G series Nikon lenses and other lenses that have no iris control ring. It does not have any electrical contact with the camera and offers no electronic iris or focus control from the camera – a “dumb” mount. I’ve tried about 10 different Nikon mounted lenses, old and new, and none of them protrude into the lens mount enough to contact the ND Slide.
I am sure that there will be interest in this built in filter system for use with other lens mounts such as the Red Digital Cine’s DSMC Nikon, Canon, and PL mounts, but at this point, I’m not sure how compatible it will be. It should be clear that a slot needs to be cut into the top of the mount and I don’t know if this will or won’t be possible with these mounts.
SHARPNESS – I’ve been doing some tests with my Epic at 5K, and I’m actually having trouble figuring out which image is with the built in ND and which is without. There is some reduction in quality but it is very slight and probably similar to using ND filters in front of the lens.
RUN AND GUN – Well, you know what this device means for run and gun – and other work. You can instantly switch on ND filters in the camera, so you can react to wildly changing light values without stopping to pull out an ND and place it over the lens. In many cases, this can mean the difference between getting the shot or losing it forever. Even for scripted narrative work, the ND Slide can speed up production and will prove invaluable, as built in ND filters have in other cameras.
CUSTOM FILTERS – not sure what other filters might find their way into the ND Slide, but if you have ideas for effects filters, etc, just let me know. I can imagine nets, UV, IR Pass, fog, double fog, low con, etc etc.
The Newsshooter view:
With a system like this focus is where things get a little tricky. Any glass placed between the lens and focal plane will shift the flange focus distance (back focus). So there will be a focus shift when you engage the filters. Every time you stack another filter then the flange focus distance will continue to change. The increase of the flange focal distance will be even more apparent when you use wider angle lenses. Another thing to keep in mind is that minimum focus distance will also increase due to the flange focal distance moving. This doesn’t mean the Slide is unusable as you can still focus on your subject by turning the focus ring on the lens until sharp focus is achieved. As long as you can still hit infinity focus then it should be ok. For those who use a tape measure and set focus using the witness mark and focus distances on the lens, it should be noted that the focus marks will not be accurate when one or both filter slides are engaged. They will all be shifted slightly.
The ND Slide is a good idea in concept but it isn’t without its problems. Just like other devices that use a similar behind the lens filter such as the Fotodiox ND Throttle, users should be aware of the potential pitfalls of having the focal flange distance change every time you ad or remove a filter. For solo operators they probably will find the ND Slide to be a workable solution, but for those working on big productions where a focus puller is required, the changing of the flange distance may end up causing too many problems and delays in filming. It will be interesting to see if a version can be made to work with PL and Canon lenses which are by far the most poplar choices among RED users.
