Filters

[Simon Wyndham]

Dunno if this has already been covered. But will there be any provision for built in ND filters in Red? Its one of the things I find very useful in run and gun. It would also help protect the sensor when changing lenses.

[Stephen Williams]

Dunno if this has already been covered. But will there be any provision for built in ND filters in Red? Its one of the things I find very useful in run and gun. It would also help protect the sensor when changing lenses.

Hi Simon,

I would second that.

Stephen

[Alex Boothby]

I think Greame once mentioned that there was not enough room between the back of the lens and the sensor to sqeeze this in. Not sure. It would be freakin' great if they could.

[chuck colburn]

Hi Simon,

Behind the lens filters have some disadvantages. First off if it's going to be more then one unit it's going to require a filter wheel with its size and mechanical requierments. Second the filters loaded into the wheel have to be carefully selected for paralelism of surfaces and thickness relative to each other. The thickness variation is important because even though the filter is plano it still has a magnification factor of appx. one third it's thickness. That is to say if the filter is .060" thick it's going to optically increase the flange focal depth about .020". Therefor the filters need to be very accurate in this measurement relative to each other especially when using lenses of short focal lengths. And at least one filter would have to be clear or at least of a uv type because if you set up the flange focal depth for use with behind the lens filters you are going to have to keep one in the optical path at all times. My choice would be to have a fixed optical window behind the lens. This optical flat could have a multi layer antireflective coating on it that filters generally don't have. This is important because every uncoated optical surface results in about three percent light loss or six percent on a given filter. And as you said, it would provide protection for the sensor surface. So I think that to put any thing between the lens and the camera that could possibly reduce the quality of the image is a no no.

Chuck Colburn

[Mike Devlin]

I would agree with the value of built-in ND filters for ENG/EFP. I would go one step further and say servo controlled filters like the Sony F950 or HDC1500 are required for the type of EFP we do. For situations such as a helicopter gimbal or simlar gyrostabilized setup (a boat in our case), or any other remote control operation (wildlife in our case) where you cannot touch (or go near) the camera while operating the camera the servo controlled filters are essential. This applies to many remote PTZ (pan/tilt/zoom) applications. This is particularly true as the light changes throughout the day and where you are using a very long lens.

Perhaps there are other solutions to get the same effect in the 35mm world and we are just used to (and own) the current EFP cameras such as the HDC1500. Worst case we will still use 2/3" HD cameras in those applications and use our RED for other shots.

[Finner]

Hi Simon,

Behind the lens filters have some disadvantages. First off if it's going to be more then one unit it's going to require a filter wheel with its size and mechanical requierments. Second the filters loaded into the wheel have to be carefully selected for paralelism of surfaces and thickness relative to each other. The thickness variation is important because even though the filter is plano it still has a magnification factor of appx. one third it's thickness. That is to say if the filter is .060" thick it's going to optically increase the flange focal depth about .020". Therefor the filters need to be very accurate in this measurement relative to each other especially when using lenses of short focal lengths. And at least one filter would have to be clear or at least of a uv type because if you set up the flange focal depth for use with behind the lens filters you are going to have to keep one in the optical path at all times. My choice would be to have a fixed optical window behind the lens. This optical flat could have a multi layer antireflective coating on it that filters generally don't have. This is important because every uncoated optical surface results in about three percent light loss or six percent on a given filter. And as you said, it would provide protection for the sensor surface. So I think that to put any thing between the lens and the camera that could possibly reduce the quality of the image is a no no.

Chuck Colburn

Hi Chuck

I will not challenge you on your math and theory here. All I can say is that I have used behind the lens filters for years with Panavision cameras and have always had great success. In fact if available to me I will always choose behind the lens over in front because the image always look better to me.

[chuck colburn]

Hi Finner,

We might not be talking about quite the same thing here. Unless I'm wrong, the behind the lens filters you are refering to are gels that are mounted in slip on holders that attach to the back of a given lens. These displaced the flange focal distance appx. .0015". This is a not a worry with standard or longer focal length lenses, but can be with the newer extreme wides and especially ones of fast neumerical aperture as it can keep the lens from reaching infinity or possibly blowing a tight split. This can be aleveated by collimating those type lenses for use with a gelatin filter at all times.

Chuck Colburn

[Finner]

Your right about them being gel filters but could this system not be used with RED?

[chuck colburn]

Absolutly.
There is no reason you can't fabricate gel holders for what ever lens you plan to use, (might be a bitch on some of the still camera types) and check them with an auto collimator to be sure they reach infinity focus. What Simon was talking about was permanetly installed (I think) glass filters. Gels would not be pratical in this application as they are supceptable to fading and also swell when subjected to moisture. On another note the reason I would not want a filter wheel is since this camera is being designed to accept different types of lens mounts, I would not want anything in the way of using rangefinder camera type optics. Some of which are of a retrofocus (inverted telephoto) design where the rear optical group extends far past the lens mount as there is no worry of it hitting a mirror like on an SLR. This type of lens has inherent design advantages such as better rectilinear correction and eveness of illumination across the field. See rangefinder lenses by Zeiss and Leica.

Chuck