Film-Tech Forum ARCHIVE: How much light do you need for 70mm?

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Posted by Paul Gordon (Member # 3320) on 06-29-2011, 12:12 PM:

Hey all,

There is a cinema uphere thinking of switching to 35mm/70mm projectors so they can host our 70mm festival. My question is how much more light do they need to project 70mm 5-perf? Right now they are getting 18-19Fl on screen in scope.

Posted by Steve Guttag (Member # 268) on 06-29-2011, 12:18 PM:

The spec is identical for 35mm or 70mm. 16fL center brightness with white light/open gate (no film but the shutter running).

-Steve

Posted by Paul Gordon (Member # 3320) on 06-29-2011, 12:43 PM:

Ok.... but if their getting 18-19fl with 35mm with their lamphouses will they get the same with 70mm? does the gate size and lens not matter much? I just want to know if they will need to upgrade their lamphouses.

Posted by Steve Guttag (Member # 268) on 06-29-2011, 02:41 PM:

Often, one gets MORE light with 70mm than with 35mm. One may need to reset the lamphouse for 70mm since the working distance changes (and/or use a beam spreader).

-Steve

Posted by Paul Gordon (Member # 3320) on 06-29-2011, 02:45 PM:

Good to know! Thanks

-pg

Posted by Frank Angel (Member # 248) on 06-29-2011, 02:49 PM:

I always wondered about that Steve -- doesn't a beam spreader, by taking the same amount of light and spreading it over a larger area, of necessity drop the light level in that larger area? An anamorphic lens does that, albeit on the other end of the aperture, but with the same results of overall brightness being reduced, no?

Posted by Stephen Furley (Member # 1277) on 06-29-2011, 03:07 PM:

The light from the lamp is spread over a larger aperture, but the light from that (larger) aperture has be be spread out less to cover the screen, if that makes sense. The screen size is what really matters. Of course, other factors can come into it, such as the aperture of the 70mm ones compared to the 35mm ones, and whether or not a beam spreader is fitted.

Posted by Gordon McLeod (Member # 33) on 06-29-2011, 04:25 PM:

I very rarely use the beam spreader as it eats light most modern lamphouses will have no isuue defocusing the beam to cover the 70mm apperture and the large f number the big apperture and large lens will work well with more overall light

Posted by Louis Bornwasser (Member # 3063) on 06-30-2011, 10:42 AM:

Typically a JJ Century with Schneider lenses and a Strong Ultra 80: 31 ft/lm on 70mm

26 ft/lm on scope

18 ft/lm on 1.85.

All else being equal, I would expect the same ratio. remember 70mm has other advantages other than grain: reduced jump/weave and better color saturation. louis

Posted by Scott Norwood (Member # 30) on 06-30-2011, 12:05 PM:

Does anyone really believe in the use of the beam spreader? The concept makes sense to me, but I have talked with a number of people who should know and they all have said that it either doesn't improve anything or that it actually makes illumination worse than simply defocusing the lamphouse.

Is its purpose simply to enable easy conversion to and from 70mm and eliminate having to re-align the lamp when changing formats?

It would be great if a lamphouse could be designed with motorized reflector adjustments that could be pre-set to recall various formats (35mm optical, 35mm silent or mag, and 70mm mag) at the push of a button, but I doubt that any theatre owner would ever pay what such a thing would cost.

Posted by Steve Guttag (Member # 268) on 06-30-2011, 01:05 PM:

My experience with Beam Spreaders varies...some did it better than others. I want to say my best experience was with Christie's on the K series lamphouses. I've had mixed results on Strong but if you get everything aligned right...one can yield more light with the beam spreader. You have to get your working distances right though.

Those that merely focus/defocus lamps are doing it wrong and likely have other artifacts that show up that they don't recognize. The reflector has one sweet spot for the arc to be and the cone of light the system puts out is fixed. You want to get the working distance right for that cone of light to just be big enough when the light hits the aperture. For 70mm, the working distance is going to be notably closer (shorter) than for 35mm.

When one focuses/defocuses the arc you will get black holes in the middle or stray rings on the side and softer pictures due to double imaging (multiple light beams are now going through the same point). It is never as ideal.

A beam spreader can go a long way to making the difference between 35mm and 70mm minimal.

Frank, with respect to a beam spreader...draw yourself a rectangle with the proportions of 2.2:1. Find the center of that rectangle and take a compass and draw a circle such that circle just touches each corner of the rectangle. All parts of your circle that are outside of the rectangle are wasted light.

Now do the same thing (draw the same sized rectangle) but this time draw an ellipsoidal around the rectangle such that the ellipse is 2.2 times as wide as it is tall but also just touches the corners of your rectangle. Which has more wasted light? The ellipse will have more on the sides but less on the top/bottom for a net improvement. However, the beam spreader is not a perfect lens and some are less perfect than others so you get some losses just by going through it. Again, if everything is not set up properly, (and the lens is not designed properly), it will not be an improvement.

On the anamorphic side...you must take into account the size of the aperture/film. A 1.85 picture will have LESS light than a Scope picture using an anamorphic lens with a common-height screen. In fact, the Scope image should be about 20% brighter since there is 20% LESS overall magnification going on (even taking into account that the anamorphic is magnifying the width by 2). This presumes the lamp is not refocused or moved...one can often eek just a little more light out of 1.85 if the spot is tightened just a bit (or better yet, lamphouse moved back).

-Steve

Posted by Monte L Fullmer (Member # 2797) on 06-30-2011, 08:28 PM:

quote: Steve Guttag
This presumes the lamp is not refocused or moved...one can often eek just a little more light out of 1.85 if the spot is tightened just a bit

Why some Christie SLC console had that autofocus with the reflector - to focus in the bulb a bit more for flat and defocus the bulb for scope.

I do the same with my SLC consoles when projecting flat: to lightly focus in the bulb for a brighter picture and defocus the bulb back for scope.

I have to leave myself a note to defocus when going back to scope when a print changes. Otherwise, I have a gradual hot spot in the middle of my scope image and the film leaving the gate is a bit on the hot side instead of a nice luke warm touch when the bulb is focused in correct proportion to the screen - which is an even field of light across the entire surface of the screen.

One can do this with any 35mm lamphouse or console when changing lens formats to get the best light out per each format.

Posted by Gordon McLeod (Member # 33) on 06-30-2011, 08:42 PM:

Get a Eprad with a bendable mirror

Posted by Louis Bornwasser (Member # 3063) on 06-30-2011, 09:43 PM:

I have NEVER seen a beam spreader that helped. All of them shifted the color temperature as well.

Steve is right, there is only one good place for the bulb. Moving the entire lamphouse forward/backward yielded a larger/smaller spot on the gate; forward for 70mm. Louis