Calculating a 'fixed' Compendium hood

[Tim Povlick]

How does one design a 'fixed' compendium hood. The meaning of fixed is the hood is CNC'ed from a solid block of material. This would be for use on a camera using only a single lens (not interchangeable), 150mm f/l and the film is 6x12.

It seems the opening should be a ratio of 12 : 6 or 2 : 1. That's straightforward. Let's say the hood will be 3" x 6" opening. How far from the lens should this opening be to have maximum shielding effect without causing vignetting?

One could calculate the angle of the opening (on the film side) knowing the 6x12 image is formed 150mm from the lens. But the hood goes on the opposite side, so does one use the thin lens equation?


I did a quick search of the web and this forum but did not see anything close to the topic.


_..--
TiM

[Jim Jones]

It is probably easier to measure the minimum front opening of the lens hood rather than calculate it. The opening will be close to, but not exactly equal to, a 1:2 ratio because the radius of the entrance pupil must be added all around a theoretically calculated opening.

[jb7]

The most efficient hood will be of a 'petal' type, you can find examples by searching on ebay...
If it's going to be made using cnc, then you will have to take into account the magnification and aperture...

[Bob Salomon]

The most effective compendium will have an opening the same size as the film format and a length equal to the distance the lens is extended from the film plane. That will only let in light that actually forms the image. Since that is impractical in use better compendiums have adjustable masks to cut off the stray light rays.

[ic-racer]

The rays enter through the front nodal point. So you need to know exactly where that is if you are going to design a lenshood using math.

[vinny]

In the motion picture world we have fixed mat boxes and "hard mats" which clip on the front which are specific to each focal length. Putting a 50mm hard mat on when using a 50mm lens only allows image forming light in like bob mentioned.
You could make the hood then add a hard mat which would have a smaller hole. I'd make a long prototype from foam core, mount it, and cut away the excess material until it doesn't vignette.

[Tim Povlick]

Thanks very much for the helpful replies.
As ic-racer points out the nodal point is needed and since this is not known toany accuracy I will skip the math exercise.

Will use some foam core pieces per Vinny's excellent idea.
Vinny's post reminds me of a post from our resident DP (MW) to some matt boxes used in the industry (Airel?), and while a work of art, the cost was nearly astronomical. Reminds me of my Father always saying don't make a Hollywood production out of things.

The petal design would be good and appreciate that suggestion as well.
Not sure but this seems like something a 3D printer would do well.

Many Thanks for the expert help and advice.


Tim

[Drew Bedo]

CNC machining? It could also be done as a one-off with 3-D additive printing in plastic or metal.

[Jac@stafford.net]

Why would anyone want to build a rigid hood when you could have a less expensive, adjustable, perfectly functional shade?

[ic-racer]

Other things not mentioned are:
1) Mount a real compendium shade and adjust it to suit and copy the dimensions.
2) Experiment with 'good guess' designs and 3D printing then CNC the final one out of metal.