>On Fri, 11 May 2001 11:05:06 +0200, you wrote: > >>In fact, at 28mm it provides some protection but a low one. >>A wide angle hood wich provides a good protection needs to be bulky. > > >Again, pardon my ignorance, but do you mean like a very wide-mouth >funnel? I'm picturing some of those huge cinema camera hoods... >would you characterize them as "optimal?" No hood is optimal. An optimal hood will be infinite ;-) Given a hood design: round, petal shaped, or rectangular, the best one is the longer oner. So, the huge cinema hoods are quite good as they are big. >I'm interested by this, and never did quite fully understand the >rationale behind the tulip design. Why are the corners cut back if, >like you say, the shade is treating the lens as though it were round? The lens is round, but the format is square. So, a square hood is not optimal (except for pinholes), and a round hood is not optimal also. It's true that a square hood is usually better, but it's not the ideal shape. It's difficult to explain without a diagram, but I will try: Imagine an ideal 35mm lens f/2.8. As it's ideal, it will have a diameter of 35mm/2.8=12.5mm. It's angle of coverage is 54, 38 and 64 degrees in the horizontal, vertical and diagonal directions. If you focus to infinite you will need to let pass all the lightrays that, coming from the framed subjects, reach the frontal lens element. Sometimes it's easier to think of the lens as it was a projector. Imagine light cylinders coming from all the framed subjects (light cones if the subject is not at infinite distance) to all the surface of the frontal lens element. 27 degrees to the left and right. 10 degrees to up and down, and 32 degrees to each of the four corners. You need to let pass all this light cylinders, but not all other light rays. Imagine you want to design a petal shaped hood. You select the diameter depending on the lens barrel diameter in order to easily pack it will in reverse position while not being used. Imagine a cilinder of the selected diameter in front of the lens. Then imagine the intersection of the light cilinders mentioned above with the hood cilynder. All the parts of the hood cylinder that block the usefull light rays are eliminated. The resulting shape will be the typical petal shaped one. The same can be done for a rectangular hood. You will use a rectangular prism instead of a cylinder for the hood shape. The resulting hood will be quite rectangular but longer in the corners than in the center regions. Rectangular hoods feature an even length, so they protect less in the corners than in the other directions. The rectangular hood gives even protection in all directions only for pinhole lenses. As the wide angle physical aperture is usually small (ratio focal vs f/ number) the rectangular hoods are suboptimal, but quite good. >The cut corners imply an allowance for the rectangular negative. Yes. >And >why the lower petal other than reversability? Seems like most of the >time light will be coming from above. In portrait mode, the lower petal is the right or left one. Under daylight the sun is usually above, but think of artificial ligths or reflecting surfaces (water, snow...). They can be everywere. > The design is so widespread >that I'm assuming actual research has been done. Sure. >I could imagine a hood designed sort of like a traffic light visor. >In fact, it seems to me that a good hood design should be shiftable >for vertical shooting, that is if it isn't a 360 design. There is no point in cutting the lower petal. It won't give any benefit. A good hood will eliminate as light rays that don't go to the film as it can. There is always a size and ease / protection tradeoff. >Thanks again > > >Ken Durling Best regards Vicente * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
