rgb@hou5g.UUCP (Rich Bantel) (02/04/85)
I'm interested in squeezing a little more light out of my medium format's viewfinder. I've seen Beattie's Intenscreen but at $130 I wonder if there are alternatives. For example, there's another company that for $60 will lamenate (?) a fresnal screen onto your own viewfinder screen. That way, you can keep whatever type of screen you currently have (e.g., split image). Has anyone tried doing this themselves? Edmund's sells fresnel lenses - but I'm confused about the fresnel lens' focal length rating. How should the f.l. be mounted to the viewing screen? Is there a danger of getting the focus point out of kilter? Also, has anyone used these intensifiers? Do they have "hot spots"? Thanks in advance. -Rich
briand@tekig1.UUCP (Brian Diehm) (02/06/85)
> I'm interested in squeezing a little more light out of my medium format's > viewfinder. I've seen Beattie's Intenscreen but at $130 I wonder if > there are alternatives. For example, there's another company that for > $60 will lamenate (?) a fresnal screen onto your own viewfinder screen. > That way, you can keep whatever type of screen you currently have > (e.g., split image). > > Has anyone tried doing this themselves? Edmund's sells fresnel lenses - > but I'm confused about the fresnel lens' focal length rating. How should the > f.l. be mounted to the viewing screen? Is there a danger of getting the > focus point out of kilter? > > Also, has anyone used these intensifiers? Do they have "hot spots"? > > Thanks in advance. -Rich I have a Beattie Intenscreen for my 4x5, and for that kind of work it is truly a help. HOWEVER, nothing can "intensify" the light available to a viewfinder! What Beattie does is not in the Fresnel lens, but in the ground "glass" beneath it. Their ground glass is ground finer than most, giving a glass closer to transparent than most. It is between std ground glass and clear glass - closer to ground though. The effect is that the bright areas are the same brightness as before - actually a little LESS bright. However, the darker parts of the image aren't "flared" by the bright areas as badly as with regular ground glass. So, effectively, you have a much greater contrast presentation, with the flare obscuring less of the shadow detail. Beattie is charging all that money for two things: 1) The high quality Fresnel lens they use, though the technological breakthrough isn't in that, and 2) the fundamental understanding of what the ground glass really does, and making a better engineering tradeoff than has been done historically. Whether this is worth $130 is really debatable. But it proves the point that there is no free lunch - the image isn't brighter, just less obscured. For your other questions, the Fresnel lens is simply laid on top of the ground glass. The ground side of the glass faces the camera lens, the ringed side of the Fresnel lens lays upon the viewing side of the ground glass. GLUE SHOULD NOT BE USED TO HOLD IT IN PLACE UNLESS ITS OPTICAL PROPERTIES ARE FULLY CONSIDERED! This would be akin to gluing lens elements together! Usually the Fresnel lens is just clamped into place. I don't know the best focal length formulation for the Fresnel lens, but you should keep in mind that it is part of an optical system from the ground glass to your eye, and has little or nothing to do with the camera lens focal length. This is because you are viewing a resolved image on the ground glass. (Aerial image focusing is another topic entirely!) The desired focal length of the Fresnel lens is modified by any other lenses in this system, including flip-up magnifying lenses (remember TLRs?), the magnifying lens of the camera itself (35mm), eyeglasses, and the lens of your eye. The focal length of the Fresnel lens is not a critical value - if you have several to choose from, lay them on a sheet of newsprint and select the one most comfortable when viewed from a similar distance as your camera viewing position. A wide range of variation is clearly acceptable with this test. You aren't looking here for sharpness or focus - the Fresnel lens position prevents it's modifying that. What you are looking for is equal clarity across the viewing area - mark the size of your ground glass on it if necessary. If areas of the print "fade out" or are darker than other areas, the focal length of the Fresnel lens isn't right. The extreme edges should be as clear as the center, and that is what the test is all about. Essentially, the job of the Fresnel lens is to make all parts of the resolved ground-glass image equally bright. If it isn't there, only the part of the ground glass directly between your eye and the camera lens has a bright image - play with a large format camera to believe this. For large format, the Fresnel lens penalty is that the eye must be in one position to see ANYTHING; for smaller cameras the eye position is fixed anyway and this isn't a penalty. -Brian Diehm Tektronix, Inc.
ron@brl-tgr.ARPA (Ron Natalie <ron>) (02/11/85)
> I'm interested in squeezing a little more light out of my medium format's > viewfinder. I've seen Beattie's Intenscreen but at $130 I wonder if > there are alternatives. For example, there's another company that for > $60 will lamenate (?) a fresnal screen onto your own viewfinder screen. > That way, you can keep whatever type of screen you currently have > (e.g., split image). > > Has anyone tried doing this themselves? Edmund's sells fresnel lenses - > but I'm confused about the fresnel lens' focal length rating. How should the > f.l. be mounted to the viewing screen? Is there a danger of getting the > focus point out of kilter? > > Also, has anyone used these intensifiers? Do they have "hot spots"? > > Thanks in advance. -Rich I don't understand this at all. Is it that applying the lens directly to the screen is collimating the light from the screen? It would seem that you aren't simply magnifying the screen since you are attaching the lens directly to the screen. In any case, I don't believe that you can get the focus out of kilter unless you actually change the focussing screens position. Any lenses that you add between your eye and the screen don't affect what is projected on the screen. -Ron