Originally Posted by
Joe Smigiel
Actually, I think Heroique provided the answer. The viewfinder/framing card helps determine where in space the picture should be taken, and also the focal length that should be used. There is really only one best position in space to photograph the scene. That spot gives the best perspective. Once it is found using the framing device, that's where the lens should be placed.
If the viewfinder opening has the same dimensions as the film format (e.g., 4x5), when you have framed the scene with the card at that spot, then the distance from the card to your eye tells you what focal length lens will give you that perspective. A card held 6" (150mm) away from your eye indicates a 6" (150mm) lens should be used on camera. If the scene looks best when the card is 12" (300mm) away, then a 12" (300mm) lens is indicated.
All the other stuff about exaggerating nearer objects relative to further objects with shorter lenses will still be true, but the card lets you see that relationship directly and also informs about the appropriate lens focal length, how much background will be seen, etc. It doesn't matter if you are doing a landscape or tabletop, the card works in both situations.
Consider what happens to the scene when two somewhat overlapping bottles at different distances are photographed with lenses of different focal lengths. If the foreground bottle is to have a certain height in the picture with the shorter lens, the lens will be closer to it and the bottle will look larger than the one in the background. There will also be more background evident overall. If you maintain the same image size of the foreground object using a longer lens, several effects will occur as you will need to back up (i.e., increase the subject distance) and thus change the perspective. First, the relative sizes of the foreground and background bottles will become more similar. Second, less background will be seen within the format. Third, the top and bottom of the foreground bottle will appear to be closer together than they were with the shorter lens relative to the same areas of the background bottle. IOW, the top of the foreground bottle will appear to be lower relative to the top of the background bottle compared to the first situation and ditto with the bases. Fourth, since you've changed the perspective by changing the subject distance, the shape of the ellipses that visually result from the cylindrical shapes will change and alter the overall impression of the bottle shapes. The ellipses become more like lines than circles, the space between the bottles appears visually reduced and the whole scene seems flatter. Substitute a landscape with trees instead of bottles and the same things happen visually.
Another thing very nice about using the card is that the card will show you where the lens should be placed in physical space. Once that is found, the big camera can be placed there. That's a lot easier than trying to find the best spot by moving the camera and tripod around and looking under the hood at an inverted image. Also, you could use a 4x5 card to determine best focal lengths for an 8x10 or any other format with the same aspect ratio. If the shot looks good with a 4x5 opening 6" from your eye, doubling that distance will tell you that a 12" lens will give the same perspective with an 8x10 camera. If you really want to get fancy, a #90 Wratten monochrome viewing filter could be added to the card for b&w work to allow tonal relationships to be estimated as well. A similar device was once marketed by Fred Picker and his Zone VI Studios.
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