Originally Posted by
GPS
So let’s put the things in some light. First of all, somehow (a strange type error?) you contradict yourself, saying first that anything less than the focal length might not cut out enough light to make enough difference and then going to say that the difference in contrast is noticeable even on a 210mm lens and 90mm length of a lens shade.
Secondly - (to go back to your first incorrect saying) - you don’t need and don’t have lens shades long 600mm or even 1200 mm for lenses of those focal lengths. Not only would it be impractical ad absurdum but unnecessary too. Why? Because the length of the shade has a very small impact after a certain «*optimal*» length. Say, to name an example, you have the Fujinon C 600 lens. If you make a lens shade of 600mm length you eliminate (under certain given conditions, without going into details) 95% of the stray light vertically on 4x5 film format. If you make the lens shade long 400mm you get the percentage down to 92%. If you make the lens shade long 200mm you get the percentage down to 83%. You see that the huge difference in the length of the shade makes just a small effect on the percentage of the offending light getting on the film. But - even 60 % (or 50% ) of the eliminated stray light makes a very important difference (clearly visible) on the picture. Usually, the «*optimal*» length after which making the shade longer is not practically important and viable is about 70%. After that, you add an unwieldy length to the shade with a small improvement in the optical performance.
So much for an example.
By now you probably understood that me too I know about lens shade construction something... Regards, GPS
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