Originally posted by carpents Absolutely fascinating. Why didn't I think to print a gradient on a transparency? (Yeah, it isn't quite the same as having a variable-density piece of glass.)
Hi,
I'm curious, though, why mounting it on the front of the lens wouldn't work. That thread talked specifically about opening up the lens and placing the transparency near the aperture blades. (Which I assume your link shows...but sadly it won't appear for me.) Maybe because I'm primarily interested in improving wide open and at most 1 stop down, whereas they were talking about getting bokeh smoothed out at f/8.
In reply #87 I mentioned that a filter just in front of the lens won't work. I also said that I don't know why it works when the extra set of aperture blades are placed closer to the standard aperture blades.
That was intuition. I don't know more about optics than a five year old kid playing with a magnifying glass in the sun.
My guess: Imagine a simple lens with two elements and an aperture mechanism in the middle somewhere, the lens focused to infinity.
Just in front of the front element we have the filter threads. Aim the lens towards the sun and we have the sun in focus at one extreme and the imagener at the other. By stopping the lens down, from, say wide open and one stop, we close the aperture rings a bit. This is done somewhere where we have "no focus" optically. We just lessen the light by stopping rays coming from the sun hitter the outer parts of the front element. The net result is that the picture gets darker.
Now the tricky part. The STF set of aperture blades makes two things: it darkens the picture, just as the ordinary aperture, and it has an pleasing effect on the OOF parts of the picture by smoothening them. My guess is that the STF aperture blades are placed pretty close to the standard ones but still at a "plane" where we have a little more focus, sort of speaking. I think you can understand how I am thinking, but I have of course no grip of this.
It would be fun to try something with a short tele lens with some air inside it between the lens groups. I'm thinking of a way to add apertures (cut out from paper possibly) in different sizes and move them a little forth and back to see what happens.
Maybe this is why the STF lens is a tele lens; they might have needed space in there to get a good aperture mechanism fit. I would like to have a FF sensor and a 24mm, a 50mm and a 90mm STF lens to go with it...
regards,