A subtle artefact of stacking leaves in an array is the issue of pseudo-shortening. Being on the continuing quest to make the perfect iris design, it’s certainly a factor to consider when creating an iris diaphragm.
So what do I mean when I use the term pseudo-shortening?
Let’s start, like all great iris designs, by drawing a circle. Looking at it straight-on, it’s perfectly round. But tilt the paper it’s drawn on slightly, and that perfect circle now starts to become oval in shape. This effect is also true with the leaves of an iris. Sure, their layouts are designed flat, but in reality, each leaf is kicked up at one end ever-so-slightly due to the way they overlap one another. This tilt has the pseudo-shortening effect of making the leaves seem slightly shorter when viewed two-dimensionally, effectively changing the shape of each leaf profile when viewed straight-on through the aperture. It also shifts the expected location of the leaf’s pins, as they also come up short.
Pseudo-shortening can be noticeable when the iris is open at maximum aperture. Despite designing to a specific size, and carefully manufacturing all the parts, you may notice your iris sometimes never quite opens to its maximum aperture. The leaves seem to – ever-so-slightly – overhang the opening when the iris is fully open. It’s almost as if the handle is being stopped a smidge too soon, when in reality, two subtle things are happening: the leaf profile has effectively become compressed, and the pins are positioned just a fraction out of alignment.
Pseudo-shortening can be particularly noticeable in dual-plane irises. Not only is the slight overhang apparent at maximum aperture – as mentioned above – but additionally they’re never quite circular at minimum apertures. With two separate planes tilted two different ways, the halves of the aperture don’t create a perfect circle in an evenly distributed manner.
Designing in a solution that counteracts this pseudo-shortening is actually fairly easy to achieve. We can continue to design the iris layout flat – as per usual – but when we come to draw up the leaves, we’ll need to factor in the angle of tilt being exerted on them. A simple way to do this is to visually count how many leaves a single leaf will traverse in its parked position, and add up all those heights. Now we know how high one end of our leaf will be, we can calculate the angle of tilt. We can then do a projection cut and flatten out the material to reveal the true leaf shape needed. Once this is done, we can place the pins on the ends, so that when the leaf is tilted in the iris, they’re actually in the correct positions in relation to the rest of the iris (i.e. their slots and pin holes).
This is perhaps too simplistic of a solution. There are some other factors happening that we should maybe consider too. Because if we look side-on at an iris at minimum aperture, we can see how the leaves are influencing one another. The inner-edge of each leaf is having to take an S-bend route through the aperture, as it goes over then under it’s neighbours. This is most significant when irises are designed with small minimum apertures, because the leaves are all fighting each other hard for that tiny piece of space. Really this kink needs to be included in our projection cut, although perhaps this may have a negative affect on the leaf’s profile at other apertures (tbd).
The pins are also not truly square with their respective holes and slots. They are constantly being forced to endure the slight angle of the leaves tilting.