Replay: Every DP is aware that the iris in a lens controls exposure and, as a side effect, depth of field. But here are five things which aren’t so commonly known about it.
Inside a lens, amongst the various glass elements, is an ingenious mechanism which we call the iris because it is modelled on that part of the human eye. It’s a critical part of exposure because, just like your biological iris, it controls the amount of light passing through the pupil to form an image. It also affects a number of other aspects of the picture.
f-stops and the entrance pupil
Many of us know that the f-number of a lens is the ratio of the focal length to the diameter of the aperture. What you may not know is that it isn’t the actual diameter of the aperture that’s used in this calculation, but the apparent diameter as viewed through the front of the lens. A lens might have a magnifying front element, causing the aperture to appear larger than its physical size, or a reducing one, causing it to appear smaller. Either way, it’s this apparent aperture – known as the entrance pupil – which is used to find the f-number.
The no-parallax point of a lens is located at its entrance pupil. Sometimes called the nodal point, although that’s technically something different, this is the point around which the camera must pan and tilt if you want to eliminate all parallax. This is important for forced perspective work, for panoramas stitched together from multiple shots, and other types of VFX.
If you need to check your focal distance with a tape measure, many cameras have a handy Phi symbol on the side indicating where the sensor plane is located so that you can measure from that point. But technically you should be measuring to the entrance pupil. The sensor plane marker is just a convenient shortcut because the entrance pupil is in a different place for every lens and changes when the lens is refocused or zoomed. In most cases the depth of field is large enough for the shortcut to give perfectly acceptable results, however.
The shape of the entrance pupil determines the shape of the image’s bokeh (out of focus areas), most noticeable in small highlights such as background fairy lights. The pupil’s shape is determined both by the number of the iris blades and the shape of their edges. The edges are often curved to approximate a circle when the iris is wide open, but form more of a polygon when stopped down. For example, a Cooke S4 produces octagonal bokeh at most aperture settings, indicating eight iris blades. Incidentally, an anamorphic lens has a roughly circular aperture like any other lens, but the entrance pupil (and hence the bokeh) is typically oval because of the anamorphosing effect of the front elements.
When the edge of an iris blade is straight or roughly straight, it spreads out the light in a perpendicular direction, creating a diffraction spike. The result is a star pattern around bright lights, typically most visible at high f-stops. Every blade produces a pair of spikes in opposite directions, so the number of points in the star is equal to twice the number of iris blades – as long as that number is odd. If the number of blades is even, diffraction spikes from opposite sides of the iris overlap, so the number of apparent spikes is the same as the number of blades.