Two different cameras filming the same scene with the same filters, aperture and shutter settings will not necessarily produce an image of equal brightness, because the ways that their electronics convert light into video signals are different. That’s why you’ll often hear the terms ISO and ASA used interchangeably. The acronym stands for International Organization for Standardization, the body which in 1974 combined the old ASA (American Standards Association) units of film speed with the German DIN standard. In this final part we’ll look at ISO, perhaps the most misunderstood element of exposure, if indeed we can technically classify it as part of exposure at all! So far in this series we have seen how we can adjust exposure using aperture, which affects depth of field, ND filters, which can help us retain the depth of field we want, and shutter angle, which affects motion blur and flickering of certain light sources. 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, as in the eight-pointed Cooke diffraction pictured above right. 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. The result is a star pattern around bright lights, typically most visible at high f-stops. 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. 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. For example, a Cooke S4 produces octagonal bokeh at most aperture settings, indicating eight iris blades. The edges are often curved to approximate a circle when the iris is wide open, but form more of a polygon when stopped down. The pupil’s shape is determined both by the number of iris blades and the shape of their edges. 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. Note also the diffraction spikes visible in the righthand image. Bokeh shape The bokeh of a 32mm Cooke S4 wide open at T2 (left) and stopped down to T2.8 (right). In most cases the depth of field is large enough for the shortcut to give perfectly acceptable results, however.Ĥ. 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. But technically you should be measuring to the entrance pupil. 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. This is important for forced perspective work, for panoramas stitched together from multiple shots, and other types of VFX. 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. The no-parallax point of a lens is located at its entrance pupil. Either way, it’s this apparent aperture – known as the entrance pupil – which is used to find the f-number. 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. The f-number of a lens is the ratio of the focal length to the diameter of the aperture, but did you know that it isn’t the actual diameter of the aperture that’s used in this calculation? It’s the apparent diameter as viewed through the front of the lens. A view through the front element would show the entrance pupil. f-stops and the entrance pupil This image shows the exit pupil because it’s seen through the rear element of the lens. But here are five things that aren’t so commonly known about irises.ġ. I’ve written about the iris’s use to control exposure before, and its well-known side effect of controlling depth of field. Just like your biological iris, it controls the amount of light passing through the pupil to form an image. Inside a lens, amongst the various glass elements, is an ingenious mechanism which we call the iris.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |