I am a little embarrassed to admit that after years of manual photography, going back to even the darkroom era, that I did not fully understand aperture. My assumption was that the aperture number (f-whatever) was an absolute value that was the same across lenses. This assumption was incorrect. Now that I think about it, this is pretty obvious. But, I just figured that it was a standard measure of the amount of light let into the lens by the blades of iris as expressed as a measurement of said iris’ diameter. Aperture is really a ratio showing the relationship between the primary focal length of the lens and the diameter of the iris. You take the focal length of your lens, divide it by the aperture setting and there you have the actual diameter of the iris.
Thus, the famed entry level Canon prime lens, the nifty-fifty or Canon EF 50mm f/1.8 II, when set to f/1.8 has an interior iris diameter of 27.78mm. Why is this important? Well, it explains why it is hard to get soft focus in your background with a wide-angle lens and also explains why longer lenses (the Canon EF 100mm f/2.8 Macro USM, for instance) make great portrait lenses and produce such a pleasing blur, often referred to by the Japanese term Bokeh. Bokeh means blurry in Japanese but sounds fancier and makes the uninitiated feel like you have some special knowledge. That’s why some photographers love to trot it out; specialized language makes you sound like you know what your talking about, just ask any lawyer, doctor, politician or clergyman.
Anyway, I put together a chart to determine what the absolute size of the aperture of each of my prime lenses was in order to figure out which had the largest opening. The larger the aperture, the thiner the depth of field (or the amount of the photo that is in focus) and when you figure out which of your lenses, regardless of f-stop setting, has the largest absolute aperture, then you can play around with that much sought after bokeh… cough… blur. To this end, I picked out my Canon EF 85mm f/1.8 USM because, when shooting wide-open or at f/1.8, it has the biggest hole through which light can pass. I have to say that I understood this to be the case in practice, that the longer lenses produced softer focus, I just didn’t know why. And as Colonel John “Hannibal” Smith taught us all, “knowing is half the battle.” Now, this actual photo is no great shakes but the blur sure is pretty. I could wax about the difference between digital and film blur (the later is some really smooth and attractive stuff) but as long as you are shooting digital and if you are looking to emphasize your subject with a shallow depth of field or just abstract your background, then these figures below might prove to be helpful.
One last point: I don’t feel too bad about my ignorance, because when I called two professional photographer friends of mine they both admitted to not knowing what f-stop really meant, in the mechanical sense, either. Ignorance loves company! Okay, that’s enough of that. I have some Thanksgiving dinner to eat.
| Aperture Diameter (mm) | |
| Zenitar 16mm f/2.8 | 5.71 |
| Canon 28mm f/1.8 | 15.56 |
| Leica 35mm f/2 | 17.50 |
| Canon 35mm f/1.4 | 25.00 |
| Voitglander 40mm f/2 | 20.00 |
| Leica 50mm f/2 | 25.00 |
| Canon 50mm f/1.8 | 27.78 |
| Canon 50mm f/1.4 | 35.71 |
| Canon 85mm f/1.8 | 47.22 |
| Canon 100mm f/2.8 | 35.71 |
| Canon 135mm f/2 L | 67.50 |
| Opteka 500mm f/6.3 | 79.37 |