I have mentioned effective maximum aperture before, but wanted to muse about it some more. Recall that f/stop is calculated as (focal length)/(aperture), so one can recover the effective aperture (abbreviated E.A. below) of a x mm f/ y lens by x / y. Thus a 50mm f/2 lens has effective aperture 50/2 = 25mm.
The effective aperture more or less determines the depth of field, although focus distance also matters - the closer you are to your subject the less depth of field there will be. This makes comparing the depth of field between lenses of different focal length more complicated, since presumably one would change the camera to subject distance to maintain the same framing. I know there are formulae for blur circles, and some day I will actually do the calculations for depth of field of various lenses assuming they are framed for say, a human head. Edit: It looks like the calculators here and here will do what I want. For the time being, effective aperture will an approximation of the possible depth of field isolation of each lens.
Here are two links which suggest that if you maintain the same framing, then depth of field does not change with focal length: Luminous Landscape
Cambridge in Colour
However, the blur in the background seems different, perhaps due to perspective? Will need to think about this some more. It would be easier if I just had all the fast lenses sitting around to try out...
In any case, here is a list of the effective apertures of various fast lenses:
| Lens | E.A. (mm) |
|---|---|
| 50mm f/2 | 25 |
| 35mm f/1.4 | 25 |
| 50mm f/1.8 | 27.8 |
| 35mm f/1.2 | 29.2 |
| 200mm f/5.6 | 35.7 |
| 50mm f/1.4 | 35.7 |
| 105mm f/2.8 | 37.5 |
| 135mm f/3.5 | 38.6 |
| 50mm f/1.2 | 41.7 |
| 105mm f/2.5 | 42 |
| 85mm f/2 | 42.5 |
| 85mm f/1.8 | 47.2 |
| 135mm f/2.8 | 48.2 |
| 50mm f/1 | 50 |
| 200mm f/4 | 50 |
| 105mm f/2 | 52.5 |
| 300mm f/5.6 | 53.6 |
| 105mm f/1.8 | 58.3 |
| 85mm f/1.4 | 60.7 |
| 180mm f/2.8 | 64.3 |
| 300mm f/4.5 | 66.7 |
| 135mm f/2 | 67.5 |
| 85mm f/1.2 | 70.8 |
| 200mm f/2.8 | 71.4 |
| 400mm f/5.6 | 71.4 |
| 300mm f/4 | 75 |
| 200mm f/2 | 100 |
| 300mm f/2.8 | 107.1 |
| 400mm f/3.5 | 114.3 |
| 500mm f/4 | 125 |
| 800mm f/5.6 | 142 |
| 400mm f/2.8 | 142.8 |
| 600mm f/4 | 150 |
A few lenses should surprise you - 135mm f/2.8 (48.2), 200mm f/4 (50), and slow f/5.6 or f/4.5 300mm (53.6-66.7) lenses have fantastic depth of field isolation possibilities and can be acquired cheaply. I'd rub this in the face of anyone who bought a $11,000 50mm f/0.95 (52.6) Noctilux. That said, there are of course advantages to the fast 50mm lens over the slower, longer lenses. The longer lenses are bigger and heavier, require more space between you and the subject, are much more difficult to hand hold, and cannot shoot in as low light.
But if you want to play with shallow depth of field on a budget, and shoot in decent light, go get a 135mm f/2.8!
It occurs to me there is a fairly natural way to measure a lenses low-light ability, accounting for both the maximum aperture and the focal length of the lens by using the "1/focal length" rule of thumb. Namely one can compare the minimum handholdable exposure value or EV one can achieve with the lens.
| Lens | EV |
|---|---|
| 21mm f/1.4 | 5.4 |
| 24mm f/1.4 | 5.6 |
| 35mm f/1.2 | 5.6 |
| 50mm f/1 | 5.6 |
| 28mm f/1.4 | 5.8 |
| 35mm f/1.4 | 6.1 |
| 50mm f/1.2 | 6.1 |
| 24mm f/2 | 6.6 |
| 50mm f/1.4 | 6.6 |
| 14mm f/2.8 | 6.8 |
| 28mm f/2 | 6.8 |
| 85mm f/1.2 | 6.9 |
| 16mm f/2.8 | 7.0 |
| 35mm f/2 | 7.1 |
| 20mm f/2.8 | 7.3 |
| 50mm f/1.8 | 7.3 |
| 85mm f/1.4 | 7.4 |
| 24mm f/2.8 | 7.6 |
| 50mm f/2 | 7.6 |
| 28mm f/2.8 | 7.8 |
| 35mm f/2.8 | 8.1 |
| 85mm f/1.8 | 8.1 |
| 20mm f/4 | 8.3 |
| 85mm f/2 | 8.4 |
| 105mm f/1.8 | 8.4 |
| 105mm f/2 | 8.7 |
| 135mm f/2 | 9.1 |
| 105mm f/2.5 | 9.4 |
| 105mm f/2.8 | 9.7 |
| 200mm f/2 | 9.6 |
| 135mm f/2.8 | 10.0 |
| 180mm f/2.8 | 10.5 |
| 135mm f/3.5 | 10.7 |
| 200mm f/2.8 | 10.6 |
| 300mm f/2.8 | 11.2 |
| 200mm f/4 | 11.6 |
| 400mm f/2.8 | 11.6 |
| 300mm f/4 | 12.2 |
| 400mm f/3.5 | 12.3 |
| 200mm f/5.6 | 12.6 |
| 300mm f/4.5 | 12.6 |
| 500mm f/4 | 13.0 |
| 300mm f/5.6 | 13.2 |
| 600mm f/4 | 13.2 |
| 400mm f/5.6 | 13.6 |
| 800 f/5.6 | 14.6 |
I think this helps explain why the super fast 50mm lenses are in such demand - they offer the highest possible shutter speed at any given light value to help stop motion, plus a large effective aperture, and no other lens except for 21mm f/1.4 and 35mm f/1.2 give you the ability to handhold in low light.
Comment: Given nominal exposure indoors (about as dark as I'd want to be able to handhold) with ISO 400 film is like 1/30 @ f/2.8 I'd say lenses with EV above about 8 are decent enough for low light handheld shooting.
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