Posts Tagged ‘focal length’

Shallow Field

16 August 2008

Dear J-

Saturdays seem to turn into photo days, for whatever reason, so let’s talk about f-stops, you and I.  F-stops follow the progression they do because they express the ratio of diameters; at f/1.0, the effective diameter of the lens matches the focal length.  Thus a 50mm f/1.0 (and they exist, at you-gotta-be-kidding-me prices) has a 50mm effective diameter.  At f/1.4, your diameter is 1/1.4-th of the focal length.  It makes more sense when you consider it this way:  since the area of the opening determines the gross amount of light transmitted through the lens (ideal lenses, here, and with modern multi-coatings, transmission ratios are pretty high); thus since area varies with the square of the diameter, it’s actually the square of the f-stop you’re interested in.

Hence, a f/1.0 lens transmits twice as much light as a f/1.4 lens, and thus you can derive the f-stop full stop scale (f/1.0, f/1.4, f/2.0, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/64, f/128, etc.) by taking the square root of the 2^n sequence (1, 2, 4, 8, 16, 32, etc.).  Consider it for a moment and it starts to make sense.  If you close down the aperture one full stop (f/1.4 to f/2.0, say), you have to double the shutter speed (1/30th at f/1.4 to 1/15th at f/2.0) to make an equivalent exposure — you show the film (or sensor) half the light, for twice as long.  All else being equal, why wouldn’t you keep the aperture as wide as possible?  Well, for one thing, lenses aren’t necessarily designed to be run wide-open, and often hit their optimum performance one or two stops down.  For another, aperture controls depth of field; low f-stops (big apertures) give shallow depth of field, which is useful for the Martha Stewart style of product photography, or if you’re trying to draw attention to one thing over another.

Isn’t it ironic, then, that in real life, opening yourself up to new experiences actually widens your depth of knowledge?  I’ve been reading about some guy out there who has some kind of beef with Democrats; he coauthored a book for the Swift Boat Veterans, blamed by some for torpedoing Kerry’s campaign in 2004 (someone please explain to me again how two draft dodgers came out looking more virtuous than a veteran, especially representing a party that prides itself on supporting the military), and he’s at it again with Obama fear-mongering.  Sadly, it’s not about the accuracy of the message, simple as it is in photography — if you’re using shallow depth of field techniques, make sure you focus on the right thing — but how repeatable the lie.

I remember from debate there were two sources you wanted to use for definitions, each useful to the affirmative (making a case for change) or negative (keeping the status quo) side.  And, depending on the side you had to argue, the standard reasons went like this:  affirmatives wanted to use a “common” dictionary like Merriam-Webster because it was the most reasonable to the greatest amount of people; negatives chose Black’s Law Dictionary because it was the most precisely focussed definition.  Neither was the best answer all the time, and you needed to carry around both, because you didn’t know what side you’d be asked to argue.  So it goes in life, so too in photography; develop your skills and make sure you use the right tools when needed.

Mike

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