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Some F-Stops Are More Equal Than Others
Posted By John Sevigny On January 5, 2010 @ 12:03 am In Art of Photography | 20 Comments
Let’s say that your 50mm lens has seven f-stops. You might think that, depth of field issues aside, your lens should be capable of making equally sharp, equally well-defined images at any of them.
And maybe that’s true — if you never print anything larger than 6×8 inches.
You see, science has its laws. Light behaves in particular ways under given circumstances, and we learn with experience that one aperture is not as good as another. Nor is it a simple question of using aperture to expand or limit depth of field.
The truth is, every lens has but a single optimum aperture that will produce maximum sharpness and definition. Not only that, but virtually every 50mm lens — Leica, Zeiss, Nikon or Canon — has at least two f-stop settings that can be counted on to produce pictures of mediocre optical quality.
All Glass Has Its Flaws
My Contax Zeiss Planar 50mm 1.7, for example, produces low-contrast, shallow depth-of-field photographs wide opened, which is to say, at f1.7. This is an expensive and high quality lens. Shouldn’t it perform well at every aperture?
No. Because light’s behavior is dictated by scientific laws, and moreover, glass — all glass — has its flaws.
A photograph shot at 1.7 relies on nearly all the glass in the lens, and any surface or engineering imperfections are going to be revealed with a wide-opened aperture. For the same reasons, I can probably forget about the idea of shooting at 2.8 if I’m looking for maximum sharpness and resolution.
Additionally, depth of field would be so shallow at f1.7 that it’s unlikely that I’d be able to get an accurately focused shot, particularly considering that this is a manual focus lens and that my eyes get worse every year.
On the other end, f16, the smallest aperture, is useless at 35mm or for DSLRs. Diffraction, a kind of distortion that happens when light passes through small holes, destroys images at f16. You might rule out using f11 for the same reason.
We are now down to three optimal f-stops on this extremely high-quality, 50mm lens — perhaps one of the best constructed ever.
With cheaper lenses, you get even fewer “good” f-stops. My advice? Don’t bother with cheap lenses. Buy the best glass you can afford and when you can, stick with the best apertures it has to offer.
So can you take good pictures at f1.7 or f16? Sure — but you can also take good photographs with a plastic Holga. We’re talking about science here, not art (though, admittedly, the art of photography is far more important than the technical crap).
From Seven to Three
So what am I to do now that I’ve narrowed down my seven f-stop lens to just three usable aperture settings?
Work with it.
Need more speed than you can get at f4? Use a faster film, a tripod to take advantage of slower shutter speeds, or a higher ISO on your digital camera.
Need more depth of field than you can get at f8? Switch to a 35mm lens, or wider if you don’t mind distortion. Photography has always been about compromises, give and take, balance in all things technical.
Don’t know the best aperture of your 35mm camera? Fall back on the old rule my father taught me back in the 1970s: the optimum aperture for sharpness and detail is about two stops away from wide opened. That is, on a lens with a maximum aperture of 2.8, you’ll probably get the best results at 5.6.
On my Zeiss 50mm, with a maximum aperture of 1.7, the “best” aperture has proven to be about f4. That doesn’t offer me a lot of depth of field — but the pictures do look punchy, bright, and tack sharp.
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