This is about why some photos are blurry. It’s aimed mostly at beginner and intermediate photographers with DSLRs. Unlike most photography tutorials, it assumes that you does not assume that you’re a three-year-old who’s scared of numbers. (Technical folk will notice some omissions and some abuses of terminology, but should find that they’re simplifications to save time, not insulting condescensions.) It’s sorted from least to most integral to the camera system, and will probably be most useful near the middle.
Let’s get some rough units in mind. A typical lens, suitable for small group portraits and so forth, has a horizontal view of about 45°, and its camera might resolve 4000 pixels horizontally, which ends up as about 0.7 pixels per square minute of arc. (Arc minutes are marked with a prime (′)).
This seems obvious, but intuition is often fooled by lighting. Think, for example, of a satellite as bright as a bright star. If you expose the bright stars correctly, for say 30 seconds, the satellite will probably be invisible. Similarly, a person standing in front of a wall for half of a long exposure might be nearly transparent if the wall is well lit and they’re in shadow, and vice versa if vice versa. It always helps to look at these things from a single pixel’s perspective: what does it see, how bright is it when it sees it, and how long does it see it for?
Your fastest shutter speed is probably about 1/4000 of a second. In this time, sound travels only about 8 cm (3 inches), and light about 75 km (45 miles). A pro baseball fastball moves roughly a single centimeter, or half an inch. So if you see blur at this shutter speed, it’s very likely not motion blur and you should start thinking about your lens.
Fog maps a scene point to a gaussian blob (see halos below), but with such a wide radius that the effect is better described as lowering the scene’s contrast than as blurring specific features. So what you can see through fog is usually fairly sharp in the sense that if you can make out an edge at all, it isn’t blurred on the arc minute scale – it’s indistinct because of low contrast, for which see sensor multipliers. Ordinary atmospheric haze and frosted surfaces like dusty windows count as fog. Fogged film
effects are usually flare (see below).
The rule of thumb for refraction is that light bends toward denser mediums. When going from air to glass, say, it heads into the glass at a deeper angle than it arrived at; when hitting air again, it deflects back. (This is only mostly true.) If you sketch a convex lens and some rays with this rule in mind, can easily see how focus works.
Because glass surfaces are imperfect, and come in pairs to boot, shooting through them is always dangerous to sharpness. A circular polarizer, for example, is only two pieces of high-quality glass, but there’s 4 opportunities for a given photon to go awry before it even hits the lens. A stack of two or three filters of less than exquisite quality will start to make the image noticeably soft. A single ordinary shop or car window is worse. Your best chance, probably, is to get close to the glass and hope that distortions will be small, isolated, and thrown out of focus. And, of course, shoot as straight through it as you can.
Shimmer is heat waves. Variations in air temperature are variations in density and so every tiny heat gradient is a weak lens. Astronomers call this scintillation, or, to mundanes, star twinkle. It’s stronger along the horizon for two main reasons: because divisions between air masses run mostly vertical, and because there’s more air between you and a lighthouse 20 miles away than between you and a star.
Incidentally, Earth’s atmosphere acts as a weak fisheye lens floating in space. Atmospheric refraction makes our field of view of the sky just over 181° – we get the celestial bodies for an extra 34′ after they’d be under the horizon if seen from an airless world. Chromatic aberration in this lens causes the green flash.
The big one. Most advice I’ve seen about this, other than practice
, is silly or superstitious. Basically, relax, wait until you’re breathing out, press the button slowly, and follow through by holding very still until a moment after the shutter is closed again.
Sharpshooting advice can be useful here, except that apparently sharpshooters squeeze the trigger so slowly that they are surprised by the shot; photographers can’t do this because we can’t twitch when the shutter’s open.
Keep in mind that simply moving the camera by n centimeters will put that much blur on a subject at its scale. If you’re taking a photo of a mountain, moving the camera ten feet to one side while the shutter’s open may well be invisible. What really matters is changes in the camera’s angle.
This is often belittled by serious , but
Diffraction is light waves bending around an object. It’s hard to think about if, like me, you usually think of light in particle mode. But if you consider the behavior of more obvious waves like sound or pond ripples, you can easily get a good model of it.
With a 50 mm lens at f/16, the aperture is only about 1 mm wide. This means
In most photography, the only place you have to think about diffraction is in the lens itself: the light At tiny apertures (f/16 or so), a big proportion of the light waves coming through the lens close enough to the edge