Stop! In the name of light...

The original motto for my weblog was to describe my findings and experiences exploring (digital) photography, hoping that other beginners would be able to benefit from it and maybe, just maybe, would be spared of one or the other unpleasant experience. :-)

Some of my recent posts may have been very camera specific (hey, I bought a D700, and I love it, it rocks!) and/or technical (the UniWB stuff). The topic I want to cover in this post is something rather "basic" - but I remember how it kept me puzzled for quite a while, and when I finally understood what it's all about (I was like "duhhhhhh!"), it helped me so much to handle my camera better, faster, and with more confidence that this post is of a personal concern. I try to give an explanation in really really simple words - I hope it works. :-)

I can't remember how many times I've read and heard about "fast" lenses, about "stopping down", about "one stop" more (or less) light, "one more stop" of usable ISO, and so on, and so on (and in German, instead of "stop" people usually say "Blende" - aperture - and it puzzled me just the same). I wondered: what the hell IS a "stop" actually?

The simple answer: a change (increase or decrease) of one stop (no matter how, I'll explain it right below) means that we allow either double or half the amount of light to work for us. Double the amount if the exposure time gets longer, and half the amount if the exposure time gets shorter.

I myself understood the idea best when I looked at the "full" aperture progression, because the aperture is clearly (more or less) "in the way" of the light as it passes through the lens onto the sensor or film. It's not so much in the way if it is wide open and lets a lot of light pass through, and it's very much in the way if we "stop down" a lot so that the opening through which the light can pass is really really small (hint: in the Nikon world, all lenses have a small lever on the end that is mounted to the camera - it can be moved with the finger and when you do that, it's very clear then what the aperture does.)

Now, if we start at f/2.8 (which is common for expensive and "fast" zoom lenses), the series of apertures that follow are 4, 5.6, 8, 11, 16, 22. Those are full stops. With the aperture blades closing farther and farther from one full stop to the next, only half the amount of light can pass through to the cameras sensor (or film) for each of these so called f-stops.

So if you "stop down" the lens from f/2.8 to f/4 (yes, this is one stop) you let only half the amount of light pass through the lens, and to get a proper exposure, you have to double the exposure time - let's say from 1/100s to 1/50s. If you stop down twice (which, starting from f/2.8, would mean you end up with an aperture setting of f/5.6) you need an exposure time that is four times as long - let's say 1/25s instead of 1/100s.

It's a bit of an abstraction when these "stops" are used for exposure times and sensor sensitivity, but the concept is just the same. If you're looking at a relatively dark scene, you might want to tell your camera to underexpose "one stop"*. So you'd set 1/100s instead of 1/50s (without adjusting the aperture), letting only half the amount of light get to the sensor (or film) because the shutter is open only half as long.

And if it's used for the sensitivity of the sensor (or film): ISO400 is one stop faster than ISO200. And ISO800 is two stops faster than ISO200. Using ISO400 instead of ISO200 means that the sensor will capture twice the amount of light (if the exposure and aperture stay unchanged).

How did this help me when I operate my camera? Well, most cameras allow adjustments to the aperture, exposure and ISO in steps of 1/3 of a stop (most Nikons can be configured to use 1/2 stop instead, but it's not the factory default).

So, if I make a photo and notice that it is underexposed (when I check the histogram on the camera's display), I simply use the main command dial of my camera to dial in one stop of overexposure, and it's 3 clicks of the dial, each 1/3 stop. Or, if I cannot use a longer exposure time, I might need to raise the ISO sensitivity by one stop: I press the "ISO" qualifier button and again it's 3 clicks of the dial, each 1/3 of a stop. Or, if I can sacrifice the depth of field, I can open up the aperture, and again it's 3 clicks for one full stop. I don't have look at the actual number, I simply go click-click-click (in either direction, and for any of the three exposure, aperture and ISO sensitivity) and I know I now have either halfed or doubled the amount of light reaching the sensor or film.

Click-click-click. That's one stop. It's really that simple. I wish someone would have told me. That's why I wrote this article. If it helps just one fellow photography fan out there, I will be very happy.

*) why? because the camera doesn't know you're looking at a relatively dark scene - it always assumes you're looking at an averagely lit scene