Choosing the Exposure Time, Sensitivity, and f-stop
Second in a series. The first is A Little Science Helps You Take Better Photographs
Exposure Time can be adjusted over a huge range. A modern DSLR camera can shoot at 1/8000 of a second, all the way up to 30 seconds (in my camera). Or longer with external help. The first thing to know about this is that without a tripod, you need to use around 1/100 of a second or faster to avoid blurring caused by the motion of our hands and bodies. There are several variables that affect this maximum exposure time, but it is almost never longer than about 1/10 of a second before you need a tripod or some equivalent support. (more about this later)
At the highest shutter speeds motion blur is not a problem, but the opposite can be. An airplane propeller taken at 1/8000 will appear stopped. This looks unnatural if the plane is in flight. It looks strangely static. A car speeding by will not show a look of speed if it and the background are both sharp. To be realistic a picture should show fast things blurred, as our eyes do. Here the propeller of the red plane looks stopped, while the other is nicely blurred.
Panning is an important photographic technique. You keep the lens aimed at the moving subject. Your eyes do this naturally. When you do it right with the camera the subject is sharp and the background is motion blurred. You need a little skill and practice to master this, but the result is worth it. The left Blue Angel is sharper than the right because I was following him / her with my lens.
Sensitivity of Sensor (or Film) (ISO):
This can be adjusted over a range of typically 100 to 3200 or more. Pictures in daylight are usually made at 100 to 400, the low end, because there is plenty of light. ISO numbers above 1000 produce “noise” or “grain”. Interestingly, high ISO film and high ISO digital images have a similar grainy look. These two completely different technologies are both affected the same by the laws of physics. In this magnified detail below, you can see the difference between ISO 200 and ISO 6400.(more about this later)
Relative Area of Lens (f-stop or f-ratio):
This can be adjusted over a range of typically f 2.8 to f 22. Most Zoom lenses are wide open at approx. f 4. Non-zoom (aka prime, or fixed focal length) lenses may open up to f 2.8, f 2, or f 1.4. (more about this later)
What’s the difference in the picture when you use a different f-stop setting? Wide open (low f-number) causes your depth of focus to be very small, meaning that only things at a particular distance are in focus. Everything closer or farther is blurred. When the iris is closed down (high f-number), much more is in focus. This is especially important for closeups. Here much more of the flower (and some background) is in focus at f 16 than f 2.
Another important thing to know about f stops is that both very large and very small f ratios reduce sharpness, for different reasons. In this picture of tree branches depth of field is not a problem, so the best image is at f 5.6. Closing the iris down to f32 causes serious blurring due to diffraction, in this magnified view. Interestingly, getting a better lens does not help, diffraction is a basic physical phenomenon. A better lens can help the blurring at wide apertures.
Please don’t be confused by the many lenses that have a (maximum opening) (minimum f-number) like 1.8, 3.5, or 4.5. These are just in-between the standard f numbers.
Lenses that have a large aperture (low f number) like f 1.4, are called “fast” lenses. That makes perfect sense if you remember that it means you can use a fast shutter speed with them.
Our eyes are very good at adjusting to a huge range of brightness. We can see in broad daylight, or in moonlight, almost 1 million times dimmer. Cameras must work hard to cover this range.
Outdoors, we usually don’t have control over brightness, but it’s always plenty when the sun shines. This means there is plenty of room to juggle the exposure variables. Indoors the light varies a lot, from bright easy conditions, to very difficult dim lighting.
Controlling the lighting indoors is the main reason for photo studios.
That is all for now. The next post will go into more detail and will be the last in this series on Science in Photography.