Saturday, September 15, 2012

The Science of Photography -- Part Four

As discussed in earlier installments of the “Science of Photography,” you set exposure using a combination of shutter speed and f/stop to obtain the correct amount of light on the camera’s film or sensor. The shutter speed controls how long the film or sensor is exposed to the light coming in through the lens. The f/stop controls how much light comes in through the lens by varying the area of the aperture. For a given film or sensitivity indicated by the ISO number and the amount of light on the subject being photographed, there is a single correct amount of light for proper exposure. This amount of light can be obtained with different combinations of shutter speeds and f/stops.

Although the f/stop is a key control in photography, it is often a mysterious value, poorly understood by many photographers. Perhaps, in this age of fully automatic cameras, this is not a problem. But, as the saying goes, it is a poor workman who blames their tools. In this installment we continue to educate users about the mystery and wonder of the f/stop.

As I described in the last installment, the beauty of the f/stop is it is a consistent measure of the amount of light, independent to the specifics of lens such as focal length. So that means that f/8 is the same amount of illumination on the film or sensor independent of the lens’ focal length.

As I also mentioned previously, that was how handheld light meters were able to indicate settings  independent of the lens used on the camera. No mater what size of film or sensor (we will talk about how sensor or film size effects lens parameters later) or what lens is being used, a 1/125 at f/8 is the correct settings for proper exposure. That will work on a wide-angle lens, a standard lens, or even a 300mm telephoto called a “long lens.”

Lenses are described by their maximum aperture. (Remember, that is the largest aperture, indicated by the lowest f/stop value.) If you look in a catalog, you might find a 50mm lens with a measure of f/1.4 or f/2.0 or even f/3.5. You will immediately note that the smaller the f/stop number, typically the more expensive the lens. That is because the lower the f/stop value, basically the bigger the lens glass -- that is the diameter of the overall lens. The bigger or wider lens lets in more light, and that is reflected (no pun intended) by the lower f/stop number.

These larger and more expensive lenses are often called “fast lenses.” That implies that, under a given set of light conditions, the camera can use a faster shutter speed by setting the aperture to the widest value. That can be an advantage when photographing subjects in motion under low light conditions.

Given that fact, why doesn’t everyone use the fastest lens they can get? Well, first of all, as I said, the faster the lens the more it will typically cost. In addition, the larger diameter aperture means larger glass which adds weight. With larger and heavier lenses, you have to use heavier materials in the barrel supporting the lens. All in all, this increases weight.

Also, the larger the lens, the more perfectly it has to be manufactured to prevent visual imperfections. In fact, many experienced photographers will always stop down the lens at least one f/stop because the outermost edges of the lens are likely to have some distortion. By stopping down, you use more of the center of the lens.

I’ve actually found cheaper lenses that had less distortion than a faster (and more expensive) lens. In other words, if you don’t need the very low f/stop for your photography, you may be carrying around a lens heavier than you need, that cost more than a smaller lens, and -- saddest of all -- the smaller lens may actually perform better at f/8 or f/16.

Issues of size and weight are even more extreme with the so-called long lenses. For example, a particular Nikon 300mm lens, even though it is only f/4.5, weighs over two pounds. Compare that to another Nikon 300mm lens that is f/2.8. It weighs over five pounds. And Nikon’s 300mm, f/2 lens? It is over fifteen pounds. How would you like to lug that around? By the way, that f/2 lens costs over $20,000!

Most modern camera buffs use zoom lenses. Those are lenses that have adjustable focal length. These can be very useful lenses, adjusting to various focal lengths to match the shooting requirements. Modern zoom lenses are typically fairly light, but a check on the available f/stops shows they are not fast lenses.

In addition, you will notice that the f/stop values vary as you operate over the range of zoom. For example, a popular Nikon zoom lens varies the focal length from 28mm to 300mm, covering everything from wide angle to telephoto. However, the aperture on maximum changes from f/3.5 - 5.6 as you zoom out. Recall the f/stop is a ratio of the aperture diameter to the focal length, so this is to be expected.

The more expensive professional zoom lenses will have a constant f/stop across their zoom range, but those designs require additional weight and cost. Everything in engineering is a trade-off.

The weight difference is even noticeable on shorter lenses. Compare the Nikon 50 mm, f/1.4 lens with the cheaper f/1.8, and there is a noticeable difference in weight which can start to wear on you if you are shooting for an extended period of time. By the way, it is possible to find lenses with the aperture so wide they are f/1.0, but those are very specially designed lenses to deal with the issues of such a large lens and you will pay accordingly. The lowest f/stop I’ve ever seen was a Canon 50mm lens with an f/0.95. But, again, you will really pay in both dollars and weight for such a super fast lens.

In the days of film photography, photographers sometimes had to pay that cost to get the very best low light performance. Most 35mm film had ISO values of 100, 200, or -- at the most -- 400. There was super fast film with an ISO of 1000, but that was pretty much the limit unless you used very exotic film. So, in the days of film, very fast lenses were useful for low light conditions, especially with subjects that were moving so the shutter speed had to be 1/60 or higher to prevent blurring.

Compare the ISO values in film to the ISO values available in today’s digital cameras. Rather than souping up the film chemicals to be especially sensitive to light, digital cameras just amplify the sensor output. By increasing the gain of the amplifier, you effectively increase the ISO.  Modern, high end digital cameras can have extremely high ISO values.

My newest Nikon digital camera can have the ISO adjusted up to 6400, and special ISO modes go to 12,800 and 25,600. This camera has a 1/8000 second maximum shutter speed to match up with these superfast ISO settings. With the sensor so sensitive to light, you can still use fast shutter speeds, even with a high f/stop number.

There are problems with fast film or sensors. With film, the faster chemical formulas which responded to lower levels of light had a tendency to be “grainy.” This grain effect was caused by the individual elements of light sensitive chemicals making up the film. The faster the film, the greater the graininess. As I said, in engineering, there is always a trade-off.

In an interesting example of “duality,” a phenomenon where two similar physical methods demonstrate matching characteristics, digital sensors will also display graininess when the amplifier gain is turn up to increase ISO. In the case of digital sensors, the grain comes from amplifier noise at high gain settings. This is similar to the hiss you hear in an audio amplifier if you turn the volume control (gain) way up.

So, there is a penalty for using high ISO settings with a digital camera, but you can often turn up the gain and thereby the ISO to well over 1,000 without a lot of noise being added.

So, modern cameras with their very high sensitivity sensors can turn a slow lens into a fast lens. In fact, given this power to shoot in low light conditions, even with a slow lens, one must ask, “Why even bother with a fast lens?” They cost more. They weigh more. They are more likely to have optical problems than a slower lens. So what is an f/1.4 lens really good for? Why would you want one?

Those are good questions. And they are questions I’ll answer in the next installments of “The Science of Photography” as we start a discussion of “depth of field.” There are still other details to describe too, including “less than full f/stop - stops” and more on shutter speed combined with f/stop. Still a lot of science to cover in this art of photography. So, until the next episode, keep your powder dry and your camera dry too.

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