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.
http://mickey-cheatham.blogspot.com/2012/09/the-science-of-photography-part-five.html
Saturday, September 15, 2012
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