Digital Photography 101
Chapter 1: Introduction to Film and Digital Cameras
1.7: Lens Technologies
The lens on a camera is what focuses light onto the focal plane
to create an image. It is a cylindrical tube that holds a series
of concave and convex lens elements. Light enters the lens through
a hole or aperture and is bent to a focal point by the series of
lens elements. Depending on the quality it will be made of plastic
or glass.
The focal distance of a lens is the distance between
the film plane and the focal point (optical centre of the lens)
when the lens is focused at infinity. The focal length of the lens
is marked in millimetres on the lens mount. The principle focal
point is the position of best focus for infinity. There are two
principle focal points; if a lens is turned around a second focus
is obtained. 'Reversed' lenses are often used in close-up Macro
photography because using a lens reversed allows a closer focusing
distance.
Confused? It is not my intention to give you a detailed science
lesson on optics used in photography but to introduce you to the
different kinds of lenses and varying quality levels used today.
A wide-angle lens will have a focal length of 12-35 mm
with a good useable average of 18mm. These lenses are great for
panorama landscapes and photos of groups of people. It is important
to remember that a cheaper lens will have more distortion issues
then a more expensive lens. For example there is a distortion in
wide-angle lenses called barrel distortion. This is the bending
of horizontal lines such as the horizon in a wide angle landscape
photo and it will also make objects towards the edge of the frame
appear wider then they are. This can have embarrassing consequences
if you are taking group shots at a wedding. The idea here is to
understand the limitations of your lens and use a longer focal length
of say 35mm on a wide-angle zoom lens. A fisheye lens will have
a very wide focal length of 10mm and will bend lines in a desired
effect.
A normal lens will have a focal length of 50mm because
it will give us a field of view, which is the same as the human
eye. You can say our eyes are calibrated to a focal length of 50mm.
Yes, that is the distance from the front to the back of your eye!
A telephoto lens is a magnifying lens with many elements
depending on the quality. The above lens is a telephoto lens or
long lens. This lens will be 50 to 500mm with a 70-200 mm zoom in
an average kit. They have a very narrow angle of view at larger
focal lengths and a very shallow depth of field. They are perfect
for separating a subject such as a person from the background and
for sports and action photography. If a basic distortion effect
is desired, they will also magnify the background in a composition
making it appear closer to the focused subject then it actually
is.
We have looked at focal length in a lens, now lets' look at the
types and quality of lens systems.
A prime lens is a lens with a fixed single focal length.
A prime lens of 100mm is excellent for portraits. The advantage
of a lens like this is that they have less elements and better quality
images as a result. They can also have larger apertures of say f2.8
or even f1.4 letting in more light and creating a very shallow depth
of field. You may have seen sports photographers using very large
lenses on the sideline? This will usually be a prime lens of 300-500
mm with a huge aperture of f2.8 for a lens of this size. Letting
in lots of light even in the dim light of a sports stadium. The
very shallow depth of field will create amazing tight action photos.
Most press photographers will be using one supported by a monopod,
which is a single leg pole.
A zoom lens is a compromise between quality and function.
It can be expected that the larger the zoom range the less the quality
of images will be. The results will not be as sharp. With the advent
of aspherical lenses we have seen very small lightweight super zooms
come on to the market. 18 to 200 mm zoom is now available which
can make for an excellent travel lens. A professional photographer
will probably be using two kinds of lenses. A wide angle to midrange
zoom such as 18-70mm and a telephoto zoom of 70-200mm. A prime lens
of 300mm may also be in the kit if wildlife and sport are on the
cards and a full frame 100 mm prime lens for portraits and macro.
Some less expensive zooms will not keep a constant aperture through
the whole range. At 70mm it may be f4.5 but at 300mm the hole will
be a lot smaller compared to the focal length and may be f5.6 and
this is typical on low-end lens systems.
Quality of the lens. They have larger apertures and better
build quality. Cheaper lens systems will have a plastic body and
sometimes plastic lens elements. The aperture will be smaller in
relation to focal length and f4.5 is typical. A professional will
want glass that lets in more light so he can use a faster shutter
speed and a lower ISO to eliminate grain. They are also known as
fast glass. A lens of say f2.8 at the desired focal length is typical.
This lens will be made of glass, carbonate and metal fittings.
Expect to pay a lot more for those extra stops of light available
in a pro lens. Most of the image quality is directly related to
the quality of the lens. It is far more desirable to use a cheaper
body and pro-glass rather the other way round. Why would you put
cheap tyres on a Ferrari? Invest in a better lens and upgrade your
camera later making sure the lens mounts will be compatible. It
is very expensive to change your whole set up. Nikon lenses are
not compatible with Canon.
Vibration reduction (VR). A new feature in Pro lenses and
is slowly being introduced to the Prosumer market. It uses a CPU
in the lens to adjust shudder when handholding a lens. This can
eliminate shudder blur at slow shutter speeds. Shudder blur is often
mistaken for out of focus images because the effects are similar.
We will look closely at shutter speeds when we do exposure in detail
later. VR means that a camera can be carefully hand held down to
30th of a second at 60mm focal length than the normal 60th of a
second, which is a huge difference in dim light. Notice how the
usual safe shutter speeds for hand holding a camera are the same
as the focal length? This is an important rule of thumb for all
photographers. Of course with VR, the shutter will open longer and
the subject will still blur if it moves. The best combination for
the professional photographer is to use a larger aperture to let
in more light and VR to eliminate shudder blur. Budget permitting
of course! Most manufacturers have a lens that has both.
Let's look at how we name a lens by looking at a labeled professional
lens.
Nikon AF-S VR Zoom-Nikkor 70-200mm f/2.8G ED-IF
This is typical of what a professional photographer will aspire
to have in his or her kit. It is a telephoto lens of 70-200mm, which
is a very useful range. It has Vibration Reduction to reduce shudder
blur and a large aperture to capture lots of light. It is an excellent
compromise to a prime lens of fixed focal length. A lens like this
will cost you.
AF-S means Auto focus (AF) and with a silent (S) fast motor
inside the lens
VR is for Vibration Reduction
Nikkor is the branding
70-200mm is the zoom focal length
f2.8 is the largest aperture opening in the lens
G means that the lens mount is a G type making this lens
compatible with all Nikon and some other camera systems using the
G type mounts
ED is glass developed and trademarked by Nikon and used
in telephoto lenses to obtain optimal correction and improve sharpness
IF or internal focus is a system used where the internal
elements in a lens are the only parts of a lens that move during
focusing. This prevents the physical length of the lens from changing
and allows for faster focusing and facilitates the use of filters
that require specific elements such as polarizers and graduated
filters. More on filters later.
Now I have introduced you to the types of SLR lenses available,
there is only one thing I need to stress and that is the difference
between lenses for digital and 35mm film cameras. A CCD is smaller
then a 35mm film frame resulting in a smaller angle of view with
the same lens. Put it simply and it's like cropping an image!
 The
conversion factor is different for each manufacturer, but as an
example Nikon have a conversion factor 1.5 in a lens built for film
used on a digital SLR camera.
A lens built for a film camera may have an angle of view of a 28mm
lens yet the same lens on a digital SLR will be 42mm.
A disadvantage if you are shooting at wide angles but an advantage
if you shoot telephoto, as your 300mm lens will now have a focal
length of 450mm getting you very close to the action.
Lens manufacturers are now making lenses only for use on digital
camera's keeping the angle of view consistent to the focal length.
A D type lens can be transferred to a film 35mm camera but you will
get massive vignette at the wide-angle settings. Vignette is like
looking down a tube and can also be caused by a combination of filters
and wide angles.
That's it for today on lens technology. We will look now at basic
colour science and pick the brains of Sir Isaac Newton.
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