How is SLR camera different from a point and shoot camera?

In an SLR camera, the image you see in the viewfinder is
exactly the image that will be recorded (after the shutter time
delay of course!) on the recording media. Whereas in a point and
shoot camera, the image seen in the view finder is an approximation
of the the actual image that will be recorded in the media (film or
ccd)

Now that we have cleared that, let me give one line explanation of
the important parts listed in the schematic above. More details
about the elements that affect the picture are discussed in detail
in latter sections.

Lens Aperture: This is equivalent to the iris of human
eye. This variable opening controls the amount of light allowed by
the lens.This is not a SLR body component, but is a lens component.
So when u change the lens, the aperture specification changes.

Focusing Aiding Screen: SLR cameras have special screens
like the split prism, ground glass screens that aid accurate manual
focusing

.

Pentaprism: This prism corrects the inverted image from
lens to upright position suitable for viewing and directs it to the
viewfinder.

Mirror: This element reflects the light from the lens to
the pentaprism. This will swing out of the way of light path when
the shutter release button is pressed so as to allow the recording
medium to be exposed.

Shutter: This equivalent to the eyelid of a human eye.
When the shutter release button is pressed to take a photograph,
this opens (or moves out) to allow the recording medium to be
exposed to  light from the subject.

Recording Media: This is a film or CCD chip depending on
whether the camera is film or digital SLR.

Light Path in SLR camera when the shutter is not released

Light from the subject is inverted by the lens and the amount of
light is controlled by the aperture area opening. This light is
reflected to the pentaprism by a mirror. The pentaprism, directs it
to the viewfinder and in the process corrects the inverted image.
The photographer sees the image sees through the lens (known as TTL
viewing) and can now correct the focus with the help of the focusing
screen. Once s/he  is satisfied with the composition, exposure
setting etc, can now press the shutter release button to capture the
image to the recording media.

Light Path in SLR camera after the shutter is released

When the shutter release button is pressed, the mirror swings
upwards and immediately the shutter contracts to allow the light
from the subject to strike the recording media. During this time,
there will be no image visible in the view finder. After the
selected shutter time elapses, the shutter expands to block light to
the recording media.

Understanding focal length is very important as it
influences the following

1. Field of viewField of view is the portion of the subject that
   will be recorded in the frame. As the the focal
   length decreases, the field of view that will be
   recorded increases. This is shown inthe following
   schematic.
2. Depth of fieldDepth of field for a given aperture increases as
   the focal length decreases Refer to the Depth of
   Field section further down for more info on this.
3. Shutter SpeedFocal length has no direct bearing on the shutter
   speed. But, in hand held conditions, to prevent
   camera shake, the shutter speed is recommended to
   be atleast 1/(focal length). This limits the
   usablity of a particular focal length depending
   on the lighting condition.
4. Chromatic Abberation:Now, The focal length can vary for different
   wavelength of light (or colors) referred to as
   chromatic aberrations. This due to the fact that
   focal length depends on the refraction property
   of glass and the refractive index varies With the
   wavelength of light used. Therefore, there is
   small change in the focal length between say,
   blue light and the red light. There is always a
   small amount of chromatic aberration associated
   with lens made of refractive elements (most
   common type). However, a lens made of reflective
   elements (like a Schmidt cassegrain or Newtonian
   reflector lenses) does not suffer from chromatic
   aberration. Chromatic aberrations can be
   corrected using low dispersion elements (Nikon
   designation is ED). Many high-end photographic
   lens have this correction incorporated.

Normal Lens

For a camera using 35 mm film, a 50 mm (well 45-55
mm) lens closely matches the vision of a human eye
and is referred to as the Normal Lens. This type of
lens used to be a part of a standard SLR packages in
earlier days. This has been replaced by consumer
level zoom lens (like 35-80 mm) in recent times. A 50
mm lens is usually very cheap and is one the sharpest
prime lens you can find.

Wide Angle Lens

A wide-angle lens refers to lens whose focal length
is smaller than 50 mm and therefore will have wider
field of vision than that of a 50 mm lens. Wide-angle
lenses are very popular with landscape photography.
As the focal length becomes smaller, there will be
barrel distortions introduced in the image. Vertical
lines will start looking curved and this can be
rectified using special corrective elements.
Wide-angle lenses with corrections incorporated for
barrel distortions are known as Rectilinear
wide-angle lenses. These lenses tend to be quite
expensive.

Telephoto Lens

A lens is referred to as a telephoto lens when its
focal length is greater than that of a normal lens
(50 mm). The field of vision in a telephoto lens is
much smaller than that of the normal lens and the
objects will appear bigger in the telephoto lens than
that of a normal or wide-angle lens for the same
subject to camera distance. A telephoto lens is
basically a telescope attached to the camera body.
Usually, lenses with focal length over 200 mm are
referred to as super telephoto lens. Super telephoto
lenses are extremely popular with nature
photographers.

Though prospect of filling the film frame with the
subject is very tempting, telephoto lenses amplify
any camera movement making the use of tripod almost
mandatory. Also, to have a reasonable shutter speed,
telephoto lenses require wider aperture for more
light gathering. Unfortunately, the increase in size
and the high cost associated with the bigger the
aperture makes buying super telephoto lenses well
beyond amateur photographers reach.

Some companies add some active elements in their lens
to counter small movements induced during handheld
photography, commonly referred as Image Stabilization
(canon) or Vibration Reduction (Nikon), that will
reduce or prevent blur even at low shutter speeds. As
you can guess, these lens are quite expensive.
“Fish eye lens” Fish eye lens are extreme
wide-angle lenses with uncorrected barrel
distortions. Any straight line other than the radial
ones will appear curved outwards (barrel
distortion).

Macro Lens

Macro or Micro lenses are special purpose lenses that
will allow the life-size or greater than life size of
reproduction of subject on the recording media (ccd
or film). They allow the subject the lens distance to
be smaller than the lens to film distance. One thing
to keep in mind is that the working distance varies
with the focal length of the macro lens. A 50 mm
macrolens and a 200 mm macro lens can produce 1:1
image, but the 200 mm lens will have much higher
working distance than the 50 mm macro lens which can
be crucia l when photographing live subjects like
insects. Also the depth of field in the macro range
is extremely limited.

Classification based on
lens element.

Refractive
Lens or
Reflective/Reflex/Mirror/Catadiaptric
Lens

Lenses can be either made up of refractive (lens) elements or
reflective (mirror type) elements

to focus an image. The most common type is of the refractive type
because of their high image quality.

Refractive lenses also tend to have very good sharpness and
contrast. The refractive lenses tend to be

very expensive as the focal length increases due to the high
manufacturing cost. Also, the refractive lenses

become bulkier as the aperture size increases. If cost is not a
concern (!), then for the same focal length

a refractive lens will outperform any reflective lens types.

Reflective lenses on the other hand are very popular in
astronomical telescopes due to their compact

size even for very high focal length. Since the lens is made up of
mirrors to gather and direct light, they

are inherently immune to chromatic aberration. Reflective lenses
also tend to be very cheap!/

The disadvantages of the reflective lens is their poor image
quality, very small maximum aperture

(photographic reflective lenses tend to have fixed aperture of f/8
or f/5.4).They are also referred to

as mirror/reflex/catadiaptric lenses. The most common design of
reflex lens is of Maksutov-Cassegrain or

Schmidt-Cassegrain type. The difference between the
aforementioned lenses is the corrector mirror

design

The progression is like this because of the diameter
is proportional to the square of area of opening! So
for a lens as the F-number increases, the diameter of
the opening decreases and light that is allowed in is
reduced.

The light allowed in at a F-Num of 2.8 is double that
of the light at F-Num of 4.0 and is four times that
of light in F-Num of 5.6 and so forth. So, the
F-number has definite value only in conjunction with
a focal length and cannot be used as an independent
measure to compare lenses of different focal
lengths.

For example, a 50 mm f/1.4 lens has an opening
diameter of 50/1.4 = 35.7mm And a 300 mm f/4 has an
opening of 300/4 = 75 mm!

F-Stops and Stopping Down

At Wide apertures (Smaller F-Num), Lenses usually
have light fall off at the corner portions. To avoid
it photographers usually recommend selecting smaller
F-Stop refers to the next (before or after) F-number.
For ex, a polarizer results in a loss of about two
f-stops” Stopping down refers to selecting a
smaller aperture (bigger F-Num). For ex, To get good
performance you should stop down to about
f/8”

Fast and Slow Lenses

We now know that smaller the F-number, More the light
that is allowed by a lens. This would mean that the
photographer could select a faster shutter speed. For
example, if the shutter speed is 1/30 of a second at
an F Number of 4, then for an F-number of 2.8 (double
light) it is sufficient to expose the film for 1/60th
of a second. Lenses that have smaller F-number (less
than are equal to 2.8) are generally referred as fast
lens. This has meaning only when used during a
comparison with another lens with same focal length
but with smaller aperture. For example, one can say
that a 300 mm f/2.8 lens is faster than a 300 mm
f/5.6 lens.

Center Weighted

In this metering technique, importance is given to
the light available in the center portion of the
frame than that of the edges of the frame. This is
based on the assumption that the object of interest
is closer to the center than the edges of the
viewfinder. Most commonly available are the 60/40 or
80/20 center weighted metering.

Matrix Metering

This kind of metering is used for complex lighting
conditions and the lighting calculation is done by
sophisticated microcomputer in the camera body.

Spot Metering

This metering takes into account of the light in a
very small portion of a frame (like 2 degrees).
Imagine a nighttime shot of full moon, any of the
above metering will result in a very slow shutter
speed (meaning long exposure) since both center and
matrix metering will result in very small overall
light values requiring long exposures to properly
(from it s point) expose the image. But, this will
result in an overexposed (burnt out) and a streaked
picture of the moon! For this situation spot metering
is ideal where the light available in a very small
portion is taken into account. When used properly,
spot meter is an extremely valuable tool.

Light meters are instruments used to measure light.
Light meters can be either incident light meters or
reflected light meters. Incident light meter
calculates the light that is falling on a subject. So
the photographer has to check the amount of light at
various points of the subject to deciding on the
aperture and shutter speed. Reflected light meter on
the other hand measures light coming into the camera
lens. This would mean one single reading that will be
used to select the aperture, shutter speed
combination.

Exposure is the also the process of judging the
combination of aperture, shutter speed and film speed
to get the desired effect. This is the control that
the photographer can use creatively to produce
strikingly different results of a same subject.

Think about a condition of photographing a waterfall
using ISO 100 Film using in-camera meter in center
weighted mode

Aperture F/2.8, Shutter speed of 1/1000 sec for
correct exposure is equivalent to using F/32 at a
speed of 1/8 sec, but the effect will be completely
different due to slow shutter speed.

Barrel distortion is when horizontal or vertical
straight lines bulge outwards like a barrel. This
type of distortion is usually found in wide-angle
focal lengths.

Pincushion

Pincushion is a type of distortion where horizontal
or vertical straight lines bend inwards. This type of
distortion is usually found in telephoto focal
lengths.