A basic view of fundamentals of lens in photography. Discusses various aspects of lens, types of lens and which lens suitable for various photography moments. Hope you find it useful

1. Fundamentals of Lenses
Anindya Das
20th October 2012

2. Introduction
 Lens is one of the most important parts of the camera
 Lens is the „eye‟ of the camera and it helps to form an
image on the sensor
 During lens design a designer have to keep the
following in mind:
1. Weight
2. Size
3. Cost
4. Amount of distortion

4. Important Lens Attributes
 Focal Length
 Speed
 Focusing Mechanism
 Lens Mount
 Image Stabilization

5. Focal Length
 Determines the size of the image that falls on the sensor
 Longer focal length – larger size, Shorter focal length – smaller
size
 Measured in mm – distance between the lens and the sensor
(where image is sharply formed) when the subject is at infinity
 Photographic lens is a combination of atleast 4 elements while
complex ones can go to more than 20.
 Multiple elements needed to correct several optical defects found
in a single lens

6. Focal Length – Images at different focal lengths

7. Focal Length – Images at different focal lengths

8. Focal Length – Angle of View (AOV)
46
deg
32
deg
 A lens is qualified – telephoto or wide angle by it‟s Angle of
View
 AOV depends on the focal length and the size of the
sensor
 Cropped sensor gives the magnification of focal length in
35mm times the “cropping factor”

9. Sensor conversion factor of various sizes
Sensor Size App. Conversion factor Used By
with reference to 35mm
4/3 System 2.00 Olympus, Kodak,Panasonic, Lecia
APS-C 1.60 All Cannon DSLRs except EOS 1D
and above
DX 1.50 All Nikon DSLRs except D3 and
above
APS-H 1.30 Cannon EOS 1D series, Leica M8
Full Frame/35mm 1.00 Nikon D3 and Cannon EOS 1Ds
film series
Film 6 x 4.5 cm 0.58 Pentax, Mamiya, Bronica,
Hasselblad
Film 6 x 6 cm 0.51 Pentax, Mamiya, Bronica,
Most of the super zoom equipped bridge digital cameras have
Hasselblad
very small sensors and thus need only a short focal length to
get a great telephoto effect

10. Lens Speed
 It doesn‟t have anything to do with the speed of the
subject to be photographed 
 Speed is an indication of the light gathering capacity
of the lens
 Referred to as the “f – number” and is denoted as “f/”
 f-number denoted by f/D
e.g. if the focal length is 16 mm and the entrance pupil
diameter is 1 mm, the f-number is 16 and the aperture
expressed as f/16
 Lens has a “diaphragm” to control amount of light
passing through the lens. It‟s set of blades make an
opening called “aperture”
 Aperture size can be controlled automatic or manually

11. Lens Speed – “Stops”
Stops are in app.
multiples of 1.4 or √2.
It is bcoz each stop
doubles or halves the
area of the aperture
opening
All DSLRs give
“automatic diapragms”
which allow clear view
from the viewfinder
while composing
regardless of the
shutter speed chosen

12. Fast Lens – A heavy price to pay
Lens Speed Weight (gms) Looks Like
f/5.6 505
f/4.0 1,440
f/2.8 2,870
f/2.0 7,545

13. Focusing Mechanism
 Since all the objects in a composition are not
equidistant a lens needs to be focussed
 “Heclical” focusing mechanism is used. Lens needs to
be moved away from focal plane for near objects and
closer for far objects
 Manual focusing done with a focusing ring and
checking sharpness in the view finder
Two different autofocus methods:
 There is a motor in the camera which drives the lens
through a mechanical coupling
 The body signals the motor inside the lens which does
the needful. When the body detects the correct focus
the driving stops

14. Lens Mount
 System of mechanical and electrical linkages that
ensures proper interface between camera and lens
body
 Propriety to each brand and cannot be changed after
purchase
 Several independent lens manufacturers who produce
lenses in different camera mounts e.g. Sigma and
Tamron
Four – Thirds System
 Allows for interchange of lenses and bodies from
different manufacturers. Standard created by Olympus
and Kodak
 Sensor sizes are as small as crop factor of 2 (i.e. a

15. Image Stabilization
 Handshake adversely affects the sharpness of a picture.
IS helps in improving the same.
 Also known as Vibration Reduction(VR) and Optical
Stabilization(OR)
 Shake is detected by a gyroscope inside the lens and
group of lens are moved to counteract this
 With this you can go 2-3 stops slower shutter speeds
(e.g. if you needed 1/500s you can g upto 1/125s or
1/60s and even get a sharp picture
 All lenses have a switch to activate/deactivate IS/VR

16. Types of Lenses
Type Diagonal Focal Length Focal Length Type of
Angle of for Full for Cropped Photography
View Frame Sensors
(AOV) DSLRs DSLRs
Ultra Wide 84 deg or 24mm or less 16mm or less Landscapes,
Angle More Architecture,
Interiors
Moderate 62– 84 deg 24 -35 mm 16 -24 mm General purpose,
Wide Angle Landscape
Normal Around 45- 40 – 50 mm 25 - 35 mm General Purpose,
50 deg Candid, Low Light
Short to 12 – 28 deg 85 – 200 mm 60 – 150 mm Portraits, Candid,
Medium Streets
Telephoto
Super 8 deg or 300mm or 200mm or Sports, Racing,
Telephoto less greater greater Wildlife, Birds

17. Normal Lens
 Focal Length equal to the diagonal of the sensor frame
 Generally they are the fastest lens around and are best
suited for low light photography
 Provide a view that is closest to the human eye
 With this you can go 2-3 stops slower shutter speeds
(e.g. if you needed 1/500s you can g upto 1/125s or
1/60s and even get a sharp picture
 All lenses have a switch to activate/deactivate IS/VR

18. Telephoto Lens
 Lenses with long focal length
 High Magnification and narrow angle of view
 Most application in wildlife, birds and sports photography
 Have an effect of compressing – distance between near
and far looks less than reality
 Due to high magnification tendency for camera
movement induced shake to show up very easily
 Use shutter speed of at least 1/(35mm focal length
equivalent). E.g. If you hold a 200mm lens on a
cropped sensor (APS sized) then minimum speed should
be 1/300s

19. Wide Angle Lens
 Shorter focal length and give wide angle of view
 They can decompress or expand the relative distance
between objects
 Can cause some problems from perspective point of view
 Best suited for landscapes, interiors etc
 Should not be used for portraits where it will make you go
close and you end up with some exaggerated features
e.g. nose
 Lens will be very close to the camera body
 At very short focal lengths the lens is so close to the body
that back of lens interferes with mirror movement
 “Retro-focus” used in wide angle lens which provides
sufficient physical length but keeping the focal length

20. Zoom Lens and Prime Lens
Zoom Lens
 Lens having a range of focal length for the user to work on
 Most popular among the DSLR users
 Wider the range harder it is to keep image quality high across
all focal lengths.
 Sweet spot in zoom lens – sharpest and clearest
Prime Lens
 Lens whose focal length cannot be changed
 Much faster than the zoom lens (e.g. 50mm f1.8). Helpful in
low light photography
 Optics in prime lens are of way higher quality
 Much better in terms of sharpness, color reproduction,
distortion etc.

21. Difference in composition at different focal
lengths

22. Zoom Ratio
 Number you get by dividing the longer focal length of a
zoom lens with the shorter one is called “zoom ratio”
E.g. for a 18-72mm zoom lens the zoom ratio is 4
 Very high ratio exist (in excess of 20). Generally such
high ratio lenses compromise on image quality
 High zoom ratios are easy to make in small digital
cameras but not possible for full frame sensors
e.g. 5-90mm on bridge cameras would correspond to 28-
500mm on full frame cameras. This will be a huge lens
and technically challenging

23. Focusing and Aperture in Zoom Lens
 Parfocal Lens: Zoom lens which hold on to the focus
when the focal length is changed. Difficult to design and
costly to produce
 Varifocal Lens: Focus changes when you zoom. Thus
need to focus (automatic / manual) after you zoom. Most
commonly used technique
 Focusing is done with turning the ring on the lens where
the focal lengths are marked.
 For P&S two way rocker switch is used to adjust focal
length
 Lens speed changes when you zoom.
 18-70mm f/3.5 –f/5.6 is a variable aperture zoom lens.

24. Macro Lens
 Allows to take pictures with high magnification
 Special helical focusing mechanism which extends lens
away from focal plane and focus very close to the subject
 Macro lens gives magnifications of >= 1:1
 Normal zoom lens produces an image as big as 1:4 on
the sensor
 Available for range of focal lengths – 50mm to 200mm
 Higher focal length macro lenses gives a greater working
distance
1. 50 mm lens
2. 300 mm mirror lens
3. 300 mm lens

25. Fish Eye Lenses
 Extremely wide angle lens with very short focal lengths
 Circular Fish-eye: Produces 180 deg coverage all round
and creates a circular image within the frame
 Full Frame: Diagonal AOV of 180deg and produces an
image that covers the frame fully

26. Tilt and Shift Lenses
 Ability to tilt or shift a part of the lens
 Allows tremendous control over the Depth of Field
 Normally tilting a camera to include a building makes it
look tilting backwards. Can be corrected with this lens
 Widely used in table top, architectural and landscape
photography
 Expensive lenses. Also requires skill and understanding

27. Tele-Converters
 Added between the lens and the body to give longer focal
length
 Cannot be individually used with the body
 Generally available with multiplication factors of 1.4X, 2X and
3X
 Increase the focal length by that much factor
 Decrease the lens speed by that much factor
 Results in loss in quality as one moves up the multiplication
factor
 Problems with autofocus. For a max aperture range of f/5.6
tele –convertors cannot be used
 Lens with max aperture of f/4.0 convertor 1.4X can be used
which will increase aperture to f/5.6. Hence autofocus is
possible

28. So which lens is suitable for you
 Lot depends on the type of photography you do and the type
of subjects
 For P&S zoom lens with range from 28mm to 105mm (35mm
equivalent) will be fine in most situations
 Bridge cameras give a lot more telephoto reach
 For cropped DSLRs two lenses will be fine : Zoom of 18-
55mm and 55-200mm . This can be covered in 18-200mm but
will be expensive and lead to loss of image quality
 For beyond 200mm lenses image stabilization is a must
 Telephoto lenses (more than 300mm) are for very specific
photography
 Image quality greatly depends on the lens and less on the
body

29. Happy Clicking