1. An Idiot’s Guide to ….. Lenses, Focal Length, Perspective and all that.
a.k.a. John’s Guide to …...

2. What’s focal length all about?
SLR
Compact
18 mm 1x
39 mm 2x
100 mm
5.5x
250 mm
14x
Wide Angle
Telephoto

3. Magnification,what’s going on?
feel free to ignore this slide if you’re not one)
(for the physics geeks like me ….
Short Focal Length
Small
Image
Long Focal Length
Large
Image
= telephoto
Object
Lens
Image plane

4. Exercise 1- Zoom / Magnification
Make sure you know how to change the focal length (zoom) on your camera.
Take a range of pictures of similar scenes at different focal lengths.
Find out how to get the focal length information from the camera or on the PC when you have downloaded the picture
How much bigger is an object if you double the focal length?
Find what is the closest distance you can focus on an object? How does it change as you zoom in and out?
What is the widest aperture (smallest f-number) on your lens? How does it change as you zoom in and out?

5. Angle of view, what’s going on?
(more geekery …. ignore if you wish)
Short Focal Length
= Wide Angle
Wide
Angle
Of View
Long Focal Length
Narrow
Angle
Of View

6. Exercise 2- Zoom / Angle of view
Experiment with angle of view:
Use something in the foreground to frame a scene (e.g. use a door, arch or a couple of trees as a frame for scene beyond).
Move towards or away from the frame & then zoom in or out to make the Frame the same size as before.
How much more or less of the scene can you see?
Take some pictures indoors (the estate agent shots)
Can you get the whole of the room in the shot?
What focal length do you have to use to get it all in?
Are the straight lines straight in the picture?

7. Digital Zoom (Sensor size)
What if instead of zooming I just expand a portion of a picture taken at wider angle?
18 mm
100 mm
Notice that the perspective is the same. Perspective does not depend on the lens it only depends on the position from where you take the picture.

8. Exercise 3- Home made digital zoom
Take a shot with a wide angle (short focal length)
Standing in the same place zoom in (long focal length) and take a close-up picture of a section of the previous scene.
On the PC crop a section of the wide angle shot which covers the same area as the close up (zoomed-in) shot
Expand the cropped picture to the same size as the zoomed in picture and compare them side-by-side:
How do the sizes of all the objects compare in the two pictures?
Is everything the same shape?
Are there any differences in perspective?
Is the resolution of the 2 pictures the same? (can you see any graininess or blurring in either of the pictures?)

9. Perspective (relative size of objects at different distances)
How can I make the bollards look bigger?
Zoom……
…. Or get closer.
Relative size of bollard and road sign stays the same.
Bollard has got relatively much larger than sign and house.

10. Perspective, what’s going on?
(the geek explanation))

11. Perspective – what happens to parallel lines
Wide angle and close to front bottle makes closer bottles appear relatively larger
so parallel lines running along top and bottom of bottles come together quickly
45mm lens
1 m distance
Long focal length and further from front bottle makes closer bottles appear relatively less difference in size
so parallel lines running along top and bottom of bottles come together gradually
140mm lens
3 m distance

12. Exercise 4- Experiment with Perspective
Experiment with taking shots with different perspective of the same scene:
Take shots closer with wider angle then move back and take shots with longer focal length.
How do the relative sizes of foreground and background objects change?
How does this change the emphasis on the objects in the picture? (Composition?)
Take pictures looking along a street. Again take shots closer with wider angle lens and further back with longer focal length.
Find a line joining points of similar heights along the street (e.g. along the top of the buildings or joining 2nd floor windows).
Compare this to a line running along the foot of the buildings.
How does the angle between these lines compare if I move back to take the shot?
How does the angle compare if you stay in the same place and change the focal length?

13. What happens when I get closer?

14. Perspective distortion, what’s going on?
(last bit of geekery)
50m
200m
206m
5.15cm
5cm
If I’m a long way away the top and bottom are both far away so seem similar sizes
50m
71m
5cm
7.1cm
If I’m close then the bottom is close but the top is still quite far so it seems smaller

15. …Remember Depth of Field?
45mm lens
1 m
distance
140mm lens
3 m
distance
F/4.5
F/22

16. Exercise 5 - Zoom & depth of field
Taking some portraits (head & shoulders).
Use a wide angle lens close-up & then move backwards and use telephoto
What happens to the background
Take some shots in the garden (flowers, birds etc.)
In some shots use the focal length and aperture to isolate the object from the background.
In other shots ensure that both the foreground and background are clearly visible.
Take some shots in which an out-of-focus foreground is used to frame your object behind.

17. Summary (what have you got to tinker with) Exposure Motion / Still
ISO
Exposure
Motion / Still
Depth of field
Brighter/Darker

18. Summary (what have you got to tinker with) Exposure Lenses
ISO
“Composition”
Exposure
Magnifi-cation
Perspective
Angle of view
Depth of field
Lenses
Focal Length (Zoom)
Distance from Subject