1. Back to Basics Ian Thompson ARPS Or, How to Take Control
of Your Camera
Ian Thompson ARPS

2. Back to Basics
Your digital camera is a very powerful piece of computing kit
But because it is so clever and automatic, we often don’t know what it’s doing

3. Back to Basics
Why am I bothered? Can’t I just let it get on with the job?
Sure you can. But as your photography improves, you will find yourself wanting more and more control. 3

4. EXPOSURE! Back to Basics Many things can make a photograph good or bad
The one thing that HAS to be correct is…….
EXPOSURE! 4

5. Back to Basics Exposure is controlled by the unholy trinity of….
Aperture
Shutter speed
ISO setting 5

6. Back to Basics APERTURE & SHUTTER SPEED
….affect the amount of light reaching the sensor:
Aperture = size of lens ‘hole’
Shutter = time the aperture is open
ISO = sensitivity of the sensor

7. Sensors

8. Sensors
A short demonstration

9. Back to Basics - Aperture
The ‘f’ number of a lens is the focal length divided by the diameter of the object glass
Focal length
Diameter
Sensor plane
e.g.
Focal length = 50mm
diameter = 35mm
Max. aperture = 50/35 = 1.4
So lens is 50mm/f1.4 9

10. Back to Basics - Aperture
This is a law of the (or at least our) universe which is constant for any lens.
So any lens at – say – f/8 will admit the same amount of light as any other lens at f/8 10

11. Back to Basics - Aperture
So what’s this thing called a ‘stop’?
‘Stops’ were originally pre-set notches where the aperture could be set to a known amount - either half or twice the next-door notch.
These days, the notches are gone…….but we still refer to ‘stops’ 11

12. Back to Basics - Aperture
Altering an exposure by a ‘stop’ is a doubling or halving of that exposure.
A ‘stop over’ or ‘extra’ = x2
A ‘stop under’ or ‘less’ = x½
Each element of the trinity can be responsible for a ‘stop’ 12

13. Back to Basics - Aperture
Perversely, a large ‘f’ number means a small aperture
A small ‘f’ number means a wide aperture.
The standard sequence of stops is
…….
From left to right, each stop allows ½ the light of the previous stop 13

14. Back to Basics - Aperture
Aperture also controls the illusion of sharpness and ‘Depth of Field’…
....but more about that next time

15. Back to Basics - Shutter
Originally, there was no such thing as a shutter.
Film sensitivity was so slow (ISO 1 or 2!) that clamps were used to keep people still!
The photographer took off the lens cap, waited for a minute, then replaced it. 15

16. Back to Basics - Shutter
These days, we have shutters that operate as fast as 1/8000 sec.
The shutter has many settings from ‘B’ = ‘Bulb’, where it will be open as long as the button is pressed, up to 1/8000 sec.
Generally speaking, each setting is twice the speed of the previous one – ‘stops’ 16

17. Back to Basics – ISO speed
In the days of film, we had ISO speeds 10 to 400
If you wanted a faster (=more sensitive) ISO, ‘push’ processing was the only way but this degraded image quality
In digital cameras, it is now possible to have ISOs of 256,000 – the black cat in a dark coal cellar can now be snapped using the light from a glow-worm! 17

18. Back to Basics – ISO speed
Double the ISO value = double the sensitivity
So even the ISO setting is associated with ‘stops’
But remember that high ISO numbers mean ‘noisy’ images.
Your camera delivers its best quality at low ISO settings 18

19. Back to Basics – Metering
In manual mode, how do we know the exposure is correct?
Correct exposure
1 stop ‘under’
1 stop ‘over’
1 stop ‘over’ 19

20. Back to Basics - Summary
Confused? Quite possibly…!
Good exposure = light is in sensor range
Choose ISO to give best quality having in mind point 1. Start with a low ISO.
Control the exposure with aperture and shutter, having in mind the type of subject……….and…….
…….consider depth of field (DOF)…….
….so many things to consider. 20

21. M Back to Basics - Exercise MANUAL MODE ONLY!!!
Switch off stabilisers! (Autofocus is OK)
Set your camera to its lowest ISO
Take pictures in the room lighting, attempting to get a correct, sharp exposure, increasing ISO until you succeed.
What’s not good about the sharpest images? 21

22. ….or the Great Focus Illusion
Depth of Field….
….or the Great Focus Illusion

23. Depth of Field means…
….the range of distance in a scene throughout which objects appear sharp.

24. ..Why? Optical illusions…. Sensor/film Lens Point source
Third-form physics tells us that a lens will form a sharp image
ONLY at the focal point….so how is Depth of Field possible??

25. The illusion..
Lens
CoC
Away from the true focal plane, the image of the point source
becomes a disc, increasing in diameter the further it is out of focus.
This disc, dramatically, is called the ‘Circle of Confusion’

26. …How?
At normal viewing distances, the average (young) eye cannot distinguish the difference between a true point and a dot or disc of diameter smaller than about .1 mm (.004”)
If we ‘downscale’ the size of this disc by an amount related to the size of the camera sensor, we can use this ‘confusion’ to our advantage and call it ‘Depth of Field’

27. Confusion reigns.. Lens CoC
For a point source, the image in front of and behind the focal plane is a disc.
The maximum diameter of this disc (after scaling down as per previous slide)
is the circle of confusion.

28. Stopping down… What happens if we ‘stop down’? Diaphragm (or iris)
Lens
CoC
For the same object distance, the CoC is smaller because the ‘angle of
approach’ of the rays of light is smaller due to the iris diameter.
If any ‘fuzzy’ image point is smaller than the CoC for the particular sensor,
then it will appear sharp on the print: this is why a smaller aperture will
allow a wider range of object distances to be acceptably sharp - DoF

29. Stopping down…

30. Landscapes go on for ever….
Infinity A
So, if we arrange that infinity is focused at the front CoC plane, everything
between it and the rear (=nearest object) CoC plane will be in ‘focus’.
The distance to ‘A’ is known as the hyperfocal distance, and depends upon
lens focal length, the aperture used, and the size of sensor in the camera.
This is far more useful than making infinity the primary focal point and
‘wasting’ the front CoC plane – it gives us a massive range of focus.

31. What about DoF for my camera and lens?
Lenses used to have DoF scales
printed on them….
On this old lens, the infinity mark on the focusing ring can be positioned adjacent to a DoF scale aperture value, meaning that everything from infinity to the distance opposite
the symmetrical aperture mark will be in ‘focus’ at that aperture. The DoF in this example (for f/22) is from 1.75M to infinity with the hyperfocal distance being opposite the orange line – about 3.5M.
Sadly, most modern lenses do not have the DoF/hyperfocal scale.
So what can we do….?

32. It gets complicated…. We need a calculator……!...
Modern digital cameras are made with 4 or 5 different sensor sizes – each has a different CoC, meaning the DoF for each is different.
A small sensor (APS-C, µ4/3) has a much smaller CoC than the full frame (35mm) kind.
We need a calculator……!...
...or a rule of thumb: use a small aperture and focus 1/3 into the scene

33. Finally…
Happy Birthday Bill!!