Exposure Settings & Lens Choices

Exposure settings & Lens choices

Graham Relf

Tynemouth Photographic Society

September 2018 www.tynemouthps.org

We will look at the 3 variables available for manual control of digital photos: ● Exposure time/duration, T ● Sensitivity, ISO ● Aperture, A

We will see what effects they have

And see how to use M Av Tv P on the dial

Canon G3X Why not just use AUTO?

The AUTO setting in your camera is probably very good but it cannot read your mind

● How can it know whether you want a fast moving object to be be sharp (frozen) or blurred?

● When you photograph a rose in a garden do you want everything behind it also to be in focus or blurred out?

● In a darkened room the automatic camera is likely to turn the sensitivity up and produce a very grainy result but you could avoid that if you take control We will discuss how to deal with these and other possible requirements

Example: control depth of field

1, In focus: snow on window pane Blurred: street lamps outside

2. In focus: a huge range

What about flash?

Most of the time you will not need it So keep built-in flash switched OFF (not AUTO) until you think it may be useful

Two points: 1. Do not expect built-in flash to illuminate anything further than a few feet away 2. Do not try to use it through glass!

Otherwise not covered here

Camera lenses are complicated

M.ZUIKO DIGITAL ED 7-14mm 1:2.8 PRO

But simple theory

For all practical purposes we can consider the lens to be one thin piece of glass, to understand how it works:

Object Image on detector

Very distant objects

Parallel rays are focussed in the focal plane of the lens

Defines f as the focal length – always in mm for photography

Ray construction

A ray parallel to the axis must go through the focal point

A ray through the centre of the lens is undeviated

Farther objects focus closer to lens

So the lens must move to focus objects at different distances onto a fixed detector in the camera. Only one distance is sharp Aperture Detector chip at back of camera ) A (

Parallel e

light from r u t

distant r e

object p A

Focal length (f)

Aperture is specified as a ratio: A = f / n where n is some number - often called the f-number

Smaller f-number means larger aperture – letting more light in f/2 is clearly a larger aperture than f/10

Aperture is NOT the filter ring diameter shown in mm on the lens f-number

Repeat:

Smaller f-number means larger aperture – letting more light in f/2 is clearly a larger aperture than f/10

Small f-number is desirable but more expensive!

Why f-number is important: Setting the same f-number on different lenses gives the same amount of light exposure

Exposure

3 factors affect the amount of light on each pixel in the detector (or on each silver grain in a film):

1. A = Aperture (f-number)

2. T = Time (exposure duration)

3. ISO sensitivity (amplifier gain)*

* previously required changing the film

Stops (EV)

Changing any of those 3 factors (A, T, ISO) is done in steps called photographic stops

In these digital days EV (exposure value) is often used instead

Increasing by 1 stop (or EV) doubles the amount of exposure

Decreasing by 1 stop (EV) halves the amount of exposure

Digital cameras usually allow finer control, by 1/2 or 1/3 stops but this may be done by processing in the camera after exposure

Reciprocity

Doubling one of the 3 settings (A, T, ISO) – up 1 stop but at the same time halving one of the others – down 1 stop results in the same amount of exposure

This is why working in stops is useful

(With film this did not always work. Very long exposures had diminishing returns. That was called reciprocity failure.)

ISO stops

The doubling/halving is easily seen in the ISO settings available:

50, 100, 200, 400, 800, 1600, 3200, 6400, 12800, …

(recent digital cameras are pushing up further)

This really controls electronic amplification as the signal comes out of the detector chip. Higher values introduce electronic noise which can be seen as graininess in the image. (Similar effect with high ISO films* but for a different reason.)

So you generally want the lowest ISO you can get away with, depending on the available illumination of the scene or object

* For films ISO was previously known as ASA Why change ISO?

Generally because of the amount of light available in the scene

But best to decide after the other two settings (T and A)

T stops

Available exposure times also double but there is a convention:

15" (with double quote) means 15 seconds but

15 (by itself) means 1/15th of a second

The scale on the camera usually goes (whole numbers only):

4000, 2000, 1000, 500, 250, 125, 60, 30, 15, 8,

4, 2, 1, 2", 4", 8", 15", 30"

After that there may be B (for Bulb): shutter remains open as long as the button is pressed

Why change T?

Is the subject moving?

If so, do you want to freeze the motion or allow some blurring?

Is it a fast swooping swallow or a gull lazily riding the currents?

If it is not moving you may want to lengthen T so you can keep a small ISO, after you have set A

Tripod needed?

Use a tripod for long exposures

Rule of thumb: if T in seconds is longer than 1 / focal length in mm then use a tripod

Eg, for a 50mm lens use tripod if T is longer than 1/50s

But image-stabilisers allow 2 or 3 stops longer (1/15s or 1/8s for a 50mm lens)

Best to switch image stabilisation OFF when the camera is fixed (some lenses are designed to detect this but others misbehave)

There may be a switch on the side of the lens (otherwise in menu) A = Aperture

Adjustable iris diaphragm:

A stops

Strange at first because A is a measure of lens diameter but the amount of light allowed in depends on area, proportional to diameter squared.

So the scale of stops as f-numbers typically looks like this:

1.4 1.8 2.8 4.0 5.6 8 11 16 22 32

(You can see doubling in alternate entries) f/1.4 is likely to be very expensive and the other end is too because the iris diaphragm for stopping down so far has to be fine

Why change A?

Two main reasons which sometimes conflict:

1. To let more or less light in

2. To control depth of field ...

Depth of field

We saw that objects at different distances cannot be in focus in one plane

When a lens has wide aperture the rays are converging steeply and so a small difference in distance causes blurring

A lens with small aperture produces less blurring and shows a greater range of distances reasonably in focus: deeper field

Example: control depth of field

1, In focus: snow on window pane Blurred: street lamps outside Used maximum aperture (smallest f-number)

2. In focus: a huge range Used minimum aperture (largest f-number)

(Camera on ground - no tripod needed)

Depth of field

Example from www.grelf.net

T up by 5 stops (8 4 2 1 2" 4") A down by 5 stops (4.0 5.6 8 11 16 22)

Exposure roughly the same The main dial on top of the camera

There is likely to be a circular dial with these (and other) positions:

AUTO – let the camera decide everything BUT: Subject moving? What depth of field do you want?

P - camera sets everything but then you can adjust aperture by turning a wheel near the shutter, while half-pressed - camera then compensates with the other settings

Av - fix aperture and let the camera decide the rest

Tv - fix T and let the camera decide the rest

M - no thanks, I want to do it all MANUALLY

Some Canon camera dials EOS 5D Mk III

S40 (from 2004)

SX240HS

G3X

Lens choices

Lens specifications show focal length, f (eg, 50mm) and aperture range (eg, f/2.8 – f/22) Zoom lenses show a range of focal lengths (eg, 24 – 100mm) and aperture range BUT in this case the aperture range will not be available at all focal length settings

Why choose different focal lengths? Mainly for wider or narrower field of view (ie, magnification) Small f (eg, 50mm or less) = wide angle Large f (eg, 100mm or more) = telephoto

Field of view Detector chip at back of camera

Focal length (f)

Field of view angle can be worked out from the rays going straight through the centre of the lens

Longer focal length means smaller field of view

Field of view

For a given lens focal length, this depends on detector size There is a calculator on the tips page of our web site: tynemouthps.org/tips.html

Detector size

Full frame: 36 x 24mm

Aspect ratio: 3 : 2

But our competitions require 1400 x 1050 which is ratio 4 : 3

We will discuss the conversion options next month (scale to 1400 x 933, crop and scale, or distort)

A small change of distance...... has a smaller f effect in a short focal length lens than in a longer one.

So telephoto lenses have f much less depth of field than wide angle lenses

Therefore choose a telephoto to blur backgrounds, if you can Quiz - 1

(a) You want to photograph a butterfly that has landed in your sunlit garden on a slightly breezy day. You want the background to be out of focus. What settings are appropriate? (b) How does this change if it is a bee buzzing around a flower?

Quiz - 2

You have been asked to photograph a long room in a museum for publicity purposes. Some objects in the room have to be protected from light so the scene is quite dark and flash is not allowed. What would you do? (Including how to get the exposure right)