New To Photography:Aperture, Shutter Speed, ISO etc. - Explanations

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This page is a chapter in the book Appendix A - Explanations of Shutter, Aperture, ISO etc.
Aperture, Shutter Speed, ISO etc. - Explanations
Refer: http://en.wikipedia.org/wiki/Photographic_lens

Try the Camera simulator Learning Centre: Practice your settings on this online camera demo.

Aperture:
A smaller aperture (larger f-number) means less light coming through the lens, so you either have to increase shutter duration or increase sensitivity (or both) to compensate.
A bigger aperture (smaller f-number) means more light coming through the lens, so you either have to decrease shutter duration or decrease sensitivity (or both) to compensate.

Shutter Speed:
Faster shutter speed means less light on the sensor, so you either have to open up the aperture (smaller f-number) or increase sensitivity (or both) to compensate.
Slower shutter speed means more light on the sensor, so you either have to close down the aperture (bigger f-number) or decrease sensitivity (or both) to compensate.

ISO Sensitivity:
Higher ISO = more gain, so the effective illumination increases, so does the noise.
Lower ISO = less gain, so the effective illumination decreases, so does the noise.

A longer explanation:
Imagine you have a wall with a hole in it and a bucket on the other side. Light is water,
Aperture is the size of that hole, shutter speed is how long you push water through it.
Only thing is aperture is backwards (inverse) so lower number is more, and higher is less.
Its a fraction. So 1/2 is larger than 1/4 which is larger than 1/8.
Correct exposure is when the bucket is nearly full (not overflowing).
If the hole is 2cm and the water is on for 10 seconds (at a constant pressure) a certain volume will flow.
If the hole is closed to 1.4cm (half the area) you will need the water to flow for 20 seconds to get the same volume.


1. Aperture (Usually Av or A on the camera dial)
Imagine pushing water though for 5 seconds with a medium size hole (f/4). Thats enough to fill the bucket so
that's a perfect exposure. Then keeping the time at 5 seconds you shrink the aperture to a smaller hole (f/8).
As the hole is smaller not enough water(light) gets through and the bucket doesn't properly fill - thats under exposure (darkness).
Then keeping the time at 5 seconds you make a really large hole (f/2). You push the water through, but because
it's a lot bigger you get too much water (light) and it overflows - thats over exposure (an over-white picture).
Aperture is what controls Depth of Field (see below).

2. Shutter Speed (Usually Tv or S on the camera dial)
Change the hole for this section to a constant (f/4). 5 seconds water is the perfect exposure from above.
Imagine only pushing water through for 1 second. Not enough water to fill the bucket so the photo under-exposes.
Then push water through for 10 seconds. Too much water, bucket overflows - over-exposure.

3. Variation
If you make the hole smaller (a larger aperture number eg. f/8) you need to leave the water speed longer.
If you make the hole larger (a smaller number eg. f/2) you need to lower the water speed. So f/2 (large)
for 1 second, f/4 (medium) for 5 seconds and f/8 (small) for 10 seconds all expose the same amount of water.

This chart helps with exposure, each row of the chart represents the same amount of light (exposure) assuming
the same ISO setting (say ISO 200).
The only difference is the DoF (depth of field) which gets smaller as you go down the list.
It turns out that each f/stop is about half the light (opening) as the next one and the shutter speed is
roughly twice as fast.
Shutter Speed f/stop
1/4 second f/45
1/8 f/32
1/15 f/22
1/30 f/16
1/60 f/11
1/125 f/8
1/250 f/5.6
1/500 f/4
1/1000 f/2.8
1/2000 f/2
1/4000 f/1.4

Explanation:
The reason that both the halving and doubling and the smaller numbers mean
more light things make sense is that the f/stop is a ratio. The ratio is between the diameter of
the aperture in the lens and the focal length of the lens. The focal length is generally measured in millimetres (mm).
On a 50mm lens, f/2 is saying that the diameter of the aperture is 25mm. The ratio is this 50/25 = 2.

Terminology: We write f/xxx (lowercase f slash number) not Fxxx or F/xxx

4. ISO Sensitivity & Noise

This is a tricky one but to bring it into this analogy, one view is that ISO (pronounced "eye so") is
the pressure you push water through at.
[Another (better) analogy is that ISO represents the size of the buckets catching the water.
See: this article for a detailed explanation of Digital ISO.

From above pushing the water through for 1 second at f/4 it underexposed. But if you double the pressure
(ISO) of the water, it allows more through and exposes correctly although it sprays around a lot more and
leaves mess around the bucket (noise). The higher the pressure (ISO) the more that misses and gets around the bucket (more noise).

5. Focal Length
The focal length of a lens determines its angle of view, and also how much the subject will be magnified
for a given photographic position. Focal length also determines the perspective of an image.

Longer focal lengths require shorter exposure times to minimise blurring caused by the shake of hands.
This tool http://www.tamron-usa.com/lenses/lea...comparison.php demonstrates focal length.

This diagram explains the fact the focal length is the same regardless of the size of the sensor.


6. Depth of Field (DoF)
Depth of Field is one of the most important aspects of photography, whether it be digital or film.
It can be the difference between an ordinary and stunning photograph. However so many photographers have
little or no idea of how to set their cameras for a desired Depth of Field effect.
Here is a DoF calculator: http://www.dofmaster.com/dofjs.html and a DoF simulator that help illustrate DoF.

Everybody has probably seen photographs in which every element from foreground to background is in sharp focus,
and other pictures in which only the subject is in sharp focus while everything else is blurry. The first picture
is said to have more depth of field than the latter, which has shallow depth of field. For those that don't know,
depth of field, or DOF, is how deep the area in focus is, when you focus on a given subject. It's a very powerful
artistic tool. You control DOF with aperture. A small aperture (high F stop number) gives a larger DOF than a
larger aperture (smaller F stop number). Refer: http://en.wikipedia.org/wiki/Depth_of_field and http://en.wikipedia.org/wiki/Bokeh

7. Colour Temperature

Colour Temperature http://en.wikipedia.org/wiki/Colour_temperature (measured in Kelvins) is sometimes used
loosely to mean "white balance". In practice this means setting your camera so that pictures look natural when taken
under various lighting conditions. You can also obtain interesting effects by varying the colour temperature setting on
your digital camera. Most camera's have an automatic setting that sorts out colour temperature, but often this
automatic setting does not work well with some lighting such as tungsten lights; this is when you should use the
manual setting.

Using raw mode and post processing using software like Lightroom and Photoshop lets you get better control of
the white balance, especially for low light work.

Lighting Source Kelvins
Match Flame 1700
Candle Flame 1850
40-Watt Incandescent Tungsten Lamp 2650
75-Watt Incandescent Tungsten Lamp 2820
100-Watt Incandescent Tungsten Lamp 2865
500-Watt Incandescent Tungsten Lamp 2960
Professional Tungsten Photo Lamp 3200
Color Photography Studio Tungsten Lamp 3350
Photoflood or Reflector Flood Lamp 3400
Daylight Blue Photoflood Lamp 4800
Daylight (Sunlight is the light of the sun only. Daylight combines sunlight and skylight.)
Sunlight: Sunrise of Sunset 2000
Sunlight: One Hour After Sunrise 3500
Sunlight: Early Morning or Late Afternoon 4300
Average Summer Sunlight at Noon in the Mid-latitudes 5400
Direct Mid-Summer Sunlight 5800
Overcast Sky 6000
Daylight Fluorescent Lamp 6300
Average Summer Sunlight (plus blue skylight) 6500
Light Summer Shade 7100
Average Summer Shade 8000
Summer Skylight (varies) 9500 30,000
Previous: New To Photography:Entering your first AusPhotography Competition Appendix A - Explanations of Shutter, Aperture, ISO etc Next: New To Photography:The Exposure Triangle - Shutter Speed, Aperture, ISO Sensitivity

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