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2018 january 07

aperture

If you're looking to gain more control over your camera then you're in the right place. Shooting full manual will require some knowledge of the parameters of your camera including shutter speed, ISO and aperture.

 

This first lesson is on aperture and hopefully you can walk away feeling a little more comfortable with what aperture is and how it affects your images...

In this tutorial we will cover:

  • what aperture is.

  • how to measure aperture.

  • primary and secondary effects.

  • stops.

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By the end of this tutorial you should be able to:

  • clearly distinguish between grain and noise.

  • operate a camera in order to avoid noise.

what is apertue

what is aperture?

An aperture is defined as 'an opening, hole, or gap' -  or - more specifically in the world of photography 'a space through which light passes in an optical or photographic instrument, especially the variable opening by which light enters a camera'.

 

Isn't that how an eye functions?

In the same way the iris has the job of changing its size to regulate the amount of light entering the eye.

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In bright light your iris (a muscle) will make your pupil (an opening) smaller and in darkness your iris will make the pupil larger. The aperture in your lens does the same job. That is to say that the aperture blades move to make your cameras aperture smaller and larger controlling the amount of light entering the camera.

primary effect

primary effect of changing aperture

The primary effect of changing the size of your cameras aperture is on the exposure of your images.

Put crudely the exposure can be thought of as the brightness of an image. A large aperture/opening is going to result in brighter and possibly over-exposed images and conversely a small aperture is going to result in darker and possibly under-exposed images.

how its measured

how is it measured

Aperture is measured in f-stops and can be controlled either by an aperture ring or - in the case of digital cameras - on the camera body with the magic of buttons and electricity.

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To check the largest aperture per lens check the front of the lens. A prime lens I own notes 1:1.8.

Zoom lenses have a different value format.

 

For example; f1:3.5-f5.6 which means very simply that when the lens is at its shortest focal length (e.g. 18mm) the largest aperture available is f3.5 and when the lens is at its longest focal length (e.g. 55mm) the largest aperture is f5.6. This it's what's called a 'Variable Aperture Lens'.

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Side Note:- Lenses with larger aperture capability are called 'faster lenses' because the more light you can get flooding in to the camera the less time required to capture the image which will in turn reduce blur.

 

Faster lenses have larger aperture capabilities allowing for very fast shutter speeds

in darker conditions. How useful!

clearing the fog

clearing the fog

around f-stops

The confusion...

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'The small f-numbers are the largest holes and the large f-numbers are the smallest holes...'

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The aperture is, as mentioned before, measured in f-stops...

 

An f-stop is actually a fraction, specifically in relation to the focal length. 

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'...It can't be that simple?' ...It is actually...

 

So, if you have a 50mm lens set to f/4, this means that the aperture opening in the lens is 1/4th of 50mm or 12.5mm.

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That explains why the f-stop scale might seem backwards... f/4 is a smaller aperture that f/2 because a quarter is smaller than a half...

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So it's not magic, it's mathematics.

Simply remember: [focal length/f-number] as a fraction and you're golden.

where did these numbers come from?

The f-number scale might seem strange but if we just focus on the whole numbers f/2, f/4, f/8 and f/16, based on what we have learned previously - that this value is a fraction - it too is a very simple concept to grasp.

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What needs to be done to each value to get the next?

- 2, 4, 8, 16... 

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They're being doubled! Which means that as a fraction the value is being halved each time...

- 1/2, 1/4, 1/8, 1/16 (half, quarter, eighth, sixteenth)

demonstration of the link between fractions and aperture size...

f-stop:  f1

ratio: 1:1

fraction:1/1

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f-stop:  f2

ratio: 1:2

fraction:1/2

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f-stop:  f4

ratio: 1:4

fraction:1/4

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f-stop:  f8

ratio: 1:8

fraction:1/8

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f-stop:  f16

ratio: 1:16

fraction:1/16

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the whole/light area represents the focal length of the lens.

dark area represents the width of the opening of the aperture in relation to it.

dim: 1x1

area: 1²

dim: 2x2

area: 4²

dim: 4x4

area: 16²

dim: 8x8

area: 32²

The area is quadrupled each time.

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Put simply - the values between the [2, 4, 8 and 16], namely [2.8, 5.6 and 11] are the steps between these fractions and as a result the aperture is no longer being quartered but instead halved.

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Remember that the f/x fraction is a measurement of the diameter of the opening in relation to the focal length. The same applies to the diameter of a circle and so the resulting area is doubled and halved using this scale.

the numbers

The whole that we're referring to is the distance across the opening - or - the diameter...

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Let's forget we're discussing aperture for a moment.

What happens when the edges of a square are doubled

each step. 

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stops

why that's useful

This behaviour of doubling and halving lights effect on an image has it's own measure called a stop...

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Stops are relevant across Apertures, Shutter Speeds, & ISOs and are useful for one fundamental reason.

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By doubling and halving the effect of light on an image across all parameters, we can stop up in one parameter and down in another and result in the same light levels.

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That's right! The same! This is useful when, for example, an image has been captured with good light levels but it's blurry.

No problem! Halve (stop down) the shutter speed to capture light over half the time and as a result halve the blur. The image will be a little dark but by enlarging the aperture by a stop the light levels we be the same as the first image. Great!

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This is known as 'stop compensation' and is very handy in many situations.

secondary effects

All of the parameters that you set have a secondary affect on the images and in the case of aperture that is 'Depth of Field' sometimes noted as 'DoF'.

 

Depth of field is the amount of depth in your image that is in focus. The Depth of Field becomes very shallow when the aperture is opened (e.g. f/1.2) and deeper when the aperture is closed (e.g. f/22).

 

It is important to take this in to account when taking portraits for example. It is common for portraits to be taken with a lot of background blur to avoid taking attention away from the subject known as bokeh (blur in Japanese).

image shot with a large aperture and resulting shallow depth of field.

notice the blurry ear!

example of 'bokeh' achieved by selecting a large aperture and making the background blurry.

As mentioned, this is achieved by shooting what is

commonly referred to as 'wide open'.

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The risk is that some facial features for instance the eyes, become very sharp while other features like the

ears and nose become wildly out of focus. It is therefore

a balance and with practice you will get used to what works and what doesn't.

secondary effects

in conclusion

Aperture does not need to be complicated. Simply remember...

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1. an f-number is the fraction of which the focal length is divided by to devise the diameter of the apertures opening.

i. Focal length/f#.

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2. the secondary effect of adjusting the aperture is adjusting the depth of field (D.O.F).

If you have found this information useful then please consider subscribing for updates. There's a lot more on the way. Don't forget to share with anyone who may also be interested.

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Until next time. 

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Elliot Sylvester.

(Amateur Photographer/Photography Blogger)

in conclusion
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