Exposure (3): Aperture

[RichardTaylor]

Aperture.
Just to recap;
Beside controlling the amount of light through the lens the aperture also controls the zone of sharpness, depth of field (DOF) in the photograph. DOF is also dependent on shooting distance to the subject, camera type and lens focal length (eg wide angle, normal or telephoto).



Apertures are known as F numbers
The wider the aperture (smaller F numbers) the less DOF.
The smaller the aperture (large F numbers) the more DOF.
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Just a note on lenses. (More to come on another tutorial)
When you look at a lens you will see numbers like.
50mm 1.8
This is a single focal length or prime lens of a maximum aperture of F1:1.8
or
24-104mm F1:4
This is a zoom lens with a maximum aperture of F4 at all focal lengths
or
75-300mm 1:4.5-5.6
This is a zoom lens with a maximum aperture of F4.5 at 75mm and F5.6 at 300mm.
The minimum aperture is lens dependant.
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What aperture to use?
Besides balancing exposure my main consideration is what depth of field do I want.
If I want lots of DOF then I will use a small aperture.

(2) Here lots is relative. I am shooting with along lens, at a very short distance. so my DOF is is approx 1/2" or 12mm

Camera Canon EOS 40D
Exposure 0.004 sec (1/250)
Aperture f/25.0
Focal Length 375 mm
ISO Speed 400
Exposure Bias 0 EV
Flash On, Fired
Exposure Program Manual
Subject Distance 1.87 m (approx 6 feet)

Do I want supper shallow DOF to blur out the background and blur out any foreground.
(3)

Camera Canon EOS 40D
Exposure 0.001 sec (1/1250)
Aperture f/2.0
Focal Length 135 mm
ISO Speed 100
Exposure Bias 0 EV
Flash Off, Did not fire
Exposure Program Aperture-priority AE
Subject Distance 1.71 m

Do I want a "general purpose" DOF for those grab shots.
This was whist on vacation and when just walking around I find an aperture of F8-F11 meets my need when extremes of DOF don't really matter

Camera Canon EOS 5D
Exposure 0.006 sec (1/160)
Aperture f/11
Focal Length 24 mm
ISO Speed 400
Exposure Bias -1 EV
Flash Flash did not fire

You may find this DOF calculator handy.
Online Depth of Field Calculator
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The main thing to remember it is a "balancing act".

ISO, shutter speed and aperture all work together and normally if you change one you change the other.
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This series

#1 Introduction to exposure
Exposure (1) A balancing act

#2 ISO and introduction to aperture.
Exposure (2) ISO + Intro to Aperture

#3 Aperture (this one)

#4 Shutter Spped.
Exposure 4: Shutter speed.

Putting it all together
Exposure 5: Putting it all together

Future posts in this series will include more detail on;
Exposure compensation
White balance

Thanks for looking.
Feel free to ask questions and feedback would be appreciated.

Richard

[FlyingKiwiGirl]

Thanks very much RichardTaylor for your this post, as a newbie to the forum and to the world of DSLR I'm cramming alot of info in and it's nice to have a clear cut and informative post like this to refresh my memory. Excellent information!

[RichardTaylor]

Good to see it came in handy.

[u8ky2]

Thank you very much for all this info. Since I am new to photography it is really helping me. But the one thing I still don't get is why in the photo of the butterfly, how you got such a shallow depth of field with a f/22? Wouldn't that give you a large depth of field? This is probably a stupid question.

[RichardTaylor]

That's a lot of DOF for a 400mm lens at near minimum focussing distance of 1.87 m (approx 6 feet)

Here is an example of what you get at F5.6 at near 400mm (380mm) and at the same shooting distance.

Camera Canon EOS 40D
Exposure 0.001 sec (1/800)
Aperture f/5.6
Focal Length 380 mm
ISO Speed 400
Exposure Bias -1/3 EV
Exposure Program Aperture-priority AE
Date and Time (Original) 2011:03:06 10:21:26.06+11:00
Subject Distance 1.87 m

DOF depends on shooting distance, focal length and aperture.
The closer you get the less DOF..
The longer the lens (from the same shooting position) the less DOF.
The wider the aperture the less DOF.

[Thomas89]

Hi! Wow, what a great resource. I bought my first DSLR about a month ago and I've been reading alot about it, especially on DPS :-)

I have a question about why a large maximum aperture is so important? The same lens with f/2.8 costs about twice as much as one with f/4... Is it really the shallow depth of field that makes a lens so much better? Or does it just mean you can generally use a faster shutter speed to get the same exposure? (and therefore it's called a faster lens?)

I'm a little confused about this and I hope to get some help from you guys. Thanks :-)

[Doug Sundseth]

f/2.8 gives you shallow DoF when you need it. It also allows twice as much light through in a given time (f/2.8 it one stop faster than f/4, because the area of the hole is twice the size). As a result, you can shoot with double the shutter speed (half the exposure time). That can be the difference between blurry and not blurry.

In addition, f/2.8 lenses are designed for use by working professionals, so they tend to be more robustly made.

Further, getting a sharp image through a wider hole at an arbitrary distance from the lens is a notably harder engineering task than with a smaller hole, so a very fast lens will tend to have more lens elements using more exotic glasses than a slower lens.

Finally, since the construction increases the cost of the lens, fewer will be sold, so the economies of scale work against the price as well.

[Thomas89]

Thank you, Doug.

I see the difference when I shoot with large aperture. But what's the difference when I shoot landscape at let's say f/11? Will I still have the advantage that I can shoot with a faster shutter speed compared to a "slow" lens?

[RichardTaylor]

If you can shoot at around F8-F11 for static subjects then fast lenses are not really necessary.

However keep in mind some lenses, the "good" ones - not only fast, may have better image quality than "cheap" lenses. This will show up as chromatic aberration which is colour fringing around areas of high contrast.
They may also have smoother bokeh.

See this for a deeper description, including examples.
Chromatic Aberration: Optical: Glossary: Learn: Digital Photography Review.

I own a Canon 70-200 F4 L (a high quality lens) and it is mostly used in good light and I am happy with it.
I do shoot in low light, events, without a tripod sometimes, and even a F2.8 won't cut it
I shoot those events with F2 lenses.

[Thomas89]

Thank you Richard, your answer explained a lot. I think I overvalued maximum aperture a little bit after reading all those comments about fast leneses.