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How to Buy a Camera Lens Pt2 - Aperture

In my last post HERE I discussed the basic fundamentals of buying a camera lens, covering different brands, and how to select a lens with the correct focal length for the type of photography you are going to use it for. This time round, I will cover the slightly more tricky, and regularly misunderstood aperture. As always, this is geared towards the amateur/ hobbyist photographer, who may be looking at replacing their old kit lens with something a bit more flashy.

I'm going to try and keep this as theory light as possible, so if there is anything to confusing here, don't worry, I will lay it lay it out in a few simple bullet points at the end.

Aperture 101

Ok, before we get this started, lets go through some ABSOLUTE basics and get a few things out of the way.

What is the lens aperture?

Simply put, the lens aperture, is a small hole at the back of every lens that lets light through. The bigger the hole, the more light, easy enough right?

When we take a picture, we want to expose it correctly. We do this in part by selecting an aperture size which lets in enough light to ensure our image is bright enough. Aperture is usually denoted as an "f" number, often written as "f/5.6" or "f:2.8", these intervals are typical referred to as "f stops".

The tricky part to remember however, is that a SMALLER "f' number, means a bigger aperture and visa versa. So f/2.8 will let in more light than f/22.

How does lens aperture relate to she shutter speed?

Every time we take a picture using our digital camera, we strike a balance between ISO, Aperture and Shutter speed.

Lets use an example where an image is correctly exposed using the following settings.

ISO: 100

F: 8

SS: 1/200s

If we make the aperture wider, lets say f2.8, then the camera would be letting in MORE light, and the picture would be overexposed using the same ISO and shutter speed. We may in this case select a faster shutter speed in order to reduce the amount of light entering the camera and therefore balancing the image.

Conversely, if we made the aperture narrower, say f/22, the camera would let in LESS light, and we would select a longer shutter speed in order to compensate.

Fast lenses

A "fast lens", is simply a lend with a large maximum aperture, e.g f2.8. As the wide aperture allows for the use of fast shutter speeds.

How to read the Aperture of a lens

Lenses on the market today, will typically denote the widest possible aperture that the lens can achieve. It is most commonly found on written on the rim around the glass on the front, or written on the barrel of the lens. Confusingly, its most often written in the form "1: X" where X is the widest available aperture.

Lets have a look at a few examples and see if we can spot the lens aperture.

Example 1:

This first one is a little tricky, but if you look closely, you can see the words "Canon Lens EF 50mm 1:1.4". This lens has a widest aperture of F1.4.

Example 2:

This one is a bit easier, we can see the words "SIGMA DC 17-50mm 1:2.8" So we know this lens has a widest aperture of 2.8.

Example 3:

How about this one?

This is a bit more tricky. The lens says "Canon Zoom Lens 18-55mm 1:3.5-5.6."

If you read my article on focal lengths, you will know that some lenses will have a variable focal length. On some lenses, no matter what focal length you select, the widest aperture available never changes, see example 2. On other lenses, the widest aperture available to you changes depending on the focal length. In example 3, if you select 18mm, you can use f/3.5, however if you zoom in to 50mm, then the widest you can use is f/5.6.

And lastly lets look at a few different lens names and see if we can guess what the widest available aperture is and weather is it fixed or variable. Ill let you make a guess, then scroll to the bottom of this article for the answers.

Example A: Sigma 150mm f/2.8 APO Macro EX DG OS Lens for Canon

Example B: Canon EF 24-105 mm f/4L IS II USM Lens

Example C: Sigma 150 - 600 mm F5 - 6.3 DG OS HSM Contemporary Canon Mount Lens

This is called Variable aperture vs fixed aperture. In most cases, fixed aperture lenses will be more expensive than variable aperture, as it is more costly and difficult to make fixed aperture lenses.

How about the maximum aperture?

We have spoken a lot about the widest available aperture, how about the narrowest? Well funnily enough it isn't usually written on either the lens or the lens name, what the narrowest is. That's because although its difficult to make lenses with wider apertures, and fixed apertures, making very narrow apertures is very easy. So in most cases lenses will go all the way up to f/22 or sometimes even higher. Although for most photographers high apertures like this are very rarely used.

Depth of field and Bokeh?

Depth of field is the distance in front and behind the subject which is in focus which will also be in focus.

A short depth of field will mean that things in front and behind the subject will be out of focus.

A long depth of field will mean things a long distance away from the subject will be in focus.

One of the most powerful tricks at any photographers disposal, is the ability to blur the background of a shot, this is most commonly seen in portraits, where we want the subject to really stand out from the background. You have likely seen it 1000 times in magazines, instagram and on the TV, but most people don't know how its done.

Credit Horst Fruchs

Simply, we use a wide aperture, usually f/4 or wider, with lenses at f/1.4 being the most extreme example. The wider the aperture, the more extreme the blur becomes. This effect is called "bokeh". In the image belowm I focussed on the cat's nose, and you can see the image is blurry even on the cat's body, this is because of the short depth of field.

For this picture I used:

Iso: 800 F: 1.8

ss: 1/80s

So to summarise all of the above:

A wider maximum aperture will allow you to use faster shutter speeds at any given ISO, this is extremely important when taking pictures of fast moving subjects such as children, sports, moving cars and moving wildlife.

Maximum aperture is much less relevant for landscapes, as we usually want long depths of field found at around f/8.

In order to get the beautiful "bokeh" effect, we need to select a wide aperture. The wider the aperture, the more pronounced the effect.

Lenses with variable aperture, are typically cheaper then those with fixed aperture.

And thats it!!!!

I hope this has been helpful. Please let me know below in the comments if you have any questions and I will try my best to answer them. If you would like to have my next articles delivered to your inbox fortnightly on a Monday morning, then please subscribe to the mailing list.

Next fortnight, I will cover all the last little tricky acronyms you see on lens names, and I will go over a few of the more obscure lens types out there.



Example A: Fixed aperture - f:2.8

Example B: Fixed aperture - f:4

Example C: Variable aperture - f5 to f6.3

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