Basics - Understanding Aperture

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This is to give you an idea of aperture sizes changing with f/number changes. These are not to size in any way.

This is to give you an idea of aperture sizes changing with f/number changes. These are not to size in any way.

It seems one of the trickiest aspects of photography to get your head around is the aperture of your cameras lens, be it a highly expensive pro styled lens or the zoom on your point and shoot (most point and shoots will control aperture for you), the aperture of the lens will control aspects of your photography in the same way. Hopefully after reading this short guide you will have a basic understanding and be able to take it further in your photographic life.

The Basics

You may have heard people talking about fast lenses or stopping down their lens, mentioning f/numbers or many other examples (I?m not going to go through all the possibilities you may have coime across). But what do they all mean? Well they are all talking about apertures.

You need to understand that photography in its base form is about capturing light; and light changes! Whether it is natural or not, different scenes or areas will have different levels of light. In order to deal with these as a photographer we need to be able to control the input of this light, and one way of doing this is with the aperture on a lens, this controls the amount of light that is taken into your camera through the lens (among other things). The other way would likely be through shutter speed (a subject not covered on this page).

Right, clear enough? No? OK, well, for your photograph to work you need a certain amount of light hitting your camera sensor (or film if you still work in that medium). Look at it in terms of putting water into a jug (a strange analogy I know but bear with me) the water represents light and the tap represents your lens. If you needed 100ml of water (1 second of light), you turn the tap on for the length of time it takes to get that volume of water, we'll call it 1 second (on your camera press the shutter release for one second). If you wanted twice as much water (light) you have two options; increase the length of time you had the tap on to 2 seconds (have your shutter open twice as long, 2 seconds); or you keep the length of time the tap was on the same (shutter open 1 second) but make the hole that the water comes out of (your aperture) larger, so that double the volume of water (Light) is dispensed.

I?m hoping that is clearer, the aperture is basically the hole in your lens through which light passes to hit your sensor/film.

The f/number represents the size of the hole. Strangely a lower number means a larger hole (so f/1.8 is a much larger hole than f/22) I know this seems odd, but this is called wide open, having your lens set to the smallest f/number. Each increase in number decreases the amount of light coming into the camera by half (an example of aperture numbers ? f/1.2, f/2, f/2.8, f/4, f/5.6, f/8 ?.) The smaller the number the more light comes in through the hole and the ?faster? the lens is. It is worth noting that many lenses use 1/3 f/stop increments e.g. f/1, f/1.1, f/1.2, f/1.4, f/1.6, f/1.8, f/2 so the hole does not grow by one stop of light for each of these (I hope that is clear).

Depth of Field Example

 Depth of Field Example

Depth of Field Example - To show you depth of field differences. İAshley Beolens

Fast Lens

A lens will be known as fast if it has a large aperture as its maximum, low f/number (f/1.2, f/1.4, f/1.8 etc.). This is because the lower the number the larger the hole and the more light can enter the camera lens, this means that shutter speeds can be much quicker (hopefully that is one question cleared up).

Fast lenses are great for low light (music concerts, indoor events etc.) or fast paced action (sports, wildlife etc.). If you find yourself photographing subjects in these situations you will be glad of a large aperture.

Depth of Field

The other aspect that is controlled by aperture is the amount of your subject that is in focus ?Depth of Field. The larger the hole the less of an area will be in focus and vice versa. Landscape photographers will quite often work with large f/numbers, smaller apertures (f/22+). This is because they get a greater depth of field (as in more of the image will be in focus). Other people like to have a solo subject really sharp while the background is blurry so they will use a smaller f/number, larger aperture, for a shallower depth of field (f/2.8 etc.).

Using depth of field is a great way to play with your photographs (for those not familiar with full manual settings try aperture priority settings; you set the aperture, your camera sets the shutter speed (refer to your manual for which setting this is). Some interesting images can be had with a manually worked depth of field (don?t just assume that shallow is best, sometimes you would be surprised).

Stopping down

'Stopping Down' is a term used quite a lot in photography circles, from forums to magazines, so it is worth me explaining this term briefly as it can be confusing. ?Stopping down? means to decrease the size of the aperture by increasing the f/number.

To sum up (and just to confuse people more) a wide open lens will have a large aperture, with a small f/number and a shallow depth of field, stopping down one or two stops will decrease the size of the aperture by increasing the f/number. All perfectly clear, I?m sure you will agree.


When it comes to using aperture you will need your camera set in manual mode or the partial manual mode of aperture priority to benefit best from the understanding you have (hopefully) gained with this tutorial.

Manual Mode speaks for itself, you control everything in the camera from aperture to shutter speed.

Aperture Priority in this mode you select the aperture you want to shoot at and the camera will choose the best shutter speed (this mode is great for action shots where you know the depth of field you want but altering shutter speeds etc while on the fly could prove complicated).

Final point

I hope that has given you a clearer understanding of Aperture, I?d love to hear from you if it has helped or if it is too complicated (I can ramble sometimes), so please get in contact.

Finally if you only remember one thing from this entire article, remember this:

Small f/number (f/1.2) = Large aperture (hole) = Shallow depth of field and more light entering the camera (so shorter shutter speed).
Large f/number (f/22) = Small aperture (hole) = Deep depth of field and less light entering the camera (so longer shutter speed).


This article was originally written as a basic understanding of aperture, I chose not to go into great detail about other variables such as focal length as I felt that while important it wasn't a great factor in the actual understanding of what is meant when people talk about aperture. So far I have only had positive comments about how it has helped people understand aperture where they couldn't before.

That being said, I have been asked a few times for a more complete description of what things mean so this update should help clear these things up.

F/Number, what it means

When looking at f/numbers we must remember that they represent a ratio where the F equals focal length and the number equals the relative diameter of the effective aperture. Think of it like a fraction, f/4 will represent one quarter so a 100mm lens set at f/4 will have a 25mm aperture but at f/2 it would have a 50mm aperture or at f/10 would have a 10mm aperture. This is why the larger the number the smaller the hole, after all 1/2 is more than 1/22. (I guess this helps explain why "faster" lenses are bigger, a much larger effective aperture is needed).

Focal Length

This factor leads to the second aspect that I did not originally cover, that focal length will play a large part in how the aperture affects depth of field. As the above equation hopefully leads you to see a 100mm lens at f/4 will have a very different aperture size to a 50mm at f/4 (100mm at f/4 equals 25mm aperture, 50mm at f/4 equals 12.5mm aperture). This then means that the 100mm will have more of the image in focus than the 50mm would.

I hope this clears up a few of the missing elements from the original, please do feel free to let me know if you prefer this or the original, or if you think I am still missing elements.

I advise reading this article in conjunction with our other basic articles ? Understanding ISO, Understanding Shutter Speed and Understanding Exposure.

Ashley Beolens