Aperture is the physical hole in the lens, changing the aperture opens and closes the hole in the lens, in the picture on the right you can see the hole and the “blades” around it that are used to make it smaller or bigger.

The Aperture size is measured in a unit called f number – a larger f number means smaller aperture (large number = smaller hole – yes, it’s reverse, but that’s how this is measured). On most DSLR cameras each “click” of the aperture wheel is 1/3 stop – so 3 clicks will double or halve the light coming into the camera (depending on the direction you turn the wheel, obviously)

The most important effect of the aperture is the bigger the hole in the lens the more light comes in – and wen more light come in we either get a brighter picture or we can compensate by changing the shutter speed or ISO – for example  by changing the aperture from f/8 to f/5.6 (3 clicks) we double the amount of light coming in – so we can compensate by halving the shutter speed, so if I go shooting handheld with my 50mm lens in f/8 and the camera tells me I need a shutter speed of 1/40 – at that shutter speed it’s really difficult to take a good picture handheld and it’s very likely I’ll get a blurry picture – but changing the aperture to f/5.6 will get me a shutter speed of 1/80 – in that settings it’s pretty easy to shoot handheld with a 50mm lens.

Quick tip – the aperture and shutter speed camera controls are usually set in the same increments so a single aperture “click” change the exposure by exactly the same amount as a single shutter speed “click”.

Aperture also controls the “depth of field” (background blur) of the picture, larger aperture (smaller f number) means shallower depth of field (less in focus and more background blur), let’s demonstrate this with some pictures.

Nothing says aperture like a stuffed lion toy  – so I’ve placed a toy lion on top of a trampoline (really) balanced on a plastic slide in front of a bamboo fence, I’ve put the camera (Canon 550D/T2i) with a 50mm f/1.8 II lens on a tripod, focused on the lion eyes and set the camera to manual focus, the white balance is set to daylight and the pictures ware taken during sunset with the sun behind the camera, I was pretty close to the lion because I wanted a very shallow depth of field in order to exaggerate the aperture effects), the pictures were taken on Av mode.

Here are the pictures, almost strait out of camera with no post processing what so ever (the pictures on this page are scaled down, click on the picture to see a bigger version that much larger but also scaled down a bit):

The first picture (right) is at f/1.8 1/100sec ISO200, the lions nose is super-sharp, the eyes are a little soft (you can see it in the full size version – I probably accidently bumped the tripod a little and moved the focus from the eyes to the nose – the depth of field is so shallow with the lens wide open) the ears are obviously out of focus even in the scaled down web version and the background is a nice undefined wavy pattern.

In the second picture (center) I stopped down to f/2.5 (1/50 ISO200), the eyes are not as soft as the f/1.8 version but still out of focus.

In the next picture (right) at f/3.5 1/20sec ISO200 the eyes are finally fully in focus, the ears are still clearly out of focus, the background is completely blurred but you can already guess it’s a bamboo fence.

Stopping down again we get to f/5 (1/10sec ISO200) on the left and then to f/7.1 at 1/2sec on the center. On the right at f/10 0.4sec ISO200 the downscaled version is already sharp with maybe just a tiny amount little blur in the background, in the full size you can see the background is still blurred (pictures become sharper when you scale them down) but the entire lion in completely in focus.

The background continues to get more details at f/13 0.6sec (left) and the last picture I took is at f/20 1.6 seconds (still ISO 200, on the right).

We clearly see that increasing the f number (or closing the aperture, also know as “stopping down”) – increases the depth of field, in each picture more of the lion is in focus and the background becomes more defined.

And we also clearly see that id we get the depth of field too shallow we take the risk of missing our focus – and your model might not be very happy you’ve chosen the tip of her or his nose as the focal point of the photo.

There’s another effect of the aperture size I want to talk about quickly – with large aperture (small f number) lights seem to glow, with small aperture (large f number) they become star shaped, I’ve taken some photographs of the lights above a highway near my home to demonstrate this.

This time I used my EFS 18-135mm f/3.5-5.6 IS lens, again the camera was on a tripod and the white balance was set to daylight (I had no idea what’s the color of the lights and didn’t care if the colors are off as long as they are consistent – but I’m happy with the results) the pictures were taken at night the lens was zoomed all the way to 135mm and on manual focus, the pictures were taken on M mode.

Here’s a picture with the aperture wide open at f/5.6 1/20sec ISO200 – you can see the strong round-ish glow around each lamp and the huge halo around the entire thing (I really love the “molten metal” look of the pole)

Here’s another picture of the same lights with the lens stopped down to a ridicules f/36 with 1 second exposure at ISO 100, now the big halo is gone and each light has a very nice “starburst” effect – in this experiment I didn’t keep the exposure constant between pictures if you do the math you can see I’ve closed the aperture by 5 and a third stops, lowered the ISO by one stop and reduced the shutter speed by 4 and a third stops – resulting in a two stop difference between the two shots (or in another words, the second picture has only one quarter of the light in the first picture)

And finally, if we combine both settings by opening the aperture to f/5.6 and setting the long 1 second shutter speed (at ISO 100) it looks like our street light is now powered by a thermonuclear device (this is 5 and a third stops brighter the previous photo – or approximately x39 the amount of light)

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