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I’ve been quite keen to understand more about how to be ‘arty’ with photography, and have been experimenting with depth of field.

From what I’ve learned so far, the easiest way to alter depth of field is to switch the camera to Aperture Priority (Av), which allows you to manually set the Aperture.

My EOS 450D with the 18-55mm kit lens allows me to set apertures between F3.5 and F22. To see what the effect is of the different apertures, I set up a simple shoot and altered the aperture to see the results.

Firstly, I took a ‘Control’ shot - this was with the camera in full auto mode - the camera picked the aperture, shutter speed, ISO and also controled the autofocus. In this case I set it to a single-point autofocus on the centre.

The resulting image shows the background slightly blurred, as the camera has focused on the Beetle in the foreground. With the full 9 autofocus points active, the background would have been sharper, but that was not the point of the shot.

Looking at the statistics for the above image, the camera calculated the following was best for the shot:

Shutter: 1/160
Aperture: F10
ISO: 200

Next, I took a second ‘control’ shot. This is the camera in Macro mode. This should focus more on the subject and isolate it. Again, a single point of Autofocus was used.

We can see already that the colour balance of the shot has changed, and the grain on the table is more pronounced. The background is more blurred - you cannot make out the grouting on the wall, nor can you easily see the palm near the back of the garden.

So what did the camera say?

Shutter: 1/500
Aperture: 4.5
ISO: 125

This makes sense - with a larger aperture, the shutter speed is higher, and also the film speed.

So, what happens when we take control of the Aperture?

In order to get the maximum aperture, I had to alter the focal length from the subject - I can only get the largest aperture when fully zoomed out, so I moved the camera slightly closer to the subject. In some ways, this nullifies the control shots, but this was never meant to be a serious experiment!

Shutter: 1/1000
Aperture: 3.5
ISO: 200

This shot is only subtly different from the Macro mode one. Without any scientific basis, the background is about the same level of burredness, but on close inspection between the two images, the rear wheelarch of the beetle has just started to lose focus. The faster shutter speed reflects the fact that there is more light getting through the aperture, but interestingly, the camera has reverted to an ISO of 200.

I then took a series of shots, reducing the aperture (or increasing the F value, depends on how you want to look at it) and recording the results:

Shutter: 1/500
Aperture: 4.5
ISO: 200

Shutter: 1/250
Aperture: 6.3
ISO: 200

 

Shutter: 1/200
Aperture: 8.0
ISO: 200

Shutter: 1/100
Aperture: 11.0
ISO: 200

 

Shutter: 1/25
Aperture: 22.0
ISO: 250

We can see that by the last photo, the entire image is in shaper focus than the autofocus shot. This is possibly partly due to the slight change in angle, but individual bricks on the wall are clearly discernable.

Interestingly, the 4.5 aperture shot, which is comparable to the macro control shot, uses a different ISO when I controlled the aperture rather than the camera. Again, this is partly down to the change of angle, but also with the change of lighting conditions.

As the aperture size decreases, the shutter speed increases. I’m not sure how the aperture rating works, but when I halve the aperture size from F11 to F22, the shutter speed increases by a scale of four - not half as one might expect.

More to come as I start to learn more about aperture - Weather permitting, I will be taking some photos up Snowdon, which from my findings here should ideally be taken with a small aperture.