Photography tips – depth of field

Today I’m going to do something a little bit different, and share some basic photography tips. If folks like this idea, I may expand it into a recurring theme. I’m going to tackle a useful subject when it comes to photography, that being depth of field and how to manipulate this when taking pictures to get the results you want.

The first question of course, is what is depth of field? Put simply, it’s all about how much of the shot you are taking is in focus. The best way to demonstrate what I mean is with a couple of photographs.

Beer bottle higher depth of field f/22 1/30th sec

Beer bottle lower depth of field f/2.2 1/3200th sec

 

 

 

 

 

 

 

 

 

 



As you can see, these images are essentially the same in terms of framing and light, with the key difference being the focus. The image on the left is largely in focus, whereas the image on the right has only the beer bottle clearly in focus.

This draws the eye away from the distracting detail in the background and toward the subject that is being captured, in this case a rather fine bottle of German beer. This technique is particularly useful for portrait shots, but can be used in a whole range of scenarios.

So how do we control focus then? Some basic camera knowledge for you first. A camera works by capturing visible light and recording this onto some medium. Much like an eye, the camera can control how much light goes in through the lens and onto the medium, to ensure that the shot is correctly exposed. Too much light, and you end up with a photo that is just washed out whiteness, too little light and the image will be too dark to use. My beer bottle kindly consented to posing for examples of under and over exposure, as can be seen below.

Overexposed beer bottle

Underexposed beer bottle

 

 

 

 

 

 

 

 

 

 



The camera has two mechanisms for controlling the amount of light it lets in. The first is the shutter speed, which controls how long the camera shutter is open. Imagine this like the eyelid – you can control how long you open your eye for. The longer the shutter is open, the more light will get in. The downside to this is that you need to hold the camera still, so unless you have a tripod you don’t want to leave the shutter open for too long or the image will end up blurry.

The second mechanism is the size of the hole that the camera opens to let the light in, which is known as the aperture. Again, using the eye as an example, this can be thought of as the pupil of the eye. When it is darker, your pupil is bigger to let more light in, when there is more light, the pupil is smaller.

A camera can use different settings of aperture and shutter speed to control how much light is let in. If you increase one, you decrease the other, and vice versa. A fast shutter speed would result therefore in a larger hole, and a lower shutter speed would result in a smaller hole.

The aperture on a camera is measured in “f” stops, a number system that goes from a smaller number to a larger number. Slightly confusingly, the smaller number means that the hole is bigger, and a larger number means that the hole is smaller. So a camera set at say f1.8 would be letting a lot more light in than a camera set at f22.

At this point you may be thinking, well, why not just leave the hole as big as possible which would let you use higher shutter speeds to avoid image shake? And the answer is: depth of field.

Depth of field is directly controlled by the size of the aperture. More depth of field (more objects in focus) can be achieved by having a smaller hole (high f-stop number), and less depth of field is achieved with a larger hole (low f-stop number). If you mouseover the two images of the first two beer bottles above you will see how the shots differ in terms of shutter speed and aperture.

So when you are taking a picture where you only want your subject to be in focus, you would choose a higher shutter speed and a lower aperture. This is not something you can directly control on many cheaper point and shoot cameras, but as you move to more expensive options you will find that you can control these options more fully. SLR cameras, for example, should let you have full control over the aperture settings.

If you have an SLR, you should be able to set the camera to “Av” mode, which allows you to set the aperture manually, and the camera will set the shutter speed according to the amount of available light. If you play around with this, you will see how changing the aperture will change the shutter speed, and the results will be images with differing depths of field.

Hopefully this post was interesting or informative. If you have any questions about what I have posted, feel free to post below in the comments and I will do my best to answer!




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