Depth of Field and Aperture Priority
A small area of focus is called a shallow depth of field and a larger area of focus is referred to as a large depth of field.
Portrait photographers who want the focus to be on their model often use an aperture that gives them a shallow depth of field. The model’s eyes are usually tack sharp as that is usually the focal point. The area around the face of the subject will often have a pleasing blur known as ‘Bokeh’. Bokeh is the quality of the out-of-focus parts of the image. It is possible to have good and bad bokeh. This is the reason why a lot of photographers will spend a lot of money on good lenses, as they tend to produce better bokeh, which is creamier and more pleasant to the eye.
Aperture priority is a great setting if depth of field is important to you, as you can focus on setting the aperture that you want and the camera will set the shutter speed for you. If you want a shallow depth of field then you need to choose a large aperture. Contrary to what you might think this will be a small number for example F1.4. This is because F-stops are fractions and so a smaller number is actually a larger aperture.
It is good to remember this:
Small F-stop number = large aperture = more light through the lens
large F-stop number =small aperture=less light through the lens
It is worth experimenting by setting different apertures to see how they affect your shutter speed. You will find that a large aperture will allow you to have a faster shutter speed than a smaller aperture and this is because the large aperture (opening) lets in more light. A smaller aperture means you will have to have the shutter open for longer as a smaller aperture only lets in a small fraction of light.
Practicing is what will help you get your head around these concepts, as it is usually something that a lot of people find confusing at the start.
Here is an exercise to start off with:
1. Mount your camera on a tripod and choose the lens you would like to do this experiment with. Choose a focal length on that lens (if there is more than one focal length)
Don’t worry too much if you don’t have a tripod. Do use one if you have one, as your results will be easier to read and more exact. It will also mean that you don’t need to worry about the shutter speed being too long and causing blur.
2. Set your camera to aperture priority and set a large aperture (small F-stop number)
3. Set your lens to manual focus and focus on one thing that you want to be the main subject in your photograph.
4. Record on a piece of paper how far you are from your subject, what you are focusing on, what lens you are using, what focal length, what image number this photo will be (for referencing later) and what aperture you are using.
5.Take your picture
6.Repeat the steps above but choose the next aperture up. Keep doing this until you have used all your available aperture settings
To see how your focal length, lens and distance to subject affect the depth of field, repeat the above experiment but change the distance to the subject or the focal length.
Make sure that you record everything so that you can later see how the differences affect your images.
Above all have fun learning and getting your head around the concept of aperture priority and depth of field.
Enjoy your creativity!
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