One of the key creative factors in photography, is Depth of Field (DoF); this is essentially all about how much of the scene is in focus, and how much of it is blurred. This blurred effect can most commonly be seen in portrait photography where the background behind the subject is commonly blurred, as this helps to draw attention away from the background and towards the subject. You may know that DoF is affected by aperture, but did you know that there were actually FOUR factors which affect the DoF? In this newsletter we look briefly at what DoF is, and the four factors which affect it.
What is Depth of Field (DoF)?
When you focus on a subject, there is an area in front of the subject (towards the camera) and an area beyond the subject (away from the camera) where the photo will be in acceptable focus. Outside of this area (towards you and behind the subject) will be areas that are increasingly out of focus, the further you move away from the subject. The area of acceptable focus is called the Depth of Field (or Depth of Focus). The DoF is created by the physics of light as it enters the lens.
In the graphic below, the camera is made to focus on the person (the focal point), creating a zone of acceptable focus in front of, and behind the person (known as the Depth of Field, or Depth of Focus). This is often referred to as being 'shallow' or 'deep'; for what this means, read on.
Aperture
Aperture is the first factor that affects Depth of Field. A large aperture (e.g. f2.8) produces a shallow depth of field (more background and foreground blur) and a small aperture (e.g. f22) produces a deep depth of field. For this reason, large apertures are frequently used in portrait photography (where a blurred background is desirable) and small apertures are generally used in landscape photography, where generally we want as much as possible to be in focus from the front to back of the photo (but not always!).
The starting point of understanding DoF is to learn how to shoot on Aperture Priority mode, where you can change your aperture and the camera manages other settings to ensure you get a well exposed image. Now you can take photos of an object with different apertures to see how the aperture changes the DoF and the amount of focus blur that you can achieve. The diagram below shows how aperture affects DoF and therefore the amount of background blur.
Lens focal length
The next factor that affects the DoF is the focal length of the lens. Wide angle lenses (generally seen as lenses with a focal length of less than the focal length of human eye sight, around 50mm) have a deep Depth of Field. Telephoto lenses (those with a focal length greater than 50mm) have a narrower Depth of Field. Note that this is ONLY true as long as the distance of the camera to the subject (the focal point) is the same. In this case, a telephoto lens will produce a shallower Depth of Field (a more blurred background) than the wide angle lens.
Notice how in the sequence below the background gets increasingly blurred as the focal length increases in stages from 24mm to 200mm.
Camera to subject distance
The next factor that affects the DoF is the distance between the camera and the subject (the focal point). As the camera is moved closer to the subject, the DoF is decreased, therefore the background will begin to appear more blurred. Note that all lenses have a minimum focus distance, so there will become a point where the lens will no longer be able to achieve focus on the object (this is why there are special macro lenses which have the ability to focus closer the subject).
This factor is the reason why macro photography can be very challenging; focussing at such close distances means that the Depth of Field is so shallow that even though the head of a bug may be in focus, its rear end may still be blurred!
Notice how in the sequence below the background gets increasingly blurred as the camera is moved from 2m away from the object (the focal point) to just 0.5m (with the focal length kept the same at 24mm)
Sensor size
The final factor that affects Depth of Focus is the size of the sensor. Essentially full size sensors have shallower Depth of Fields than crop sensors (such as APS-C or Micro 4/3 sensors). Note that this is ONLY true when the other factors are the same, including the focal length.
Note that the effective focal length is also affected by the sensor size (i.e. a 24-70mm lens is NOT a 24-70 lens on a APC-C sensor camera; a 24mm focal length lens on the APS-C sensor, is actually a focal length of 35mm: The convention is that lenses are marked with focal lengths as if they are on a full size sensor, even if they were designed for a crop sensor camera and therefore in reality achieve different focal lengths! Confusing eh?
When I am doing portrait or pet shoots, I therefore use a full frame camera, and a telephoto lens; because the combined affect achieves a much more effective blur in the background (see image below).
Distance between subject and background
Hold on, I thought there were only 4 factors and this is a 5th?!?! Actually, this one doesn't change the DoF, but it does affect how blurred the background is behind the subject. As blur increases the further behind the subject you go, this means that the further the background is from the subject, the more blurred it will be. So, if you are taking a photo of a person, or a flower, and you want a blurred background, you can increase the amount of blur by choosing a location where the background behind the subject is as far away as possible.
Final words
Depth of Field is quite a complicated topic, and is best understood by experimenting. Hopefully it far easier for you to experiment now you have a basic understanding of what's going on!
Why bother? because selective blurring is a powerful tool for creative control in photography; both in portraits which are significantly enhanced by background blur, but also in macro and landscapes, where selective blur can help to simplify a scene and enhance the main subject. This was so important, that the people who design phone cameras, which don't have widely variable apertures, wrote software to identify the subject and blur the background - it's called Portrait mode! You will notice though, that unlike a proper camera, phones can't blur the foreground....
Want a learn more about photography?
If you're a beginner photographer who is local to East Devon and want to learn the art of landscape photography, check out my range of local photography courses. Alternatively why not make a weekend break of it and attend one of my residential photography workshops - either my beginners landscape photography masterclass for those starting out in photography, or my intermediate photography masterclass for those who already understand the basics and want more practical help progressing their landscape photography.
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