In the first of the Back to Basics series of posts we looked at what the numbers on the front of your lens stood for and we came to the conclusion that these help us to understand the lens’ focal range and maximum aperture at any given focal length. This led to the question of what is Aperture…
Keeping things simple, aperture is essentially the size of the opening through which light passes and is a function of the lens. There are a series of blades inside each and every lens which based on either a value you dial in (In aperture priority and manual modes) or which the camera sets (In Shutter Priority) constricts or dilates to effect the amount of light entering through the lens and onto the sensor of the camera.
We can get very technical on this but I’ll keep some of that for later on.
Yes, the aperture (opening through which light passes) controls the amount of light entering through the lens but this is not the most important aspect of aperture.
Aperture, through changing the manner in which light passes through the opening, ultimately determines how much of a scene is rendered as being sharp and in focus and how much is soft and blurred. This range of acceptable sharpness is what we refer to as depth of field and is one of, if not the most, powerful tools you have as a photographer.
Essentially, you have the choice to either completely isolate your subject from a busy background (Shallow Depth of Field) or include more detail in the background (greater/larger Depth of Field).
Notice how much depth there is in the second image compared to the first image where only a small portion of the scene is sharp and in focus?
Small Number = Shallow Depth of Field
Most photographers go weak at the knees when we see images with that creamy blurred background (technically referred to as bokeh) which is created as a result of aperture values which result in a shallow depth of field. Photographic subjects appear to pop out from the background and leave our viewer with almost no distracting elements to draw their eye away from the subject.
Does The Resultant Depth of Field Remain the Same at Any Given Aperture?
No. This is where things become a bit more interesting as there are a couple of variables which have an impact on the depth of field you’re able to achieve at any given aperture value.
- Your Focal Length. Greater focal lengths (eg 400mm) essentially compress a scene and result in shallower depths of field when compared to wide angle lenses (eg 35mm). F8.0 on a wide angle lens may give you a couple of meters of DOF on a 16mm lens whilst F8.0 on a 600mm lens may result in a DOF of a couple of centimetres.
- Your proximity to the subject. A bit like focal length really but the closer you get to your subject, the shallower the resultant depth of field given a constant aperture. Think about macro photography where even aperture values of 22 result in a fraction of a centimeters worth of DOF. This is because of the combination of focal length (usually 105mm) and the proximity to the subject (usually a couple of centimeters).
It should make sense now why lenses with a maximum aperture of F2.8 and F1.8 are so popular for portraiture and wildlife photography. The small number (Maximum aperture which infact represents a large opening) allows for beautifully blurred backgrounds (and foregrounds) which isolate the subject from the background.
However, these lenses come at a cost and are not always accessible to everyone. Understanding that you can maximise the blur (achieve a shallow depth of field) by dialling in the smallest number possible, getting as close to your subject as possible and zooming in as much as possible will help you achieve similar results even at a maximum aperture of say 5.6.
Since everything behind your subjects becomes progressively more blurred the further away it is, it also makes sense that you place your subject as far away from the background as possible to try and blur it out as much as possible.
You can also check out this post to see how to achieve similar results to those beautiful lenses with a maximum aperture of F2.8.
Does the DOF start at the exact point where Focus is Achieved?
Yes and no. Once the camera has achieved focus, the point where focus has been achieved will be inside your range of acceptable sharpness. However, the total DOF is actually distributed 1/3 in-front of this focus point and 2/3 behind this focus point. It looks something like this:
So, it is vital, especially when you are shooting with larger focal lengths or are very close to your subject that you focus in the EXACT right spot to ensure that your subject is rendered sharp and in focus.
This also gives you something to think about in terms of where you should focus if you have multiple subjects at differing focal planes (eg two people standing behind your first subject). Would you choose to focus on the front or back subjects?
Give it some thought!
That leads us to the next topic:
How does the camera achieve focus and how can I control this?
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