Have you ever noticed that professional photographers seem to be able to create imagery with only a very small area that is in focus? We call that a shallow depth of field (DOF) and sometimes, that area of critical focus can be as narrow as only a millimeter or two.
Utilizing a shallow depth of field in an image requires the photographer to be very careful about dialing in the focus because even a minor adjustment can make the difference between a beautiful portrait and one that is destined for the recycle bin. When it all comes together, the background is pleasantly out-of-focus with pleasing bokeh, and that is a tried and true way to simplify what might otherwise be a busy background. The key to creating this type of imagery comes down to just a few factors: the type and setting of the lens, the distance from the camera to the subject, and the sensor size. Last month I wrote about how the lens aperture plays a role so today I’d like to follow that with a look at the other factors.
To begin, there is a broad range of sensor sizes in cameras on the market today. For example, my point and shoot camera (Canon PowerShot G16) measures in at 7.44 mm x 5.58 mm while my pro camera (Canon 5DS r) is 36 mm x 24 mm. It is possible to create images that have a similar depth of field regardless of the camera, but unfortunately, it isn’t simple to make all things equal between cameras – especially as the sensor size changes. Practically speaking, that means it is much easier to create images with a shallow depth of field as the sensor size increases.
The difficulty with the smaller sensor size comes in the way that we frame our compositions. That is, in order to fill the frame, we use an entirely different combination of factors to achieve a similar result. Even when we use the same wide aperture setting, we either have a difference in the distance to the subject or a change in the relative focal length of the lens, both of which can have an enormous impact on DOF. In the two examples of Spring below, I used a consistent aperture setting of f/1.8, I used a similar framing for each composition, and I held the camera a similar distance away from her face yet the difference in both the depth of field and perspective is quite noticeable. The G16, with a smaller sensor, rendered the scene with quite a bit more detail than the 5DS r. The difference comes down to the effective focal length. Effective focal length is a way to compare the angle of view captured by two camera systems when they have different sized sensors. This difference is calculated as a ratio and is often referred to as crop factor.
I used a 50 mm lens on my 5DS r, but to achieve a similar composition, the focal length on the G16 was equivalent to a 28 mm on the 5DS r. A 28 mm (or 28 mm equivalent) focal length is considered wide angle and one of the characteristics of a wide angle lens is that it has a relatively greater DOF when compared to lenses in the normal to telephoto range (50 mm and beyond).
I’ll leave you with one last thought – an image doesn’t need to have a super shallow depth of field in order to be a great portrait. The important point is that you need to understand and embrace the limitations of your system. Once you do that, you’ll be better prepared to create your best photography.