As I'm updating the Buying Guide and expanding the information on MFT, one of the most frustrating aspects is the lack of great portrait lenses for the system. It's keeping me from recommending micro four-thirds for portrait work, but it's not a problem inherent with the system--it's just that manufacturers haven't bothered to make a real portrait lens. I actually feel like the problem is caused because many camera buyer's don't understand how sensor size and crop factor affect background blur.
Anyway, I'd love to get your thoughts (and some double-checking on my math) on this new section of the Buying Guide, which I'm writing because I've heard from so many people frustrated by their inability to get the same background blur that they see from the full frame cameras.
Depth of field factor
Different sensor sizes change the field of view provided by a focal length. We refer to this as the “crop factor”, and it allows us to quickly determine that a 45mm micro four-thirds lens is equivalent to a 90mm full frame lens.
While the crop factor works for determining the field of view, it does not work for determining the depth of field and background blur that you’ll get from any given lens. While camera manufacturers often provide a “35mm equivalent” when describing a lens, they don’t tell you that you won’t get the same background blur when using smaller lenses, and this has led to many frustrated portrait photographers.
For example, consider two portrait lenses that seem very similar:
· The full frame Canon 85mm f/1.8 ($400)
· The micro four-thirds Olympus 45mm f/1.8 ($400)
Knowing that you double the focal length of micro four-thirds lenses to determine the 35mm equivalent, the Olympus seems to compare favorably to the Canon. You might see portraits taken with the Canon 85mm f/1.8 (such as the following), and assume that you’d be able to achieve similar background blur.
The Olympus can’t achieve the same background blur, however, because you must apply the crop factor to the aperture to calculate depth-of-field (and thus background blur). In this example, the Olympus 45mm f/1.8 lens is equivalent to a full-frame 90mm, f/3.6 lens when considering both field of view and background blur.
You can also multiply the depth-of-field by the crop factor. Therefore, a micro four-thirds camera with a crop factor of 2x, has about twice the depth-of-field (and thus half the background blur) of a full frame camera, even after you multiply the focal length by the crop factor. An APS-C sized DSLR has 1.5X to 1.6X more depth of field, or 50-60% less background blur than a full frame camera.
For calculating the shutter speed you’d need in any given lighting scenario, you wouldn’t need to multiply the aperture—the Olympus would still have the same shutter speed as a full frame 90mm f/1.8 lens, or any f/1.8 lens, for that matter. However, for portrait work, lenses for smaller sensors have far less background blur. To achieve similar background blur to the Canon 85mm f/1.8, you would need a 45mm, f/0.9 lens, and nothing like that is currently available.
Currently, the best micro four-thirds lens for achieving a nice background blur is the Olympus M Zuiko ED 75mm f/1.8 ($830). For calculating background blur, this lens is equivalent to a full-frame 150mm f/3.6 lens. Unfortunately, that doesn’t compare favorably to traditional 35mm portrait lenses. My budget full frame portrait recommendation, the Tamron 70-200 f/2.8 ($750), is just as sharp, less expensive, offers much better background blur, and provides a very useful zoom range.
Let’s consider that lens on crop and full-frame DSLRs. On a Nikon DSLR with a compact sensor, it becomes equivalent to a 105-300 f/4.2. On a Canon DSLR with a compact sensor, it becomes equivalent to a 112-320 f/4.5. Only on a full-frame body will you be able to achieve the full potential of the lens’ ability to blur the background.
I don’t want you to feel bad about purchasing a micro four-thirds or APS-C camera; they’re very capable cameras, and background blur is only one aspect of photography. Smaller sensors, and their large depth of field, actually show you much more of a scene, making them ideal for landscapes. There are also other ways to control background blur, including moving your subject further from the background. For detailed information, refer to Chapters 4 and 6 of Stunning Digital Photography.
I hope that highlighting this weakness of smaller sensor designs for portrait work will help push lens manufacturers to offer faster lenses for the smaller sensors. The first manufacturer to give fair treatment to APS-C sized sensors is Sigma, with the Sigma 18-35mm f1.8 lens. When calculating background blur, this lens is roughly equivalent to a 27-52mm f/2.8 lens. It’s still too wide angle to make a viable portrait lens, but it’s an amazing lens for general photography with compact sensors, and I'm glad to see Sigma manufacturing faster lenses for smaller sensors.