Model Georgia – D750/Sigma 135mm f1.8 ART @ISO50, f2.2, 1/4000s
One of the main reasons that photographers prefer larger sensors, is that gives them the ability to create a very shallow depth of field (DoF) affect, so that portraits in particular really ‘pop’ and separate from a creamy out of focus background. It gives photos that professional ‘3D look’ that.
What is ‘bokeh’?
In photography, bokeh is the aesthetic quality of the blur produced in the out-of-focus parts of an image produced by a lens. Bokeh has been defined as “the way the lens renders out-of-focus points of light”. Differences in lens aberrations and aperture shape cause some lens designs to blur the image in a way that is pleasing to the eye, while others produce blurring that is unpleasant or distracting—”good” and “bad” bokeh, respectively. Bokeh occurs for parts of the scene that lie outside the depth of field. Photographers sometimes deliberately use a shallow focus technique to create images with prominent out-of-focus regions
There are many factors that affect the amount of bokeh. They are:-
- Aperture of lens (wider makes more ‘blur’)
- Focal length of lens (longer makes more ‘blur’)
- Distance of camera to subject (closer makes more ‘blur’)
- Distance of subject to background (greater makes more ‘blur’)
- Size of sensor (larger makes more ‘blur’)
This doesn’t take account of ‘quality of bokeh’, which is influenced by additional factors like the number of aperture leafs (more are generally better) and the micro contrast of the lens, which is down to the grade and quality of the glass. These last factors are not not part of this first test, as I want to focus on the different amounts of bokeh you can achieve from various lenses of various focal lengths across the MFT and FF formats.
Crop Factor – How it affects DoF
The size of the sensor is the main reason why full frame cameras are capable of a shallower depth of field in photos. In the past few years, the micro four thirds producer (mainly Olympus and Panasonic and some third party producers like Sigma) have been trying to reduce the difference, by releasing a series of super fast (f1.8 ~ f1.2) high performance prime lenses. Most people appreciate that on a ‘crop sensor’ like MFT, the full frame equivalent focal length of an MFT lens is double. In other words, in my 42.5mm Nocticron lens on my Olympus, produces an image in the viewfinder that looks a similar size as my 85mm lense on my full frame Nikon. This is because the full frame sensor (36x24mm) is exactly twice the size of my Olympus MFT sensor, so this is known as a 2x crop.
However, that same crop factor also affect the depth of field by an equivalent of 2 STOPS. What does that actually mean? In terms of the two lenses I just mentioned, the 42,5mm Nocticron, which has a maximum aperture of f1.2, will produce very similar images to the 85mm Nikon when the Nikon is stopped down to f2.4. A little known fact is that although the progression of f stop numbers advances in a seemingly random fashion to anyone other than seasoned photographers, if you want to add 2 stops to any f stop number, you just double it. So 1.2 x 2 = 2.4. There are some pro build MFT lenses that offer a constant maximum aperture of f2.8, which is certainly fast enough to let lots of light in, but in terms of depth of field, it’ll produce a shallowness similar to a full frame lens set to f5,6 (2.8 x 2), which is similar to most DSLR kit zooms.
While shallow DoF is not everything in photography, for example in landscape photography having more DoF to play with is actually a benefit, in portraiture, in most cases, you do want shallow DoF, and so the only way MFT can take the fight to FF is by going the fast prime route.
For this test I used my Nikon D750 and Olympus OMD E-M1 (mk1)
The lenses I used here on my Nikon D750 were the Sigma 50mm f1.4 ART, Nikon 85mm f1.8, and Nikon 105mm f1.4 . For the Olympus, I used the Panasonic Leica 42.5mm f1.2 Nocticron and the Olympus 75mm f1.8. All 5 of these prime lenses are very highly regarded and score top marks for sharpness and clarity on independent lens review sites like DxO.
Because the lenses are of differing focal lengths, I adjusted my distance to the model so that the framing was similar in all shots. The subject to background remained constant as the model was stood on the dockside with the a large expanse of water behind her. However, the background appearance changes quite dramatically, not only due to bokeh differences, but also due to the changes in perspective of the different focal lengths.
Full Length Portraits
For this first test, the full frame 105mm f1.4 produces the most ‘pop’ by a considerable margin, with the 85mm Nikon and 75mm Olympus vying for second place. Although this is subjective, I think it is quite clear.
Head and Shoulders Portraits
By moving closer to the subject, the amount of ‘pop’ has imprived across the board. To my eye, all of these are now very pleasing. For sure the 105mm f1.4 Nikon is still the one with the most extreme background blur, but both of the MFT contenders now give a very good account of themselves, and for 99% of photographers is probably ‘enough’, particularly when you factor in cost saving, and the bulk/weight benefits of MFT over FF.
One caveat to this test. These shots were taken in London on a pleasant, but freezing cold winter morning. My long suffering g/f agreed to model for me while I messed around changing lenses and cameras. She is not a professional model, so many thanks to her for putting on a brave face in testing conditions.
I hope you found this little test interesting. Since shooting it, I’ve acquired the brilliant new Olympus 25mm F1.2 lens. I plan to re visit this topic later in the year so I can include this new lens.