Even though you (or your DoP/lighting cameraman) might know how to get the look you want in terms of selecting a format, lens, aperture and camera position you might be hazy on the details of their relationship to each other. So here’s my take on the technical and aesthetic aspects of how large sensor cameras achieve that lovely ‘soft bokeh’ look.
Bokeh is a Japanese word that refers to the out of focus parts of an image. However, for the purpose of this post I’m using bokeh to refer to the ‘blurriness’ of a scene’s background, which is typically the look of movies, commercials and big-budget TV dramas. Now, more and more of us are trying to achieve an attractive bokeh in more budget-constrained genres like corporate video.
Depth of field (DoF, or depth of focus) is a term any photographer/cinematographer understands as the amount of a scene’s depth that is in focus. ‘In focus’ is actually an objective assessment of the sharpness of an image decided by its circle of confusion (CoC). In photography, the CoC diameter for an image is often defined as the largest blur spot that will still be perceived by the human eye as a point. I’m not going to go into any detail about CoC here but there’s a link at the end of this post if you’re interested. For the purpose of clarity, I’m also not going to get into hyperfocal distance and diffraction which are relevant to any discussion on sharpness and depth of field as these are also covered in the links below.
Despite what you might think, the amount of depth of field you have is not directly affected by the focal length of a lens (providing the camera moves accordingly in order to maintain the subject’s size in the frame, thus altering the point of focus). There are only two factors that affect DoF to a significant degree: aperture and distance to subject (point of focus). However, sensor size will affect the field of view (FoV) and therefore also distance to subject, so sensor size affects DoF as a result of maintaining the desired field of view or subject size in the frame by moving the camera nearer or further away. Of course, in the real world you change focal length to get a wider of narrower FoV instead of moving your position. It then becomes intuitive to think that the amount of DoF is somehow inherent to the focal length.
The following two images (shot with a Canon 5DMK11) illustrate this and were shot at focal lengths of 24mm and 70mm, with the camera further away in the 70mm shot to maintain the size of the foreground object.
You’ll notice from the cropped shots below that even though the perspective differs markedly between the two shots, the amount of DoF and background blur is actually the same. Why? Because the point of focus moved between the two shots as the camera moved back to maintain the subject image size and the DoF for the two different focal lengths became the same.
Here’s another example where the subject (the coffee mug) is kept at the same size when doubling the focal length by doubling the distance of the camera to the subject. Again, the cropped shots show that the depth of field and the background blur are almost identical apart from the magnification effect of the longer lens:
In my own work, where I do a lot of interviews in small rooms with boring backgrounds, throwing the background way out of focus was always really difficult with video cameras – unless I could get either myself a long way from the subject, or the subject a long way from the background – neither of which was usually practical.
But now a super 35mm or full-frame 35mm sensor video camera with a medium focal length lens (such as a 50mm at wide apertures) and at a subject distance of, say, two meters will render even a close background out of focus without any magnification effects, producing a natural looking bokeh and perspective.
As I said, bokeh like this is impossible to achieve with small sensor video cameras – which was all of them just a few years ago. To get the same degree of softness in the background the camera would need to be several meters further back (in the next room!) and the perspective would be much more compressed with the background features greatly magnified.
Let’s look at another example, showing the difference between a large and small sensor camera in the same setup, without changing the distance of camera to subject. These shots were taken with two stills cameras: a Canon 5D MK11 and a Sony DSC F-717. The Canon has a full-frame 35mmm sensor measuring about 40mm diagonally and the Sony’s sensor measures about 10mm. Incidentally, the sensor in the Sony is around the same size as those in standard professional 2/3inch video cameras that have been the workhorses of the TV industry for decades, so it’s a nice demonstration of the differences between ‘traditional’ cameras and the new breed of large sensor video cameras.
The Canon is set to 70mm @ f/2.8 and the front of the bus is about one meter away. To match the shot size with the camera in the same place the Sony is set to 18mm @ f/2.8:
So whilst the increase in DoF is apparent with the Sony image at 18mm, this is only because the sensor is smaller and the FoV is narrower, and the camera is much further away at 18mm than it would be if it had a much larger sensor. If I had a 18mm lens for the Canon (I don’t) and matched the subject size by going closer, the DoF characteristics would look very similar to the 70mm shot (as shown with the earlier examples), though of course the perspective would be different.
To sum up there are several factors that affect the DoF and the bokeh:
DoF is independent of focal length – when the size of the image on the sensor is held constant by moving the camera and therefore the point of focus nearer or further away.
Distribution of DoF – does change with focal length so that at, say, 100mm the ratio of distribution of DoF is roughly 50% in front of the point of focus and 50% behind; whilst at 20mm it’s more like a 60:40 ratio. This may or may not affect the background bokeh depending on the situation.
Aperture – larger apertures = less DoF and more blurred bokeh. The amount of DoF roughly doubles or halves for every two stops smaller or larger respectively.
Sensor size – affects FoV but not DoF or magnification directly (magnification not to be confused with crop factor). However, the sensor size is going to determine your FoV and therefore your lens choice and subject distance – which will affect DoF. So you could say sensor size affects DoF but only with regard to these other factors.
Focal length – affects FoV, magnification and perspective but only affects DoF if the subject distance (point of focus) remains the same as focal length changes – in other words if the subject size changes. If you move the camera forwards and backwards to compensate for changes in focal length to maintain the same subject size then DoF will also stay the same. However, the distribution of the DoF will change with focal length, as stated above.
Subject distance (point of focus) – affects DoF in combination with focal length as described above.
So in a nutshell, large sensor cameras – due to the much wider FoV and closer point of focus with any given lens – allow you to achieve a significant degree of background bokeh in many situations that would be impossible with small sensor cameras.
Links and online tools
Here is a link to a more detailed explanation that also covers hyperfocal distance, diffraction and CoC. There is also another nice visual demonstration here that proves focal length does not, by itself, affect the amount of DoF all that much: http://toothwalker.org/optics/dof.html
Useful calculator showing the FoV with different combinations of camera and lens from Abelcine, equipment sales and hire outfit in New York: http://www.abelcine.com/fov/
From Stu Maschwitz at the Prolost website, a very nice graphical demonstration showing the FoV and DoF characteristics of video cameras with different combinations of sensor and lens: http://gallery.me.com/prolost
One of the better online DoF calculators and information resource: http://www.dofmaster.com/dofjs.html