# Depth of Field, and Hyperfocal Distance

Depth of Field, and Hyperfocal Distance
By Robert Render Harrison
Depth of field means how much in front of and behind the precise point of focus (the subject distance) appears to be in focus. The more critical you get in determining this – for example, with very sharp lenses, and a big magnification, like projection on a cinema screen – the narrower the depth of field. When Canon calculate depth of field, they use a circle of confusion of 0.03mm, as does the DOFMaster website: dofmaster.com. What is a circle of confusion? A rather confusing name that in essence means the size that a point on the subject, when focused by a lens, can become before being noticeably out of focus in the resulting image.
Using this circle of confusion of 0.03mm, the depth of field when using a 50mm lens set at f/4 and a subject distance of 10 feet, when mounted on a full-frame HD-SLR like the Canon 5D MkII or MkIII, is 8’9” to 11’8,” a total of 2’11.”
Mounting this 50mm lens on an APS-C sensor camera, like the Canon 60D or 7D, it becomes 9’2” to 11.’ Wait a minute, you say, it’s less! Isn’t the depth of field a physical property of the lens? Well, no. The depth of field changes because an image from a smaller sensor needs a higher standard because it will be magnified more when viewed on the same size output, be it a computer, TV, or cinema screen. The circle of confusion for an APS-C sensor comparable to 0.03mm on a full-frame sensor is 0.019mm, obtained by dividing 0.03mm by the 1.6, the same factor used when calculating full-frame equivalent focal length for an APS-C sensor. In the case of the circle of confusion you divide by 1.6. For focal length equivalent, you multiply, so the 50mm has an equivalent angle of view to an 80mm lens on a full-frame camera. Easy, yes?
Micro Four Thirds cameras have a 2x factor in determining focal length equivalents (meaning a 50mm lens on a MFT camera is equivalent in angle of view/magnification to a 100mm lens on a full-frame sensor camera) so depth of field calculations should be calculated using 0.03mm divided by 2, thus 0.015mm. Easy, right?
The 50mm lens mounted on an MTF sensor camera, like the Panasonic GH3, would have depth of field at f/4 and 10 feet of 9’4” feet to 10’9” feet, a total of 1’5.” Even less depth of field than on the APS-C sensor. And, a 50mm lens on an MTF camera, applying the 2x factor to equate the focal length to the full-frame camera, gives 100mm equivalent, a 2x telephoto.
So let’s see what happens if we fit a 25mm lens on our MTF camera: first, we get the same angle of view as 50mm on the full-frame camera. At the same f/4 and 10 feet, the depth of field is 7’9” to 14’1,” a total of 6’4,” a tad more than double that of the 5D, which is what we’d expect: smaller format cameras provide more depth of field for the same angle of view, f/stop and subject distance. To get about the same depth of field on an MTF camera using a 25mm lens, with the equivalent angle of view to using a 50mm lens on a full-frame sensor camera, both lenses being set to a subject distance of 10 feet, open the iris on the MTF camera by two whole stops to f/2. So taking the same shot, with the same depth of field, the MTF camera can operate in one quarter of the light, which provides some interesting possibilities for available light shooting.
On the DOFMaster website there’s also a calculator for hyperfocal distance, a setting you’d use when you need the maximum depth of field, for example when doing a Steadicam shot where the talent may vary the distance from the camera, and you don’t have a focus pull option. Hyperfocal distance is the setting that gives the maximum depth of field, from half the hyperfocal distance to infinity. I mention this because I once heard a DP say that for Steadicam shots he set the lens at infinity. He was ‘throwing away’ depth of field, and should have been setting his lens to the hyperfocal distance. This is easily obtained from the DOFMaster website, and there are also simple calculators available. Due to the complexity of marking zoom lenses, they aren’t marked with the depth of field indicators familiar to stills photographers using fixed focal length (prime) lenses. The advent of the digital SLR as a tool for HD acquisition has given a new lease of life to prime lenses, both 35mm and other formats, and it’s worth studying simple optics again, just like we did in ‘the old days’ of 35mm (still) film cameras. Of course, this was part of the training of cinematographers in the days of film, so it’s just come, if I may be permitted the pun, full circle again.
DOFMaster.com

Article Posted: 8/19/2013