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Quote: I'd use the 85mm for my wife, the 50mm for my young son, the 28mm for my grandfather, with the 20mm reserved for my little brother when Gurning.
..... and if you framed them the same size in each photo, and used the same aperture, the depth of field in each case would be exactly the same.
As stated, the depth of field is a function of distance and focal length (magnification at the plane of focus) and aperture. The other properties are irrelevant when comparing lenses on the same camera, as the other factors will remain constant. Construction of the lens will affect how the out of focus areas are rendered, not how out of focus they are.
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Thanks, hadn't realised that....
the depth of field is a function of distance and focal length (magnification at the plane of focus) and aperture.
This information is incomplete. Depth of field is also partly a function of hyperfocal distance.
Hyperfocal distance is closer on a 50 mm lens than on an 85 mm lens, aperture for aperture
When focus is closer than one third of hyperfocal distance, as it usually is in portraiture, the shorter focal length lens has more depth of field and less background blur.
Looked at another way the changed perspective and narrower angle of view at 85mm means more magnification of the background detail increasing the blur effect when focussing closer than 33% of HD.
Not the whole story though, as the hyperfocal distance itself is a function of the lens focal length and the lens aperture (assuming circle of confusion the same in all tests, as we are using the same camera), both of which you can substitute into the original equation anyway, so calculating the hyperfocal distance is unnecessary. The hyperfocal distance is the focussing distance where the far limit of acceptable focus is at infinity, and for the 50 and 85 lenses in question are around 200' and 600' respectively, so there is little practicable difference when considering the distances involved are of the order of a few feet.
However, using the information from the OP:
Using two different lenses on the same body, at typical distances for head portraiture, both at the same aperture and both with the same magnification gives:
(using a constant circle of confusion of 0.00079")
85mm lens at 85 inches gives a depth of field of 2"
50mm lens at 50 inches gives a depth of field of 2"
So TBH, at the distances concerned, there is no practicable difference in calculated depth of field. More important is the rendering of the out of focus detail for the aesthetic appeal of the photograph, which is more a function of the construction.
Quote: Longer focal lengths may also appear to have a shallower depth of field because they enlarge the background relative to the foreground (due to their narrower angle of view). This can make an out of focus background look even more out of focus because its blur has become enlarged. However, this is another concept entirely, since depth of field only describes the sharp region of a photo — not the blurred regions.
- Cambridge in Colour website. The best summary I found in 5 mins of googling for one!
FFS people, get a life!! Just stick the bloody lens on the camera & try it.
I have studied optics (a long time ago, admittedly), and I don't like people getting the wrong information. There are a number of misconceptions propagated on websites like this, and if nobody adds the correct information, the less experienced will never learn. Rules of thumb get us through most situations, but sometimes you need to be more precise. For instance, the "rule" that depth of field extends 1/3 in front and 2/3 behind the plane of focus only applies exactly at 1/3 the hyperfocal distance, but it is 'close enough' in most circumstances.
The reason Ray Mears teaches indigenous tribespeople how to make fire, is because basic information is lost when people think it is no longer necessary. Yes, you could just "stick the bloody lens on", but who really wants to waste time experimenting to see the different effects of lens/aperture/distance for each focal length when the simple knowledge and calculations will give you the information in seconds? Maybe if lens manufacturers started putting depth of field scales back on their lenses, DoF would become much easier again.
It might seem pedantic, but millions of people consult depth of field tables every day, most without considering how they work. Knowing simple things might help clarify that.
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