Originally posted by Pål Jensen This is true for every proponent of "equivalence"
You can use any format as reference and misuse the law of reciprocity to suit you argument. .
---------- Post added 02-25-15 at 12:05 AM ----------
The problem is that there is no DOF equivalence among formats when comparing different focal lengths yielding the same angle of view on their respective formats. It is a misunderstanding based on mixing what the various terms actually mean, and an oversimplification of DOF.
The DOF of a lens is not defined by a single number calculated from the F.stop and the focal length in use as proposed by equivalence ; DOF is based on aperture, focal length, focus distance and subject magnification. DOF of a lens forms a spectrum of possibilities or a space if you like. If you compare "DOF space" of two lenses of different focal lengths but similar angle of view on their formats, they will overlap a lot, but some areas will not; they will never totally overlap. For some reasons, never explained, only a single point outside this overlap, and only a point not covered by the smaller format is of value for the equivalence proponents. All other DOF characteristics are ignored. We are never given a reason for this. Nor, for that matter, why DOF wide open is the only factor of importance among the factors that make up reciprocity. Nor are we told why everything must be "normalized" to FF to be of any value.
When you take into calculation that DOF is dependent on focus distance and magnification as well, it is obvious that there are images you can shoot on APS which cannot be done on FF even by cropping when using "equivalent" lenses. In fact, in cases I have calculated, the APS "equivalent" lens will have thinner minimum DOF than the FF (even if they have the same max aperture) simply because it has inherent larger magnification (this is why you can take fantastic macro images with the Q).
There is obviously always an equivalence. The deph of field lenses provide is continuous. Focus a little far a away, you get more. Get a bit longer/ Go more near to the subject, you get less. Only apperture can't be continous but with 1/3 of stop you have a good enough approximation. 1/3 a stop is just arround 10% difference in deph of field.
For dof, as long as it is in the range of thing the lens can do at all, you be able to get the one you want. For framing, that true of course too.
Eventually the perspective would be different, but if you change the focal just the level needed as the same time as the sensor, the perspective could be kept the same. The bokeh/rendering would be different anyway.
But yes you can get same framing, same dof, same perspective compression by choose the right corresponding focal length, apperture and keeping the same subject distance by changing format... As long as the corresponding lense exist (or near enough for that purpose) you'd get it.
Difference in bokeh/rendering would stay. They would be subble in many case and has much can be set as being part of the precise model of lense you use than the format too.
So mathematically ou can do it, of course, this is even basic math.
Your idea is like because 2 vehicule have 2 different wheels diameter, they obviously can't run at the same speed. Ultimately, this might be true, but in many case even between a bicycle and an airplane, there a shared range of speed, here in 0-40km/h range. The one that look at appertures and say f/2.8 stay f/2.8 and look at focal length and say focal lens stay the same are the one that would conclude that obviously because bicycles and trucks have arround the same wheel diameter they would drive the same speed and that most cars because they come with smaller wheels obviously comes with more limitation and can't go has fast. Reality is that 3 kind of vehicule have a shared range of speed even if the formula in term of wheel diameter and turns per minute to know the vehicule speed might appear a bit complex. Hey there even a "Pi" irrationnal number in it!