Originally posted by Jimfear I think you'll have to explain a little more how you have come to these conclusions. Where have you found the equation, why does it work that way? And are you sure about measuring the distance from the subject to the lens, and not to the sensor or some nodal point in the lens?
I've only read the most basic optical physics and I don't think too many of us are optical engineers so this might need some explaining to make sense.
A little change in focal length would be logical but cutting it in half that just seems strange. That should show in quite a drastic way regarding DOF and FOV, have you tried this practically or is it just a theoretical experiment? Forgive me for being sceptical but at the moment it just doesn't make sense.
Originally posted by Laurentiu Cristofor I don't think it means that. For an IF lens, S2 would be constant in that formula, because IF lenses don't extend or contract. Which means that when S1 gets smaller, f needs to get smaller as well for the equality to continue to hold. Which means that the focal length must decrease as you focus closer.
correct. but it is even more complex because in an IF lens, usually the rear element is also fixed, so the equation really falls apart.
For the sake of measurement, I have elected the "nodal point" to be the frong element. While this may not be 100% accurate, for the purpose of estimating the focal length of a lens, based upon the measured magnification ratio, and this distance it is good enough.
As to the source of the formula,
M = f / (f-d) where D is the distance to the lens (S2) this is from the same wikipedia page, just a little further down than the formula 1/S1 + 1/S2 = f/F
The issue was discussed, but not really properly answered both in magazines and on the forum over a year ago with issues and questions about super zooms having different field of view than primes at any equal focal length. The same issue holds true there, the lens makers have found it easier to modify focal length to achieve focus when not at infinity.
The issue was also discussed with respect to 1:1 macro using newer IF macro lenses, and the fact that they do not achieve 1:1 at a working distance of 2 x focal length, and the pros and con's of this. The pro is since focal length changes and reduces, the aperture may be increasing such that with magnification the image remains brighter, as opposed to using extension tubes and a true externally focused (entire lens group moves) lens, where light fall off at extreme magnification is an issue
All I am doing is proposing , through this thread, that we attempt to characterize our Internally focused lenses, to get a better understanding of lens performance.
Also if focal length loss at close focus can be as severe as my tests with the vivitar suggest, this can change one's approach in a studio. It is very clear that in the 1-2 meter working distance, I can do things with my vivitar 85 that I can't do with my super tak 85. On the other hand, if magnification is what I am looking for, my super tak 85 would be the lens to select.