DavidBM
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 Re: FM Review: Sigma 35mm f/2 DG DN Contemporary | |
chiron wrote:
DavidBM wrote:
chiron wrote:
DavidBM wrote:
chiron wrote:
DavidBM wrote:
chiron wrote:
DavidBM wrote:
chiron wrote:
DavidBM wrote:
Steve Spencer wrote:
QuietOC wrote:
Steve Spencer wrote:
Most lenses (for stills anyway) change focal length at least a bit depending on focus distance (in the video world they call this focus breathing).
This conflates angle-of-view and focal length/power. A lens without focus breathing must reduce its focal length in a specific manner with close focus. A lens with fixed focal length necessarily reduces angle-of-view with close focus.
Ok, but can you explain how that matters for the general point that a 35mm lens' focal length is typically measured at infinity and doesn't necessarily have that same focal length at shorter focus distances.
He’s just being picky about terminology, Steve.
A unit focus 35 or any other lens doesn’t change focal length as if focuses closer, but it *does* change field of view. Of course for practical purposes (holding format fixed) photographers often use focal length and field of view interchangeably, as though they were measures of the same thing (one measured in mm and one in degrees) and this rarely is problematic.
I think the terminology may be confusing. As I understand it (to a very limited degree), the focal length of a lens is defined as the distance from the rear nodal point of a lens to the plane of focus. Lenses are notionally identified and named by what this distance is when an object at infinity is brought into focus (e.g., 50mm). However, the focal length does change when an object closer than infinity is focused because in order to bring the closer object into focus the nodal point may be moved further away from the plane of focus and thus the actual non-notional focal length becomes a bit longer. This change is largest when the object is closest.
Maybe it is confusing, I was taught that the focal length is defined as the distance from rear nodal point to pane of focus when focussed at infinity. From which it follows that it doesn’t change if it’s unit focus as you focus closer.
So why does it change if there’s IF or FE? Because if you focus close, and the elements move relative to each other *and then refocus without changing the arrangement of the elements* (which of course you can’t in practice do) the focal length as defined would be different.
But of course nine of this matters much in practice (except perhaps to encourage us to think in terms of AOV rather than FL)
Yes, confusing terminology. I think it is the notional focal length of the lens that is the distance from nodal point to the plane of focus when the lens is focused at infinity. But the actual or empirical focal length may vary a bit as the nodal point moves further away from the plane of focus when focusing on a close object, and with the change in focal length comes (I think) a change in magnification and angle of view.
I don’t think it’s merely notional (although the definition in terms of plane of focus, infinity and nodal point is not quite precise)
Here’s why
Strictly speaking, focal length is the reciprocal of optical power. And optical power is an angular measure...it the degree to which the lens converges or diverges light. That is something which is an intrinsic property of the lens regardless of how it’s focussed, even if focussing closer will change magnification and hence AOV. So focal length should not change with focus, even if AOV does, unless the elements move relatively as in IF or CRC lenses, where the intrinsic shape of the lens changes.
I think that calling a lens a specific focal length based on infinity focus is notional--there is no reason why the focal length at infinity focus is any more real a physical fixed entity than is the focal length at minimum focus distance. The power, magnification, and angle of view do all change as the nodal point moves closer or farther from the plane of focus because they are actual empirical entities and they are changing as the focal length changes. In this sense, most primes lenses are very limited range zoom lenses! 
Here's why I think it's more than notional. You are trying to define a property of an optical system partly independent of it's current extrinsic state. Optical power is a function of the refractive powers of the lens system itself. Using the infinity measurement standard gives you a way of defining things so that you can say, of (to use the example of a diopter) that its optical power is +4 diopter and thus that its focal length is 250mm. And this is meant to be a property of the refractive power of the lens itself. It it has an infinity focal length calculated the way we were discussing before *because it has that power itself*.
The upshot is that there are two ways to define focal length, one in terms of the inverse of the refractive power that a lens has in itself )even just sitting in a box), the other how it behaves in an optical system. Both ways of thinking of it might be useful in some contexts. Of course how it behaves in a system is explained in part by the first, but not vice versa.
None of this is of course to deny that angle of view, and what people sometimes call "focal length" (in one sense correctly I suppose) vary as you focus a lens.
Perhaps "teminological" is a clearer word for what I mean than notional. The measurable focal length of a lens is of course actually a certain length at infinity. But it may well also be a different measurable length at minimum focus distance, and in terms of magnification, power, and angle of view it will behave differently and accordingly at each of its actual focal lengths. These changes will occur as the distance of the nodal point from the plane of focus changes thereby altering the optical performance of the lens.
I don’t think we disagree about anything important, and I agree that this is just a terminological matter.
But here’s how I was taught it: the measurable focal length of a lens at infinity is not what “focal length” is.
Rather focal focal length is the physical property of the element or compound elements (1/d) of the system that causes the measurement to come out that way at infinity. So having a focal length of 250mm will cause you to be able to measure 250mm from node to focus plane when focussed at infinity. But that same physical property (having a “focal length” of 250mm) will cause different measurements, not 250mm, focussed closer.
Of course we could use “focal length” to mean the measurement that we get from node to sensor rather than the physical property which makes it come out a certain way at infinity. And some do, and with terminology there’s no point in my just stamping my feet.
One advantage of using it the way it is often technically defined as 1/d of the glass, regardless of how it’s focussed is this: we can distinguish between block focus lenses whose focal length in sense never changes (though angle of view and magnification certainly do with focus) and CRC or IF lenses where the AOV and Mag also change, but in part for different reasons: the underlying physical property, the focal length in this sense, changes with the reconfiguration of the elements. The other advantage is that focal length becomes a property of lenses in themselves, off the camera, not just a fact about how they would behave in certain circumstances.
Yes, I agree that we are not really disagreeing about anything much, mostly about two different ways of saying something. An advantage of thinking of the focal length changing at MFD for some prime lenses is that it explains focus breathing as a variation in power/AOV/magnification, all of which result from a slight variation in the focal length of the lens. The convention of identifying a lens by its focal length at infinity focus obscures more than it clarifies with zoom lenses, while thinking of lenses as potentially having a variable focal length as the nodal point moves fits the zoom case very nicely. A zoom, after all, is a single lens with a variable focal length--QED.
Pace.
Maybe then that is the only thing we do disagree about, just what is most explanatory. The convention of identifying focal length as the intrinsic property which explains the measurement at infinity has the advantage of clarifying that focus breathing has two separate and importantly different causes depending on the kind of focal length: changes in “true” focal length in the case of crc and some IF lenses, and changes in magnification and apparent focal length caused by focussing closer in block focus lenses.
But I think we understand each other now!
Cheers
D
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