DavidBM wrote:
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....Show more →
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!
In the fascinating video to which Michael (@zhangyue) linked recently, the ARRI representative explained that their Signature Primes have a kind of “internal zoom” that eliminates focus breathing, maintaining a constant focal length from infinity to MFD.
genji wrote:
In the fascinating video to which Michael (@zhangyue) linked recently, the ARRI representative explained that their Signature Primes have a kind of “internal zoom” that eliminates focus breathing, maintaining a constant focal length from infinity to MFD.
I remember watching that video when Potato Jet released it and it blew my mind. Art Adams explains things in such an accessible and direct way...it's like a mini class on lens design. You begin to understand why ARRI Signature Primes cost $25k...
chiron wrote:
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!
Focal length is but one property of a lens, it gives us an idea of what sort of field of view to expect on a particular format. It is not 'notional' it is measured, and done st infinity as thst is a constant and is at a distance where it is most useful to know.
As you focus closer then the field of view changes. By how much depends on the lens design, usually it is not significant until you get really close but became of concern I think when the latest exotic zoom made life easier for sports photographers but the change in field of view as you focused closer was extreme enough to be noticeable.
My micro Nikkors have a focus distance scale and also a reproduction ratio, both useful for the sort of work you might do with such a lens and measurable.
It might be useful if testers gave us a table of changing field of view with focus distance for a lens, but it usually isn't of significance. But describing one property of a lens, changing field of view at close distances, by a term properly defined at an irrelevant distance doesn't help proper understanding of the subject. Much better to use the proper terminology and avoid ambiguity. It isn't really of significance in everyday out and about photography but it might as well be right.
chiron wrote:
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.
DavidBM wrote:
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.
chiron wrote:
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.
DavidBM wrote:
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....Show more →
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.
chiron wrote:
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....Show more →
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 lens: 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.
Thanks very much! The last one has almost no post processing so I was very surprised by how well it turned out.
I like them all, but I especially like the first and the fourth. They have a sense of deep space and dark colors and variation rather than uniformity which makes them complex to look at. In the first picture especially, the sense of space is so cut up and varied that I kept thinking that I was looking at a collage. I had to make myself see that it was a single photograph. It really works.
Got this with a7c to replace RX1Rii. Love the image quality. I won't be able to tell the difference between transition areas. Both the Sigma and Sonnar are buttery smooth. The a7c combo is still quite bigger than the RX1Rii, but "compact" in the sense of full frame E mount f2.
chiron wrote:
I like them all, but I especially like the first and the fourth. They have a sense of deep space and dark colors and variation rather than uniformity which makes them complex to look at. In the first picture especially, the sense of space is so cut up and varied that I kept thinking that I was looking at a collage. I had to make myself see that it was a single photograph. It really works.
One might be tempted to say... 3D pop
The results from this lens are so good that I am really second guessing whether I should go for the 35 GM, as f/1.4 won't blur the background significantly more than the f/2 and I could instead pick up the Sigma 65 f/2 or Zony 55 f/1.8 and keep the Sigma 35 all for the same price (roughly) as the GM... And those longer lenses will supply more DOF control than the 35 GM.
lukecookphoto wrote:
One might be tempted to say... 3D pop
The results from this lens are so good that I am really second guessing whether I should go for the 35 GM, as f/1.4 won't blur the background significantly more than the f/2 and I could instead pick up the Sigma 65 f/2 or Zony 55 f/1.8 and keep the Sigma 35 all for the same price (roughly) as the GM... And those longer lenses will supply more DOF control than the 35 GM.
I have the Sigma 35mm 1.2 Art, which is big and heavy, but superb. I'm torn between adding this impressive little Sigma 35mm F2 to give me a lightweight alternative to the massive 1.2 or just selling my 1.2 and going with the GM. Excellent options all-around, but I have to confess that the extraordinary (to my eyes) rendering of the GM is extremely appealing.
Anyway, loving all the images in this thread and I'm looking forward to continuing to enjoy the images in here whether I add this lens to my kit or not.
lukecookphoto wrote:
One might be tempted to say... 3D pop
The results from this lens are so good that I am really second guessing whether I should go for the 35 GM, as f/1.4 won't blur the background significantly more than the f/2 and I could instead pick up the Sigma 65 f/2 or Zony 55 f/1.8 and keep the Sigma 35 all for the same price (roughly) as the GM... And those longer lenses will supply more DOF control than the 35 GM.
If you shoot close though the GM looks to be the better lens. I read the Sigma f2 doesn't get very sharp up close unless you stop it down a few clicks.
fotografur wrote:
If you shoot close though the GM looks to be the better lens. I read the Sigma f2 doesn't get very sharp up close unless you stop it down a few clicks.
That's true, from MFD the Sigma is not so great! If you get a bit further back from MFD it sharpens up nicely.
I'll have to look at some f/1.4 vs. f/2 examples on a 35mm and see how I feel. Seems like it'd be better to have a fast 55 or 65 for when I want to blow out backgrounds though as a 35 is still a 35.
fotografur wrote:
If you shoot close though the GM looks to be the better lens. I read the Sigma f2 doesn't get very sharp up close unless you stop it down a few clicks.
Good point. I personally don't shoot a 35mm up close because I don't like distortion in my images, but for some people this would be a bit of difficulty. But the Voigtlander 40/1.2 had the same issue, and that didn't stop it becoming a favorite for many people.
