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11-27-2016, 12:20 PM   #16
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QuoteOriginally posted by stevebrot Quote
That is why they call it focus "breathing". The FOV appears to "breathe" in and out as focus is racked far/near. Traditional non-internal-focus primes will breathe "long" meaning that FOV will become narrower when focusing to near objects. This is due to increased effective focal length due to extension.
This is where I disagree. I do not believe that any camera lens has a longer effective focal length (narrower FOV) at short distance than it does at long distance. My A 135mm f2.8 shows an effective focal length of 136mm according to the thin lens equation. That makes perfect sense. Until I see a concrete example that a lens has a longer effective FL at close distances, I will not accept your assertion.

QuoteOriginally posted by stevebrot Quote
Lenses having floating elements and/or internal focus are less predictable and may sometimes breathe "short" meaning that FOV increases as one focuses nearer. As a result, many modern zooms may produce far less magnification at maximum focal length and MFD than anticipated despite very short focus distance. The equation used in the comments above will often detect the fault, but the calculated focal length is often simply wrong even when it is in the correct direction. That being said, the actual number generally makes little difference.
That lenses with floating elements show a wider FOV at close distances is not controversial. I will agree that thin lens may not be an accurate predictor of actual focal length at MFD. But it's the best we have without a lab or empirical test and is far better than believing manufacturers' stated focal lengths. The Sigma 150-500mm cannot produce a 500mm FOV at any distance below infinity. Thin lens theory says 295mm at MFD, the empirical test below claims 267mm. That's not far off. It would be interesting to see how others fare.

I am busy with a home reno right now, but I propose to test some zooms vs. my A135. My A 135 at MFD, on tripod, will be the control. Framing will be set with a ruler. I will then change lenses and zoom until I match the width of the control shot and see what focal length the camera reads. Sound reasonable? I can test the following lenses; DA 18-135, DA 18-250mm, DA 55-300 WR.

11-27-2016, 02:23 PM - 3 Likes   #17
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QuoteOriginally posted by audiobomber Quote
This is where I disagree. I do not believe that any camera lens has a longer effective focal length (narrower FOV) at short distance than it does at long distance.
Welcome to the Twilight Zone. The rest of us live in a world with conventional optical physics. There is nothing revolutionary about the idea. Change of FOV with extension (aka focusing near with a non-internal focus lens) is a fact of life given that effective focal length (regular focal length too) is measured from the rear principle plane to the focal plane. FWIW, it also happens with a thin lens.

From the link I provided in a previous comment, below is the thin lens diagram:



Distance b is the rear focus distance and f is the nominal focal length. When focused to infinity, b = f and is equivalent to the focal length. The quoted text below describes what happens when focused to closer than infinity (lifted from the previously linked site (LINK)).

QuoteQuote:
For example, take a hypothetical 35mm camera lens, with only a single thin lens with a fixed focal length of 50mm. If you want to focus on objects at 'infinity', then the distance to the in-focus image on the film plane, b, becomes (b=1/(1/50-1/∞)), or 50mm, with a diagonal angle of view of 46.8 (on a full frame 35mm sensor).

Thin 50mm lens focused to 137mm from principal plane

QuoteQuote:
But if you take the same fixed 50mm lens and want to focus it on a very close object, say at 137mm from the lens (v=137; seen above), then the distance to the in-focus image (the film plane), b becomes 78.7mm (b=1/(1/50-1/137)), with a diagonal angle of view of 30.7. In other words, to focus the image of a close subject on the film plane, the lens needed to be moved 28.7mm further away from the film plane, which reduces the angle of view recorded by the photo by a third.
Note: There is a typo on the original article where "b" is printed instead of "v" when describing the distance from lens to subject. I corrected in the text above.

If one needs a concrete example, one may put the calculator away and put your camera to the eye. Frame a prominent vertical object (I used a fir tree) at say 200m distance at the edge of the frame with focus at infinity. Rotate the focus ring to MFD and watch that object disappear from the frame.

Below is the thick lens diagram. The front and rear principal planes are labeled H and H' respectively.



Internal focus works by moving the rear principle plane relative to the front principal plane rather than by moving the lens. As such the effective focal length as well as FOV changes according to the position of the rear principal plane. Move the plane back and FOV increases.

Why the thin lens formula does not properly calculate focal length at MFD is easy to see if one considers that focus distance is from the plane of focus (subject) to the focal plane and includes the distance between the front principal plane and rear principal planes. On a thin lens, the two are coincident and usually just referred to as "the principle plane". On a thick lens the distance between the two principal planes can be several centimeters depending on lens design. Using the thin lens formula also brings with it the assumption of symmetrical design. Most (all?) SLR lenses are asymmetrical* to some extent which complicates things. As a result Thom Hogan's famous formula will usually understate or overstate the rear focus distance depending on lens design.

In practice determining if a lens has deleterious focus breathing is easy enough by direct examination through the viewfinder. Alternative, a little logic will also work. One need not be virtually touching the subject in order to get 1:2.8 at 70mm (my Sigma 17-70). If one needs a quantitative measure for comparison purposes, it is much easier to simply measure the FOV at MFD directly as opposed to locating the principal plane locations and calculating the effective focal length.

Your project to actually do the FOV measurements should prove useful.


Steve

* The most common forms of asymmetry are true telephoto and retro-telephoto designs. True telephoto lenses place one or both principal planes forward of the front element to allow for compact design. Retro-telephoto (aka retrofocus) place the rear principal plane behind the lens rearmost element to allow for mirror clearance with short focal length lenses.

Last edited by stevebrot; 11-27-2016 at 02:52 PM.
12-02-2016, 12:03 PM   #18
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Another way to find the effective focal length at MFD is (for Pentax lenses) to look in the EXIF the value that is transmitted to the Shake Reduction system.
e.g. for the 60-250 at 250mm and MFD it is 148mm, and for the 18-135 at 135mm and MFD it is 100mm.
03-11-2017, 08:49 PM   #19
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QuoteOriginally posted by stevebrot Quote
Welcome to the Twilight Zone. The rest of us live in a world with conventional optical physics. There is nothing revolutionary about the idea. Change of FOV with extension (aka focusing near with a non-internal focus lens) is a fact of life given that effective focal length (regular focal length too) is measured from the rear principle plane to the focal plane. FWIW, it also happens with a thin lens.

From the link I provided in a previous comment, below is the thin lens diagram:



Distance b is the rear focus distance and f is the nominal focal length. When focused to infinity, b = f and is equivalent to the focal length. The quoted text below describes what happens when focused to closer than infinity (lifted from the previously linked site (LINK)).




Thin 50mm lens focused to 137mm from principal plane



Note: There is a typo on the original article where "b" is printed instead of "v" when describing the distance from lens to subject. I corrected in the text above.

If one needs a concrete example, one may put the calculator away and put your camera to the eye. Frame a prominent vertical object (I used a fir tree) at say 200m distance at the edge of the frame with focus at infinity. Rotate the focus ring to MFD and watch that object disappear from the frame.

Below is the thick lens diagram. The front and rear principal planes are labeled H and H' respectively.



Internal focus works by moving the rear principle plane relative to the front principal plane rather than by moving the lens. As such the effective focal length as well as FOV changes according to the position of the rear principal plane. Move the plane back and FOV increases.

Why the thin lens formula does not properly calculate focal length at MFD is easy to see if one considers that focus distance is from the plane of focus (subject) to the focal plane and includes the distance between the front principal plane and rear principal planes. On a thin lens, the two are coincident and usually just referred to as "the principle plane". On a thick lens the distance between the two principal planes can be several centimeters depending on lens design. Using the thin lens formula also brings with it the assumption of symmetrical design. Most (all?) SLR lenses are asymmetrical* to some extent which complicates things. As a result Thom Hogan's famous formula will usually understate or overstate the rear focus distance depending on lens design.

In practice determining if a lens has deleterious focus breathing is easy enough by direct examination through the viewfinder. Alternative, a little logic will also work. One need not be virtually touching the subject in order to get 1:2.8 at 70mm (my Sigma 17-70). If one needs a quantitative measure for comparison purposes, it is much easier to simply measure the FOV at MFD directly as opposed to locating the principal plane locations and calculating the effective focal length.

Your project to actually do the FOV measurements should prove useful.


Steve

* The most common forms of asymmetry are true telephoto and retro-telephoto designs. True telephoto lenses place one or both principal planes forward of the front element to allow for compact design. Retro-telephoto (aka retrofocus) place the rear principal plane behind the lens rearmost element to allow for mirror clearance with short focal length lenses.
I can't help but be swayed by the preponderance of complementary angles. But the important point here is really not the absolute magnitude of discrepancies in FOV, but the mailing address for the entity which owes us all $ubstantial monie$ for product not delivered. And I would settle for a tablet of 50% off coupons, being a reasonable person.

Carry on...

03-28-2017, 09:23 AM   #20
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QuoteOriginally posted by Ptitboul Quote
Another way to find the effective focal length at MFD is (for Pentax lenses) to look in the EXIF the value that is transmitted to the Shake Reduction system.
e.g. for the 60-250 at 250mm and MFD it is 148mm, and for the 18-135 at 135mm and MFD it is 100mm.
This agrees with another common-sense observation -- at minimum focus, this is approximately where the zoom ring starts to have an effect with the DA*60-250. I have seen people say 125 or 135mm but it appears to me that the angle-of-view approximately agrees with the markings for 150mm.

I'm not sure just how much to trust calculations based on quoted specifications because I don't completely trust that manufacturers are consistent with how they quote these specs.
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