p.1 #4 · Post true focal length of lenses you tried
dclark wrote:
Would you describe the method you use to measure the focal length of the lenses.
Most of the times I owned multiple lenses at same FL. Sigma 50 1.2 is consistently shorter than Sony and Nikon 1.2 in my testing. It's bit tighter than Sigma's i Series 45mm.
35 1.2 DN was so close to Art 40mm F1.4 making me sell 40. It was longer than 35 GM. But GM suffers from horrible FB and it evens out.
I switched from A7RV to Z8 and before buying current 180-600 I was using 200-600. 180-600 is true 600 at minimum focus, when I zoomed out Nikon 180-600 to 560 it matched Sony at 600, both at MFD.
p.1 #5 · Post true focal length of lenses you tried
Outstanding wrote:
Would you describe the method you use to measure the focal length of the lenses.
Manufacturer angle of view (diagonal FOV) spec matched to its corresponding value in mm at https://vahonen.info/fov. E.g., the 32.6° angle of view provided by Voigtlander for the 75/1.5 Nokton corresponds with the 32.59° diagonal FOV of a 74mm lens.
p.1 #6 · Post true focal length of lenses you tried
Does the measured focal length change as the focus distance changes?
Should the measured focal length change as the focus distance changes?
Does the Angle of View change as the focus distance changes?
When a lens manufacturer specifies the focal length of a lens, what is the focus distance for which that is the focal length?
p.1 #7 · Post true focal length of lenses you tried
Focal length is determined by the distance between the sensor and rear principal plane of the lens when the lens is focused at infinity. The focal lengths given by most manufacturers are approximate. The 40/1.2 Nokton's design is noticeably a little "in" from other 40mm lenses I've used, and I like it even more for that.
p.1 #8 · Post true focal length of lenses you tried
nbaronzzi wrote:
Focal length is determined by the distance between the sensor and rear principal plane of the lens when the lens is focused at infinity. The focal lengths given by most manufacturers are approximate. The 40/1.2 Nokton's design is noticeably a little "in" from other 40mm lenses I've used, and I like it even more for that.
Since lens focal length is defined for focus at infinity, how do you measure the focal length?
The web site you linked is set up for a focus distance of 320cm (10' 6"). From where on the lens do you measure the focus distance?
p.1 #9 · Post true focal length of lenses you tried
nbaronzzi wrote:
Manufacturer angle of view (diagonal FOV) spec matched to its corresponding value in mm at https://vahonen.info/fov. E.g., the 32.6° angle of view provided by Voigtlander for the 75/1.5 Nokton corresponds with the 32.59° diagonal FOV of a 74mm lens.
That only works for accurately rectilinear lenses like a fine macro. Most lenses have some distortion and some have so much that they don't even cover 43mm diagonal and have 10% or more barrel.
p.1 #11 · Post true focal length of lenses you tried
EB-1 wrote:
That only works for accurately rectilinear lenses like a fine macro. Most lenses have some distortion and some have so much that they don't even cover 43mm diagonal and have 10% or more barrel.
EBH
Ah, that makes sense. I've seen people on here using FOV calculations trying to optimize gaps between primes and such; gave me the impression that it was much simpler than it actually is. Then, is it true that different optical designs demonstrate different variations in FOV depending on their focus distance?
p.1 #12 · Post true focal length of lenses you tried
nbaronzzi wrote:
Then, is it true that different optical designs demonstrate different variations in FOV depending on their focus distance?
How else could different Focus Breathing be explained?
p.1 #13 · Post true focal length of lenses you tried
dclark wrote:
Does the measured focal length change as the focus distance changes? Yes
Should the measured focal length change as the focus distance changes? Yes. Otherwise the lens has focus breathing, which is undesirable for video.
Does the Angle of View change as the focus distance changes? For an ideal lens, no, but most lenses do show change in the angle of view as focus distance changes. That is, they show focus breathing.
When a lens manufacturer specifies the focal length of a lens, what is the focus distance for which that is the focal length? Infinity
p.1 #14 · Post true focal length of lenses you tried
dclark wrote:
Yes Yes. Otherwise the lens has focus breathing, which is undesirable for video. For an ideal lens, no, but most lenses do show change in the angle of view as focus distance changes. That is, they show focus breathing. Infinity
This is helpful, thanks. So, the "true" focal length of a lens is measured at infinity focus, as most lenses decrease to whatever extent in focal length as you focus closer. And when I see people discuss the varying FOVs of lenses spec'd at the same focal length, it's kind of dubious because it depends on their distance from their typical subject. Am I understanding correctly?
p.1 #15 · Post true focal length of lenses you tried
Here's a great resource for patent information that gives actual tested focal lengths and, in some cases, you can see the lens as tested both at infinity and MFD.
...is a 72-194 f/2.88 at infinity. If you advance through the scenarios using the buttons in the top right corner, you'll see it's a 66.71mm f/2.9-126mm f/3.0 at close working distance. You can also see the distortion and SA graphs by pushing those buttons in the middle-right of the options.
It's not a comprehensive list, but it does take the guesswork out of the venture. Plus, it's fun to see all the optical drawings.
p.1 #16 · Post true focal length of lenses you tried
nbaronzzi wrote:
This is helpful, thanks. So, the "true" focal length of a lens is measured at infinity focus, as most lenses decrease to whatever extent in focal length as you focus closer. And when I see people discuss the varying FOVs of lenses spec'd at the same focal length, it's kind of dubious because it depends on their distance from their typical subject. Am I understanding correctly?
Yes, but you also need to consider some of the distortion factors mentioned by @EB-1.
p.1 #17 · Post true focal length of lenses you tried
freaklikeme wrote:
Here's a great resource for patent information that gives actual tested focal lengths and, in some cases, you can see the lens as tested both at infinity and MFD.
...is a 72-194 f/2.88 at infinity. If you advance through the scenarios using the buttons in the top right corner, you'll see it's a 66.71mm f/2.9-126mm f/3.0 at close working distance. You can also see the distortion and SA graphs by pushing those buttons in the middle-right of the options.
It's not a comprehensive list, but it does take the guesswork out of the venture. Plus, it's fun to see all the optical drawings....Show more →
Wow, visualizing the position & movement of the element groups and their effect on transmitting light is super cool. The 'Optics Primer' page is also very thorough. Bookmarked for future reference; thanks for this!
p.1 #18 · Post true focal length of lenses you tried
nbaronzzi wrote:
Wow, visualizing the position & movement of the element groups and their effect on transmitting light is super cool. The 'Optics Primer' page is also very thorough. Bookmarked for future reference; thanks for this!
Happy to share the find. If you pano at all, you can use it a reference to help find the no parallax point on which the lens should rotate. It'll show you the entrance and exit pupils and the point will be very close to the entrance pupil. It can be helpful if the lens is on there.
p.1 #19 · Post true focal length of lenses you tried
freaklikeme wrote:
Happy to share the find. If you pano at all, you can use it a reference to help find the no parallax point on which the lens should rotate. It'll show you the entrance and exit pupils and the point will be very close to the exit pupil. It can be helpful if the lens is on there.
Are you sure about the "no parallax point"?
I take some panos and always thought the lens should rotate around its entrance pupil. It doesn't matter much as I always find the correct camera position by experimenting (exact position doesn't take long now with digital photo). My main reason for asking is I want to get the facts and nomenclature right.
p.1 #20 · Post true focal length of lenses you tried
Jonas B wrote:
Are you sure about the "no parallax point"?
I take some panos and always thought the lens should rotate around its entrance pupil. It doesn't matter much as I always find the correct camera position by experimenting (exact position doesn't take long now with digital photo). My main reason for asking is I want to get the facts and nomenclature right.
Oops. Sometimes I think I type faster than I can think. Thanks for catching it. Corrected.
