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caesar2164
7th of January 2009 (Wed), 14:27
just out of curiosity I had my 28-135 on my elan 7e the other day trying to figure out what the focal length of my eye is...

I was wearing my glasses and I saw that at about 70mm what I saw through the camera was roughly the same as without.

does that sound about right or did I get that result from my glasses?

rdompor
7th of January 2009 (Wed), 14:35
I've always been told that ~50mm on a FF body is very close to what we see.

FlyingPhotog
7th of January 2009 (Wed), 14:37
Think Wider FOV:
http://www.clarkvision.com/imagedetail/eye-resolution.html

Ob Com
7th of January 2009 (Wed), 14:38
For me its 35mm.

50mm is too tight and 24mm way too wide and perspective is unnatural (but its my favorite FL).

Perspective on 35mm (on FF) replicates how my eyes see the world

dpark
7th of January 2009 (Wed), 14:40
About 17mm or 22mm, depending on how you trust.

http://hypertextbook.com/facts/2002/JuliaKhutoretskaya.shtml
http://www.clarkvision.com/imagedetail/eye-resolution.html

FlyingPhotog
7th of January 2009 (Wed), 14:42
The FOV of the eye Vs degrees of what we actively perceive is different.

We can actually "see" much more on the periphery than we "see" in the brain.

dpark
7th of January 2009 (Wed), 14:43
For me its 35mm.

50mm is too tight and 24mm way too wide and perspective is unnatural (but its my favorite FL).

Perspective on 35mm (on FF) replicates how my eyes see the world

That's not the focal length of your eye, though. A 50mm lens looks much different on a crop frame than on a full frame. Likewise, a 35mm focal length would be much different on a full frame than on your retina.

KarlosDaJackal
7th of January 2009 (Wed), 14:45
On full frame/film I've found similar to you.

That is 70mm is pretty close to what I see with one eye open. 50mm is probably closer to what I see with both eyes open.

richardchoi
7th of January 2009 (Wed), 14:51
Remember, your brain won't let you "see" the periphery (like FlyingPhotog said), so your vision is actually quite wider than you think it is. I mean, if you tried looking it up on google you'd probably find 17~24mm.

Familiaphoto
7th of January 2009 (Wed), 14:55
Remember, your brain won't let you "see" the periphery (like FlyingPhotog said), so your vision is actually quite wider than you think it is. I mean, if you tried looking it up on google you'd probably find 17~24mm.

So does this mean our brain is a crop sensor and our lens (eye) is full frame.

Sounds like using an EF lens on a 40D. :D

FlyingPhotog
7th of January 2009 (Wed), 15:00
So does this mean our brain is a crop sensor and our lens (eye) is full frame.

Sounds like using an EF lens on a 40D. :D

In a sense yes. But, because the brain is an active player and not just a passive receptor like a camera sensor, you're still aware of what is outside the cropped image.

EF-S lens on a 5D would be more correct as the 40D can take an EF lenses.

Wilt
7th of January 2009 (Wed), 15:04
FL equivalence to the eye is a question with no valid answer...

The eye sees very sharply only within a very narrow AOV, outside that is quite blurry, and outside that is perception of motion only!

The eye sees a sharp image over a large area only by virtue of the integration of the images being performed in the brain, not the eye!

The size as seen in the viewfinder is very dependent upon viewfinder magnification designed into a camera...the magnification of a 20D is very different than the magnification in a 40D, for example, using the same lens! So what is meant by 'same as ' ???

The actual FL of the lens of the eye is equivalent to the depth of the eyeball...if the eyeball is too shallow, it is far sighted; if it is too deep, it is near sighted. And since muscles shape the lens of the eye, to focus, it has no single FL.

Perry Ge
7th of January 2009 (Wed), 15:06
I posted something like this before - the exact specs of the eye. Let me try dig it up. It's something like 17mm.

Edit: Found it, here we go, it's here: http://photography-on-the.net/forum/showpost.php?p=4982151&postcount=2

Odd way of thinking about it but here goes.

The eye is roughly 25mm in diameter, so the focal length is definitely smaller than that, probably about 15mm give or take. The pupil varies from about 1.5mm-8mm, making it something like an f/2 aperture that you can stop down to about f/10. Not sure how big the sensitive portion of the retina is (the whole thing is about 35mm, but it's curved), but it'd have a pretty decent 'crop factor' for sure. I did a google search and the fovea has a diameter of 1.5mm.

Ok I looked it up. Focal length is 17mm, retina size is 35mm, fovea 1.5mm. So there's no crop factor in terms of our field of view - a raw 17mm. But since the fovea is only 1.5mm, the area that we can actually focus on is really small, that's a 23x crop factor, so about 400mm. That seems about right really, the field of view we have is very much like a 17mm lens but we can only focus our attention on a little portion of the field, like a 400mm FOV.

Pretty cool that our eyes are pretty much 'full frame'. Don't get the 50mm part confused, a 50mm focal length is said to have a 'perspective' similar to the human eye, not a field of view similar to it. [This isn't quite right, since perspective is to do with distance and position]

The cool thing is that our rods and cones are kinda like pixels, and our AF system works in the same way, by detecting contrast.

But our dynamic range is insane and we can crank our ISOs to like, a million, hahaha.

Familiaphoto
7th of January 2009 (Wed), 15:10
In a sense yes. But, because the brain is an active player and not just a passive receptor like a camera sensor, you're still aware of what is outside the cropped image.

EF-S lens on a 5D would be more correct as the 40D can take an EF lenses.

I understand, just playing around.

oaktree
7th of January 2009 (Wed), 15:18
I've worn glasses since I was 5, so take the following with a grain of salt. Went shooting at our local zoo with a 1.6 crop body and the 50/1.4 and 70-300 DO. When I first looked through the viewfinder with the 50/1.4 on, I did a double take since nothing changed. The apparent size of the eagle and tree I was focusing on looked the same with or without the camera.

So for my eyes, 50 x 1.6 = 80mm looks the same (magnification wise anyway; view wise the XTi's viewfinder cuts off a lot of "peripheral" vision).

dpark
7th of January 2009 (Wed), 15:32
The actual FL of the lens of the eye is equivalent to the depth of the eyeball
I'm pretty sure that's not correct. First off, the lens of the eye isn't of negligible thickness. Second, the eye isn't truly a single-lens system. The cornea (and anterior chamber) and lens work together. These two aspects work together to produce an effective focal length that isn't the same as the depth of the eyeball.

Wilt
7th of January 2009 (Wed), 15:40
I'm pretty sure that's not correct. First off, the lens of the eye isn't of negligible thickness. Second, the eye isn't truly a single-lens system. The cornea (and anterior chamber) and lens work together. These two aspects work together to produce an effective focal length that isn't the same as the depth of the eyeball.

Inherently itne distance from the image formation layer of the eye to the lens is the FL, or else you always have an out of focus system!

dpark
7th of January 2009 (Wed), 16:23
Inherently itne distance from the image formation layer of the eye to the lens is the FL, or else you always have an out of focus system!
That definition only holds for thin lenses. The eye (like a camera lens) is not a single thin lens. It's a system of lenses (the cornea has a lens effect). How do you measure distance from the lens when there's more than one lens?

For a thick lens, or for a lens system, the effective focal length is not simply the distance from the rear lens (or front lens, or any other arbitrary point) to the focal plane. It's the distance from the principle point to the focal plane. The principle point (actually points, there's two) in a way describes the effective location of an equivalent thin lens, and does not necessarily lie inside a physical lens at all.

joedlh
7th of January 2009 (Wed), 16:37
There are really two questions here.

Most people are answering the implied question, which is: "What focal length of a lens on a 35mm camera gives a field of view similar to the unaided human eye?"

The literal answer to your question the way you actually phrased it is related to the optics of the human eye: "For an object distance of infinity, the focal length of the eye is equal to the fixed distance between the lens and the retina, about 1.7 cm"
http://hypertextbook.com/facts/2002/JuliaKhutoretskaya.shtml

SkipD
7th of January 2009 (Wed), 16:38
Something that most folks claiming that "XX focal length looks like what I see with my eyes" are totally missing is that the camera has additional optical components which are part of what they are seeing. The viewfinder optics play a VERY LARGE part in forming the image that you see through the viewfinder. Thus, you could have several cameras - all with the same focal length lens attached - that all show you different sized images through the viewfinder.

Wilt
7th of January 2009 (Wed), 16:43
That definition only holds for thin lenses. The eye (like a camera lens) is not a single thin lens. It's a system of lenses (the cornea has a lens effect). How do you measure distance from the lens when there's more than one lens?

For a thick lens, or for a lens system, the effective focal length is not simply the distance from the rear lens (or front lens, or any other arbitrary point) to the focal plane. It's the distance from the principle point to the focal plane. The principle point (actually points, there's two) in a way describes the effective location of an equivalent thin lens, and does not necessarily lie inside a physical lens at all.

Ah, you're getting very nerdy in your answer! ;) I don't disagree on that point...it is not merely the removeable lens of the eye (which is replaced in cataract surgery) by itself. The effective FL of the eye's 'lens system' is still the distance to the back of the eyeball where the image is formed, or else you are myopic or presbyopic

Wilt
7th of January 2009 (Wed), 16:44
Something that most folks claiming that "XX focal length looks like what I see with my eyes" are totally missing is that the camera has additional optical components which are part of what they are seeing. The viewfinder optics play a VERY LARGE part in forming the image that you see through the viewfinder. Thus, you could have several cameras - all with the same focal length lens attached - that all show you different sized images through the viewfinder.

Sentence #4 in message #12 of this thread! :D

bric-a-brac
7th of January 2009 (Wed), 16:46
why? did you pick one up for cheap on ebay? I didn't think they made EOS conversion mounts for those. :p

I studied this over the summer, and a "standard" lens is designed to reproduce approximately the same field of view as the human eye. as most people are familiar with from experience comparing digital and full frame bodies, the same lens has a different field of view depending on the size of the image plane used. the quick "official" rule of determining the "standard" lens focal length which achieves the human eye field of view is focal length = the diagonal length of the image plane (distance from one top corner to opposite bottom corner). On a full frame, this is approx 46mm, and historically, 50mm has been considered close enough. But if you're working with medium format? closer to 80mm, and 150mm for a 4x5 camera. all reproduce a field of view similar to the human eye.

I realize this doesn't exactly help explain what the focal length of the human eye actually is, but I hope it might have answered the question on an applicable level.

SkipD
7th of January 2009 (Wed), 16:47
Sentence #4 in message #12 of this thread! :D... And I did say "most folks". ;)

joedlh
7th of January 2009 (Wed), 16:49
So does this mean our brain is a crop sensor and our lens (eye) is full frame.



Not quite. It's the other way around, and then some. The fovea of the retina is the only part of the eye with good resolution. It's only a small percentage of the surface area of the retina. If you hold a dime at arm's length, it will cover the approximate retinal area of the fovea. The eye compensates for this limited field of view by making saccadic movements at a frequency of around 5 per second. During those movements, successive copies of the image "captured" on the retina are sent to the occipital lobe of the brain, which builds an integrated image of the visual environment. That image has a much larger field of view -- more properly called a field of perception -- than the eye itself.

The analogy is like painting with light with a camera.

Take the integration a step farther and the image is integrated with data in the parietal lobe which tells you things like "where am I?".

dpark
7th of January 2009 (Wed), 16:57
Ah, you're getting very nerdy in your answer! ;)
True enough. :)

The effective FL of the eye's 'lens system' is still the distance to the back of the eyeball where the image is formed, or else you are myopic or presbyopic
Yes, it's the distance to the retina, but from where? From the principle point, not the lens (though the principle point might be located within in the lens).

Photography circles use a really simple (and inaccurate) definition of focal length, which is strange to me, since the even crappy photographic lenses are too complex for the definition to apply. If the effective focal length were really just the distance from the rear lens (or another arbitrary lens) to the focal plane, then that would me the rest of the internal lenses would be doing absolutely nothing.

Wilt
7th of January 2009 (Wed), 16:59
I studied this over the summer, and a "standard" lens is designed to reproduce approximately the same field of view as the human eye. .

This is one of the biggest urban legends of photography! Read first 3 sentences of message 12, for an explanation of why it is hard to even characterize 'FOV of the human eye' in any meaningful manner. Joedlh provides a lot more detail about these points.

Wilt
7th of January 2009 (Wed), 17:00
True enough. :)


Yes, it's the distance to the retina, but from where? From the principle point, not the lens (though the principle point might be located within in the lens).

Photography circles use a really simple (and inaccurate) definition of focal length, which is strange to me, since the even crappy photographic lenses are too complex for the definition to apply. If the effective focal length were really just the distance from the rear lens (or another arbitrary lens) to the focal plane, then that would me the rest of the internal lenses would be doing absolutely nothing.

I refuse to play any longer into this debate. FL is the distance from the rear node of the lens system to the focal plane.

JeffreyG
7th of January 2009 (Wed), 17:02
just out of curiosity I had my 28-135 on my elan 7e the other day trying to figure out what the focal length of my eye is...

I was wearing my glasses and I saw that at about 70mm what I saw through the camera was roughly the same as without.

does that sound about right or did I get that result from my glasses?

All you measured was the magnification of the viewfinder. This is how it is defined BTW, by comparing the relative appearance of a scene at a given focal length.

As was covered, the true focal length of the eye is very short, and the effective AOV (don't forget that the eye is not 135 nor APS-C format!) is very, very wide.

Returning to format, also realize that of course the eye works nothing like film. There is a center 'sharp' region as well as a large unsharp region. Also, the mind can selectively focus on very small areas. When reading this text you are probaly 'seeing' the effective AOV of a very long telephoto lens and ignoring the rest.

tonylong
7th of January 2009 (Wed), 17:03
The eye sees very sharply only within a very narrow AOV, outside that is quite blurry, and outside that is perception of motion only!

The eye is a Lensbaby:)!

Wilt
7th of January 2009 (Wed), 17:06
The eye is a Lensbaby:)!

There is probably a bit of truth to that statement! It certainly does not have a single angle to the subject, having a variable lens plane; although it cannot vary the angle between the lens plane and focal plane, like the Lensbaby can.

tonylong
7th of January 2009 (Wed), 17:13
There is probably a bit of truth to that statement! It certainly does not have a single angle to the subject, having a variable lens plane; although it cannot vary the angle between the lens plane and focal plane, like the Lensbaby can.

You just have to do some tilting and shifting, that's all!

Wilt
7th of January 2009 (Wed), 17:32
You just have to do some tilting and shifting, that's all!


I can't alter the relationship of the lens of my eye to the retina. Have you got some muscles in your eyes which permit the relationship to be altered? ;)

dpark
7th of January 2009 (Wed), 17:33
I refuse to play any longer into this debate. FL is the distance from the rear node of the lens system to the focal plane.
So you're telling me that when you adjust a Bigma, the rear lens actually moves from 50mm away from the sensor all the way to 500mm away from the sensor? You're saying that the rear element has 450mm of travel? The Bigma isn't even that long fully extended.

Also, can you please explain how a 2x teleconverter turns a 50mm focal length into a 100mm focal length, and a 300mm into a 600mm, even though the teleconverter length is constant?

Your definition of focal length simply doesn't make sense with practical lenses.

http://en.wikipedia.org/wiki/Focal_length#General_optical_systems
http://en.wikipedia.org/wiki/Cardinal_point_(optics)#Principal_planes_and_point s

JeffreyG
7th of January 2009 (Wed), 17:39
So you're telling me that when you adjust a Bigma, the rear lens actually moves from 50mm away from the sensor all the way to 500mm away from the sensor? You're saying that the rear element has 450mm of travel? The Bigma isn't even that long fully extended.

Also, can you please explain how a 2x teleconverter turns a 50mm focal length into a 100mm focal length, and a 300mm into a 600mm, even though the teleconverter length is constant?

Your definition of focal length simply doesn't make sense with practical lenses.

http://en.wikipedia.org/wiki/Focal_length#General_optical_systems
http://en.wikipedia.org/wiki/Cardinal_point_(optics)#Principal_planes_and_point s

The lens you are talking about is a 'telephoto'. This means the rear nodal point of the lens is technically in front of the front element. This can only be done with multiple elements.

Let me back up. The focal length of a lens is the distance from the rear nodal point to the focal plane. For a simple convex lens this is simply the distance from the center of the lens to the focal plane. If you were to use a simple lens as a telescope for instance, the focal length of the telescope would be the length of the body.

All modern lenses are no simple, and some have more than 20 elements in them. Most of the long focal length ones are 'telephoto', which I defined above. Even stranger, not all of the long lenses that you can buy are technically telephoto lenses, but they are all described as such as photographers have gotten in incorrect idea that 'telephoto = long'.

I digress. The human eye is two elements in one group. As such, it is not capable of being retrofocal or telephoto, and so the focal length of the lens is very nearly the true distance from the center of the lens to the focal plane.

Wilt
7th of January 2009 (Wed), 17:43
So you're telling me that when you adjust a Bigma, the rear lens actually moves from 50mm away from the sensor all the way to 500mm away from the sensor? You're saying that the rear element has 450mm of travel? The Bigma isn't even that long fully extended.

Also, can you please explain how a 2x teleconverter turns a 50mm focal length into a 100mm focal length, and a 300mm into a 600mm, even though the teleconverter length is constant?

Your definition of focal length simply doesn't make sense with practical lenses.

http://en.wikipedia.org/wiki/Focal_length#General_optical_systems
http://en.wikipedia.org/wiki/Cardinal_point_(optics)#Principal_planes_and_point s

No I did not state that at all. You did.

A 'telephoto' design optically is shorter than the FL of the lens, and a 'retrofocus' design (wide angle) is longer than the FL of the lens. A 'long focus' lens would have the same distance from the rear node to focal plane as its FL designation.

tonylong
7th of January 2009 (Wed), 17:51
I can't alter the relationship of the lens of my eye to the retina. Have you got some muscles in your eyes which permit the relationship to be altered? ;)

OK, I'm trying, and after a few hours working at my computer today I feel like I'm succeeding!

dpark
7th of January 2009 (Wed), 17:57
No I did not state that at all. You did.

A 'telephoto' design optically is shorter than the FL of the lens, and a 'retrofocus' design (wide angle) is longer than the FL of the lens.
What? What does "optically shorter" even mean? You said "FL is the distance from the rear node of the lens system to the focal plane." That definition has nothing to do with whether a lens is telephoto or wide angle.

If focal length is truly just the distance from the rear element of a lens system to the focal plane (the sensor), then the rear element of the Bigma must move 450mm from its shortest to its longest focal length. If focal length is not the distance from the rear element to the focal plane, then your definition is not correct.

Wilt
7th of January 2009 (Wed), 18:01
What? What does "optically shorter" even mean? You said "FL is the distance from the rear node of the lens system to the focal plane." That definition has nothing to do with whether a lens is telephoto or wide angle.

If focal length is truly just the distance from the rear element of a lens system to the focal plane (the sensor), then the rear element of the Bigma must move 450mm from its shortest to its longest focal length. If focal length is not the distance from the rear element to the focal plane, then your definition is not correct.

'telephoto' and 'retrofocus' are optical designs that alter the distance from the lens to focal plane, so that they are NOT determined by the focal length of the lens. For example, I can mount an 8mm fisheye on a 35mm SLR, even though the reflex mirror would hit the back of the lens if it was not a 'retrofocus' design! The position of the rear node, in the design, is same as the FL. Similarly, I must lock up the reflex mirror if I wanted to use a superwide lens on a Hasselblad, because the lens is not a 'retrofocus' and the mirror would destroy itself on the swing upward, so I lose the ability to look thru the lens but must use an auxiliary viewfinder.

Wilt
7th of January 2009 (Wed), 18:14
If focal length is truly just the distance from the rear element of a lens system to the focal plane (the sensor), then the rear element of the Bigma must move 450mm from its shortest to its longest focal length. If focal length is not the distance from the rear element to the focal plane, then your definition is not correct.

It is NOT, literally. It is the distance from the 'rear node', a position defined by the optical design, and very often 'inside the lens' -- which is NOT 'the rear of the lens'

JeffreyG
7th of January 2009 (Wed), 18:32
What? What does "optically shorter" even mean? You said "FL is the distance from the rear node of the lens system to the focal plane." That definition has nothing to do with whether a lens is telephoto or wide angle.

If focal length is truly just the distance from the rear element of a lens system to the focal plane (the sensor), then the rear element of the Bigma must move 450mm from its shortest to its longest focal length. If focal length is not the distance from the rear element to the focal plane, then your definition is not correct.
You are using 'node' and 'element' interchangeably which is incorrect. The node is not a physical part of the lens, it is a part of the physics of the optics of the lens assembly. The 'rear element' is just that, the last piece of glass. The 'rear node' can be anywhere depending on the design of the lens.

If the rear node is located behind the rear element of the lens assembly, then the lens is considered a 'retrofocus' design.

If the rear node is located before the front element of the assembly then the lens is considered a 'telephoto' design.

If the rear node is located inside the lens assembly then the design is neither.

You are arguing this somewhat belligerently for someone who is reading stuff on Wikipedia and not understanding it correctly.

mehran.mo
7th of January 2009 (Wed), 18:35
I felt compelled to give a brief explanation of focal length of a single lens. Think of a lens as the boolean intersection of two fused spheres. Your focal length is the radius of that sphere. Now that applies to a single, simple lens. There are different ways of calculating the FL of a lens system. And yes at infinity focus the rear nodal point of a lens is separated from the sensor/film by the distance of the FL.

http://img399.imageshack.us/img399/9124/95036991pc5.jpg

Also, comapring a human eye to a camera system doesn't work very well. Our retina has different concentration of cones and rods at different areas, thats why the peripheral area is not clear, not just because the brain renders it that way. Also our retina is a curved plane, therefore we do not get the wacky distortion of a wide lens. It is also said that the sharp area of the human vision is equivalent to about 100mm on35mm basis. But the whole vision is muuuch wider.

tonylong
7th of January 2009 (Wed), 18:36
It is NOT, literally. It is the distance from the 'rear node', a position defined by the optical design, and very often 'inside the lens' -- which is NOT 'the rear of the lens'

You are using 'node' and 'element' interchangeably which is incorrect. The node is not a physical part of the lens, it is a part of the physics of the optics of the lens assembly.

If the rear node is located behind the rear element of the lens assembly, then the lens is considered a 'retrofocus' design.

If the rear node is located before the front element of the assembly then the lens is considered a 'telephoto' design.

If the rear node is located inside the lens assembly then the design is neither.

This here what you guys're sayin' is an important point that can clear up a lot of confusion, as well as confuse a lot of clear points.

SquareOne
7th of January 2009 (Wed), 18:43
So does this mean our brain is a crop sensor and our lens (eye) is full frame.

Sounds like using an EF lens on a 40D. :D
Great analogy. It really is pretty close to that from what I've read.

Wilt
7th of January 2009 (Wed), 18:48
This here what you guys're sayin' is an important point that can clear up a lot of confusion, as well as confuse a lot of clear points.


A lens can be a 'telephoto' or it can be 'retrofocus' or it can be neither of those.

The rear nodal point can be in front of the front element in a 'telephoto' because it is designed for compactness so that the distance from the front of the lens to the film plane is less than the focal length of the lens...the rear node distance to film plane can be farther than the length of the lens.

In the 'retrofocus lens the rear node distance to focal plane is the FL, but the rear of the lens is in front of that; again the node position is outside the lens.

Or, in a non-telephoto and non-retrofocus design, the nodal point could be inside the lens, like it might be with an 70mm lens on 35mm camera.

Wilt
7th of January 2009 (Wed), 18:50
Great analogy. It really is pretty close to that from what I've read.

The reality is closer to the lens being a telephoto and the brain is Photoshopping small sections into a larger total image.

mehran.mo
7th of January 2009 (Wed), 18:52
bingo ^

Also, the human eye has about 200 degree viewing angle which renders it very very wide. something like 3mm in terms of 35mm.

dpark
7th of January 2009 (Wed), 19:10
You are using 'node' and 'element' interchangeably which is incorrect. The node is not a physical part of the lens, it is a part of the physics of the optics of the lens assembly. The 'rear element' is just that, the last piece of glass. The 'rear node' can be anywhere depending on the design of the lens.

If the rear node is located behind the rear element of the lens assembly, then the lens is considered a 'retrofocus' design.

If the rear node is located before the front element of the assembly then the lens is considered a 'telephoto' design.

If the rear node is located inside the lens assembly then the design is neither.
So basically the term "rear node" is synonymous with "principle point" (presumably with "front node" referring to the other principle point). I was assuming node was just being used as a synonym for element, as we'd been talking about physical lenses up until that point. Can I assume that "rear node" is a term used pretty much exclusively in photography circles? It's not something I have come across until now.

You are arguing this somewhat belligerently for someone who is reading stuff on Wikipedia and not understanding it correctly.

Eh. I'm a bit embarrassed about not knowing what "rear node" means, but that's an issue of differing terminology. I don't think my posts were especially belligerent. I gave concrete examples (Bigma lens moving 450mm) because I wanted either confirmation that I was right or an explanation of why I was wrong. You've now provided both, so thank you. :)

skygod44
7th of January 2009 (Wed), 19:11
...If you hold a dime at arm's length, it will cover the approximate retinal area of the fovea....

How many Yen is that? ;)

Oh and here's a factor just to screw around with all you optics dudes. We have TWO eyes! So the field of "perception" (as opposed to field of view) is utterly dissimilar to that of a SINGLE lens reflex camera.

And if you add in optical illusions, such as the Hermann grid illusion (below) wherein we see blobs that don't exist (which at any focal length, our dSLRs don't do!) you can see that our eyes, in conjunction with the big blob of grey and white matter in our heads makes "focal length" the easiest area to discuss.

btw, I always thought that "focal length" has a definition: it's the distance from the centre of a convex lens or concave mirror to the point where parallel rays of light (ie, at infinity) converge. Just because our lens can change shape, doesn't alter the focal length.

You're also missing out that the cornea has a fixed, but modest role in the initial stages of convergence. But the sake of ease, I think we can all-but-ignore this, as it would only move the measurement outwards a little.

So, I'm gonna say the FL of the human eye is......

20mm, plus/minus a few mm! Oh and enjoy the grid....there are no dots at the intersections of the horizontal and vertical lines!!!!!

dpark
7th of January 2009 (Wed), 19:11
It is NOT, literally. It is the distance from the 'rear node', a position defined by the optical design, and very often 'inside the lens' -- which is NOT 'the rear of the lens'
Thanks. That was the point of confusion. I thought you were using rear node to refer to the rear element. Apparently we were just using different terms for the same thing.

Wilt
7th of January 2009 (Wed), 19:17
So basically the term "rear node" is synonymous with "principle point" (presumably with "front node" referring to the other principle point). I was assuming node was just being used as a synonym for element, as we'd been talking about physical lenses up until that point. Can I assume that "rear node" is a term used pretty much exclusively in photography circles? It's not something I have come across until now.



Eh. I'm a bit embarrassed about not knowing what "rear node" means, but that's an issue of differing terminology. I don't think my posts were especially belligerent. I gave concrete examples (Bigma lens moving 450mm) because I wanted either confirmation that I was right or an explanation of why I was wrong. You've now provided both, so thank you. :)

Right.

And on your second paragraph, I have to agree with Jeff about my assessment of the general tone in your messages, even if you didn't mean it. I was thinking "Smart college kid who thinks he knows it all better than the 'these idiots' who have been around for years" :cool:

dpark
7th of January 2009 (Wed), 19:31
Right.

And on your second paragraph, I have to agree with Jeff about my assessment of the general tone in your messages, even if you didn't mean it. I was thinking "Smart college kid who thinks he knows it all better than the 'these idiots' who have been around for years" :cool:
Sorry if I came off rude. I was a bit annoyed by your comment: "I refuse to play any longer into this debate. FL is the distance from the rear node of the lens system to the focal plane."

Not knowing what "rear node" meant, this seemed like you were making a really outlandish claim and refusing to consider that you might be wrong. If you go back and swap "rear node" for "rear element", your posts look really silly. Of course, since everyone else apparently knows what rear node means except me, I'm the one who looks silly.

Wilt
7th of January 2009 (Wed), 19:47
Sorry if I came off rude. I was a bit annoyed by your comment: "I refuse to play any longer into this debate. FL is the distance from the rear node of the lens system to the focal plane."

Not knowing what "rear node" meant, this seemed like you were making a really outlandish claim and refusing to consider that you might be wrong. If you go back and swap "rear node" for "rear element", your posts look really silly. Of course, since everyone else apparently knows what rear node means except me, I'm the one who looks silly.

Water under the bridge. FWIW, my comment about refusing to play was prompted by what seemed to be endless efforts to be 'more right', rather than taking an earlier statement for it fundamental soundness...the 'I know it better' college age retorts, along with the "Are you telling me (idiot non-fact)", reinforcing my idiocy! :)

dpark
7th of January 2009 (Wed), 19:59
Well, next time I'll try to be more polite with my incredulity* when I totally misunderstand what you're saying. :)

*bonus know-it-all word!

Wilt
7th of January 2009 (Wed), 20:06
I am over 40 and I already get my share of 20-something wisdom from sons-in-laws and daughters. I have a thick skin already! :cool: Fortunately for most, POTN is an information exchange, not a bunch of self-bolstering ego opportunities like seen on some newsgroups

joedlh
7th of January 2009 (Wed), 20:20
How many Yen is that? ;)



The full Moon. And let's not get into the illusion of Moon's angular size at the horizon and directly overhead. :lol: