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Meaning of Xmm for lens?
- Thread starter katatonia
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hbx2004
Senior Member
As I see it, equation above confirms, that focal length must change, when focusing (subject) distance changes.
From http://en.wikipedia.org/wiki/Focal_length -section "In photography":
"As (u) is decreased, (v) must be increased. For example, consider a normal lens for a 35 mm camera with a focal length of f = 50 mm. To focus a distant object (u=infinity), the rear nodal point of the lens must be located a distance v=50 mm from the image plane. To focus an object 1 m away (u=1000 mm), the lens must be moved 2.6 mm further away from the image plane, to v=52.6 mm."
Resulting from above: 1/1000+1/52.6=1/f ; thus f=49.97... mm
IMO, this is especially true when optical centre of the lens doesn't change (whole lens assembly is moved when focusing).
Greetings,
Bogdan
--
My pictures are my memories
http://freeweb.siol.net/hrastni3/
sherwoodpete
Veteran Member
No, you've muddled the mathematics here.
We have the known information:
Focal length = f = 50mm.
Subject distance = 1m = 1000mm.
The unknown quantity is the image distance v.
f = 50
u = 1000
v = ?
1/u + 1/v = 1/f
1/1000 + 1/v = 1/50
1/v = 1/50 - 1/1000
1/v = 0.02 - 0.001 = 0.019
v = 1/0.019 = 52.631579
So now we know that the distance from the optical centre of the lens to the sensor is 52.63mm (or it has been moved by 2.63mm from the infinity focus position).
Regards,
Peter
BA ba, that's a good reference. He understands.
--
Charlie Davis
Nikon 5700, Sony R1, Nikon D300
HomePage: http://www.1derful.info
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'Experience: Discovering that a claw hammer will bend nails.
Epiphany: Discovering that a claw hammer is two tools...'
David Hughes124436
Veteran Member
chuxter wrote:
... Everybody is wrong...
Next people will be saying that the front element of a zoom doesn't do the focussing in a lot of zooms, as I said and was apparentl wrong.
Regards, David
PS Most primes move the entire group but when the Olympus XA, XA2 etc came out in 1979 onwards the internal focusing (group) gave the compact a lot of advantages and things started to change but a lot of cameras with fixed or non-zoom lenses (as opposed to "primes" which I use to mean part of an interchangeable lens system on say a CRF or SLR) still focus by moving the entire group back and forth.
... Everybody is wrong...
I said that but I was wrong?
Luckily I said for simple lenses and 1/u + 1/v = 1/f and that's wrong?
This is a generalisation; you are saying lenses when you mean lens systems. At least I said I was talking about a simple lens.
Next people will be saying that the front element of a zoom doesn't do the focussing in a lot of zooms, as I said and was apparentl wrong.
Regards, David
PS Most primes move the entire group but when the Olympus XA, XA2 etc came out in 1979 onwards the internal focusing (group) gave the compact a lot of advantages and things started to change but a lot of cameras with fixed or non-zoom lenses (as opposed to "primes" which I use to mean part of an interchangeable lens system on say a CRF or SLR) still focus by moving the entire group back and forth.
No.
Yes, you were. But the OP has a complex lens. You were just showing off for remembering where your college physics book was. ;-)
Nope you were right.
But you also said, "But the focal length hasn't changed because it can't." which is wrong in 2008. I didn't have room to say that not everything you (and others) said was wrong. Sorry.
--
Charlie Davis
Nikon 5700, Sony R1, Nikon D300
HomePage: http://www.1derful.info
Bridge Blog: http://www.here-ugo.com/BridgeBlog/
'Experience: Discovering that a claw hammer will bend nails.
Epiphany: Discovering that a claw hammer is two tools...'
David Hughes124436
Veteran Member
- Snip! Snip! - - -
Yup, that's the trouble with generalisations.
BTW, I did optics at school in the 50's when I was about 13 or 14 not at college and it seemed normal at the time. Does the USA system not touch optics until 7 or 8 years later, and at college? (Quoting 1/u+1/v=1/f seems to have caused this cultural problem for me. I'd just assumed everyone had done it at school.)
BTW, are you sure the lens we're talking about, as opposed to answering a later point as I was, focuses by movement of one of the internal elements? Primes usually move back and forth... And as I said, most zooms move just the front element.
BTW 2, I'd love to have copies of the Physics paper (plain ordinary level at that point) that I took then and try it out on a class of 15 yr olds...
Regards, David
Yep...
The USA does not have ONE educational system. Never did. Never will. I took Physics when I was about 13 (like you), but most kids didn't. I took it again when I was about 17 (in high school). I took it again when I was in college (I was a Physics major).
Prolly 90% of the people in this thread have no idea what you were talking about. Most can't even divide w/o a calculator and then 50% would get the wrong answer. :-(
Not any more. The new lens designs don't do either of these things! Read the review here on dpr about the new Sigma 50mm f/1.4 prime...just as an example.
It would be depressing. As smart as some humans are, as a group of animals we continue to regress...at least in the USA we do. :-(
--
Charlie Davis
Nikon 5700, Sony R1, Nikon D300
HomePage: http://www.1derful.info
Bridge Blog: http://www.here-ugo.com/BridgeBlog/
'Experience: Discovering that a claw hammer will bend nails.
Epiphany: Discovering that a claw hammer is two tools...'
JulesJ
Forum Pro
That is not strictly true (below). The focal lens of a (single) lens is the distance from the centre of the lens to the point behind the lens at which parallel rays of light shone from the other side converge, This may or may not be the sensor depending on a few things.
Jules
Why can't you blow bubbles with chewing gum?
Jules
--
Why can't you blow bubbles with chewing gum?
hbx2004
Senior Member
Hi Jules,
Thank you for correcting my (simple) answer version
Anyway, my impression was, OP asked what defines FL for given lens (assembly), which is AFAIK (per definition):
Distance between optical centre of the lens (assembly) and sensor plane, when subject distance is at infinity distance, and subject projected on sensor is in-focus.
If subject isn't focused on sensor plane, it's (obviously) OOF... but FL is defined for in-focus state (and by subject distance) only.
Of course 50mm lens will, per it's basic/general definition, always be 50mm. But when we change subject distance (and we focus on it), we actually changed condition (of FL definition), where:
-we changed subject distance,
-to get subject in focus (on sensor's plane), we must change distance between optical centre of the lens (we can't change sensor's position, though),
-because of that (according to "famous" equation) FL changes (according to given situation: subject isn't at infinite distance anymore).
Ok, as I'm no expert in optics, there might be other special cases, I have no idea about.
Time for me to go to bed
Greetings,
Bogdan
--
My pictures are my memories
http://freeweb.siol.net/hrastni3/
Thank you for correcting my (simple) answer version
Anyway, my impression was, OP asked what defines FL for given lens (assembly), which is AFAIK (per definition):
Distance between optical centre of the lens (assembly) and sensor plane, when subject distance is at infinity distance, and subject projected on sensor is in-focus.
If subject isn't focused on sensor plane, it's (obviously) OOF... but FL is defined for in-focus state (and by subject distance) only.
Of course 50mm lens will, per it's basic/general definition, always be 50mm. But when we change subject distance (and we focus on it), we actually changed condition (of FL definition), where:
-we changed subject distance,
-to get subject in focus (on sensor's plane), we must change distance between optical centre of the lens (we can't change sensor's position, though),
-because of that (according to "famous" equation) FL changes (according to given situation: subject isn't at infinite distance anymore).
Ok, as I'm no expert in optics, there might be other special cases, I have no idea about.
Time for me to go to bed
Greetings,
Bogdan
--
My pictures are my memories
http://freeweb.siol.net/hrastni3/
Alan Martin256506
Forum Enthusiast
A better definition is: "The distance behind the lens' rear nodal point where a focused image would be formed, for a subject at infinite distance." That is, both the subject and the image are hypothetical, and focal length is a characteristic of the lens alone.
If we actually do have a subject at infinity, and a sensor receiving its focused image, then we can measure the distance between rear nodal point and sensor and say that it is the focal length.
But if we have a subject at some other distance, then it doesn't fit the definition. We can still measure the image distance, but it is no longer called the focal length.
The lens still has a focal length, we just haven't set things up properly to measure it. And that focal length cannot change unless the lens elements or their spacings are changed. (The field of view imaged by the sensor can change, because it depends on image distance rather than focal length.)
--
Alan Martin
sherwoodpete
Veteran Member
Note that the wikipedia article has stated the result v=52.6 mm accurate to 3 digits. We can if we wish verify that this is consistent by attempting to re-calculate the focal length. This gives a value of 49.97149914 mm. But since the input value was rounded to 3 digits, the result cannot be more accurate than 3 digits. When it is correctly rounded, it becomes 50.0 mm.
If we use the a more precise value of v=52.63157895,
then 1/1000+1/52.63157895=1/f ; thus f=50.000000002375 mm, but we can only use ten digits in this case, so f=50.00000000 mm.
Regards,
Peter
David Hughes124436
Veteran Member
The system here was Biology, Chemistry and Physics taught as seperate subjects for three years (from 11 -12 yrs old) then one or two dropped. Each subject taught by someone with a degree in that subject. Nowadays, they "do" "Science" meaning someone who hated biology at school and dropped it in favour of chemistry will be teaching biology. And in those days degrees involved exams rather than attendance. Worse still, the last two years of school are now done in a college where they have exam managers :-(
Worse still, you can get them by collecting box tops and have a degree (well, they call it a degree) in flower arranging, French without verbs and remedial maths part 1...
I'll join you in that collective sigh. The trouble is the failed snake oil salesmen go into politics and then run the country for the benefit of snake oil salesmen. What we'll do when the only ones who know what they are doing are all Chinese, worries me but, otoh, I'll be long gone then. And my grand children will be living in a world where understanding screwdrivers is a lost art...
Regards, David
hbx2004
Senior Member
Hi Peter,
I have somehow "dropped the ball" on this... having the feeling, nobody really understand what I'm trying to say -on top of that, my english isn't the best.
To be honest, when reading your "clarification", I simply couln't believe... like fatamorgana.... needless to say, I've checked what you've written
Thank you very much for insisting staying on topic and explaining the "catch" (to me).
Wish you the best,
Bogdan
--
My pictures are my memories
http://freeweb.siol.net/hrastni3/
I have somehow "dropped the ball" on this... having the feeling, nobody really understand what I'm trying to say -on top of that, my english isn't the best.
To be honest, when reading your "clarification", I simply couln't believe... like fatamorgana.... needless to say, I've checked what you've written
Thank you very much for insisting staying on topic and explaining the "catch" (to me).
Wish you the best,
Bogdan
--
My pictures are my memories
http://freeweb.siol.net/hrastni3/
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