Reading mode: Light Dark

Lens physical length vs mm.

Go to last post
N

nickolas84

Well-known member
Messages
161
Solutions
1
Reaction score
29
In general there is a relationship at least above 35mm between lens length and lens mm.

But there is quite a bit of variation as well.

For example the FUJINON MK 18-55mm T2.9 is really long physically.

On the contrary a lens like the Sony 16-50mm F3.5-5.6 is much shorter even zoomed out!

Why is there such a discrepancy?
 
Differences in design. I don't know the specifics of either lens, but there are a lot of different ways to make a zoom. Or a prime lens for that matter.
 
But the lens mm is supposed to be a physical thing. The actual distance between the iris opening and the sensor surface area...
 
nickolas84 said:
But the lens mm is supposed to be a physical thing. The actual distance between the iris opening and the sensor surface area...
Click to expand...
No it isn't. The nominal focal length of a lens is the distance from some point to the sensor at which parallel light is focused to a point. But that point isn't the actual plane of the iris.

Just where that point is depends on the design of the lens. It can move about for zoom lenses - an internal zoom lens stays the same physical length but its focal length changes as some of its elements move inside the lens.
 
nickolas84 said:
But the lens mm is supposed to be a physical thing. The actual distance between the iris opening and the sensor surface area...
Click to expand...
It's the distance between the lens optical center (not iris opening) and the sensor surface.
 
My understanding is that it is from the 'point of convergence' to the imaging sensor that defines the focal length of a camera lens.



98b72bd42a814d5fbdca0f1bb5d720e3.jpg



--
www.flickr.com

The Colour of Night

Explore this photo album by Pete Oliver on Flickr!
www.flickr.com www.flickr.com
 
There you go. And it's all the other bits and pieces inside the lens that fill up the rest of the space. The number of elements, their size and position, and their range of motion and the mechanisms that allow them to do so, can vary a lot from lens to lens even if the focal length remains the same.

Lenses with larger maximum apertures are bigger, as are macro lenses with close maximum focus distances. And of course autofocus lenses need room for the mechanical bits, and maybe a motor to drive them.
 
Here is an other drawing to illustrate the point :

0c5a249fa70240539777041f641834d3.jpg

note the "lens focused at infinity"

2606041bd468439f8d7a0f54d57467f5.jpg.png

with a zoom lens , the "focusing lens" moves back and for to change the focal length (simplified version...)

Another illustration to show how how it is that from the external size you can't determine the focal length :



df8e599da4314c6ca89c3f061471b327.jpg
 
Last edited:
It's one of the most basic concerns of lens design, retrofocus & telephoto, making the lens longer or shorter than you'd expect from the focal length. It's easy to look up a beginner's guide.
 
nickolas84 said:
But the lens mm is supposed to be a physical thing. The actual distance between the iris opening and the sensor surface area...
Click to expand...
As others have pointed out (although some of those diagrams look screwy to me), the focal length at infinity is the distance from the sensor/film to the exit pupil.
The exit pupil is a virtual image, not a physical thing.
It doesn't have to be inside the physical lens.

There is also a (virtual) entrance pupil that determines how much light enters and is related to the F-number.
The entrance and exit pupils don't have to be at the same location.
How far apart they are determines pupil magnification (can be less than 1).

The pupils are located at the front and rear principal planes.
If you want to learn more try searching for something like "thick lens" and "principal planes" (or "cardinal points")
 
You anglo-saxons do not make a clear difference in language difference between a simple lens and what you call a "camera lens"

If you had a flat lens in front of your sensor/film and if you were taking a picture at infinity, the distance between your simple lens (could be a standard double convex, a achromatic doublet or an apochromatic triplet, doesn't matter) and your sensor/film would be the focal length of the said lens. If you were taking a picture of something closer to your lens than infinity, you would have to raise the distance from your sensor to get it in focus, hense you would get longer than the focal length.

A camera lens or "objectif photo" in french, is a complex assembly of lenses elements that allows to correct for many defects: light fall-off, astigmatism, spherical aberration, etc... The physical length doesn't have much to to with the effective focal lens especially if it's a rear focussing lens and more specifically if it contains a Fresnel element.
 
Tourlou said:
You anglo-saxons do not make a clear difference in language difference between a simple lens and what you call a "camera lens"

If you had a flat lens in front of your sensor/film and if you were taking a picture at infinity, the distance between your simple lens (could be a standard double convex, a achromatic doublet or an apochromatic triplet, doesn't matter) and your sensor/film would be the focal length of the said lens. If you were taking a picture of something closer to your lens than infinity, you would have to raise the distance from your sensor to get it in focus, hense you would get longer than the focal length.

A camera lens or "objectif photo" in french, is a complex assembly of lenses elements that allows to correct for many defects: light fall-off, astigmatism, spherical aberration, etc... The physical length doesn't have much to to with the effective focal lens especially if it's a rear focussing lens and more specifically if it contains a Fresnel element.
Click to expand...
I don't think this is helpful. We are obviously talking about a thick lens.

In any case, even with a thin lens it's not clear whether to measure from the vertex or some other point.
 
Bill, many people here have no clue what a focal lens is. Not everybody studied optics! Going down to the basics with a standard DC lens to explain what a focal length is compared to an effective focal length is, IMHO, relevant. Some extra knowledge never hurts.

Regards.

Rene
 
nickolas84 said:
In general there is a relationship at least above 35mm between lens length and lens mm.

But there is quite a bit of variation as well.

For example the FUJINON MK 18-55mm T2.9 is really long physically.

On the contrary a lens like the Sony 16-50mm F3.5-5.6 is much shorter even zoomed out!

Why is there such a discrepancy?
Click to expand...
Because photographic lens's focal length is nothing but imaginary number, while the size is real.

This single one-dimension number represents the lens's true characteristics : Angle of View (which is three dimensions) and the (interaction with) sensor size.

This imaginary number is used for the (easier) communication and design.

A bit something like the numerical value of Automatic Shift Gear of the car. The 4th auto gear is not the same as the 4th manual gear.

+++

Headache? Then, just accept it as it is :)
 
Last edited:
Tourlou said:
You anglo-saxons do not make a clear difference in language difference between a simple lens and what you call a "camera lens"

If you had a flat lens in front of your sensor/film and if you were taking a picture at infinity, the distance between your simple lens (could be a standard double convex, a achromatic doublet or an apochromatic triplet, doesn't matter) and your sensor/film would be the focal length of the said lens. If you were taking a picture of something closer to your lens than infinity, you would have to raise the distance from your sensor to get it in focus, hense you would get longer than the focal length.

A camera lens or "objectif photo" in french, is a complex assembly of lenses elements that allows to correct for many defects: light fall-off, astigmatism, spherical aberration, etc... The physical length doesn't have much to to with the effective focal lens especially if it's a rear focussing lens and more specifically if it contains a Fresnel element.
Click to expand...
In English we call that sort of possible confusion " double entendre"

What is the French equivalent ?

Anyway....

If you read the context , it is pretty obvious we are not talking about a single piece of glass but a bunch of glass elements inside a tube that we also call lens.

From the OP :

For example the FUJINON MK 18-55mm T2.9 is really long physically.

This is the lens :

495e004a61e848d28d60ff65a682c0d5.jpg

it happens to be a cine lens, the T stop could have also been a clue.

BTW, in Italian it is the same as in French (lente e obbiettivo) however often lente is used to mean obbiettivo.
 
Last edited:
Tourlou said:
Bill, many people here have no clue what a focal lens is. Not everybody studied optics! Going down to the basics with a standard DC lens to explain what a focal length is compared to an effective focal length is, IMHO, relevant. Some extra knowledge never hurts.
Click to expand...
Your extra knowledge was not helpful, IMHO, because you didn't state from where in the lens the distance would be measured and this is exactly what the Original Poster (OP) raised.
Your distinction between thin and thick lens is not helpful because all practical lenses (and that's what we are discussing) are thick.
Sorry, it's not "extra knowledge" when it is unclear; it's "extra confusion".
 
If you're worried about precision, just use a f/10, 1 meter focal length PC lens and measure it from anywhere, error will be much less than 1%. I'm not after precise numbers here, more after the concept itself.
 
Tourlou said:
You anglo-saxons do not make a clear difference in language difference between a simple lens and what you call a "camera lens"
Click to expand...
You Franks do not make a clear difference in language between the relatively few native English speakers of Anglo-Saxon descent and the majority whose descent is French, Norman, German, Irish, Scottish, Spanish, Greek, Italian, Norwegian, Swedish etc. And, of course, much of modern English stems from those non-Anglo-Saxons.

Some of that majority (especially, perhaps, Irish and Scottish) would vigorously resent being lumped in with Anglo-Saxons.

--
---
Gerry
___________________________________________
First camera 1953, first Pentax 1985, first DSLR 2006
http://www.pbase.com/gerrywinterbourne
[email protected]
 
Last edited:
The simple answer is you can make the lens any physical length (within reason) irrespective of its focal length.

The two simple examples are telephoto (the lens is made a good deal shorter than its focal length for portability) and retrofocal (the lens is made so it is bigger than its focal length, usually so there is room behind it for the reflex mirror, though the Fuji cine lens you mentioned is made that way so it is easy for cine operators to use in the same way as all their other lenses.





To answer your basic question needs one to go no further than this. This is the limit of my optics (almost) so I am confident in saying it is enough.

--
Andrew Skinner
 
You must log in or register to reply here.

Similar threads

R
Lens for Sony a6500?
2
Replies
30
Views
7K
Phaser
P
E
Nikon Z7 II vs Sony A7RV for an extra €500?
2
Replies
27
Views
776
enfranchise
E
Z
A7R2 - one (or two) good everyday lens in range about 15-60mm
2
Replies
23
Views
775
Impulses
I
Johnnew Archibald - SMGJohn
  • Locked
Whats the point of crop sensors when we have 60mp full frame??
3 4 5
Replies
97
Views
5K
Mako2011
Mako2011
D
The "most used lens" a different question for prime shooters
Replies
19
Views
5K
Impulses
I

Keyboard shortcuts

f
Forum
m
My threads
Mobile site
About
Editorial content
Cameras & Lenses
Community
All content, design, and layout are Copyright © 1998–2025 Digital Photography Review All Rights Reserved.
Reproduction in whole or part in any form or medium without specific written permission is prohibited.
When you use DPReview links to buy products, the site may earn a commission.
©GPS Media - Guides, Products, Services.
Back
Top