Focal length confusion

[GaryPolter]

I'm a little confused by what is meant by focal length. Wikipedia says it's both how strong the light converges and the distance from the lens to the image plane. They also give an example that is not quite clear to me:

Assume the camera is set to a focal length of 50mm, and now I want to focus on an object 1000mm away. Wikipedia says "the lens must be moved 2.6mm further away from the image plane". Does this mean I change my focal length to 52.6?

The thin lens equation says 1/u + 1/v = 1/f. u is the distance from the lens to the object. v is the distance from the lens to where the image is formed. f is the focal length. So does the image sensor go on v or f?


Focal length - Wikipedia, the free encyclopedia

 

[Big Mike]

I'm no lens engineer, but you need to consider the 'focal point' of the lens. Which is the point where the light converges. This may change as the lens is focused, zoomed etc.
So it's not the whole lens that has to physically move, just certain elements, that will adjust the focal point (or not). :scratch:

 

[GaryPolter]

So I did a bit more digging and came across a post on this forum (Difference between focal length and focussing distance??? - The Photo Forum - Photography Discussion Forum). In this post, one person says focal length is fixed for a lens, and you change the focal ring to move the lens closer and further to the image sensor.

Say I buy a lens with a focal length between 80mm and 210mm. What does changing the focal length do compared to changing the focal ring (zoom)?

 

[Polyphony]

Focal length is the distance from a lens to the point at which light going through it converges.

Doesn't get much simpler than this: http://images.trustedreviews.com/images/article/inline/3973-FocalLength_diag.gif

 

[Big Mike]

In practical terms, I think you may be way over-thinking this.

This isn't the type of thing you need to be thinking about when considering which lens to buy. This is the stuff you think about when you are really bored and need to satisfy a geeky curiosity.

 

[Polyphony]

In practical terms, I think you may be way over-thinking this.

This isn't the type of thing you need to be thinking about when considering which lens to buy. This is the stuff you think about when you are really bored and need to satisfy a geeky curiosity.

Or took years and years of physics classes.

 

[GaryPolter]

Focal length is the distance from a lens to the point at which light going through it converges.

Doesn't get much simpler than this: http://images.trustedreviews.com/images/article/inline/3973-FocalLength_diag.gif

Say I have a lens set to a focal length of 110mm. Effectively, this means that the lens is 110mm away from the image sensor, right?

Now say I have an object 300mm away that I want to focus on. Using the lens equation, I know that the distance from the lens to the object image plane is 173.68mm.

How would I focus on this object? If I adjust the focal length, then the object image plane will change...

My overall goal is to understand this diagram: http://http.developer.nvidia.com/GPUGems/elementLinks/fig23-02.jpg

 

[AdrianC]

I don't know much about lenses, but I think you're confusing things.

The focal length tells you how much the lens zooms, basically. Then you have the focus ring which allows you to focus on whatever you want. There are two rings.

 

[prodigy2k7]

I'm a little confused by what is meant by focal length. Wikipedia says it's both how strong the light converges and the distance from the lens to the image plane. They also give an example that is not quite clear to me:

Assume the camera is set to a focal length of 50mm, and now I want to focus on an object 1000mm away. Wikipedia says "the lens must be moved 2.6mm further away from the image plane". Does this mean I change my focal length to 52.6?

The thin lens equation says 1/u + 1/v = 1/f. u is the distance from the lens to the object. v is the distance from the lens to where the image is formed. f is the focal length. So does the image sensor go on v or f?


Focal length - Wikipedia, the free encyclopedia

The beginning of that quote says "To focus a distant object" Its referring to the focusing, not the focal length. Focusing is just a small movement of lense(s) to ...focus

Im no expert but that sounds right doesn't it? i hope so ha

 

[prodigy2k7]

Focal length is the distance from a lens to the point at which light going through it converges.

Doesn't get much simpler than this: http://images.trustedreviews.com/images/article/inline/3973-FocalLength_diag.gif

Ya thats what I thought, but how do you explain the EF 24-70 F/2.8L USM?

"Of note is that the 24-70mm L reverse extends - it becomes longest at 24mm as shown above. Most lenses including the 24-105 L become fully extended at their longest focal length setting."

Canon EF 24-70mm f/2.8 L USM Lens Review

 

[clanthar]

Focal length is the distance from a lens to the point at which light going through it converges.

Doesn't get much simpler than this: http://images.trustedreviews.com/images/article/inline/3973-FocalLength_diag.gif

This is absolutely correct. I think what you're missing is this additional information:

Lenses have a specific "focal length at infinity" where they are as close to the film/sensor as possible. A lens closer to the sensor than it's focal length at infinity isn't in focus on anything. As you focus a lens on subjects increasing closer to the sensor the focal length increases. At the point where the lens focal length is double it's infinity focal length it will be equal distant between the sensor and the subject and you'll be into the macro range of photography.

Joe

 

[clanthar]

[/quote]
Ya thats what I thought, but how do you explain the EF 24-70 F/2.8L USM?
[/quote]

Focal length as the distance from the center of the lens (nodal point) to the film is the basic principle for simple lens designs. That all changed when this happened: P. Angenieux 35mm f/2.5. Lens designers discovered how to move the nodal point behind and/or in front of the lens center.

Joe

 

[Helen B]

Assume the camera is set to a focal length of 50mm, and now I want to focus on an object 1000mm away. Wikipedia says "the lens must be moved 2.6mm further away from the image plane". Does this mean I change my focal length to 52.6?

No. The focal length does not change as you focus (move) the lens (except for some special lenses that shorten in focal length when focusing on close objects). In the above case the focal length stays at 50 mm. The image distance changes from 50 mm (when the lens is focused on infinity) to 52.6 mm.

The thin lens equation says 1/u + 1/v = 1/f. u is the distance from the lens to the object. v is the distance from the lens to where the image is formed. f is the focal length. So does the image sensor go on v or f?

The sensor should be v from the thin lens. Notice that when u = infinity, f = v.

Best,
Helen

 

[clanthar]

Yes. The focal length does change as you focus the lens on subjects increasing closer to the camera. The lenses "focal length at infinity" does not change but it's effective focal length does. If it didn't change it wouldn't be necessary to calculate a bellows compensation factor for exposure in macro photography.

I used to have a very nice Nikkor 55mm f3/5 micro lens in which you could watch the aperture change size as the focus ring was turned -- they built the bellows factor adjustment in mechanically to compensate for the increased lens focal length.

Joe

 

[Helen B]

What you call the 'effective focal length' is usually called the image distance. The focal length does not change as you focus (no matter how close), unless the lens elements are designed to move with respect to one another to alter the focal length as the lens is focused.

The effective aperture is calculated from the entrance pupil diameter and the image distance in macro work, or more properly, from the true aperture multiplied by (1+m/mp) where m is the image magnification and mp is the pupil magnification.

Best,
Helen