Focal length confusion

Derrel

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Quite a LOT of lenses lose effective focal length as they are focused closer...many,many,many lenses lose effective FL in fact...not sure where the idea that loss of focal length is "rare" or "uncommon" is coming from...
 
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GaryPolter

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Yes. The focal length does change as you focus the lens on subjects increasing closer to the camera.

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)

Thanks to everyone for the great discussion on this topic.

I'm confused as to whether the focal length changes as you focus the camera.

If have a lens with 110mm focal length, and one object 300mm away and another 150mm away how will I know which one is in focus? How do I control the focus without changing the focal length?
 

Derrel

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Yes, typically in the extreme close-up range there WILL be a loss of effective focal length with many,many lenses. Your last question, about two objects, one 300mm away and another 150mm away...you're asking how you will know which one will be in focus????

What do you mean? Only ONE object will be in focus...you can focus by moving the camera closer or farther away, or by using the focusing ring on the lens...you will not be able to bridge a distance of 150mm/300mm with depth of field to cover with a "fixed" camera...you should be able to see through the viewfinder with an SLR Camera what is in focus...
 

Helen B

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I'm confused as to whether the focal length changes as you focus the camera.

For a simple lens for which all parts (elements) of the lens move together, the focal length never changes. There are some lenses, becoming increasingly common, which deliberately shorten in focal length as they focus closer by rearranging the lens elements in a slightly similar way to what happens inside a zoom lens when it is zoomed. This allows them to have a greater focus range (magnification) without having an excessively long focus travel; better corrections at close distances; and less loss of effective aperture as they focus close. I called such lenses 'special' to try to avoid confusion with the fundamental optical principles under discussion here.

If have a lens with 110mm focal length, and one object 300mm away and another 150mm away how will I know which one is in focus? How do I control the focus without changing the focal length?
You move the lens in and out - ie you change the image distance. This is all the focusing helicoid does on a normal lens with no floating elements or groups.

Best,
Helen
 
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clanthar

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For a simple lens (designed so that the nodal point is at the center of the lens) the focal length increases as you focus the lens on increasingly closer subjects. The lens will have a focal length at infinity that is fixed. Let's make it practical:

You have a 150mm lens on a 4x5 camera. Point the camera at the mountains and focus the lens and the distance from the center of the lens to the film will be 150mm. (At that point the f/stops for the lens can be calculated as: f number = focal length/diameter of the aperture. Let's assume the maximum diameter of the aperture then is 26.5 mm. The lens then is a 150mm f/5.6).

As you focus the lens on increasing closer subjects it physically moves away from the film. If this is a simple lens then the elements of the lens remain in a fixed relationship. Focused on a subject 10 feet from the camera the lens will be approx. 160mm from the film. It will not be a 160mm lens at infinity but in this case its focal length will be 160mm -- you can hold up a ruler if you have to.

Focus this lens at a subject 2 feet away and the lens will be 265mm from the film. It will have a focal length of 265mm. (AND it's fixed maximum aperture of 26.5mm means that the lens will be set at f/10 when the inscribed setting on the f/stop ring is 5.6. This is why it's helpful to think of focal length in this way.)

In the end this get's down to definition of terms. I'm defining focal length as no more and no less than the Webster dictionary definition: "focal length: the distance from the optical center of a lens or curved mirror to the point where light rays from a distant object converge." An understanding that takes that literally works well in the practice of photography.

Joe
 

Helen B

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In the end this get's down to definition of terms. I'm defining focal length as no more and no less than the Webster dictionary definition: "focal length: the distance from the optical center of a lens or curved mirror to the point where light rays from a distant object converge." An understanding that takes that literally works well in the practice of photography.

That is a correct definition. The idea that the focal length changes as the whole lens is simply moved as a unit is not correct, and is a cause of confusion.

Best,
Helen
 

AdrianC

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Yes. The focal length does change as you focus the lens on subjects increasing closer to the camera.

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)

Thanks to everyone for the great discussion on this topic.

I'm confused as to whether the focal length changes as you focus the camera.

If have a lens with 110mm focal length, and one object 300mm away and another 150mm away how will I know which one is in focus? How do I control the focus without changing the focal length?

As I said, there are two rings around the lens. One controls the focal length, one the focus point. You use focus ring to decide what you want to have in focus.

Also, if its a 110mm prime lens, then its focal length will not change. It will always be 110mm.

Do you happen to own just a kit lens? Take your camera, and examine it closely. You have the big ring around the lens. When you turn that you will "zoom". Now set the lens in manual focusing mode, grab the outer end of the lens, and turn it. Not the zoom ring, just the end of the lens. Your focus will change. (Note, this should move very smoothly. If it feels like its stuck, do NOT force it to turn.)
 

clanthar

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In the end this get's down to definition of terms. I'm defining focal length as no more and no less than the Webster dictionary definition: "focal length: the distance from the optical center of a lens or curved mirror to the point where light rays from a distant object converge." An understanding that takes that literally works well in the practice of photography.

That is a correct definition. The idea that the focal length changes as the whole lens is simply moved as a unit is not correct, and is a cause of confusion.

Best,
Helen

Looks like maybe agreeing to disagree is our option out here. I of course would say that not understanding how and why the focal length changes is what causes confusion. 30 years ago in a classroom I tried to explain to a group of students that as they focused the camera ever closer the f/stop was changing even though it was (like the focal length) engraved into the lens barrel. f/stop = focal length/aperture diameter. When I gave them that formula and a tape measure and told them to measure the focal length the confusion went away.

Take Care,
Joe
 

Helen B

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Marked 200mm on the new 70-200 VFR-II Nikkor zoom is really:
1.4m>>>134mm
2m>>>>147mm
3m>>>>164mm
5m>>>>176mm
10m>>>186mm
Infinity>>192mm

It would be an interesting exercise to mount that lens onto bellows, set the focus ring to infinity and the zoom to the nominal 200 mm then rack it out to the same magnification that is achieved at 1.4 m with the lens in 'normal' use. The image distance can then be measured (bellows flange-flange distance plus 192 mm) and the focal length calculated. One would expect that it is still 192 mm.

Best,
Helen

Edit: I was curious to know how Thom calculated those 'effective focal lengths' because it isn't straightforward. He appears to have used the formula
[FONT=Verdana, Arial, Helvetica, sans-serif]"You can calculate the focal length of a lens at its closest focus distance by using the formula minimum_focus / ((1/reproduction_ratio) + reproduction_ratio + 2)."[/FONT]

It should always be remembered that that formula applies only when the nodal space is zero or very close to zero. It often isn't for a zoom, telephoto or retrofocus lens. The assumption that it is zero when it isn't zero in reality will not have much effect when applying the formula on distant subjects, but the closer you get the more the error grows.
 
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Helen B

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Looks like maybe agreeing to disagree is our option out here. I of course would say that not understanding how and why the focal length changes is what causes confusion. 30 years ago in a classroom I tried to explain to a group of students that as they focused the camera ever closer the f/stop was changing even though it was (like the focal length) engraved into the lens barrel. f/stop = focal length/aperture diameter. When I gave them that formula and a tape measure and told them to measure the focal length the confusion went away.

Take Care,
Joe

Joe,

I have no confusion about why the image distance changes, why the effective aperture changes and why the focal length stays the same as long as the lens moves as a unit. I think that you have been teaching your students using optical terminology that is not in agreement with normal terminology in photographic optics. That can be a cause of confusion, as shown in this thread. Had you taught what effective aperture really is, you would not have had to invent the idea that the focal length changes.

Best,
Helen
 

clanthar

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Helen,

I'm not confused either. I understand that I'm not using common terminology, but I did not invent it (thanks though). I learned a very long time ago that this way of thinking about it can be useful -- got it from this guy Ernst Wildi at a seminar in a session about macro photography. What I've learned is that having multiple ways to understand things usually helps rather than confuses. I say "effective aperture" to my students as well and I also show them how to manage bellows factor by measuring magnification -- the more ways to think about it the better. I found the way Mr. Wildi explained was very useful.

Joe
 

Derrel

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Marked 200mm on the new 70-200 VFR-II Nikkor zoom is really:
1.4m>>>134mm
2m>>>>147mm
3m>>>>164mm
5m>>>>176mm
10m>>>186mm
Infinity>>192mm

It would be an interesting exercise to mount that lens onto bellows, set the focus ring to infinity and the zoom to the nominal 200 mm then rack it out to the same magnification that is achieved at 1.4 m with the lens in 'normal' use. The image distance can then be measured (bellows flange-flange distance plus 192 mm) and the focal length calculated. One would expect that it is still 192 mm.

Best,
Helen

Edit: I was curious to know how Thom calculated those 'effective focal lengths' because it isn't straightforward. He appears to have used the formula
[FONT=Verdana, Arial, Helvetica, sans-serif]"You can calculate the focal length of a lens at its closest focus distance by using the formula minimum_focus / ((1/reproduction_ratio) + reproduction_ratio + 2)."[/FONT]

It should always be remembered that that formula applies only when the nodal space is zero or very close to zero. It often isn't for a zoom, telephoto or retrofocus lens. The assumption that it is zero when it isn't zero in reality will not have much effect when applying the formula on distant subjects, but the closer you get the more the error grows.

The photographs show a 29% loss of focal length compared with the older 70-200mm VR Nikkor....all one has to do is look at the images to see that there is a HUGE loss of focal length as the lens is focused closer...the mannequin head is vastly,vastly smaller with the new lens....134mm is a far cry from 200mm, or even the 182mm of the old lens at MFD...

SOme lenses, as he mentions, like the 18-200 VR Nikkor lose a HUGE amount of focal length when focused closer....the Tamron 90mm f/2.8 AF-SP drops to ~73mm at 1:1..

The Nikkor 50mm f/1.8 AF-D OTOH, increases in FL to ~59mm from its normal Infinity focal length of 51.6mm when racked out to its .45m MFD...

70-200mm AF-S VR II Lens Review by Thom Hogan

I'm still a bit unclear on how/what/why the OP is asking such an odd question...it would seem that a few minutes actually focusing on close-up items would answer his questions.
 
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GaryPolter

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I'm still a bit unclear on how/what/why the OP is asking such an odd question...it would seem that a few minutes actually focusing on close-up items would answer his questions.

I wish I could do this, but I don't actually have an SLR camera. I came here because I believed (and was proven correct) that all of you are very knowledgeable about lenses.

The main purpose of me asking this question is because I'm trying to create a realistic depth of field technique for a 3D computer application, but I was not clear on what the difference between focal length and focus was. All literature I've read on depth of field assumes knowledge of these two items.
 

Helen B

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As far as modeling a simple lens goes, and for applying the simple 1/f = 1/u + 1/v formula you mentioned in your original post, you can safely assume that the focal length is constant. This applies to all lenses that focus as a unit, without exception. Using any other theory will get you into trouble if you try to apply optical formulae.

The complication caused by lenses that alter the arrangement of their internal elements as they focus is probably not worth incorporating into a model, unless you wish to model the behaviour of a particular lens. In that case you would have to have information about the way that particular lens changes its focal length, because there are no fixed rules.

Best,
Helen
 

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