Focal length of Nikkor 55-300mm lens

[Panama Hat & Camera]

In the manual of my Nikkor 55-300mm lens is written: "The focal length of this lens decreases as the focus distance shortens".
Is this a rule for all lenses? It is possible to calculate the true focal length if the focus distance is known?
Cheers,
Antonio.

[Shadowman]

In the manual of my Nikkor 55-300mm lens is written: "The focal length of this lens decreases as the focus distance shortens".
Is this a rule for all lenses? It is possible to calculate the true focal length if the focus distance is known?
Cheers,
Antonio.

The statement may or may not matter to you after reading this.

https://www.cambridgeincolour.com/tu...era-lenses.htm

[royphot]

Does it really matter?
It is a Zoom lens, so even if you wanted to calculate the real focal length at various focussing distance, you would need to do it for multiple zoom settings as well.
Then of course when lenses do get measured the stated focal lengths are often not the same as the actual focal length.

Does it really matter because you are using the lens to take photos, not make scientific measurements.

Roy

[Manfred M]

The focal length for any zoom lens is only the stated value when focused at "inifinity", as you get closer the focal length will indeed be different. The well regarded Nikkor f/2.8 70-200mm VR II (i.e. expensive pro glass) was reported to have a focal length of 135mm when set at 200mm and focused to the closest distance it can focus on.

Lenses (even primes) are not exactly the stated focal length and maximum aperture...

[Saorsa]

Using values at infinity give us a common basis for comparison and setting expectations. Aperture also varies. I've often seen folks complain about the 105mm Micro-Nikkor since the aperture changes at closer focus. One fellow claimed that his 3rd party lens didn't do that. It does but doesn't report a derived aperture, only the nominal at infinity focus.

[Panama Hat & Camera]

John,
I read the tutorial, and it is a good article, but I didn't find the answer there.
Thanks for your advice
Antonio.

[Panama Hat & Camera]

Roy,
My question really matters.
After reading what is written in the manual of my lens, I read the manual of other nikkor zoom lenses and there was nothing written about this subject.
What I would like to know was if the reduction of focal length is significative when using a small focus distance with my lens.
All the best,
Antonio.

[Panama Hat & Camera]

The focal length for any zoom lens is only the stated value when focused at "inifinity", as you get closer the focal length will indeed be different. The well regarded Nikkor f/2.8 70-200mm VR II (i.e. expensive pro glass) was reported to have a focal length of 135mm when set at 200mm and focused to the closest distance it can focus on.

Lenses (even primes) are not exactly the stated focal length and maximum aperture...

Manfred,
Thank you for your explanation. Because the principles of optics, I knew that focal lenght should decrease as the focus distance shortens, but I found odd when I read the statement only in the manual of my lens.
Again, thank you very much.
Antonio.

[Panama Hat & Camera]

Using values at infinity give us a common basis for comparison and setting expectations. Aperture also varies. I've often seen folks complain about the 105mm Micro-Nikkor since the aperture changes at closer focus. One fellow claimed that his 3rd party lens didn't do that. It does but doesn't report a derived aperture, only the nominal at infinity focus.

Brian,
Thank you for your explanation.
It is helpful.
Cheers,
Antonio.

[tclune]

If I understand your question, you're asking about focus breathing. A nice short article on it can be found here: http://www.bobatkins.com/photography...h_changes.html FWIW

[Acorn]

Does it really matter?
It is a Zoom lens, so even if you wanted to calculate the real focal length at various focussing distance, you would need to do it for multiple zoom settings as well.
Then of course when lenses do get measured the stated focal lengths are often not the same as the actual focal length.

Does it really matter because you are using the lens to take photos, not make scientific measurements.

Roy

Your reply, Roy, sounds like common sense to me!

[ajohnw]

Ken Rockwell isn't a bad place to look for an over view of problems like this. From his site on the 55-300mm

Even though Nikon cautions that the effective focal length shortens as one focuses more closely, I don't see any shrinkage. 300mm looks like 300mm, even at 4.6 feet (1.4 meters).

By comparison, the 28-300mm VR is nowhere near 300mm at its 300mm setting at 5 feet.

This 55-300mm lens shows the same tight image at 4.6 feet as the 28-300mm shows at 1.5 feet!

This can be a rather extreme effect on some lenses. The F2.8 70-200mm for instance. The VR II version shortens to 135mm at 200mm close up. The effect is no where near as noticeable on the earlier version. I'm fairly sure i have read a review on one lens where the opposite happens but don't quote me. Where it's most likely to matter is when trying to do semi macro work with a telephoto zoom. At a guess telephoto distances will be 20 to 30 or more times the focal length. A sort of part educated guess. I've definitely not noticed any zoom rate drop off on my 55-300mm at that sort of range. Not really tried it closer.

Actually I suspect this is another instance of web tom tom drums and very little factual information. You will probably find that at telephoto distances the effect is relatively small on most lenses. I have seen comments that lenses only have the stated focal length when focused to infinity - true if the designers choose to go that way. What may happen on lenses like the 70-200 is that they have chosen to do this to maintain high image quality.

It should be possible to work things out roughly from magnification and focus distances but the manufacturers don't give sufficient information. No saying where the max mag is or that min focus distance is the same at all focal lengths so it would all need to be physically measured.

John
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[Panama Hat & Camera]

If I understand your question, you're asking about focus breathing. A nice short article on it can be found here: http://www.bobatkins.com/photography...h_changes.html FWIW

Tom,
Thank you for article about focus breathing. It has everything I would like to know about the subject.
Again, thank you very much.
Antonio.

[Panama Hat & Camera]

Ken Rockwell isn't a bad place to look for an over view of problems like this. From his site on the 55-300mm



This can be a rather extreme effect on some lenses. The F2.8 70-200mm for instance. The VR II version shortens to 135mm at 200mm close up. The effect is no where near as noticeable on the earlier version. I'm fairly sure i have read a review on one lens where the opposite happens but don't quote me. Where it's most likely to matter is when trying to do semi macro work with a telephoto zoom. At a guess telephoto distances will be 20 to 30 or more times the focal length. A sort of part educated guess. I've definitely not noticed any zoom rate drop off on my 55-300mm at that sort of range. Not really tried it closer.

Actually I suspect this is another instance of web tom tom drums and very little factual information. You will probably find that at telephoto distances the effect is relatively small on most lenses. I have seen comments that lenses only have the stated focal length when focused to infinity - true if the designers choose to go that way. What may happen on lenses like the 70-200 is that they have chosen to do this to maintain high image quality.

It should be possible to work things out roughly from magnification and focus distances but the manufacturers don't give sufficient information. No saying where the max mag is or that min focus distance is the same at all focal lengths so it would all need to be physically measured.

John
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John,
I agree with you. At normal telephoto distances, I haven't noticed any zoom rate drop off (focus breathing) on my 55-300mm. I liked the article about the 55-300 in Ken Rockwell's site.
Thank you for your explanation,
Antonio.

[Manfred M]

Antonio, focus breathing is something completely different. That is the effect where the image changes size when you change your focus point WITHOUT changing the focal length.

This is usually not noticed by photographers, other than perhaps when macro work is being done and one is shooting for focus stacking. It is a real issue when pulling focus in a scene when shooting video.

[Dave Humphries]

We're at risk of getting lost in the semantics here, but I think it is important we are accurate.

I believe the effect being referred to is Focus breathing - i.e. the change of image magnification resulting from focus distance being altered when the zoom control (focal length) is not.

I would suggest we are talking about the 'effective focal length', determined by image magnification (or field of view if you prefer) and how that changes when focussed closely.

This is most relevant when image stacking (e.g. making successive small focus changes at close to minimum focus distance) - and note this does apply even to a prime lens (esp. a macro one).

Or comparing one lens with another e.g. (apocryphally) the 70-200mm, fully zoomed to telephoto, but focussed closest, compared to a prime 135mm at same focus distance - and there apparently* being little difference in image magnification (field of view).
* Although I haven't seen it myself.

I would say the above are the same thing, but ...

In use; Almost all DSLR 'zoom' lenses need to be refocused after zooming, which isn't a problem with AF.

As Manfred mentions though, for video zoom lenses, this lack of "focus tracking" is a problem (and a separate issue) - they need to retain focus on the subject while creative zooming is employed during a 'take', that's why those (professional TV/movie) lenses are a lot more expensive - cheaper/modern ones may AF during the zoom, which is possible if the motor is quiet enough - another case of technology rescuing the laws of physics.


Going back to Antonio's posts:

In the manual of my Nikkor 55-300mm lens is written: "The focal length of this lens decreases as the focus distance shortens".
Is this a rule for all lenses?

and

After reading what is written in the manual of my lens, I read the manual of other nikkor zoom lenses and there was nothing written about this subject.
What I would like to know was if the reduction of focal length is significative when using a small focus distance with my lens.

I would say it is common to all zoom lenses, especially the modern IF (internally focussed) ones - I would not put much faith the concept of lenses that don't mention it in the handbook not having this effect, they're just being 'economical with the truth' - or trying not to confuse their owners with matters that 99% won't notice

Unless you're macro stacking images, in the practical world it does not matter.

Cheers,

[ajohnw]

I thought I had seen one that went towards higher magnification closer up. The Nikon F4 70-200mm VR. Surely that is the desirable characteristic for normal use.

I searched photozone.de for focus breathing. Found no mention of Canon, only Nikon, Tokina and Sigma.

John
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[ajohnw]

Noticing stacking mentioned. I intended to do some on a flower that disappeared by the time a macro slide arrived in the post. It will have to wait for another subject and me in the right mood.

Unless I had no choice I wouldn't refocus the lens for stacking just either move the camera or the subject. The people who probably make most use of dof stacking techniques are often using microscope objectives and may only have dof of a few um (0.001mm's). So many shots are taken at times that some buy equipment to automate it. Usually the subject moves between shots. The software that is designed for stacking images works remarkably well when shots are taken like this.

There is another aspect that is off topic but worth mentioning on the same subject. One of the main points of stacking is that it allows lenses to be used that have F ratios that are optimum for the detail in the subject. In camera lens terms this means using lenses at their best aperture to get the maximum possible resolution with minimum contrast loss which unlike microscope objectives for instance is unlikely to be when they are wide open.

Yeh. I know certain people reckon diffraction effects don't matter but they really do in this area. Not much in many others though.

John
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[Glenn NK]

Unless I had no choice I wouldn't refocus the lens for stacking just either move the camera or the subject.

John
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Unfortunately changing the camera/subject distance (particularly if the subject is close to the camera), changes the perspective (parallax shift). It's less of a problem for a very shallow subject, but for many flowers, the change in perspective really screws things up.

In the attached example, I attempted to use focus stacking, but the image posted here is a single shot because of the parallax shift, the stacked image was useless.

I think the best solution is to use as long a FL as possible; however I've found that even if I keep the camera/subject distance constant, moving the focus ring on my 100 mm macro causes some parallax shift.

Addressing Antonio's situation, I think the most practical solution is to take test shots to determine how any one particular lens behaves - or as Manfred has said on more than one thread, "know your equipment".

[ajohnw]

Unfortunately changing the camera/subject distance (particularly if the subject is close to the camera), changes the perspective (parallax shift). It's less of a problem for a very shallow subject, but for many flowers, the change in perspective really screws things up.

In the attached example, I attempted to use focus stacking, but the image posted here is a single shot because of the parallax shift, the stacked image was useless.

I think the best solution is to use as long a FL as possible; however I've found that even if I keep the camera/subject distance constant, moving the focus ring on my 100 mm macro causes some parallax shift.

This thread might help

http://www.photomacrography.net/foru...er=asc&start=0

Couple of points though. There is mention of aberrations - rather likely with 2 close up lenses.

The point not mentioned is that in an ideal situation each image that is stacked needs a degree of sharp detail overlap otherwise the software is likely to finish up stacking mush as there is nothing else there. Sharpness needed depends on image size so if full sized images are needed then the overlapping sharp parts need to be sharp at that size. Low contrast detail can be a problem. The person who wrote one of the software packages recommends 70 - 80% of the available depth of field as an increment between shots. Unfortunately that is for microscope objectives which is a bit different to dof calc results. It's a visual thing though a microscope. He also reckons there is no significant penalty in taking too many images other than time to process them. Also suggests taking care of colour balance before stacking. I would suspect a certain amount of reduction and other PP batch wise might be a good idea too. Some do stack full sized images but the shot counts tend to be huge. Lots of work is done with jpg's.

There is at least one macro rather than micro result here - the stack of coins for instance.

https://www.flickr.com/search/?q=zer...ll&adv=1&s=rec

There are also some high mag images stacked from photo's taken in the field. I wish I had a kept a link to a video of some one doing this in a field by hand with the Canon high magnification macro lens purely through the viewfinder. Not sure I believe really but it seems some do it and results are good at say 800 - 1200 px odd wide web shots.

Some people insist on using Helicon Focus. I don't think anybody who does this sort of work seriously uses anything like this that may be in PS.

John
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