Drdale55 Loc: Prescott, AZ

Hi,
Wasn't sure if there is a term that describes this issue. Some like my Nikon 200-500 extend when zooming. Others like my Nikon 70-200 and my 2 Tokinas (12-24 and 20-35) do not. Can't find a term in the lens description that describes that. Was actually wondering about the Tamron 24-70 f2.8. Thanks.

speters Loc: Grangeville/Idaho

Sorry, I have no idea what you're asking! (What issue)??

chaman

Issue?? What?

haroldross Loc: Walthill, Nebraska

I think you may be referring to an internal focus lens (the lens does not extend out when focusing) as opposed to an external focus lens (the front group of lens elements move away from or closer to the rest of the lens elements).

larryepage Loc: North Texas area

I cannot answer concerning the Tamron lens, but can give a general answer to the first part of your question.

With a very simple lens, which has a well-defined "optical center," the only thing that needs to be done to focus is to change the distance between the lens and the film. You can demonstrate this with a simple magnifying glass. Change the distance to whatever you are looking at. and you will need to change the distance from your eye to the lens to see the object clearly.

Lenses that seek to correct the normal kinds of distortion and error that any lens naturally introduces have more elements. The optical center is also more complicated (it may not be a single point), and because there are more lens elements, more things have to be moved to focus. Sometimes, this means that the entire barrel of the lens has to move. If the lens is designed to focus without moving externally, it is generally called an "Internal Focus" lens. And some lenses, especially very inexpensive ones, also rotate when being focused, which can really complicate the use of some polarizers. (There are also 'rear focus' lenses, where all the movement is at the back...sometimes into the camera body.)

Zoom lenses, like you are asking about, are even more complicated. They can also change their focal length, which is a very fundamental lens characteristic. This is done by moving the lens elements in relation to each other inside the lens. Some zooms, including intermediate cost lenses, and especially includin lenses which zoom both to wide angle and telephoto lengths, have to change their overall length to accomplish this. As an example, my Nikon 70-200, which is just telephoto, does not change its overall length. The technologically equivalent 24-70 zoom, is shortest at 50 mm and extends when moving either to the telephoto or wide angle side of 50mm. (I have a generally equivalent 17-55 DX zoom which extends on both sides of 35mm, which is the base focal length for that lens). And you can tell that different things are happening on the two sides...both the 'feel' and the appearance, as you look into the lens, are very different.

This may be more than you wanted to know, but perhals it will help your understanding.

LP

robertjerl Loc: Corona, California

The two terms that apply are: internal focus and internal zoom.
In both operations the distance between the lens elements and the film/sensor plane changes. (other stuff like what larryepage mentions also)
A lens with internal focus or zoom (two different things) has a large enough body that the movement is all inside the body. This makes a lens that is easier to seal against moisture and dust. It also means you don't have to watch out for banging the front against something when it becomes longer than it was. But it also means the body is bigger and heavier. More (sometimes much more) expensive in most cases also.
For those reasons most companies reserve those two features for their high end lenses.

Drdale55 Loc: Prescott, AZ

Thank you all .... mystery solved. My apologies for the word "issue" implying something was wrong. Not an issue, just an observation. Internal focus/internal zoom describes the "observation." I also meant Tokina 24-70 not Tamron and I have determined that it is of the internal focus/internal zoom type (does not change length). Unfortunately the use of the term doesn't seem to be consistent. For example (in B & H) the Nikon 70-200 2.8 is described as having internal focusing and "does not change length." In the description of the Nikon 24-120 4.0 it also says it has internal focusing but it, in fact, does change length when zooming (I have them both).

At any rate, thanks to haroldross, larrepage and robertjerl for the explanations!!

Bart1

My wide-angle 18-35mm zoom lens only moves internally but, most outside that do extend i think.

chaman

What? What is wrong with people in this thread?

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

Internal.

imagemeister Loc: mid east Florida

The more expensive/better zooms are non-extending "internal ZOOM" - and as far as I know, they are all constant aperture.

Extending Zooms are all variable aperture with the exception of the Nikon 200-500 - and I do not know ( a mystery) how they can make it constant aperture ??

cthahn

You seem to think purchasing all these lenses is going to help your photography. Take some photography classes and learn about the basics of photography. Start taking pictures and quit worrying about what new zoom lens to purchase.

Eddy Vortex

Seems the word "issue" means "problem" these days. Whats wrong with the word "problem"

JCam Loc: MD Eastern Shore

I think most zoom lenses extend, but some to a much lesser extent than others and some with shorter ranges may do it internally. Most--Canons, Tamrons, Sigma, etc.--use a rotary action to zoom, and separate rotary focus ring if set for Manual; some, mostly older I believe, use a push/pull sliding motion to zoom. Years ago I had a Vivitar that had the slide action; as I recall the extension was longer and it worked fine, but took a bit longer to achieve the amount of 'zoom' & composition I wanted.

imagemeister Loc: mid east Florida

As a side note, internal zooms have a greater propensity for losing focal length as you focus closer ("focus breathing") .......