another lens question

[Nikon Collector]

I've always used mostly primes but have picked up the odd zoom here and there. Now that I'm retired and sitting on my deck watching the sun set between the oak and maple in my back yard, I have time to contemplate the mysteries of lif, like why is my 200mm prime 75mm longer to the middle of the lens than my 75-210 zoom? Both focused to infinity and the zoom set at 200mm. I know there are people out there that know the reason, but can you put it in terms that an old soldier can understand?

 

[MattKing]

Because the zoom is more of a telephoto design than the fixed focal length lens.

 

[Nikon Collector]

Ok, you lost me there Matt, isn't my 200mm a telephoto?

 

[MattKing]

Ok, you lost me there Matt, isn't my 200mm a telephoto?

Might not be. It might just be a long lens that doesn't make use of any telephoto effect in its design.

A true telephoto lens can actually be shorter than its focal length. The design of the lens moves the nodal point - the point from which one measures the focal length - forward to a point nearer the front of the lens. It is actually possible to have a nodal point that is effectively outside of the lens itself.

That sort of design makes for a smaller lens, but of course involves some other compromises.

Wide angle lenses often involve something similar (but in reverse). A retro-focus lens is designed to move the nodal point back, so as to allow it to be closer to the film surface then the lens elements themselves.

You cannot do this with a single element lens, but once you start using multiple elements of different shapes and types ....

 

[Nikon Collector]

Interesting, I always thought the focal length was the distance from the middle of the lens to the film plane, didn't know it was variable

 

[markbarendt]

SLR's also have the similar problem at the short end. Short lenses like 20 & 28mm are retro focus in design and actually focus behind the lens. This is done to make room for the mirror to swing up and out-of-the-way.

 

[Xmas]

Interesting, I always thought the focal length was the distance from the middle of the lens to the film plane, didn't know it was variable

The problem arose with large format cameras when a long (sic focus) lens was used and needed triple extension racks and bellows, the wind resistance ment the tripod could need guys.

So the optical people put a negative (concave) lens behind the long focus lens to telescope the bellows.

Today the design computers agonize about optimising the whole design but the rear of a telephoto lens is still essentially negative.

The SLR wides and even many of the Leica wides are inverted telephoto lenses which are longer (further away) from the film plane than a simple short focus lens.

Some of the pancake lenses are 'squeezed' optically.

Any of these unnatural lenses are more complex than they would otherwise have been, more $...

 

[ckpj99]

The focal length is measured from the focal plane to the "optical center" of the lens. Using multiple elements and groups, and using a variety of types of glasses makes the center shift. One of the early lens innovations was the discovery that mixing crown glass elements and flint glass elements in lenses produced better images. The classic tessar design does owe some of it's success to it's triplet design, but it owes a lot to its implementation of mixing glass types as well.

 

[AgX]

Interesting, I always thought the focal length was the distance from the middle of the lens to the film plane, didn't know it was variable

The focal lenght won't vary for a prime lens, but the built length can vary.
The focal length is the distance between the so-called nodal point of a lens to the film plane, when focusing something at infinity.
(Actually there two... we are concerning the rear nodal point.)
That point not necessarily is located in the middle of a lens, it can even be located outside the lens assembly.

The latter is the case with retrofocus lenses (wide-angle lenses designated for SLRs). Where the nodal point should be positioned there is still the swinging mirror. Thus a lens type had to be made that mechanically is further off, but optically still in the mirror box.

 

[E. von Hoegh]

The focal lenght won't vary for a prime lens, but the built length can vary.
The focal length is the distance between the so-called nodal point of a lens to the film plane, when focusing something at infinity.
(Actually there two... we are concerning the rear nodal point.)
That point not necessarily is located in the middle of a lens, it can even be located outside the lens assembly.

The latter is the case with retrofocus lenses (wide-angle lenses designated for SLRs). Where the nodal point should be positioned there is still the swinging mirror. Thus a lens type had to be made that mechanically is further off, but optically still in the mirror box.

Standard and some 'short tele' lenses are retrofocus as well. It's not until you get into the 85 - 100mm range that you can use a 'straight' (i.e. non retro- non-tele-) lens on most SLRs with a flange focus of 44~ - 48~mm.

 

[RalphLambrecht]

the multiple-lens design needs to be packaged somewhere;used or not,in other wordsinherentlens design