Overall lens length differences

4 years 2 months ago #676454 by Alfonso Camil
I think I might have a interesting question for you and I'll do my best to explain.  I was wondering how does the overall physical length of a lens impact the focal length?  

Does it have any impact on it?  Let's say you compare a 35mm prime lens to a 18-200mm.  And if the 18-200mm is set to 35mm, does having a longer lens tube like the 18-200mm reduce how much light is needed?  

I hope this makes sense.   


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4 years 2 months ago #676458 by Nikon Shooter

Alfonso Camil wrote: … how does the overall physical length of a lens impact the focal length?


There was a time when a direct correlation was consequent
but no more since many decades. 

Alfonso Camil wrote: …compare a 35mm prime lens to a 18-200mm. And if the 18-200mm is set to 35mm, does having a longer lens tube like the 18-200mm reduce how much light is needed?


If set at 35mm, the zoom will render the same image than the
35mm prime but will need more light to do so… like you said,
because of the longer tube.

Light is free… capturing it is not!
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4 years 2 months ago - 4 years 2 months ago #676482 by Ozzie_Traveller
G'day Alfonso

Geez mate - it's been over 50 years since I did Photographic Optics as part of my tertiary studies :(

Originally, all "big" lenses were regarded as 'long' lenses. ie: a 400mm lens was 400mm long, until post-war maths allowed better designs and glass compounds, when the industry was able to shorten the physical length via better optics. With the telephoto design of lenses a 400mm lens became around 300mm long ~ and we loosely refer to all lenses today as being a telephoto lens without realising what it really refers to

As to the Aperture and its possible variability during zoom operations, I do not think that it is an issue. For example - someone using a 100mm macro set to F11 should have the same image brightness as the user of a 70-200 zoom at the 100mm setting and also using F11

If this were not the case, the outcry from users world-wide would kill off zoom lenses ~ surely?

Hope this helps
Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/

Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/

The following user(s) said Thank You: KCook
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4 years 2 months ago #676491 by garyrhook
If a lens is designed to function at a given focal length and a given aperture (keeping the lens mount the same for comparison, of course), then that will produce the same image as any other lens with the same focal length and aperture.

The physical changes to the lens body would be necessary to accomplish the above. That's a given for the lens design.

Also: look up "telephoto". The term means that the lens is physically shorter than the focal length. At least, if Wikipedia is to be believed. 

To answer your question, the 18-200 would probably need more light than the prime. An 18-200 is kinda fat, whereas a fast 35mm prime is pretty short and small.


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4 years 2 months ago #676499 by Alfonso Camil
Super awesome, tons of thought here for me.  Going to take your advice and look up these items!


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4 years 2 months ago #676582 by fmw
Focal length is an optical measurement.  It is the distance from where the light begins to converge at the front element to the focal point.  This is true despite the lens design or the tube in which the lens elements are assembled.  As an example, a catadioptric or mirror lens will be physically shorter than, say, a normal telephoto of exactly the same focal length.


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