Question: I accidentally saw Nikon 17-35mm f/2.8 lens and noted that it is rather large for its focal length. Here's [the first review of this lens search offered](https://kenrockwell.com/nikon/1735.htm) - looks like it's 107 millimeters long. Given its focal length of up to 35 millimeters I would expect it to be more like a pancake lens but 107 millimeters is very far from pancake lens.

I guess there're some design reasons for that.

Why is this lens so long given it's rather short focal lengths?

Answer: 
There are different optical designs. From [Wikipedia](http://%20A%20collection%20of%20telephoto%20lenses%20In%20photography%20and%20cinematography,%20a%20telephoto%20lens%20is%20a%20specific%20type%20of%20a%20long-focus%20lens%20in%20which%20the%20physical%20length%20of%20the%20lens%20is%20shorter%20than%20the%20focal%20length.[1]):

> 
> a telephoto lens is a specific type of a long-focus lens in which the
>  physical length of the lens is shorter than the focal length.
> 
> 
> 

What you are seeing is the reverse though which is a retro-focus design, also from [Wikipedia](http://The%20retrofocus%20lens%20solves%20this%20proximity%20problem%20through%20an%20asymmetrical%20design%20that%20allows%20the%20rear%20element%20to%20be%20farther%20away%20from%20the%20film%20plane%20than%20its%20effective%20focal%20length%20would%20suggest.):

> 
> The retrofocus lens solves this proximity problem through an
>  asymmetrical design that allows the rear element to be farther away
>  from the film plane than its effective focal length would suggest.
> 
> 
> 

This amounts to having a design where a reversed telephoto lens is placed at the back of elements to allow a wide-angle lens to be built for a large sensor.
There seems to be a fundamental miss understanding that some how the focal length of a lens represents its physical length.

The focal length of a lens refers to the distance it sees or its field of view and is not a reference to its physical size. Although a longer focal length telephoto is physically longer then a shorter focal length lens, the physical length of the lens is not equal to the focal distance number. I.E. a 400 mm lens is not physically 400 mm long. How does one "accidentally" see a lens?