Question: In general I have found that as the focal length of 35mm (or APS-C) lenses increases, the physical length of the lens increases as well. However, my large format lens is much thinner than other 35mm lenses of equivalent focal length. Here is a picture:

[![Two lenses](https://i.stack.imgur.com/oNMHx.jpg)](https://i.stack.imgur.com/oNMHx.jpg)

The large format lens is a Wollensak Raptar 135mm f4.7, and it not more than 5 cm long. The lens next to it (albeit slightly unfair as it is a zoom) is a Sun telephoto zoom, 85mm-210mm f3.8. In general all large format lenses seem to be rather diminutive in length. Why is this?

Answer: 
The focal length is the distance from the (theoretical) center of the lens to the image plane. On the large format camera, there's a lot more *camera* between the lens and the film.

The lenses are also often relatively simple — there's no need for a focusing mechanism in the lens itself, for example. 

@osullic gives the example of the [Schneider PC TS Makro-Symmar 90mm f/4.5](http://www.bhphotovideo.com/c/product/818360-REG/Schneider_06_1064382_PC_TS_Super_Angulon.html) for 35mm and [Schneider 90mm f/4.5 Apo Digitar N](http://www.bhphotovideo.com/c/product/400328-REG/Schneider_03_017780_90mm_f_4_5_Apo_Digitar.html) for large format. They appear to have similar optical design (six elements in four groups), and this is particularly interesting, because the former is a *tilt/shift* lens. Tilt lets you change the angle of the focal plane for depth-of-field effects; shift changes the appearance of parallel lines. As with focus, tilt and shift are both adjustable in large format by adjustments *to the camera*.

So, basically, with 35mm, much of what would be in the camera in large format is *moved to the lens*, which of course makes the lens bigger.
Take a look at these two Schneider lenses that both have 90mm focal length:

![](https://i.stack.imgur.com/yztbd.jpg)
![](https://i.stack.imgur.com/RhrEg.jpg)

The first has coverage for "35mm" format, the second coverage for large format. I am not sure, but I think the main reason for the difference in size is the fact that the large format lens is a "simpler" design, i.e. fewer elements/groups. The reason a large format lens can use a simpler design is the distance between the lens board and the film plane - the lens need not project light at extreme angles out its rear element, whereas a 35mm-format lens must do so, and so requires a more complex design. Also, the fact that large format is so much bigger than 35mm format might have something to do with it; because 35mm format is smaller, any distortion/imperfection is "magnified" to a much greater degree, so more elements are incorporated to correct for any distortion/optical aberration.