[Brightcolours]

f/4 is f/4 is f/4 all day.  The f number is arbitrary as is the focal length. 

 

Three things drive the design complexity of wide-angle lenses:

 

* angular coverage (sensor size does not matter - field of view does)

 

* f number

 

* telephoto ratio.  If the BFL is >> the EFL the design is very difficult.  Increasing this is what drives the difficulty of wider and wider angle lenses.  Because the angle on the back is fixed (i.e sensor size) the only way to widen the field is to shrink the efl.  The reason full-frame SLR wide angles tend to be better than smaller format ones overall is because the EFL is longer to achieve the same field, thus the telephoto ratio is closer to 1.0 and the design is easier in that respect.

Focal length is focal length is focal length all day too.

 

So, the lens just is a 10-24mm lens, not a 16-35mm lens.

 

However:

Want to know what the lens is equivalent to in 135 format terms? -> 15-36mm f6.

 

What does the f-number signify? focal length divided by the number = apparent aperture size. 10 / 4 = 2.5mm

Lets see if that works the other way around? 15mm FF equivalent / 2.5mm = 6mm. Oh nous.