[Rover]

Not the design, of course, but the corresponding specs (when you adjust them to the format).

[Brightcolours]

Not the design, of course, but the corresponding specs (when you adjust them to the format).

When you adjust them to the format you get this:

10 x 1.5 = 15mm

24 x 1.5 = 36mm

f4 x 1.5 = f6

 

The corresponding specs are not really equal then, compared to full frame 16-35mm f4.

[Scythels]

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.

[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.

[Scythels]

The f number is defined as 1/2u where u is the marginal ray angle.  It is only defined as efl/edp (effective focal length / entrance pupil diameter) at infinity where first order approximations are valid.

 

The entrance pupil diameter is not a particularly useful specification because it says almost nothing about the system.  Look up the lagrange invariant and conservation of etendue.

 

field*fno is what drives design difficulty.  if field*fno is the same across different format sizes, the lenses are equivalents. 

[Brightcolours]

You make me chuckle. Now give a similar "rant" about the focal length number. And then realize that indeed the focal length and f-number stay the same no matter which size sensor one puts in the camera, and that it was about equivalents. 

Equivalent focal lengths give a similar FOV. Equivalent f-values give a similar DOF (and similar apparent aperture, which lets through a similar amount of light).

[Scythels]

Of course the focal length is independent to the sensor behind it.  But also realize that you can make a 100 degree field of view 50mm lens and you can also make a 1 degee field of view 50mm lens. 

 

Total luminance flux (aka the etendue of the system) is not the same as the energy density of the system which is described by the f number. 

[Rover]

Haha here we go again

[Brightcolours]

Fine, you go ahead and make a 1 degree FOV 50mm lens. What I wrote is not that difficult to comprehend.

[felix]

Haha here we go again

 

The same old equivalence discussion, but on a whole new level