On Thursday, June 14, 2018 at 2:28:51 AM UTC-4, RichA wrote:
Now with cameras and their exceptional high ISO capability, these
slower but high-quality telephotos are now good and perhaps, preferable.

https://www.dpreview.com/news/7758816413/nikon-developing-compact-and-lightweight-500mm-f5-6-lens

The option has been around for "long enough", but one of the ways
that the OEMs has functionally obstructed it is through their dSLR
specifications.

Specifically, when Autofocus capability is disabled when a certain
f/stop value is exceeded. If memory serves, the common "upper limit"
value had been f/5.6, with some exceptions at f/8 (such as offered
only on higher end bodies, and/or only at the center focus point).

From a designer policy standpoint, what's needed to really enable
this is (enabled by said higher ISO sensitivity) is for the camera
to be designed & allowed to (try to) AF at f/8, f/11 and/or even f/16.

For example, if we use lens filter diameters as a convenient
surrogate, a 200mm f2.8, a 300mm f/4 and a 400mm f/5.6 all have
primary apertures of 77mm, whereas when we increment up by one
stop (eg, a 300mm f/2.8, 400mm f/4, or 500mm f/5.6) the aperture
area needs to double, which is ~120mm.

Conversely, if we 'speed up' the dSLR's Autofocus system to allow
for an even slower lens, a 500mm f/8 should drop to be only 77mm.


-hh