Depth of focus is an informally coined term, but it is a good term to avoid confusion.
Depth of field, as it applies to all optics, and especially cameras, is correct,
but ends up mangled in with "apparent 3D depth" which is totally different.
Field depth should be used for focus, but is mangled by some using binoculars.
So...I like depth of focus!
Sometimes, focusers with a fine pitch can seem like they have a greater depth of focus,
but that would be some confusion...
Still, depth of focus meaning "the range over distances over which things look sharp"
is vey high in 7x50 binoculars. Since marine binoculars are rarely used for less than
75 ft, they hardly ever need to be focused...for one person. So...indpendent focus
is no problem, and it's far easier to seal and has better alignment.
You just focus for something far away, and everything from ~70ft to inifinity
is focused well enough for your visual acuity...and your eye's ability to adjust a bit.
There is no debate that the lower the power, the greater the depth of focus.
I take depth-of-focus to apply to the outside world, not the focuser travel....as with cameras.
(the two depths might mean the same thing though)
There are arguments over whether the optical design or the f-ratio (stopped) have an effect.
Experimeting with many binoculars and masks (lens covers with reduced diameter holes in them),
I have found that the depth of focus can be extended, but nowhere near as much as with cameras...
...not entirely sure why.
Experimenting with dozens of models, I have found that some models do stand out
from others. B&L 7x35 Zephyrs go from infinity down to 60 feet, and the current
best 7x35 is a pair of Limer IFs, with a stunning 45 ft to infinity, box marked "XXFD".
They use an aspheric EP that can do 55 degrees but it field-stopped to 50deg.
The deepest are a pair of 6x30 Kendons Imarked "XXXFD", which can take you from
a mind-boggling 353ft (approximately) to infinity. Not sure exactly how this happens,
but there is a very slight defocusing at about 100ft, and then things crisp up again coming in.
I suspect the makers played the objectives and prisms and eyepiece off against each other...somehow.
So, empirically, I must conclude that some binoculars of the same power and apreture can have
a different depth of focus than others. I have them. I can't say why this happens, though.
Many cognoscenti say this isn't possible......but here they are.
One experiemwnt I haven't done yet:
to take extra-wide-field (and not mushy) binoculars and field-stop them narrower,
to see if that increases the depth-of-focus. The idea would be that the phase response
of the extra-wide can translate into a better distance response if the whole field isn't assembled.
Edited by MartinPond, 24 November 2017 - 01:25 PM.