[John P]

I feel sure this must have been noted before, (but not by me).

I was out last night looking for Nightjars, the full moon rose over the horizon of trees shortly before sunset and was looking particularly fine.

The trees were about 2-300 yards away and the adjustment on my binocular focus to change view between the moon and the treetops was negligible.

I'm aware of the optical illusion of the moon looking closer than it is at times, but how does it fool a piece of glass?

[dantheman]

Transparent sheet silicone and solid lactulose-based animal growth-inducing derivatives (cheese) have a well-known affinity to each other in both traditional faerie and modern plasticene lore. The use of the correct grade of sheet silicone product in the objective would of course lead to a negligible difference in the apparent distances involved; the moon, when viewed through a glass, changes its molecular constitution instantaneously, and for the time-span involved it also occupies a spatial placement only a short distance from the viewer; the moon really is made of cheese and it really is only just above the treetops ...

You weren't imagining it!

[John P]

Originally Posted by dantheman

the moon really is made of cheese and it really is only just above the treetops ...

You weren't imagining it!

In my best imitation of Stephen Fry imitating Robert Robinson: "Would that it were Dan, would that it were............."


cos then I'd have eaten it all

[ronh]

Here is how magnification alters perspective.

Imagine two distant objects, the same size, one 50 yds away and one 100 yds away, in the same direction from you. If you walked 45 yds towards the objects, their distances would diminish to 5yds and 55yds, respectively. So, the apparent size of the close object would increase by a factor of 50/5=10, while that of the more distant would only increase by the factor 100/55=1.8.

Compare this to what happens when you view the same scene with a 10x optic, rather than walking closer. The closer object again increases size by a factor of 10. But now, the more distant object also increases size by a factor of 10.

So, relative to near objects, magnification appears to be stronger on more distant objects. This has the effect of appearing to bring far and near objects to more nearly the same distance, making the moon look bigger with respect to the hillside.

A concise mathematical explanation is that longitudinal magnification is the square of the lateral magnification. Which as almost as weird as the cheese theory. Of course cheese is quite different in England!

I love to watch nightjars, nighthawks, bullbats, goatsuckers, whatever, myself. Aren't they some flyers!
Ron

[spyglass]

John, you saw it close, yes? Is it really green cheese?

On a more down-to-earth note, a good way to illustrate what Ron described is to look thru yr bino at a car 20-25m away, front to rear (or v-v) at perhaps 10* off parallel. You'll notice the wheel farthest away appears slightly larger (more apparent on an lwb auto, e.g. Rolls, Bentley, Jag sedan, et al). That used to puzzle me, until I applied optical arithmetic (and formulae) to it, then, voila!

[spyglass]

OTOH, Dan has a clever way with the words......

[Kevin Purcell]

Originally Posted by John P

In my best imitation of Stephen Fry imitating Robert Robinson: "Would that it were Dan, would that it were............."

Of course, most American's have no idea what you're on about. But I happened to rewatch that episode of QI over the weekend where he makes that comment!

The illusion is a consequence (as ronh explains) of the flattening or foreshortening effect you get with magnification. Telephoto lenses on cameras do this too. It's like everything gets "squished" into a short distance. Then your brain goes to work on comparing relative sizes of objects and assigning "distance independent" sizes to those objects. And you think the moon looks bigger than it should (a conscious bit of thought).

And on Common Nighthawks I watched several them flying during the day in Oregon at the beginning of the month. I suspect they're feeding nestlings and are driven to bring in more food just before the fledge so they hunt during the day (and interact with others). But they look more interesting in the daylight than those W shaped sillouettes at dusk (though seeing those is fun too).

I'm sure Robert Robinson had something to say about Nightjars too ...

[John P]

I understand all the stuff about the illusion, and it's not really a question of the moon looking bigger.

I have about 1.25 turns on my bins from close focus to far, when I'm seawatching I can see the Needles on the Isle of Wight, they're about 4 miles from Milford beach, when the Needles are in focus in my bins I would have about a quarter turn left on my bins to get to the furthest focus.

Last night I could see the moon ( which is roughly a further 250,000 mile away) in sharp focus and I still had one full turn left on my bins before going to the furthest focus.

That's the bit I don't understand.

[ronh]

Imagine a Porro prism bino with external focusing, the kind where the eyepieces move forward for viewing more distant objects and vice versa.

Let's let O stand for the "object distance" from the objective lens out to the object, and I for the distance behind the lens to the image. The eyepiece must be positioned so that the field stop coincides with the image from the objective lens, to achieve focus. The equation that describes the position of the eyepiece vs the distance to the object can be written if we know the focal length of the objective, F, which will be around 6 inches. The equation is

1/F = 1/O + 1/I

As O approaches infinity, I must approach F. At great distances, 1/O will become zero, and then even large changes in O, like from 1 mile to 250,000 miles, hardly effect I. This defines practical "infinity" for a binocular: distances many times F, like roughly 1000 times.

Conversely, for closer objects, as O approaches F, I must approach infinity. Of course, O never gets closer than about 6 feet with normal binoculars, largely because it's impractical to design an eyepiece carriage with more than about a half inch of motion. But, for the near range of objects, a change in O will cause a considerable change in I to be made, that is, a considerable focus adjustment.

For example, for a 6" F, and the two near range cases O=10 feet and 20 feet, the corresponding I values would be 6.32 and 6.44 inches, and 0.12 inches is a goodly crank on the knob.

For this same F, a far range case might be 500 feet and 500 miles, which would require I values of 6.006 and 6.000 inches, a very small change.

Of course, an equation describing something you already know happens really only precisely quantifies it, but does not tell "why". It's "because" that's how lenses work. You already observed the essence of this deep and wondrous thing. So if you are not a techno nerd, and never fool with equations, no worries.

The reason you still had a full crank left when viewing "infinity" is probably to allow very nearsighted people to focus on infinity without their glasses. Nearsightedness is by far the most common focus malady these days, so most makers allow some headroom there.

Don't you think this calls for a piece of cheese?
Ron

[John P]

Originally Posted by ronh

Don't you think this calls for a piece of cheese?

Ron

I feel that is my best option right now, I can feel a couple of Cream Crackers and Stilton coming on.

I'll come back to your explanation for another look in the morning, my doodles of O's and I's are confusing me now.

Many thanks for taking the time and trouble to try and get through to me, (but I'm still trying to understand why my bins think the Needles at just 4 miles distance are further away than the moon).

[looksharp65]

John P

if you are absolutely confident about your need to focus further for the needles than for the Moon, I might have a plausible explanation.

Actually, they are two, but also likely to co-operate.

1) The strong moonlight causes your pupils to constrict, hence the perceived sharpness of the image is still satisfactory.
When you focus your binoculars from closer to further away, the increased depth of field deriving from your constricted pupil lets you reach zone of sharpness at an earlier focus point.

2) The Moon's image size is vast compared to the needles.
I imagine you involuntary struggle with your ocular accommodation and convergence to achieve full sharpness on the needles. If you accommodate excessively, you will have to turn the focusing knob further towards infinity before you experience full sharpness.