ot optics question for Phil

Thanks, but that doesn't answer my question.

I found it online.

My lens is 2" wide and sunlight projects a 4" wide image at 11" from a screen. So the fl of the lens is -11".

That's bad, since that eye focuses at about 17". These glasses are over-correcting.

You're pulling some weird NASA moves by asking "what's the diopter of a lense" and then concluding a focal length of -11 inches.

Next question is where the hell did you get those glasses in the first place, if you have no idea of what their prescription even is?

That conversion is not difficult.

I don't have the prescription handy. And I like to measure things.

I think my eye has changed since I got the glasses.

Are you always so rude?

Nothing weird here. Just some simple optics.

This well predates NASA, having been developed by Isaac Newton in the

16th century:

.

Diopters are the inverse of the focal length in meters. Yes, a lens can have negative power expressed in diopters:

.

Joe Gwinn

You'll get a better measurement by making the aperture much smaller than the focal length. (The magnitude of the FL, that is.)

An object at a distance d_o from the center of a thin lens of focal length f produces an image at a distance d_i, where

1/d_o + 1/d_i = 1/f.

The sun is very distant, so we can ignore 1/d_0, so that the image of the sun is at f, which for a concave lens is on the side towards the sun.

You figure the FL using similar triangles as usual, except backwards--given two projections, find the focal point. That takes one more line of algebra.

The sun 's angular subtense is about 0.5 degrees, so you have to take that contribution out somehow--if you measure the disk diameter out to where the intensity falls to nearly zero, your measured angle will be

0.5 degrees too large, and your measured FL correspondingly too short.

Cheers

Phil Hobbs

It is weird. He's going on about diopters, but they're not a part of either known or unknowns in what he's trying to calculate or confirm.

I have no issue with inch focal lengths by themselves. Tons of "standard" sizes in mm for objective lenses are based off the original ones in inches.

Eyeglasses prescriptions are in dioptres.

Don't worry, CD, JL is very unlikely to crash into Mars this week. ;)

Cheers

Phil Hobbs

Glasses are prescribed in diopters. I measured the focal length of that lens as -11 inches. That's about -3.5 diopters.

Is that weird?

He doesn't know his rx, or that of the glasses. It's all pointless.

Ha.

If the resting focus of that eye is +2.3 dioptre (17 inch FL), then a

-3.6 dioptre corrective is indeed too strong. (This is after cataract surgery, so there's no accommodation to take into account.)

Cheers

Phil Hobbs

Well, the diopter is just the reciprocal of the focal length in metres, so it is relevant. Opticians use diopters because lenses can be stacked in front of each other and the powers in diopters simply add or subtract as appropriate. This is much easier than working directly with focal lengths.

John

It's just like resistors in parallel.

When is the last time you use mhos in a calcuation?

I type

rval

1/x

(RPN) often. I don't call them mhos; they are actually Siemens.

What resistor do you put across 5 ohms to get 3 ohms?

3 1/x 5 1/x - 1/x

Seems like the answer is never. I see lots of repetetive 1 over calculations.

There really is no going back once you learn RPN. Even a fancy calculator where you can see rows of nested braces just doesn't compare.

I always though mhos were just odd. So when did you last use susceptance, in siemens of course? Maybe it's time to hawk special "ultra high susceptance" cables to the audiophiles.

Conductance, susceptance, admittance, they have their uses. In the design of matching networks, filters and pulse forming networks, it's common to switch back and forth between those and their reciprocals all the time.

Jeroen Belleman

I have an app on my Android phone, Free42, which looks and feels a lot like a classic HP calculator. Very nice.

Are kids learning RPN these days?

Do you have an example? I'm just curious. Say you're making a 2 stage type E network, where do the weird units come into play?