LED headlights?

Quoted message said:

Quoted message said:

Quoted message said:

>David L. Johnson wrote:

-snip-

Quoted message said:

>>Are there LED lights that really do the job? How many watts would it
take?

Quoted message said:

Quoted message said:

Quoted message said:

"Zog The Undeniable" <[email hidden]> wrote in message "]news:[email hidden]...
>The Cateye EL-300 passes British Standards for front lights and is wildly popular over here.
>Personally I don't think it throws a real beam, so I'll stick with my dynamo for now.

Quoted message said:

Quoted message said:

Bruce Lange said:

I would say that for routes you are familiar with, the

EL300 is fine, -snip-

Quoted message said:

I'm waiting for is a good LED light designed to work with dynamos. That would be cool.

Quoted message said:

A Muzi <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

I was intrigued. So I clipped a Cateye EL-300 light to my handlebar and ran a couple of wires from
my dynamo through a diode bridge to it. Amazing! It's very bright. Brighter than with batteries.
The dynamo-powered EL-300 works fine in both constant and flashing modes.

-snip-

Gary Young said:

How does it compare in brightness with an incandescent light?

I do not have a light meter but it seemed less than my regular lamp. The blue color can be deceiving
-I really cannot say.

--
Andrew Muzi www.yellowjersey.org Open every day since 1 April, 1971

Dave wrote:

This makes sense, but there is a nagging question I have. When I ride on dark country roads, my eyes
adapt to the dark quite well. But when I meet the occasional car, I am blinded temporarily by the
lights. This can be pretty scary on a descent around a curve. Any hints on avoiding this problem? I
wonder if "dark adaptation" (i.e. dilated pupils) adds to that problem.

Ted Bennett wrote:

Dark adaptation is a retinal phenomenon. Pupil size is related more to focusing than to
light control.

Peter <[email hidden]> wrote:

Both pupil size and retinal sensitivity increase in response to low light levels. In bright light
the pupil size is around 3 mm and after some time in darkness it increases to about 5-7 mm. The
maximum size tends to decrease with age. I've never noticed any significant change in pupil size
based on focusing distance. Various drugs, either artificial or naturally produced by our bodies,
will also affect pupil size. The change in retinal sensitivity is over a much larger range than that
produced by changes in pupil size.

Ted replies:

Yes, that is why the variation in pupil size is more for focusing than for light control. As you
say, the sensitive range of the retina is several orders of magnitude greater than the light
variation due to pupil size.

Pupil size certainly does change with accommodation, however, and I think it is significant. Under
constant lighting conditions, the pupil contracts as the eye accommodates, or shortens its focal
length, and this is readily observable. The benefit is the same as with any variable aperture: a
greater depth of focus is obtained with a smaller aperture.

--
Ted Bennett Portland OR

Benjamin Lewis said:

Ted Bennett said:

Dark adaptation is a retinal phenomenon.

Yes.

Quoted message said:

Pupil size is related more to focusing than to light control.

Can you explain this? It seems counter-intuitive, given that pupil size varies with changed light
levels at a fixed focal length, but does not if you change your focus from near to far objects.

The retina responds to light stimuli over a range of around 7 orders of magnitude, if I am
remembering correctly. I'd have to look it up to be sure. A pupil, when changing from 3 to 8 mm
diameter, increases its area by a factor of about 7, which is less than one order of magnitude. So
changing pupil size has little effect on the sensitivity of the eye to light.

Quoted message said:

Pupil size changes your depth of field of course, but I would think that this is a side effect.

It well could be a side effect, but it is certainly more useful than the light control effect. And
the original impetus for us to evolve a variable pupil? Who knows? Ask your god, or your dog,
depending on your point of view of those questions. Since the pupil obviously reacts to a change in
light level, it's easy to assume that the primary use of the variable aperture is to control light,
but that isn't borne out by sensitivity measurements.

Quoted message said:

I can't decrease my pupil size by attempting to focus on near and distant objects at the
same time.

It is not possible to focus at two different distances at the same time. When you shift focus from
distance to near, the pupil does contract. It's part of the accommodative reflex.

--
Ted Bennett Portland OR

Ted Bennett said:

Ted Bennett wrote:

Dark adaptation is a retinal phenomenon. Pupil size is related more to focusing than to light
control.

Peter <[email hidden]> wrote:

Both pupil size and retinal sensitivity increase in response to low light levels. In bright light
the pupil size is around 3 mm and after some time in darkness it increases to about 5-7 mm. The
maximum size tends to decrease with age. I've never noticed any significant change in pupil size
based on focusing distance. Various drugs, either artificial or naturally produced by our bodies,
will also affect pupil size. The change in retinal sensitivity is over a much larger range than
that produced by changes in pupil size.

Ted replies:

Yes, that is why the variation in pupil size is more for focusing than for light control.

This is the opposite of what I've observed directly or read in other literature. It's clear that
pupil size varies rather dramatically with light level - I can readily observe this effect on my own
eyes while looking in a mirror and it is widely documented (e.g.
exploratorium.edupupil.htmlOpen ↗). And it is in a direction that makes sense for dark
adaptation by allowing more light to enter under dim light. OTOH, variations in pupil size due to
focussing distance appear to generally be very small.

Quoted message said:

As you say, the sensitive range of the retina is several orders of magnitude greater than the
light variation due to pupil size.

Sure, but the pupil size change is still significant in dark adaptation even though the chemistry of
the retina produces most of the effect. Going from 3 mm to 7 mm changes the amount of light admitted
by more than a factor of 5 which is certainly of benefit even though the retinal sensitivity varies
over many orders of magnitude.

Quoted message said:

Pupil size certainly does change with accommodation, however, and I think it is significant.

I just tried to observe such an effect by focussing on different distances over my full range of
accommodation and checking my pupil diameter. The effect was too small for me to see - in clear
contrast to the situation when I change the light level.

Quoted message said:

The benefit is the same as with any variable aperture: a greater depth of focus is obtained with
a smaller aperture.

Unlike photography where a wide field of view is frequently captured in sharp focus, depth-of-
field is less significant in human vision since only an extremely narrow field is in sharp focus
at any one time.

"David L. Johnson" <[email hidden]> wrote in message news:<[email hidden]>...

Quoted message said:

Bill Cotton said:

I need to know the DC power plug/jack size that the Night Rider uses. I like to put that size
jack on my 30 LED bicycle light.

Bill: I just measured mine, and it was 5.4 OD and maybe 2.4 ID by my caliper, which is not real
great. You can drop by my house sometime and check it out. Mine is the NiMH battery type; I don't
know the output of it, either, but we can also check that.

Thanks Dave, I will contact you direct.

Ted Bennett said:

Dave wrote:

When I ride on dark country roads, my eyes adapt to the dark quite well. But when I meet the
occasional car, I am blinded temporarily by the lights. This can be pretty scary on a descent
around a curve. Any hints on avoiding this problem?

I hate to interrupt the scientific discussion to deal with practicalities, but here goes! ;-)

Yes, I have a hint. Wear a cycling cap, or some other hat with a bill or visor. when a car
approaches, tip your head forward so the visor just blocks the headlights.

If you're riding without a hat, it can help to close one eye until the glare is gone.

--
Frank Krygowski [To reply, omit what's between "at" and "cc"]

Peter <[email hidden]> wrote in message news:<SuGMb.36457$na.27823@attbi_s04>...

Quoted message said:

Ted Bennett said:

Ted Bennett wrote:

Dark adaptation is a retinal phenomenon. Pupil size is related more to focusing than to light
control.

Peter <[email hidden]> wrote:

Both pupil size and retinal sensitivity increase in response to low light levels. In bright
light the pupil size is around 3 mm and after some time in darkness it increases to about 5-7
mm. The maximum size tends to decrease with age. I've never noticed any significant change in
pupil size based on focusing distance. Various drugs, either artificial or naturally produced by
our bodies, will also affect pupil size. The change in retinal sensitivity is over a much larger
range than that produced by changes in pupil size.

Ted replies:

Yes, that is why the variation in pupil size is more for focusing than for light control.

This is the opposite of what I've observed directly or read in other literature. It's clear that
pupil size varies rather dramatically with light level - I can readily observe this effect on my
own eyes while looking in a mirror and it is widely documented (e.g.
exploratorium.edupupil.htmlOpen ↗). And it is in a direction that makes sense for
dark adaptation by allowing more light to enter under dim light. OTOH, variations in pupil size
due to focussing distance appear to generally be very small.

Quoted message said:

As you say, the sensitive range of the retina is several orders of magnitude greater than the
light variation due to pupil size.

Sure, but the pupil size change is still significant in dark adaptation even though the chemistry
of the retina produces most of the effect. Going from 3 mm to 7 mm changes the amount of light
admitted by more than a factor of 5 which is certainly of benefit even though the retinal
sensitivity varies over many orders of magnitude.

Quoted message said:

Pupil size certainly does change with accommodation, however, and I think it is significant.

I just tried to observe such an effect by focussing on different distances over my full range of
accommodation and checking my pupil diameter. The effect was too small for me to see - in clear
contrast to the situation when I change the light level.

Quoted message said:

The benefit is the same as with any variable aperture: a greater depth of focus is obtained
with a smaller aperture.

Unlike photography where a wide field of view is frequently captured in sharp focus, depth-of-
field is less significant in human vision since only an extremely narrow field is in sharp focus
at any one time.

Dear Ted and Peter,

Actually, you're both right.

The primary focus for the human eye is the lens, the clear, somewhat flexible structure dangling
behind the opening known as the pupil, the dingus which eventually goes cloudy with age as you begin
to suffer from cataracts.

Our eyes do not, in fact, work like cameras, as anyone can see at a glance. The inflexible glass
lenses in cameras, spyglasses, and binoculars cannot change their shape to focus, so instead they
move back and forth (think of the modern camera snout), something not seen in people outside of
"Roger Rabbit" but curiously developed in many fishes and some lizards, whose lenses are inflexible
and move in and out for focus. (Eyes in the animal world offer a bewildering variety--my favorite is
the squid eye, which has a W-shaped pupil.)

Our eyes mostly focus by muscles that pull and relax at the edges of the lens, making the lens
thinner or thicker. With age, our lens loses its flexibility and even near-sighted people like me
lose our ability to focus up close because our lenses are no longer flexible enough to relax into a
fat enough shape. "Presbyopia" merely means the bad sight of age and time for reading glasses.

So yes, the lens is what does most of the focus work in our eyes. But the pupil also plays a small
role. If you've ever had your pupils dilated with drops so that an ophthalmologist can get a better
look into the interior of your eye, you were probably given some cheap sunglasses to cut the glare
and then found that you simply couldn't focus well enough up close to read.

The reason that your up-close focus went to hell when your pupils were unnaturally expanded was that
your eyes are also crude pin-hole lens cameras. Just as a beautifully ground glass lens will focus
an image, so will a tiny hole. If the hole gets too big, the extra focus-effect is lost, and there
you sit, fuming because you can't read until the drops wear off.

If you're near-sighted enough, you can test the pinhole focus trick quite easily right now. Take off
your glasses and this post will become even fuzzier than usual. Now pinch your thumbs and
forefingers together to form a tiny, irregular hole and peek through it. You should be able to bring
the text back into focus through the pinhole and then relax your thumb and forefingers and watch the
text go fuzzy again. The same principle is used by peephole rifle sights and lets Mr. Magoos like me
shoot quite nicely at .22 targets without glasses.

(If you're not near-sighted enough to need thick glasses, how simply terrible for you.)

So most of our focus lies in the stretching and relaxing of the clear lens behind the pupil, but
some of our ability also comes from the aid of the pinhole effect governed by the pupil.

Carl Fogel

Tim McNamara said:

Peter said:

Sure, but the pupil size change is still significant in dark adaptation even though the chemistry
of the retina produces most of the effect. Going from 3 mm to 7 mm changes the amount of light
admitted by more than a factor of 5

Retinal illuminance is calculable by the formula:

(cd/m^2)(pa)=td

(in English: luminance * pupil area = troland)

where

luminance = candela per meter squared (cd/m^2) and pupil area = area of pupil in mm^2 troland =
retinal illuminance

A white page of textbook paper typically has a luminence of 34 cd/m^2 in "normal" room lighting.
Therefore, for 3 mm and 7 mm pupils:

34 * 9 = 306

34 * 81 = 2,754

or a difference factor of 9 (I erroneously posted a factor of 4 in another post for 2 mm and 8 mm
pupils, when the correct difference would be 16).

In your calculation you seem to be using 9 mm pupils to get your factor of 81 and ratio of 9 -
that's way on the high side for most humans. Using 7 mm instead gives a factor of 5.4 as I stated
above. BTW, your formulas are also lacking a factor of pi/4 (since our pupils are round rather than
square) if you really want the numbers to represent candelas - but that doesn't matter if you're
only concerned with ratios.

I don't know the formula- my perception class having

Quoted message said:

been long ago- but the number "td" can be used to calculate the number of photons falling on the
retina. Compare this to the difference factor of 1,000 in the sensitivity of the light-adapted and
dark-adapted human eye.

Sure, I had already said the retinal sensitivity changes by many orders of magnitude. But this is
not an either/or situation. Both factors apply multiplicatively to give the total increase in
sensitivity and that makes the contribution from the dilating pupil still significant even if it's
not the dominant effect.

If I can cut way back on my spending I might be able to increase my savings for retirement by a
factor of 1000 (assume it was very low to start with). Now say my employer and the government come
along and tell me that by putting that money in a special account it could be twice as big through
matching funds and tax-free status. Am I going to say - 'Naw, don't bother, a factor of 2 is
insignificant compared to the factor of 1000 I get by myself'? I don't think so - I'm going to be
happy to get that extra factor of 2 and will consider it to be quite significant.

Since the Cateye Micro Halogen is so common, how about using it as a basis for comparison of the new
LED models? Are they brighter? How do the beam shapes compare?

Matt O.

Tim McNamara said:

Your first point is correct, the pupil changes size much more rapidly than the retina becomes dark-
adapted. The former takes a few seconds, the latter takes about 40 minutes. This gives us two
systems for adapting to variable brightness- one very rapid but of limited range (about 4 times
for a minimum pupil size of 2 mm and a maximum of 8 mm), one of great range (1,000 times) but slow
to adapt.

As I point out in a subsequent post, I screwed up the math here. The difference for a 2 mm pupil vs
a 4 mm pupil would be a factor of 16.

Peter said:

Sure, I had already said the retinal sensitivity changes by many orders of magnitude. But this is
not an either/or situation. Both factors apply multiplicatively to give the total increase in
sensitivity and that makes the contribution from the dilating pupil still significant even if it's
not the dominant effect.

Good point. The two systems interact; one provides rapid adaptation and the other more range of
sensitivity. The combination allows us to see effectively in an stonishing variety of ambient
conditions.

Quoted message said:

I may have been that guy. The light is the Planet Bike Super Spot
(planetbike.comfeature.htmlOpen ↗) at ~$35. Performance was selling it for ~$30 as an intro
price. A call to Planet Bike confirmed that they use the Lumileds Luxeon Star 1W part. They say
price is the only thing keeping them from using the higher output Luxeon Star parts. One of the
frustrating things about the rating of bike lights is that everyone seems to quote watts as the
yardstick. Watts tells you how much power the unit consumes, it doesn't tell you how much light (in
lumens) is output.

This is very true. However when comparing bulbs of the same technology, with similar lenses, wattage
is one indicator of the comparative illumination.

What I did on my web site was to also include the lumens of each lamp, as far as could be determined
from the literature. This is in the section "Watts Versus Lumens." I've added Luxeon LEDs to these
charts, as well as HID.

But you know of course why they use watts; it's easy to measure. I always warn people about not
confusing light output with wattage input.

Quoted message said:

believe that LED lights have higher efficiency as they produce more lumens/watt than your standard
halogen type bulb so using watts isn't a good unit of comparison. For the future, Lumileds
propaganda talks about output going from about 20 lumens/watt to around 50 lumens/watt in a couple
of years so the future of LED lighting for bicycles might be promising.

Higher power LEDs will all depend on thermals. They have to figure out a method of cooling the LED
as the wattage goes up. HID is more promising for bicycle lights since it's more efficient AND easy
to get up to the levels of illumination needed for headlights. Prices should come down--eventually.
I' ve added HID to the lumens/watt table on my web site. Surprisingly, you can get to the efficiency
of HID by over-voltaging regular halogen lamps, but you'll pay a penalty in lamp life.

Quoted message said:

The other challenge in LED light design seems to be the optics; standard halogen reflectors don't
seem to be optimal.

Reflectors are always difficult. It's especially difficult to build a reflector when you have
several LEDs that you want to combine into a single beam. The greatest thing since sliced bread was
the invention of the MR11 and MR16 lamps with matched precision matched bulbs and reflectors. Yeah,
you have to throw away a perfectly good reflector when the bulb burns out, but the increase in
illumination from the matching of the bulb and reflector are significant.

Quoted message said:

I currently have a 1W LED flashlight which is pretty amazing, but I don't believe it has enough
light for cycling at any significant speed. I have one of these Planet Bike units on order, and
plan to modify it using the 3W Luxeon Star if possible ($13 from www.elekrolumens.com). If it works
out, I'll post my findings.

Watch the heat when you do the conversion. There is a thermal design guide on the Luxeon web site.
But thermal management of LEDs is very different than thermal management of quartz-halogen lamps.

"Ron Hardin" <[email hidden]> wrote in message
"]news:[email hidden]...

Quoted message said:

That would be nearly enough light if it's not generating heat; but the LED

reputation

Quoted message said:

for lasting a long time comes from its not drawing much current, which

until now

Quoted message said:

has meant not much light either. So waiting for LEDs to throw out more

light so

Quoted message said:

as to get a longer runtime is doomed to failure from the start, except for

some

Quoted message said:

constant factor from LED efficiency, say two, from not generating heat as

well.

This is true. The Luxeon web site goes into extensive detail about dealing with the thermal issues
of their LEDs. IF, and it's a big IF, the thermal issues can be solved, and the optics issues can be
solved, then the high efficiency of the LEDs will enable their use in higher brightness lamps.
Portable applications are especially difficult since you can't have a big fan blowing on them.The
thermals always get you! I spend half of my time dealing with managing thermal issues of
semiconductors. There's a good layman book on this, "Hot Air Rises and Heat Sinks." ISBN 0-7918-0074-
1. amazon.com0791800741Open ↗.

The lumens/watt of the LEDs is slightly impressive at the lower lumen levels, but HID lamps are much
more efficient than LED lamps. It's actually easier dealing with the thermals of quartz-halogen
lamps because of the way the heat radiates out through the glass, and then out to a metal enclosure
exposed to the air. You can't get around the thermals. An LED is still just a semiconductor, and the
more current through it the hotter it gets.

Ted Bennett said:

Benjamin Lewis said:

Ted Bennett said:

Pupil size certainly does change with accommodation, however, and I think it is significant.
Under constant lighting conditions, the pupil contracts as the eye accommodates, or shortens its
focal length, and this is readily observable. The benefit is the same as with any variable
aperture: a greater depth of focus is obtained with a smaller aperture.

This may be a benefit, but is it the cause? Under constant light conditions, with constant pupil
size, more light strikes one's retina under shorter focal lengths (which is why zoom lenses
require larger apertures). Is there any reason to believe that pupil size is determined by
anything other than the amount of light impinging upon the retina? (Not counting dilation induced
by drugs, of course.)

Pupil size is affected by much more than photons. Specifically, pupils contract at shorter focal
lengths, which has the beneficial effect of making it easier to focus at closer distances. That
can't be said to be a result of the amount of light, although more light does make reading easier.
I suspect I may be misunderstanding what you wrote, but with constant light conditions and
constant pupil size, the amount of light in the eye will not change with various focal lengths.
The amount of light is determined by the aperture.

Oops, I think you're right. Mea culpa. I see that the zoom lens logic doesn't apply here.

However, I still think that the main "purpose" of pupil dilation is controlling the amount of light
that enters the eye, since adaptation of the retina occurs too slowly for quick fluctuations in
light levels.

Otherwise, if the primary purpose had more to do with focusing, we would expect to see a much larger
range of dilation when changing focus between infinity and nearby objects, and a much smaller range
when looking at light fluctuations at a fixed distance.

--
Benjamin Lewis

Although the moon is smaller than the earth, it is farther away.

Ted Bennett said:

All right, let's try it this way: Variation in pupil size is more effective for focusing than for
light control.

Then why don't we *always* have small pupils? Isn't it desirable to have increased depth of field at
any focal length? What is it about the *variation* that is good for focusing?

--
Benjamin Lewis

Although the moon is smaller than the earth, it is farther away.

Tue, 13 Jan 2004 06:10:01 GMT, Steven M. Scharf:

Quoted message said:

The Luxeon web site goes into extensive detail about dealing with the thermal issues of their LEDs.
IF, and it's a big IF, the thermal issues can be solved, and the optics issues can be solved, then
the high efficiency of the LEDs will enable their use in higher brightness lamps. Portable
applications are especially difficult since you can't have a big fan blowing on them.

The situation on a bicycle is perfect in this regard: You don't need much light on the front when
stationary and you have lots of cooling air when riding. The combination of a current limited (hub)
dynamo with a 700mA Luxeon LED in a metal housing (added with a capacitor based standlight) sounds
promising. Someone still has to develop some really efficient optics. The small, rotational
symmetric plastic things are far from what could be done.

Andreas

"Peter Cole" <[email hidden]> wrote in message

<snip>

Quoted message said:

An ideal set up for me would be a low power continuous light augmented with a quick operating, much
higher power light, much like auto high/low beams. I've yet to see one on the market like that.

These are available. A low power 5 watt light and a high power 20W light in the same fixture, or in
two separate fixtures, but powered by the same battery. The consensus seems to be that it's better
to have two separate fixtures, since you want to aim the "being seen" light higher, and the "seeing"
light lower.

The 5W MR16 lamps are available from: e-go.gb.comparts.aspOpen ↗, while 10W and 20W MR16 lamps
are widely available.

I try to make things simple, and carrying only one battery is simpler than having two batteries or a
battery and a dynamo. However the advantage of a dynamo lamp is that you are not at the mercy of
batteries.

frkrygow' 'frkrygow said:

rosco wrote:

Quoted message said:

Quoted message said:

Just curious - has anyone ever made a 12VDC florescent bike light? Are they more fragile?
Certainly ones I've seen for home use aren't exactly small and light.

There are small, 12V flourescent lamps aplenty in RV and marine stores. Be prepared to pay the
"yachtsman's discount," though.

Quoted message said:

Back in the 1970s, someone manufactured a generator-powered flourescent light. I saw only one
in action. It cast a very diffused glow around the rider, and wasn't very useful, from what I
could see.

Quoted message said:

I confirmed what Jobst later posted here: there was no practical way to get a focused beam out of
that diffuse source.

That's the problem. Even if a suitable reflector could be designed, it would be rather large for a
bicycle. For a 12" lamp it would look like half a pony keg. For a smaller, "light bulb" style
flourescent, it might be like one of those $5 clamp-on work lights from Home Depot.

Quoted message said:

I've recently seen small flashlight/lanterns with miniature flourescent tubes, roughly 1/8"
diameter by 2" long, IIRC. Those might be focused with moderate success. But I though I read that
their lumens/watt efficiency wasn't all that great. Can anyone confirm?

Lumens/watt is fine, probably the best this side of an LED. The problem is focusing all that light.

Matt O.

Benjamin Lewis said:

I've only once had my eyes dilated chemically, but I recall being especially sensitive to bright
lights afterward. If your hypothesis is correct, this must be an additional side effect of the
drug, rather than an effect of the pupil size as I've always presumed. Perhaps this is correct,
but it sounds a bit fishy to me.

I see the misunderstanding. You are talking about sensitivity in a different way that I meant.
Having artificially dilated pupils is certainly uncomfortable, but that has little to do with
the amount of stimulus that prokes a visual reaction, which is what I meant by the eye's
sensitivity to light.

A lot of the discomfort with artificial dilation is due to imaging problems. The eye has a number of
aberrations, all of which are much worse with a big pupil.

Quoted message said:

I have another hypothesis:

- retina response to light change is much greater than pupil size, but the former is much slower
than the latter.

- in constant light conditions, changing the focal length of the eye changes the amount of light
that reaches the retina.

No. Only the aperture changes the amount of light that enters.

Quoted message said:

- thus, being able to change sensitivity rapidly over a smaller range is
useful. A quick change in pupil diameter allows us to do this.

Are there any experts here who can confirm or deny?

"Expert" is a relative term, but I have been interested in certain aspects of vision for decades,
involved in research and eye health care. This getting pretty off topic for r.b.t. If you or anyone
else wants to continue, it's off-line only for me.

--
Ted Bennett Portland OR

In article <[email hidden]>,

Tim McNamara said:

Normal attribution styles would make reading your posts much simpler.

Like this? OK.

Quoted message said:

For those who are unfamiliar with the visual system, here some details. <big snip of true stuff>

Quoted message said:

Ted Bennett said:

Pupil size certainly does change with accommodation, however, and I think it is significant.
Under constant lighting conditions, the pupil contracts as the eye accommodates, or shortens
its focal length, and this is readily observable. The benefit is the same as with any variable
aperture: a greater depth of focus is obtained with a smaller aperture.

While this makes sense from what we know about camera optics, I can't find anything to support
this in any of the sources I've got on my bookshelf. Do you have a reference?

It has been taught in medical school for a while ;-) -actually the mechanisms are not identical, and
in certain diseases the pupil may not react to light but still react to accommodation (You can
search for Argyll Robertson or Adie on the net).

Quoted message said:

I found this interesting: from Gibson (1966, p. 172):

"Accommodation can be achieved by either moving the lense toward or away from the retina, as in
fish, or by changing its curvature, as it is in mammals... As Walls (1942) points out, "the small
eye needs no accommodation at all" and not all vertebrate eyes accommodate for distance. The
comparison of eyes with cameras or projectors is misleading, and the investigators of human
vision, where the eye is not very large, have overstated the importance of focusing the lens, as
if it were like the focusing of a picture on a screen. The human eye loses most of its power of
accommodation at the age of 50 or 60, but if it was normal to begin with it remains a fairly
efficient organ."

Walls seems to have an incomplete understanding of the optics of eyes or of cameras. There are
differences, naturally, but both eyes and cameras do work a lot better with a good image at the
right place. There's plenty of focusing of light going on in presbyopic eyes. Power of accommodation
has nothing to do with best acuity at the focal point.

Quoted message said:

Which must be whey I wear bifocals. ;-) But then my eyes weren't normal to begin with (myopia).

Consider the alternative. Hyperopia is worse.

--
Ted Bennett Portland OR