Eye Relief: Will more nearsighted people need more eye relief? (1 Viewer)

Laughing Kookaburra · Apr 24, 2015

Will more nearsighted people need more eye relief, or is it just a function of how far your glasses are from your face?

Will someone wearing contact lenses still need eye relief? I've noticed I benefited from eye relief even with contact lenses on.

OPTIC_NUT · Apr 25, 2015

Just a function of the glasses.
However, larger glasses frames need more eye relief than smaller frames..
that's a function to be aware of.

looksharp65 · Apr 25, 2015

Laughing Kookaburra,

I'm an optician and optometrist. If the frames are positioned exactly the same and the lens apex is at the same distance from the eye, the negative lenses for myopes will require less minimum eye relief than the positive lenses for hyperopes.
However, this effect will be amplified due to the center thickness of positive lenses and the base curves of the lenses. Strong positive lenses can have a considerable center thickness (in particular if the spectacle frame is large, which is the only situation when OPTIC_NUT's line of reasoning is correct).
Strong negative lenses can have a nearly totally flat front curve, which means that they can come closer to the oculars.
However, there are even more things to complicate it. Spectacles are usually inclined inwards in the lower part, and have some angle between the front lenses as well.
This means that the actual distance between the eye and the ocular lenses is not decided by the distance between the cornea's apex and the spectacle lens's apex, since the contact points between the spectacle lens and the binocular's eyecups are located at about 1 to 2 o'clock for the left side and 10 to 11 o'clock for the right side.

I was close to selling my Nikon E II but decided to make a dedicated pair of spectacles for it. Apart from being without the customary nose pads, I reshaped the frame so the front lenses are not inclined towards the cheek and not to each other. This helps, but they are a bit strange to look through outside the binoculars.

//L

Binastro · Apr 25, 2015

. Dear looksharp65,

Sorry to introduce a somewhat different topic here, but I haven't heard from you for a while on the forum.

You may be interested in the ongoing thread ' How Long Has Each Manufacturer Been Around? ', where I have found and mentioned on the thread a lens and spectacle lens maker whose family has been making lenses in Sri Lanka since 1340. These are made using nearly clear amethysts, which are mined locally and of high quality.

But what I would like to ask is, I met my optician yesterday, and I need some prism correction only for my reading glasses. I think that in the past there was actually some prism in each lens, which gave some colour effects if you looked for them. However, yesterday he mentioned that there is another way of correcting for prism by de-centring the lenses, and he may use this method for my new reading glasses.

Also as the prescription is a little bit stronger, only about a quarter dioptre, he will look into high index lenses, but I mentioned that I've seen mentioned on this forum that these could give rise to chromatic aberration affects. But he wasn't sure if this is always the case. The hgh index lenses would be a bit lighter weight as they are thinner, but anyway my reading glasses are quite small and I don't think that high index lenses are a necessity.

I did think that I could answer correctly the original poster here, but I wasn't sure as I'm no expert, so I waited for somebody professional to answer.

It is nice to hear from you again.

P. S.
I showed my optician the 6.5×21 Papilio binocular and he used it, but he was surprised by his hand tremor while holding it and looking at close objects. I did not have the accessory 18g post on it which screws into the tripod socket and helps to stabilise it.

I don't quite know why it works so well as the objectives close on each other, but parallel to the optical window, as you focus to near 0.5 m. There is a slightly curved track at an angle of about 20° for each objective as they get nearer at the closest point. The optical system does not seem to incline at all as a Minolta patents seems to suggest for another similar binocular.
With the Papilio binocular the IPD stays constant from the infinity focus to closest focus. It is a nice binocular in its mark II form.

perterra · Apr 25, 2015

looksharp65 said:
Laughing Kookaburra,

I'm an optician and optometrist. If the frames are positioned exactly the same and the lens apex is at the same distance from the eye, the negative lenses for myopes will require less minimum eye relief than the positive lenses for hyperopes.
However, this effect will be amplified due to the center thickness of positive lenses and the base curves of the lenses. Strong positive lenses can have a considerable center thickness (in particular if the spectacle frame is large, which is the only situation when OPTIC_NUT's line of reasoning is correct).
Strong negative lenses can have a nearly totally flat front curve, which means that they can come closer to the oculars.
However, there are even more things to complicate it. Spectacles are usually inclined inwards in the lower part, and have some angle between the front lenses as well.
This means that the actual distance between the eye and the ocular lenses is not decided by the distance between the cornea's apex and the spectacle lens's apex, since the contact points between the spectacle lens and the binocular's eyecups are located at about 1 to 2 o'clock for the left side and 10 to 11 o'clock for the right side.

I was close to selling my Nikon E II but decided to make a dedicated pair of spectacles for it. Apart from being without the customary nose pads, I reshaped the frame so the front lenses are not inclined towards the cheek and not to each other. This helps, but they are a bit strange to look through outside the binoculars.

//L

Thank you, interesting post.

OPTIC_NUT · Apr 26, 2015

Dear looksharp65,

Excellent post, and thanks for the insight!

In my (and my mother's and brother's and wife's) experience, however,
large frames seem to place the lens further from the eye than small frames.
This uses up valuable eye relief. I have special glasses too, and that's the main reason.

However, I didn't mean my little rule of thumb covered everything, so
what you posted still applies, as a seperate factor.
Maybe the actual spacing to the lenses is
different in different places and styles. The eye cannot see through all of
a big frame unless there is a little distance added.

I tack a -5D lens in some binoculars, ones that give 'blackout' problems.
Since I have very little astigmatism, this is practical.

looksharp65 · Apr 26, 2015

The power of a lens is decided by the base (front) curve and the rear curve together. The distance between these, i.e. the lens's thickness, has negligible effect on its power.

But since in a convex lens the curves will meet at the lens's edge, the curves and the diameter will decide how thick the lens will be. If you imagine pushing the front curve and the back curve together, they will meet at a shorter distance from the center.
In reality, this means that a larger frame may mean thicker lenses. Usually, there are different lens diameters available so when a frame is small enough for a smaller lens, that lens is preferred before the larger one.

My drawing uses copied elements, the front and the rear curve are identical for both "lenses" and so are their powers.
This also explains why loupes have so small lenses, which often are bi-convex as well. At some point, the maximum size will be reached and then we find a crystal ball.

//L

looksharp65 · Apr 26, 2015

Binastro said:
. Dear looksharp65,

Sorry to introduce a somewhat different topic here, but I haven't heard from you for a while on the forum.

You may be interested in the ongoing thread ' How Long Has Each Manufacturer Been Around? ', where I have found and mentioned on the thread a lens and spectacle lens maker whose family has been making lenses in Sri Lanka since 1340. These are made using nearly clear amethysts, which are mined locally and of high quality.

But what I would like to ask is, I met my optician yesterday, and I need some prism correction only for my reading glasses. I think that in the past there was actually some prism in each lens, which gave some colour effects if you looked for them. However, yesterday he mentioned that there is another way of correcting for prism by de-centring the lenses, and he may use this method for my new reading glasses.

Also as the prescription is a little bit stronger, only about a quarter dioptre, he will look into high index lenses, but I mentioned that I've seen mentioned on this forum that these could give rise to chromatic aberration affects. But he wasn't sure if this is always the case. The hgh index lenses would be a bit lighter weight as they are thinner, but anyway my reading glasses are quite small and I don't think that high index lenses are a necessity.

I did think that I could answer correctly the original poster here, but I wasn't sure as I'm no expert, so I waited for somebody professional to answer.

It is nice to hear from you again.

P. S.
I showed my optician the 6.5×21 Papilio binocular and he used it, but he was surprised by his hand tremor while holding it and looking at close objects. I did not have the accessory 18g post on it which screws into the tripod socket and helps to stabilise it.

I don't quite know why it works so well as the objectives close on each other, but parallel to the optical window, as you focus to near 0.5 m. There is a slightly curved track at an angle of about 20° for each objective as they get nearer at the closest point. The optical system does not seem to incline at all as a Minolta patents seems to suggest for another similar binocular.
With the Papilio binocular the IPD stays constant from the infinity focus to closest focus. It is a nice binocular in its mark II form.

Hi Binastro and thank you!

Please follow the link in my signature. Those lenses are far older than the 14th century.

It is correct that a decentered lens induces a prismatic effect. The effect can only be controlled with spherical lenses, not aspherical. The principle is easy: the prismatic power is measured in prism diopters. A 2.00 D lens decentered by 0.5 cm induces a prism power of 1.0 prism diopters. This can only be done with single-vision lenses, not bifocals, near-progressives or progressive lenses.

For single-vision lenses, there is no technical difference whatsoever between a decentered standard lens and a custom made.
Only the delivery range with respect to sufficient lens diameter decides whether a standard or a custom made lens can/must be used.

I don't know about your reading powers or which convergence direction your prism prescription is supposed to correct. If you over-converge and have convex lenses (like +2.00), the lenses should be moved outwards. If the frame size (the distance from the outer edge of right lens to the inner edge of the left lens) is bigger than your near IPD, this might mean that a smaller/thinner lens could be used.
OTOH, if you under-converge, the lenses must be decentered towards the nose. If their powers are quite weak, the decentration must be longer in centimeters and vice versa. But if the powers are stronger, less decentration could be used. However, if the frame is big, your IPD tight and your prism prescription is strong, this means the lenses can become very thick.
Very few customers complaint about coloured edges, that's all I can say.
Double vision is more annoying for anyone than some extra colours

//L

OPTIC_NUT · Apr 27, 2015

Throughout my life, I have had large-frame glasses that rode farther from my face,
and small-frame glasses that rode closer. I actually purchased smaller lenses to
place them closer. The theoretical drawing doesn't seem to have a human face
in it, which would block rays for some large glasses. Heck, big glasses cannot physically
fit some cheekbones unless the are moved out. It seems obvious. The drawing doesn't
account for a face at all.

Not sure what else to say. Maybe glasses are different in different places (??)
Elton John type glasses certainly ride farther out than John Lennon glasses,
and contacts use up zero relief. Can't make it plainer.

looksharp65 · Apr 27, 2015

OPTIC_NUT,

the simple drawing was made only to show that larger diameter convex lenses will be thicker with larger diameter.
Larger frames require larger lenses, so if the powers are positive, the increased thickness may induce the need for a greater eye relief.

You can rest assured that I'm very familiar with spectacle lenses, frames and fitting of the lenses.
I have spent at least five years of my career in the optical workshop.
So I'm trying to figure out what you actually mean. If the size difference is considerable, like in your examples, I can agree to some extent. Very large glasses can touch the eyebrows and/or the cheekbone, but they will need to be a lot smaller to allow for a closer fit. This connection is not linear - you can't make a 20 mm lens sit closer to the eye than a 30 mm because the eyelashes will decide how close the lens can sit. The size of a contact lens is irrelevant in this case since it's behind the eyelids. A 9.5 mm diameter RGP lens will also not get closer to the eye than does a 14 mm soft lens, for obvious reasons.

It is not difficult to construct a spectacle frame that allows for a very close fit. However, while a small/ish size of the lenses is necessary, it takes some more.
The nose bridge has to have a pronouced crescent shape to follow the nose's shape. This means that the bridge protrudes in front of the attachment points to the lenses.
Furthermore, the distance between the inner edges of the lenses should be elongated to avoid contact with the nose's sides. The nose pad arms may need to be compressed or removed or the nose pads removed (not recommended with alloy frames).
When all this is done, the distance between the eyes and the glasses can be very small or too small, when the eyelashes touch the inner sides of the lenses.
And if all the above-mentioned is done, the outer (lateral) width of the lenses is unimportant for the distance between the eyes and the lenses.

//L

looksharp65 · Apr 27, 2015

Ordinary frame and adjusted for close fit

Attached is an image of two of my spectacles that are very similar in size.
The upper is my ordinary frame, and the lower is my E II dedicated spectacles.

Notice the difference in nose width, nose bridge curvature, the presence/absence of nosepads and the curved vs. almost straight inclination between the lenses.

The yellow dots indicate where the frame's outer contact point with the binocular's eyecups is located. The straight shape of the dedicated spectacles may not be very convenient for ordinary viewing, but it allows viewing the full FOV with the E II.

//L

OPTIC_NUT · Apr 28, 2015

I have some big older B+L Aviators and some recent 'eyebuydirect', which are
actually very good..
I had some old horn-rims (Clark Kent style) that sat pretty far away.
Perhaps the style has more of an effect than the size? not sure.

Come to think of it, my eyelashes do brush the lenses, though I have set the nosepads
so I can get close.

This is strange, but my eybuydirects look just like your specials in profile.
I got lucky perhaps?
I suppose when one is shopping you can look out for the curved bridge.

Binastro · Apr 28, 2015

. Hi looksharp65,
. It is good that you have found a way of using the E II binocular while getting the whole field of view.

An older distance glasses kept losing the nose pad, and finally I could not find the nose pad. However, I find it extremely comfortable to use without the nose pad. It is a thin plastic frame with reasonably large lenses, I think multicoated plastic. For some reason I find these glasses very good for normal use even though they are now not the correct prescription.

As to crystal balls or spherical lenses. I think it might have been William Herschel who used tiny spherical glass lenses for very high magnification use on his telescopes. I think he used magnifications up to a thousand times or more. I think that other astronomers also used these tiny spheres as high magnification eyepieces. The field of view was tiny, but for planetary observations and observing double stars this didn't matter.

Swedpat · Apr 28, 2015

looksharp65,

Thanks for your sharing your knowledge of optics. Around 15 years ago I decided to have only binoculars with eye relief usable with eyeglasses. Earlier I was pretty demanding when it comes to eye relief, but since a few months I got new eyeglasses who work better. The design and construction of the eyeglasses really makes sense for the use of binoculars. Here is a picture of my old eyeglasses compared to the new. The new eyeglasses to right let my eyes to come, I would think, ~2mm closer the ocular lens. A really important difference.

OPTIC_NUT · Apr 28, 2015

This is interesting....that 2mm does make a world of difference!
I can use anything that claims 13mm er, but others can't make use of 15mm....

Maybe people can report 'close frames' for binocular use.
A rough standard would be, 'can you press in and brush with eyelashes'.
Then you just use the nosepad for fine-tuning.

Eye Relief: Will more nearsighted people need more eye relief? (1 Viewer)

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