-
Welcome to BirdForum, the internet's largest birding community with thousands of members from all over the world. The forums are dedicated to wild birds, birding, binoculars and equipment and all that goes with it.
Please register for an account to take part in the discussions in the forum, post your pictures in the gallery and more.
Eye Relief (1 Viewer)
- Thread starter fornacino
- Start date
I am by no means an expert on this but generally speaking I think it is the size (focal length) of the oculars and the design of the oculars that determines the length of the eye relief. Wide field oculars also usually have shorter ER than standard size oculars.
My experience is that most 42mm binoculars have longer ER than 32mm binoculars.
A 7x42 binocular usually has longer ER than an 8x42 and 10x42 binocular. Probably because, all things being equal, the oculars for the 7x have a longer focal length than those for an 8x or 10x.
Oculars for 32mm binoculars are smaller and have shorter focal lengths than oculars used on 42mm binoculars and and this results in shorter eye relief.
Bob
Last edited:
With f/4 objectives.
7x42.
Focal length of objective 168mm.
Divide by 7.
Gives 24mm focal length eyepiece.
10x32.
Objective focal length 128mm.
Divide by 10.
12.8mm eyepiece focal length.
For the 7x42 a simple eyepiece will give a long eye relief.
For the 10x32 a simple eyepiece will give short eye relief.
Because the eyepiece of the 10x32 is much smaller than the 7x42 it can have a much wider apparent field of view, but this needs extra elements.
The 7x42 may have a 3 element eyepiece.
The 10x32 5 or 6 elements.
To get longer eye relief the 10x32 also needs a more complex eyepiece, so maybe actually 6 or even 7 elements.
Maybe 6 to give a wide AFOV and reasonable eye relief.
Because I don't wear glasses with binoculars, I don't like long relief eyepieces. I much prefer medium to small eye relief, but not very small as this means I lose some of the wide field, if the view is at an angle to straight ahead.
7x42.
Focal length of objective 168mm.
Divide by 7.
Gives 24mm focal length eyepiece.
10x32.
Objective focal length 128mm.
Divide by 10.
12.8mm eyepiece focal length.
For the 7x42 a simple eyepiece will give a long eye relief.
For the 10x32 a simple eyepiece will give short eye relief.
Because the eyepiece of the 10x32 is much smaller than the 7x42 it can have a much wider apparent field of view, but this needs extra elements.
The 7x42 may have a 3 element eyepiece.
The 10x32 5 or 6 elements.
To get longer eye relief the 10x32 also needs a more complex eyepiece, so maybe actually 6 or even 7 elements.
Maybe 6 to give a wide AFOV and reasonable eye relief.
Because I don't wear glasses with binoculars, I don't like long relief eyepieces. I much prefer medium to small eye relief, but not very small as this means I lose some of the wide field, if the view is at an angle to straight ahead.
Foracino,
I'll see if can spexplain some of the aspects of binocuar design that that are important here.
Long eye relief is often listed as a desirable feature for those who wear spectacles. Usually around 16mm is adequate for close fitting designs but some individuals, with the frames of their choice, may need 22mm or more. If we use a 65° apparent field of view as an example. With a 16mm ER the diameter of the eyepiece lens would need to be at least 20.4mm in diameter. At 22mm ER the eyepiece lens would need to be at least 28mm in diameter. That is about double the area and could result in more than double the weight. It's one of the reasons why manufacturers may choose a shorter ER for their more compact models.
Another obvious factor governing weight is the size of prism required to funnel the light from the objective to the focal plane. For roof prisms it is common for 8x and 10 x models to share the same objective. For example if the prism was sized to allow a 6.5° angle of view then then either of those 65° eyepieces with the appropriate focal length to give 10x magnification would utilise the full 65° AFoV. However if we use an 8x eyepiece instead, then that 6.5° FoV times 8x is only 52° so some of that eyepiece diameter is wasted. Instead of a 20.4mm diameter lens at 16mm ER only a 15.6mm lens would be required. The manufacturer could potentially offer a 20.1mm ER for the 8x instead amongst other options.
In practice the designers will juggle with these and several more parameters to come up with the best specification they can offer for the price, so things are rarely quite that simple.
I know some are confused by how the eyecup extension interplays with the ER. The ER is simply the distance the eye should be from the lens for best performance. The eyecup meerly bridges the gap.... or not, between the binocular, your face or your glasses. With the current trend towards longer ERs it seems the eyecups often do not extend enough to satisfy those who do not wear glasses in particular. So is the eyecup design too short, or the ER too long? Opinions will differ.
David
I'll see if can spexplain some of the aspects of binocuar design that that are important here.
Long eye relief is often listed as a desirable feature for those who wear spectacles. Usually around 16mm is adequate for close fitting designs but some individuals, with the frames of their choice, may need 22mm or more. If we use a 65° apparent field of view as an example. With a 16mm ER the diameter of the eyepiece lens would need to be at least 20.4mm in diameter. At 22mm ER the eyepiece lens would need to be at least 28mm in diameter. That is about double the area and could result in more than double the weight. It's one of the reasons why manufacturers may choose a shorter ER for their more compact models.
Another obvious factor governing weight is the size of prism required to funnel the light from the objective to the focal plane. For roof prisms it is common for 8x and 10 x models to share the same objective. For example if the prism was sized to allow a 6.5° angle of view then then either of those 65° eyepieces with the appropriate focal length to give 10x magnification would utilise the full 65° AFoV. However if we use an 8x eyepiece instead, then that 6.5° FoV times 8x is only 52° so some of that eyepiece diameter is wasted. Instead of a 20.4mm diameter lens at 16mm ER only a 15.6mm lens would be required. The manufacturer could potentially offer a 20.1mm ER for the 8x instead amongst other options.
In practice the designers will juggle with these and several more parameters to come up with the best specification they can offer for the price, so things are rarely quite that simple.
I know some are confused by how the eyecup extension interplays with the ER. The ER is simply the distance the eye should be from the lens for best performance. The eyecup meerly bridges the gap.... or not, between the binocular, your face or your glasses. With the current trend towards longer ERs it seems the eyecups often do not extend enough to satisfy those who do not wear glasses in particular. So is the eyecup design too short, or the ER too long? Opinions will differ.
David
Last edited:
mooreorless
Well-known member
David very good explanation! This should help a lot of people on here.
Thank you all for your kind explanation. Like Enrico Fermi once said:"I'm still confused, but at a higher level."
If I understand correctly...can I say that with long ER, the image comes into focus further away from the eye piece; which benefits those who wear glasses. For those who don't, the eye piece extension is there to provide the required distance. If the extension is not sufficient, then the image will be somewhat vignetted or blacked out. I say this because I just tried it and that's what happened.
An unexpected result of designing for long ER is added weight and optical complexity. This makes me feel much better about choosing a lighter pair on binoculars that has a relatively short ER. Knowing this I'm now more willing to put up with occasional black outs out of the lighter binos.
If I understand correctly...can I say that with long ER, the image comes into focus further away from the eye piece; which benefits those who wear glasses. For those who don't, the eye piece extension is there to provide the required distance. If the extension is not sufficient, then the image will be somewhat vignetted or blacked out. I say this because I just tried it and that's what happened.
An unexpected result of designing for long ER is added weight and optical complexity. This makes me feel much better about choosing a lighter pair on binoculars that has a relatively short ER. Knowing this I'm now more willing to put up with occasional black outs out of the lighter binos.
Yes.
Not necessarily added weight, more likely smaller field than a binocular with short eye relief.
Do you wear glasses with binoculars? I presume not.
The long or not long enough eyecups are only there because the makers are trying to sell to everybody.
For me this is just not on. I do not want excessive eye relief.
However, nowadays more people seem to need glasses, maybe because of the use of computer screens etc.
Not necessarily added weight, more likely smaller field than a binocular with short eye relief.
Do you wear glasses with binoculars? I presume not.
The long or not long enough eyecups are only there because the makers are trying to sell to everybody.
For me this is just not on. I do not want excessive eye relief.
However, nowadays more people seem to need glasses, maybe because of the use of computer screens etc.
Binastro,
I do not wear glasses. I used up until the age of 58; since then my vision has improved sufficiently to let me get away with not wearing them.
On the subject of glasses: I don't seem to gain any visual advantage by wearing corrective lenses when looking through my binos. Is this normal or is it my inability to see critical details?
I do not wear glasses. I used up until the age of 58; since then my vision has improved sufficiently to let me get away with not wearing them.
On the subject of glasses: I don't seem to gain any visual advantage by wearing corrective lenses when looking through my binos. Is this normal or is it my inability to see critical details?
Hi fornacino,
Whatever is most comfortable for you, either wear glasses or don't.
Some people prefer to wear glasses with binoculars even though they don't gain much.
A yearly eye test is useful, or at least after age 65.
It may be that acuity is not as good as when young.
Tiredness affects me more than anything nowadays.
Regarding eye relief.
Simple classic eyepieces such as Kelners, achromatic Ramsdens, orthoscopics and Plossls have an eye relief that is a fraction of the eyepiece focal length. Typically around half the focal length, sometimes a bit more.
The Erfles and Berteles have rather short eye relief.
The longest eye relief was probably the Kepler, just one piece of glass, but the sharp field of view is tiny, a few degrees. With say Jupiter, which is around 1/80th degree across, then using 200x on an astro telescope increases the apparent size to 2.5 degrees. Even this may not be critically sharp, but the area studied would be a fraction of this.
With a clockwork mechanical drive for the telescope to correct for the Earth's rotation and the planet dead centre, then fine observations resulted.
With a single piece of glass the transmission is high even uncoated.
Herschel used spherical tiny balls of glass that gave very high magnification.
He was able to determine that Uranus had a disc and was not a star. First he thought it was a comet, then realised that it was a new planet.
Previous astronomers including Galileo had seen Uranus, but not realised it had a disc. They thought it was a star.
But this eyepiece is useless with a binocular.
Usually three or more elements are used.
To get long eye relief eyepieces extra lenses are used to throw the eye position well behind the eyepiece.
With Vixen Long eye relief eyepieces I think that the eye relief was about 20mm for all focal lengths even 2.5mm focal length.
I will check as this is from memory.
But the fields are typically smaller with these long eye relief eyepieces.
Whatever is most comfortable for you, either wear glasses or don't.
Some people prefer to wear glasses with binoculars even though they don't gain much.
A yearly eye test is useful, or at least after age 65.
It may be that acuity is not as good as when young.
Tiredness affects me more than anything nowadays.
Regarding eye relief.
Simple classic eyepieces such as Kelners, achromatic Ramsdens, orthoscopics and Plossls have an eye relief that is a fraction of the eyepiece focal length. Typically around half the focal length, sometimes a bit more.
The Erfles and Berteles have rather short eye relief.
The longest eye relief was probably the Kepler, just one piece of glass, but the sharp field of view is tiny, a few degrees. With say Jupiter, which is around 1/80th degree across, then using 200x on an astro telescope increases the apparent size to 2.5 degrees. Even this may not be critically sharp, but the area studied would be a fraction of this.
With a clockwork mechanical drive for the telescope to correct for the Earth's rotation and the planet dead centre, then fine observations resulted.
With a single piece of glass the transmission is high even uncoated.
Herschel used spherical tiny balls of glass that gave very high magnification.
He was able to determine that Uranus had a disc and was not a star. First he thought it was a comet, then realised that it was a new planet.
Previous astronomers including Galileo had seen Uranus, but not realised it had a disc. They thought it was a star.
But this eyepiece is useless with a binocular.
Usually three or more elements are used.
To get long eye relief eyepieces extra lenses are used to throw the eye position well behind the eyepiece.
With Vixen Long eye relief eyepieces I think that the eye relief was about 20mm for all focal lengths even 2.5mm focal length.
I will check as this is from memory.
But the fields are typically smaller with these long eye relief eyepieces.
I have an observation that I'd be interested if others find it as well. I wear glasses and with a couple of my binoculars I find if I pull the eye-cups out a bit I gain a bit of sharpness in the view although this can mean I then can't see the whole field of view.
These are binoculars with a wide field of view anyway. When the eye cups are fully in they view is still sharp, relaxed and without blackouts (with good eye placement), but still a bit of sharpness seems to be gained by bringing the eye cups out a bit.
So does that mean for me with the eye cups fully in I'm closer than the optimum eye relief? However this would mean the binoculars have been designed such that at the optimum eye relief you don't get the full field of view.
These are binoculars with a wide field of view anyway. When the eye cups are fully in they view is still sharp, relaxed and without blackouts (with good eye placement), but still a bit of sharpness seems to be gained by bringing the eye cups out a bit.
So does that mean for me with the eye cups fully in I'm closer than the optimum eye relief? However this would mean the binoculars have been designed such that at the optimum eye relief you don't get the full field of view.
It wouldn't be at all surprising to need to extend the eyepiece a little when using glasses, but as the light path is afocal it's hard to see how altering the eye to eyepiece distance a little would alter the effective resolution in anyway. Could your perception of sharpness increase sharpness could be due to something else, like an increase of contrast? A certain amount of vignetting might reduce stray light?
David
elkcub
Silicon Valley, California
This all gets very complicated because the eyeglass prescription itself makes a difference in the eye's effective pupil size and location. It would be very hard to make sense of everyone's idiosyncrasies, although there is a difference between near- vs. far-sighted people.
Ed
Ed
Thanks Binastro, David and Ed for your views. BTW I'm short sighted. It's interesting that it doesn't seem to be something that happen to any of you (with or without glasses). David you're definitely right that it's a perception of sharpness thing rather than resolution (I've fallen into that trap before and have been corrected on here). I'm happy to put it down to my glasses and own idiosyncrasies.
WJC
Well-known member
You have been counseled (or, being British, counselled) by some very informed people. But this is the short route:
1. Evaluate eye relief in the store of a well-stocked and established dealer.
2. If the image, ergonomics, and eye relief suit your taste, buy it and use it.
3. The End.
One year later:
4. If, during the year, you have come to notice shortcomings not related to the opinions of the ultracrepidarians, use the information gleaned to buy what YOU consider to be a more suitable instrument. :cat:
Just a thought.
Bill
