Nearsightedness with refrlectors and refractors

[AlcorAlly]

Not sure this is the right section of the forum to post this question but this is something I would really like to understand. 

I'm nearsighted and wear glasses or contact lenses. When I look through my reflector (Heritage 150) without glasses, I need to refocus. The focuser needs to go inwards a little more than with glasses. 

I also have a very basic refractor (Celestron Travel Scope 70) which doesn't require any change of focus when I take my glasses off. The view through the refractor is always the same - with or without my glasses.

Now the Celestron is only used for bird watching so perhaps this is something to do with the proximity of the object? Or perhaps refractors focus things differently to reflectors.

Either way I would like to know why this happens. 

Edited October 6, 2022 by craftui

[vlaiv]

It has to do with the fact that you are using refractor for close objects and not because there is some difference between refractor and reflector in focusing - there is none.

There is something called depth of focus and there is also ability of our eye to accommodate different focus positions.

Fact that you are near sighted means that you can't "relax" eye enough for infinity focus and you need to compensate with glasses (or refocus when you remove glasses).

When focusing on close object - people with normal vision when relaxed will focus as if object is at infinity - meaning exit rays will be parallel.

Given that you are near sighted - for you relaxed observing is when exit beam is diverging. When looking at close object - you will set focus position differently then someone without nearsightedness, and when you take glasses of - you eye will be able to accommodate change in focus without you realizing (like when someone with good vision switch from near to far object - their brain refocuses without them really doing anything or noticing that).

Bottom line - you will always have different focus position then person that uses no glasses (has no nearsightedness) but your eye can't compensate at infinity focus and can compensate for near focus, while people without nearsightedness have range of compensation at both points - at infinity and at close focus (younger people more so than older because eye lens hardens with age and it is harder to refocus with muscles).

[AlcorAlly]

16 hours ago, vlaiv said:

It has to do with the fact that you are using refractor for close objects and not because there is some difference between refractor and reflector in focusing - there is none.

Thank you! I thought the reason was the proximity of the object. 

I'm still not sure I quite understand the mechanics of what happens exactly. 

How far would an object need to be be to require refocusing? For example, if I observe the moon without glasses and someone who is not nearsighted joins me, can we both observe it without having to change focus on the telescope? Or is it too far? I imagine it depend on the level of my nearsightedness? So what is the relationship between:

• the level of nearsightedness
• focus of the telescope
• focal length of the eyepiece
• proximity of the object

Perhaps this is too complicated to explain. If someone could direct me to a clearly written article or a book that explains this topic I would really appreciate it 😊

Edited October 7, 2022 by craftui

[vlaiv]

6 minutes ago, craftui said:

How far would an object need to be be to require refocusing? For example, if I observe the moon without glasses and someone who is not nearsighted joins me, can we both observe it without having to change focus on the telescope? Or is it too far? I imagine it depend on the level of my nearsightedness? So what is the relationship between:

To be honest, I have no idea.

Much of it is related to medical terms like diopter / dioptre

https://en.wikipedia.org/wiki/Dioptre

Ideally, eye should be able to focus on range of distances by itself - form infinite focus down to say arms length (although my arms appear to be shorter as I get older ).

I can draw you a diagram of what geometry of rays look like in this case:

Left is object at infinity and right is object closer. When object is at infinity - this simply means that lines are parallel and that they eventually come to same point - but at infinity.

You can't focus when rays are like in left part of this diagram without corrective optics (glasses / contact lens). You however have no problem focusing on something that is closer even without glasses (there should be a distance where you can read without any issues even if you don't wear glasses at that moment).

This means that your eyes also have range of focus - but that range is shirted compared to person that does not need glasses. They can focus from infinity to some distance, and you can focus from some distance closer than infinity to some distance closer than that other person.

Ok, back to scope.

Scope focused at infinity for person that does not need glasses has following ray diagram:

Both exit and entrance beam have parallel rays - as if both images are at infinity (real and projected image).

When you are wearing glasses - you can also observe in above configuration, but when you remove glasses, you must have following configuration in order to be able to observe:

This happens when eyepiece is moved forward to focus point - exit rays are no longer parallel and it looks like object is closer. Most people have to strain their eye to be able to look like that - but this comes naturally to you as you are nearsighted.

In relaxed state our eye is tuned for far and we need to work with our muscles to focus to near (this is why there is old age farsightedness - lens gets less elastic and muscles weaker and can't bend it into shape to focus real close).

When observing local objects - above ray diagrams are just a tad different

People that don't need glasses usually focus like this:

Or maybe with slightly diverging outgoing rays. That depends on how their eyes are tuned and how they observe - if you look at target without using spotter scope and then switch to spotter scope - I think that eye never really relaxes like in night time when you don't have reference - it is always tuned to some distance.

Similarly - when you observe - telescope is focused like this:

Now when you remove glasses - rays are not parallel, and but are already a bit bent. Now you can refocus with your eyesight as it does not require you to do something that is physically impossible for your eyes - to "over relax" in order to achieve infinity focus.

This is "conceptually what is happening - but actual angles and when you can refocus with your eyes and can't - that bit I don't know. It depends on distance to object, focal length of telescope and eyepiece - your diopter (and range of distances you can focus to) and so on, but should not be hard to find the answer as long as you apply basic geometric optics formulate (see thin lens formula) and figure out what diopter really means in term of lens power.

[AlcorAlly]

@vlaiv wow thank you so much. I will take the time to study the diagrams and to read a little more. I might have more questions later! Thanks again for the clear and detailed explanation. 

[allworlds]

When you don’t wear your glasses, are you wearing nothing, or are you wearing your contacts?

It could be down to differences in your vision in bright and dim light. IIRC some glasses-wearing astronomers get eye tests and glasses done for their night vision and it might be a different prescription to daytime.

Could be the Travelscope’s chromatic aberration making its focus less precise?

It could be that you are focusing differently. The telescope focuser enables you to make an object look like it’s closer than, at, or beyond infinity in terms of how your eyes (with or without glasses) focus it. Maybe by day you’re focusing the objects so they look like they’re near (in focus terms, not the magnification). See if you get different results depending on whether you start with the focuser retracted and extend it until you reach focus, versus starting extended and retracting it.

[AlcorAlly]

Just now, allworlds said:

When you don’t wear your glasses, are you wearing nothing, or are you wearing your contacts?

Nothing

[AlcorAlly]

4 minutes ago, allworlds said:

Could be the Travelscope’s chromatic aberration making its focus less precise?

The birds look crystal clear, no chromatic aberration - with or without glasses, exact same view with no need to refocus when taking glasses on and off. 

[vlaiv]

16 minutes ago, allworlds said:

It could be down to differences in your vision in bright and dim light. IIRC some glasses-wearing astronomers get eye tests and glasses done for their night vision and it might be a different prescription to daytime.

This is actually another thing to consider.

Quite true - in daytime pupils contract and there is less curvature of the lens that can be "wrong" so prescription indeed changes - which can shift focus range of eye alone (but it still does not let you focus to infinity - it won't correct your eyesight completely).

[Gfamily]

It is possible that the need to re-focus is exacerbated for astronomical observations because the iris is wider at when you are making nighttime observations compared to your iris size when birding during the day. 

Have you tried using the Travelscope at night, to see if the 'no need to refocus'  applies there as well? 

It is also possible that for some reason when you use the Travelscope for birding, you are automatically putting the final image at a virtual position that is within the accommodation distance for your uncorrected vision. 

[AlcorAlly]

1 minute ago, Gfamily said:

Have you tried using the Travelscope at night, to see if the 'no need to refocus'  applies there as well? 

Will try! 

[DaveL59]

24 minutes ago, Gfamily said:

It is possible that the need to re-focus is exacerbated for astronomical observations because the iris is wider at when you are making nighttime observations compared to your iris size when birding during the day. 

Have you tried using the Travelscope at night, to see if the 'no need to refocus'  applies there as well? 

It is also possible that for some reason when you use the Travelscope for birding, you are automatically putting the final image at a virtual position that is within the accommodation distance for your uncorrected vision. 

that was my thoughts also, in brighter light your pupil will close and as with a camera that'll increase the depth of field of you eye. Hence you may find more accommodation when daytime viewing. If you used the scope on the moon which is quite bright then again your pupils will close a little and you may find no need to adjust focus but on the planets/stars you may need to adjust.

[Louis D]

In the daytime, you will definitely be benefiting from the pinhole effect.  The smaller the pupil opening, the greater the depth of focus.

I also wonder if the spotting scope is slightly slower (higher f-ratio) and as such has a slightly greater depth of focus.  I know if you stop down a telephoto lens, it will have greater depth of focus.

I would try both scopes under identical viewing conditions.  Same lighting, subject, etc.  I would also try an off-axis aperture mask on the H150 to see if depth of focus improves any.