Parallax/Erector tube size Relationship

[TheOtherAndrew]

@koshkin


From the cursory searches Ive done to date it seems reticles are etched on generally speaking 3 sizes (lenses at least) - 18/19mm (30mmtube) or 21/22mm (34mm tube?) and 25/26mm (35-40mm tubes).

My question relates to how parallax is inherently introduced/removed from a scope system by its overall design (as in no dialing of parallax knob, just inherent quality of entirety of scope design.

For instance TT and ZCO are known to be great at minimizing parallax without having to move the parallax dial from 3-900yd (100yd shot need a change in dial still just due to how light bending works, but generally I consider those 2 as most "forgiving parallax.")

Im wondering what is the major player, optically, that makes this [forgiving parallax/minimized need to dial] possible? I believe it has something to do with the size of the erector tube/reticle lenses.

In 34mm scopes Vortex/NF where parallax needs more dialing (you can still remove all parallax, you just need that extra bit of dialing for each distance). To include more elevation travel companies can use smaller reticle lenses and get edge usefulness through design of rest of scope (distance between erector-reticle and objective/ocular, number/type of lenses, etc). But to make a clear, parallax free image in this design requires more detailed setting of diopter AND dialing of parallax from my experiences.

Tangent - Id hypothesize has a larger reticle and erector tube. This allows greater image quality (less distance from optical center) but because of 34mm main tube in TT means less overall travel. Less travel from center and a larger/(fatter erector lenses allows for good depth/contrast), less distortion -> less inherent parallax introduction and overall more 'forgiving parallax' meaning no need to dial it 3-900yd.

ZCO - Using the same 25/26mm reticle and perhaps shorter erector tube/higher index lenses (overall length) affords similar parallax forgiveness as TT but because of the 36mm main tube ZCO allows for more elevation travel relative to TT. ZCO isn't AS forgiving as TT (close though) which could be due to more minor details (better ocular/diopter, lens index, further distances between erector and ocular/objective to allow cleaner light paths), but in general I think has to do with the fact that more travel means further off center means less forgiving parallax even though larger reticle lenses give larger/flatter central spots, introducing less distortion effects in general.

I have terrible prescriptions so have dealt a lot with the high index lenses - they are thinner but as a result cram more (image density) into a unit area which causes headaches for me more so than thicker/low index lenses - trade off is the thickness and weight of the lower index...

Its all a balancing act; I want to know more about how that act takes place though.


If I had to guess generally Id say:

1)TT - has 25/26mm reticle (the length of erector tube probably also plays an optical role (not sure what that is but 'everything matters' - maybe TT has longer erector tube than ZCO with same reticle size - lower index lenses (thicker, more depth/contrast and easier for end user to SEE the quality))
2)ZCO - similar 25/26, shorter erector tube, but 36mm main tube so increased travel vs TT, with tad higher index lenses inherent to going 36mm
3)NF - they reduced to 19 to optimize elevation travel made up for with longer erector tube (scope length) and low index lenses. But could easily be 21/22 with high index lenses haven't used NF much - Im basing answer primarily on NF advertised travel.
4)Vortex - 21/22 with shorter tube length but more/thicker lenses(APO?) of lower optical quality but APO/better optical design?; hence more weight (what is main reason for Razors huge weight, all else the same - Im guessing lower index lenses and APO/more lenses)
5)Leupold 25/26 reticle w/ short erector, 35mm main for some travel (but overall less than a Vortex because vortex smaller reticle size) but somehow made it very light (shorter distances between, taller/thinner/flatter (highest index) lenses reducing contrast quality (increased 'image density' - harder for eye to resolve well)?

(All guesses)


*Anybody have real info on the internals of scopes? I figured DLO would if anybody..... and in a perfect (tele)scope design with no other parameters to follow than make image perfect across entire range of motion (magnification and travel) - what alteration/deviation from [that design] introduces the most inherent need for user adjusted parallax?

(may need to add "as observed through a computers eye" in order to remove the obvious variability in each persons eye- though I think that is the purpose of the ocular no? so either computer observer or perfect ocular setting)

 

[ArTeeKay]

Man, that's a lot of words for a question... Especially when you're both simultaneously throwing out WAGs, and asking for proprietary optical engineering data that is likely the subject of a non disclosure agreement.

Instead of asking the dark lord to spoon feed you, why not find his blog and do some reading first? And then maybe watch some of his videos. And then, perhaps subscribe to his paid content. Ilya is super knowledgeable, why not throw him a couple bucks for his work?

 

[TheOtherAndrew]

Please go troll someone else. I know what I asked, why I asked it and worded it how I felt best suited that.

I have a question, if anybody has a sliver of an answer I encourage them to make their own 'WAG' better known as a hypothesis - reasoning on why your guess may have merit.

DLO may even have facts. That'd be awesome.

 

[ArTeeKay]

I'm telling you where to find the information you're looking for. It's there, if you look.

Something about horses and water...

 

[TheOtherAndrew]

I only asked [DLO] specifically because I admire his past works Ive ran across and think he may have good info; others on here may be even more knowledgable on this topic - thats why its a thread not a DM.

If you actually have something worthwhile to add/answer or you have certain access that makes the answer obvious to you - I dont see why you dont just tell me/us?

 

[TheOtherAndrew]

 

[ArTeeKay]

You know, you are showing that you have all the tools and info at your disposal. My depth of knowledge of the material isn't great enough that I can present a neat and tidy analogy for what's going on that allows the uninitiated to have a good handle on the optical phenomena in question. And since you're displaying what appears to be a gross conceptual misunderstanding of what is happening between the target and your eyeball, I'm gonna quit making the effort to point you in a righter direction.

Someone besides Ilya may come along with a satisfactory answer to your question. But based on what I know, I doubt it. Best of luck.

 

[TheOtherAndrew]

So they all can play A role in why the image collects in the eye incorrectly or with parallax.

Ocular adjustment may be the biggest reason the numbers on parallax read differently for different people and are rarely ever correlated to actual distance to target.

Erector tube/first plane reticle will obviously create distortions of some kind (Im not learned enough in the subject matter to know what)

Magnification - distance between 'picture reversal assembly' and field lens being adjusted can be done so the distortions are minimized but you sacrifice on overall scope parameters (weight, length, travel, mag range, et al) these parameters and corresponding implications could be mitigated by using different 'non-perfect' lenses which help you reach parameters but introduce more distortion issues at the fringes (i.e different indexed lenses like the prescription eyeglass thing I referenced above).

I thing the second plane aperture may play a big role in how the parallax is inherently introduced into the overall system disregarding the other variables already discussed. But again Im not skilled enough in the trade to actually know - that is a guess.

I again assume the parallax knob itself is correlated to the movement of the first plane lens (reticle in most of my cases)? But a picture included lends some useful info in that its not only the reticle that needs be in alignment its also the eye and/or ocular set up that can mess with this (and experience/manuals indirectly tell us that) - which is why I said the computer vision thing or perfect eye with perfect ocular.

But honestly I do not know the real answer. Perhaps I am slightly correct in some areas, wildly off base in other areas. Thats the most likely case IMO, but I still made the assertions I did in order to spur those that do know the difference and have enough knowledge of the trade to be confident in pointing out exactly where Im assuming incorrectly.

What makes Tangent so good optically with parallax?

What makes ZCO almost as good, but not quite with parallax?

The rest is me really just going with the flow of thought I was in about the other companies/heavy/light/ better worse in xyz.

 

[beetroot]

The base parallax is set by the grade of inerting gas that is put in the scope.

As you probably know Argon gas is used in most scopes these days, whereas histrically Nitrogen was used. Both gases perform the task of keepingo out moist air which can/will cause internal foging of lenses but Argon has the other distinct advantage.
The refractive index of gases can vary alot, much like the different shaped lense elements will bend light in different ways and cause/rectify particular light issues, so can the gas that is used.

By varrying the isotope of Argon that is used manufactures adjust the base paralla setting.
If you send a non adjustable scope to Leupold and ask to have the parallax changed from say 100 yards to 200yards (or ay 50 yards) they will use a different isotope of Argon.

You may also notice that some scopes are very picky on parallax settings where others are not.
Expensive scopes like Tangent Theta and Scmidts use the highest quality glass Schott glass in the world which requires very little tweaking to iron out lighting artifacts.
Whereas cheaper scopes use lesser quality lenses to save money, so instead they try tweaking the grade/Isotope of inerting gas that is used to imporove the image quality at the expense of parallax sensitivity.
Most manufacturers are are pretty secretive about the gas mixes they use but typically it's some other Nobel gas (such as Xenon, Krypton, neon etc).

You may also have heard of flourine coated lenses or flourite lenses, this was also an older technology that was used along with flourine gas to help improve light transmission and decrease parallax sensitivity.
As flourine gas is poisionus, even though the chances of it escaping the scope are rare it stopped being use.

 

[TheOtherAndrew]

Much appreciated, that is phenomenal info.

I always only thought the inert gases were used simply to keep internal fogging and pressure effects minimized (like when you use nitrogen to fill a tire instead of air) NBD. But I have noticed a lot of lower end scopes using Ar and higher end scopes using N2 even though argon is more 'stable.' But the way you describe that makes perfect sense.

A follow up question Id ask is why a high end company like S&B and ZCO say they use N2 not Ar? Maybe they are manipulating the meaning of 'fill gas' when they label it as N2 and its really a more complex mixture like you were saying (Xe, Ne, Kr, et al).

 

[koshkin]

Much appreciated, that is phenomenal info.

I always only thought the inert gases were used simply to keep internal fogging and pressure effects minimized (like when you use nitrogen to fill a tire instead of air) NBD. But I have noticed a lot of lower end scopes using Ar and higher end scopes using N2 even though argon is more 'stable.' But the way you describe that makes perfect sense.

A follow up question Id ask is why a high end company like S&B and ZCO say they use N2 not Ar? Maybe they are manipulating the meaning of 'fill gas' when they label it as N2 and its really a more complex mixture like you were saying (Xe, Ne, Kr, et al).

I am pretty sure he is messing with you. The whole business of depth of field varying with different fill gasses is hillarious nonsense. The refractive index difference between them is in the fifth digit.

ILya

 

[koshkin]

On the rest of it. You are thinking about parallax adjustment, while you should be thinking about it image focus and depth of field. The side focus turret is not a parallax turret. It is an objective focus turret. It happens to also help with parallax correction, but that is a side effect of its main function.

I covered it somewhat in a podcast episode with Modern Day Sniper guys and also talked about it a little with Silvercore.

I have a livecast on Youtube and Facebook tonight. The primary conversation topic will be precision rimfire and optics for it (Greg from Vudoo Gunworks will join me), but I'll open it up to Q&A. If you remind me I can cover this topic to some degree.

ILya

 

[TheOtherAndrew]

Ya, I just wanted to have a quick answer too badly. Will tune in tonight at 8pm CST? (Thats what my facebook is saying)

 

[koshkin]

Ya, I just wanted to have a quick answer too badly. Will tune in tonight at 8pm CST? (Thats what my facebook is saying)

Correct

 

[Denys]

When I first read this thread, I thought it was a parody. When I read betroot's post, I was convinced it was a parody. I hadn't laughed that much in a while; that was truly inspired.

Then it dawned on me that the thread really wasn't meant to be a parody.

I'm glad koshkin rectified the issue with parallax adjustment versus image focus and depth of field. I will address the choice of fill gas.

Contrary to what was stated by TheOtherAndrew, the vast majority of riflescopes are filled with nitrogen, the most common gas in Earth's atmosphere, at 78%. Oxygen is next at 21%. Argon is the third gas at 0.9%. The remaining 0.1% is made of humidity, clouds, SUV emissions, and cow farts.

Nitrogen is used to fill most riflescopes because it's cheap, plentiful, and mostly inert.

Deon elected to use argon to fill their scopes mainly because the atom is much larger and thus somewhat less prone to leaking out over time. Conversely, another noble gas is helium and it's not use for this purpose because the atom is so small it leaks out of most everything. Argon is several times more expensive than nitrogen but it's still not a big expense. Argon is used as a fill gas when looking for the most inert atmosphere. It also has a low thermal conductivity, which is why it's used in double and triple pane windows.

If you're looking at a fill gas in a riflescope for the long term, argon is the way to go. It's more expensive than nitrogen, but I think it's worth it.

I know that all March scopes have argon in them, I have heard there are some other high-end riflescopes filled with argon, but I can recall which ones.

 

[TheOtherAndrew]

These images don’t accurately depict the internals of a scope with side focus. Their is more going on.
And @koshkin is correct.

Can you elaborate a bit?

When I first read this thread, I thought it was a parody. When I read betroot's post, I was convinced it was a parody. I hadn't laughed that much in a while; that was truly inspired.

Then it dawned on me that the thread really wasn't meant to be a parody.

I'm glad koshkin rectified the issue with parallax adjustment versus image focus and depth of field. I will address the choice of fill gas.

Contrary to what was stated by TheOtherAndrew, the vast majority of riflescopes are filled with nitrogen, the most common gas in Earth's atmosphere, at 78%. Oxygen is next at 21%. Argon is the third gas at 0.9%. The remaining 0.1% is made of humidity, clouds, SUV emissions, and cow farts.

Nitrogen is used to fill most riflescopes because it's cheap, plentiful, and mostly inert.

Deon elected to use argon to fill their scopes mainly because the atom is much larger and thus somewhat less prone to leaking out over time. Conversely, another noble gas is helium and it's not use for this purpose because the atom is so small it leaks out of most everything. Argon is several times more expensive than nitrogen but it's still not a big expense. Argon is used as a fill gas when looking for the most inert atmosphere. It also has a low thermal conductivity, which is why it's used in double and triple pane windows.

If you're looking at a fill gas in a riflescope for the long term, argon is the way to go. It's more expensive than nitrogen, but I think it's worth it.

I know that all March scopes have argon in them, I have heard there are some other high-end riflescopes filled with argon, but I can recall which ones.

I said N2 is what ZCO and S&B advertise...I know Riton for instance uses argon; seems mostly like a marketing thing to allow companies that use lesser glass to justify higher pricing/better margins for their products because higher end companies still advertise using N2 and they're just fine because of higher optical quality- Koshkins point IMO was it (fill gas) really doesn't matter (5th decimal); only matters that it is filled/sealed with an inerting gas of some kind and is only really meant to keep internals from fogging/exreme pressure changes - allow the internals (inherent glass and build quality) to 'shine.' Doesn't mean March et al. cant incorporate both though...
------------------------
I studied to be a chemist, no training in optics past physics 101/2. So the 'isotope' bit hooked me good (along with my desire to get a simple answer quickly). I was wrong - but I am here to learn.

So please elaborate on how the pictures are a false depiction in general - they obviously aren't complete depictions they are simple/general depictions. I can add more elaborate ones but idk how that helps the conversation.

For instance I know the reticle is what is moved by turning the turrets, but the designs above (perhaps incorrect) make it seem like the erector tube is anchored on the ocular side(second focal aperture), either the tube becomes angled relative to main tube or there is something missing - like reticle part moves independently of rest of erector tube...or it isn't anchored and erector includes mag lenses and first/second focal glass, but pushing on one end (first focal side) seems like you'd be asking for problems - opinion.

I want to learn, but cant spend 3 weeks deep diving into optical telescope design patents and reverse engineer what ppl do and why - if you have answers give em up, it aint top secret!

 

[TheOtherAndrew]

Another thing Id guess, is that the parallax/side focus knob moves the first focal plane glass? hence Koshkins "it really is meant to focus the objective"

 

[Denys]

Another thing Id guess, is that the parallax/side focus knob moves the first focal plane glass? hence Koshkins "it really is meant to focus the objective"

Wow. No. The side focus knobs moves a focusing lens inside the objective bell forward and backward, to focus the image on the first focal plane.

Also, I checked the Riton site, and they only use argon for their more expensive scopes. The ones less than about $1200 use nitrogen.

 

[TheOtherAndrew]

Okay, so the second focal lens to first focal lens = erector tube?
1)inside these are multiple lenses depending on the magnification ratio desired (3:1, 5:1 etc.); smaller ratios = less lenses?

Is entire erector assembly fixed as its own piece?
2)Do the turrets move entire erector tube assembly or just the reticle? Considering second focal reticle scopes have same adjustability it was always my assumption the entire erector tube moved from turrets, maybe the 'pressure' is applied in middle of erector tube not directly under turret?

Is the erector tube's length a function of the magnification range desired? more magnification range = more movement needed?
3)8-40 (5x)[32] vs 3.6-18 (5x) [14.4] vs 5-25 (5x) [20] - all same length or different length between first and second plane lenses?

 

[TheOtherAndrew]

As I now understand it from others' input...

Blues = fixed does not move independently
shades denote 3 different sections of scope

Grey = diopter lens movement corresponds to diopter ring movement

Green = magnification lens assembly has lateral movement corresponding to mag ring movement

Red = elevation/windge (erector tube) contained by first and second focal lenses that only move indirectly corresponding to turret movements

Orange = a focusing lens that moves between objective lens and erector tube assembly (which happens to also correspond to parallax) corresponding to the parallax knob movements

 

[TheOtherAndrew]

 

[TheOtherAndrew]

I saw the phrase "depth of focus" a couple times. Trying to decipher what that means exactly; guess-> inherent ability of system (picture at observer) to retain focus throughout the [first plane reticle's] movements from spinning turrets? Jimmy Jr seemed to imply a smaller objective helps boost depth of focus which makes me think that as the ratio between objective lens and first focal lens gets smaller, there is less inherent stretching/compression of original image so less inherent distortion at observers eye??

*Is this on the right track or 'depth of focus' means something else entirely?

 

[TheOtherAndrew]

 

[koshkin]

This is depth of focus. For example, maybe you see clearly from 75 to 125 yards. Arbitrary numbers there.
Here is an image, compliments of Canon.
View attachment 7873670

Guess I should be using the term depth of field. You do photography much? There is many similarities when it comes to image.
Also you make assumptions (I assume ) about reticle size, which I am guessing doesn’t really matter. It is multiple lenses working together to gather an image, bending light from various distances that you want to shoot at, so it focuses on the same plane as the reticle, then magnified in the erector, and finally straightened out in the ocular lenses so it is again in focus for your eye. Lots going on to effect the depth of focus. Some companies focus (pun) on different things and do a better or worse job.
Also, lense thickness doesn’t much matter, as it is the shape of the surface that determines the index. Another guess of mine is that might be how leupold makes their scopes so light, by using thinner lenses.

Depth of field is correct, although the two are related. Depth of field is in the object space and depth of focus is in the image space.

ILya

 

[TheOtherAndrew]

Looking at a few Youtube vids mainly about cameras/depth of field- Im trying to relate that info to scopes. LargerDOF would correspond (from my understanding) to having a 'forgiving parallax' and apparently a lot of this relates back to the second focal aperture (and the lens' inherent design -focal length/chemical compound).

1) Something commonly brought up is F-stop and "aperture" (iris)- just so happens Ive seen this word before as the second focal plane of an erector tube....Are these functionally the same thing?

2) Is focal length in cameras equivalent to in scopes the distance between eye (sensor) (or 'prediopter' since diopter is mainly to fix individual eye differences) and the lens being controlled by 'parallax' adjustment knob, or the first focal plane of erector tube?
*I heard it defined as "Imaging plane and optical centerpoint when lenses are focused at infinity with collimated light"

So a revisited explanation would be along the lines of:
To get a more forgiving parallax you need a smaller (second focal) aperture, this will collect less light and mean you'll need [better] light transmitting lenses (objective et al.) aka better glass = more expensive scope. How long the scope is in general relates to the focal length of lens used which correlates [amount of light let through] with [SFaperture] and has many combinations to get desired effects
(low cost, optically 'bright' w/ shallow DOF vs high cost, optically clear, and deep DOF). Tweaks to outer dimensional parameters can be made using indexed-fast/slow lenses.

My Takeaways:
I think aperture/f-stop size and price point (glass used) is the big difference in modern scopes. Id assume also plays a role in a scopes 'eye box' where lower end/larger aperture scopes may be easier to 'get behind' but not a useful picture for making a precision shot (reticle/image misaligned off center vs higher end scope with smaller aperture may be harder to get behind but once you see a picture the reticle and image are useful for taking a precision shot....a side effect really.


**I think the top alpha+ tier scopes use a 'telecompressor' on ocular side of aperture to essentially get best of both worlds.**


Barium Oxide glass = Schott glass (by Zeiss)

(Credit to Dylan Bennet and John Hess - YT)

 

[TheOtherAndrew]

A good nugget of info from DLO regarding the [angle of the light beam going through the parallax/objective focus lens and the FFP lens] - a larger angle corresponds to a more 'finicky' parallax adjustment for a given scope (the shallower the angle (smaller the objective/further distance between the objective and parallax/obj focus lenses = shallower angle = larger DOF or more forgiving parallax.

Id guess that this may be why (most) LPVO optics dont really require a parallax adjustment (its set at 150/200yd). Smaller objective because of lower magnification range, light is therefore more 'collimated (shallow angle/flat)' and light from say 300yd vs 600yd hitting the LPVO has a relatively similar slope and therefore a very minimal difference in where the objective focus/parallax lens needs to be in order for acceptable image quality.

Additionally this could be what is meant by the infinity symbol on your parallax - it is the position where the light beams from extended ranges begin overlapping each other with the same slope (because of how far away they are 1300/1700yd) so the change required in obj focus/parallax approaches zero.

*I am not yet clear on if 'afocal telescopes' have any kind of 'f-stop' feature in the SFP (like cameras)- or anywhere for that matter. my guess now is they probably dont, even though I previously assumed they did.... so Im not at all confident in either answer (if anybody with knowledge of that aspect wanted to chime in and answer that specifically).

 

[Convex]

"1) Something commonly brought up is F-stop and "aperture" (iris)- just so happens Ive seen this word before as the second focal plane of an erector tube....Are these functionally the same thing?"..........

No they are not....

As with any concept in optics, this can be discussed in general terms, the problem w/optics in practice, is that it amounts to endless/almost infinite detail which is just as important as the general concept being asked about like your question.

To give U the answer U need, sometimes you have to give a TLDR response to the give the folks who're trying to figure all this out/unfamiliar w/all this an idea of what they need to know.

The term "F-stop" is a calculation/the relationship between the hole at the back of a lens, and the ratio of the diameter of that hole to the focal length of the optic. A 100mm lens set @ F4 means the diameter of that hole/aperture is 25mm.

Why does F4 mean 25mm on a 100mm lens, Bcuz the diameter of the aperture set @ F4 means it's 1/4th the diameter of the focal length.


An "F-stop is a calculation, and an "aperture"/iris diaphragm is an actual "thing".


You calculate in "F-stops" for exposure, and adjust your iris diaghragm to "F-4".

When you refer to a 100mm lens, you're refering to it's focal lenth (focused @ inf) not it's actual dimensions because those two things can be different.

A 100mm lens can be 50mm in actual length, and attached to a view camera w/a bellows and that combination focused @ inf equals 100mm, or the optic can have an additional amount of barrel w/a helicoid enabling it to focus @ inf.

Remember what I said about detail. There's the issue of F-stops and T-stops, and there's usually a difference between them.

You calculate your exposure, and your meter tells you need to open up your 100mm lens to F-4 (widen/close down the aperture to 25mm).

Problem is that some light is reflected off the front objective that should've gone through the lens, some of that light, bounces around inside the optic and is lost, and some more light is lost because of dispersion.

Long story short, this light loss means you're not getting the exposure you calculated for, so they calibrate the lens/opening up the aperture to 28mm (from 25mm) to compensate for the light loss and mark that on the aperture ring as "T-4".

"F-stops" are what's calculated for, a "T-stop" is the setting reflecting the actual transmission of what was calculated for, and you saw mostly motion picture lenses calibrated in "T-stops", because their exposure settings had to be exactly "on the money".

How big or how small the aperture is at the back of the optic influences not only exposure, but DOF.

When the hole/aperture gets too small, then you have "diffraction limiting", but that's a conversation for another day.

What I just said is simple to those who know it, and a lot to digest to folks unfamiliar with it to get you over the "you don't know what you don't know" hump.


This is my Toyo 810 MKII, an 8x10 inch technical field camera, or a fairly modern version of the view camera w/a negative of 80 square inches.

Large format photographers will call the lens I have on this camera a "12 inch lens", which means the actual lens in combination w/the bellows (focused @ inf) equals 12 inches.

A six year project, this is my creation, the "Silhouette 612" which does it the other way. The lens/helicoid is combined w/a lens cone which when focused @ inf. is the optics focal length.

 

[Convex]

*I am not yet clear on if 'afocal telescopes' have any kind of 'f-stop' feature in the SFP (like cameras)- or anywhere for that matter. my guess now is they probably dont, even though I previously assumed they did.... so Im not at all confident in either answer (if anybody with knowledge of that aspect wanted to chime in and answer that specifically).

What would come anywhere close to answering your question, bearing in mind that an afocal telescope and say a camera system do different things, is that you can "knock down" the light entering any optic, like w/the disc March provides you for cutting down the light entering one of their scopes by 50% which is 1 stop, but you can't apply what I said about the focal length aspect of a camera system to an afocal optic like a telescope/microscope.

Koshkin has explained the major difference between an afocal optic like a telescope and a camera system which ironically is the only way combining the telescope w/either your eyeball or a cell phone/camera behind it is going to work.

If telescopes don't work the way Koshkin explains, you wouldn't be able to place your eyeball or a camera/cell phone behind it and see an image.

That combination wouldn't work if the telescope doesn't work the way it does. I would suggest you peruse his video some more and I'm sure more and more info will soak in.



Here's a diagram....

This is how you combine a telescope w/a 2nd optical system, which is either your eyeball or a camera system, and as Koshkin explains, why it works.

In the diagram there's 2 optical systems,

"1" is the afocal optic/the telescope

"2" is the camera system/cellphone, or your eyeball.

Now look at the light traveling to the front of the telescope which enters the front objective. This light enters the front objective, and the telescope turns that light into a projected image it sends out of the eyepiece as a virtual image in light traveling in a paralell path (collimated).

Said another way, the telescope takes light traveling along a paralell path that's entered the front objective, and produces an image that's traveling along a paralell path coming out of the eyepice. The light travels the same way in and then out of the telescope only now the light exiting out of the scope includes an image.

The light travels THE SAME WAY IN AND THEN OUT OF THE TELESCOPE.

Important because then the 2nd optical system BEHIND the telescope can now take that light entering it's front objective (just as it entered the telescope) and turn it into a FOCUSED image.

Look at the diagram, the light path entering "1" and the light path between "1" and "2" are the same path; again this enables the 2nd optical system behind the telescope to produce a focused image.

It's set up this way because if it ins't set up this way, it won't work.


The trouble w/this diagram is the same thing as w/many diagrams; the folks who drew this should spell out that the "dotted line" going through the center of all this is the "centerline optical axis".

The reason I bring this up is that each of the 2 optical systems has its own centerline optical axis, so the most accurate/nuanced description of this dotted line would be that it indicates that the centerline optical axis of each optical system have merged together as one line/along a common axis.

 

[Convex]

If U want to throw out all the minute details, then think of system "1"/a telescope-microscope as an optic "waiting" for a system "2"/camera-cell phone-your eyeball to be lined up BEHIND IT so the 2nd optical system can bend the light produced by the front optic into a focused image.


An afocal optic can't/doesn't produce a focused image.

Cameras/cell phones/your eyes can produce a focused image, by themselves, or lined up behind an afocal optic.