Reply With QuoteI had a look through a 1.5-5x20WTC and could see didly squat but it was a realy warm day. All the other scopes seemed fine to me.
I'm a bit confused about the eye relief and exit pupil size of rifle scopes. The exit pupil size is simpley the diameter of the front end divided by the magnification. So a 50mm on 5 times magnification will have an exit pupil of 10mms. This is something of a problem as the maximum diameter of the pupil in the eye is about 8 mmms in the dark and maybe 2 or 3 or so in daylight so you can't see the whole field of view the outer parts will be black. On this basis the minimum usefull magnification is at least 20 for a 50 mm scope on a dull day. You can get round this problem by moving your eye back away from the scope and I assume that when you do this the eye is no longer focused on infinity. I have even had this happen on a 20 mms scope at 2.5x on a bright day. Are there any rules about the variation in eye relief needed to make full use of a given scope? Do some of them need less head movement? Any pointers on scope choice? They will all have this problem to a certain extent as we are talking physics.
I had a look through a 1.5-5x20WTC and could see didly squat but it was a realy warm day. All the other scopes seemed fine to me.
i always thought the bigger the object lens the more light it gathered and the size of what you can see is governed by the tube dia duz this help
Zeiss & the like say the optium scope for eye releaf is an 8x56 I do know the reason behing this but, it's late & the MEDS kiked in <img src="http://jsramsbottom.co.uk/bbs/e/icon_wink.gif" alt="Wink" width="15" height="15"><!--graemlin:-->
Hope it an't confused?
ATB
Darren
I've been as confused as you over the years about scopes. Trying to get an optics engineer to explain something without contradicting himself is difficult, probably because it's so hard to find a competent one. The worst are amateur astronomers, who have no idea of anything that focusses at less than infinity. Some camera buffs know a bit, but even they fall by the wayside when the conversation gets down to more specific questions.
You might get some answers to your questions from this book: Optics for the Hunter, by Richard Barsness. It leans towards the US deer hunter but there's lots of useful information, which dispells many myths, and it doesn't get too technical. From what you've written, you're on the right track and will understand it.
Regards
Andy
Thanks Andy.
Afraid I'm one of those amateur astronomer buffs from time to time but I've spent some time on general optics.
Having said it's physics a further thought occures. It may be possible for zoom scopes to be designed so that the exit pupil at a set eye relief stays the same as the magnification is decreased.
I've gone for low diameter scopes to try and avoid the effect but even have trouble geting my head far anough away from 1.5 to 6 by 20 WA. Are there any scopes out there that do minimise this effect? Going closer is ok but moving my head back wrecks my shooting. I like to get down on a gun.
I'm getting a bit confused now. I thought the basic 7 rule as in earlier post- Zeiss 8x56- was just the light gathering quality. The lens size divided by magnification gives the optimum pupil size of 7mm and just gives the best low light target view and nothing else a 6x42 is just as good. The scope relief distance was explained to me as simply the larger any object is the further back you have to be to get the full view of it and this transfers to the larger the lens the further back your eye has to be able get the full view through the scope with slight variations depending on magnification. Probably a bit too simplistic and would take some professor to give the proper explanation but it is generally right and once you get used to a particular scope it is done automatically anyway.
OOps, sorry this is my first post, just been lurking a bit getting the feel of things, damn good forum
I would expect the ziess to give the brighter image. The area of the front lense sets the ammount of light that enters the optics. The magification then dims the image as it gets higher. Trouble here though is the quality of the optic design will also have an effect as will the losses at each of the lense surfaces even when coated. Given that our sense of light is logrithmic things get quite complicated.
I can't stand the black ring you see through a scope that means that you need to move your head back. Leaving it where it is drastically reduces the field of view. I also can't see the point in specifying the exit pupil at 3 ins of eye relief when it is larger than your pupil is likely to be. 7mms at 3 inches is only likely to be of any use on a dark night. It would be better to specify the eye relief variation at a 2mm exit pupil over the magnification range of the scope. This whole area is made worse by the miserable scope rails on most air rifles. The only one I have seen with a full length rail was a huge HW underlever. I'm 6ft2 but I have spoken to a number of much lesser mortals that have excatly the same problem.
The advantage of a larger lenses is that they're easier to make, so they're more likely to give a better image. Then there's the quality of the glass, and the coatings etc, which all contribute to the quality of the image which lets you see in poor light.
It's not just about quality of image either - if the scope has very forgiving eye relief then you'll get easier target acquisition. And, come to mention it, if the exit is larger than your pupil, doesn't this mean that you get better target acquisition too?
Andy
Making lenses is an interesting subject. They can be made individually but they are usually made on a sort of mushroom shaped block with loads of them on it. In general the larger the lense the more accurately it has to be made and this plus the cost of the glass makes them more expensive. I'm surpised that optical plastics haven't appeared in rifle scopes yet. They can have a huge impact on both cost and weight.
You can move your eye around an exit pupil that is larger than your own pupil - try it. Don't think it will help with target aquisition though as with some of the figures quoted here you would loose sight of the cross hairs even on a dull day. On a bright day forget it. So the best answear is to move your head back so you can see the whole field of the scope. The amount will vary from scope to scope - so which is the best in this respect?
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by A John:
Making lenses is an interesting subject. They can be made individually but they are usually made on a sort of mushroom shaped block with loads of them on it. In general the larger the lense the more accurately it has to be made and this plus the cost of the glass makes them more expensive. I'm surpised that optical plastics haven't appeared in rifle scopes yet. They can have a huge impact on both cost and weight.[/quote]
I suspect we're trying to say the same thing here, in different ways. What I'm trying to say is that a defect on a small lens will show up more than the same defect on a large lens.
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>
You can move your eye around an exit pupil that is larger than your own pupil - try it. Don't think it will help with target aquisition though as with some of the figures quoted here you would loose sight of the cross hairs even on a dull day. On a bright day forget it. So the best answear is to move your head back so you can see the whole field of the scope. The amount will vary from scope to scope - so which is the best in this respect?[/quote]
I think it does help, although I'm willing to be convinced that it doesn't. I'm not sure what you mean by moving your head back - don't you find that there's an optimum position and you just have to get the scope in the right place? I think one of the general rules is that the longer the scope, the less critacal your eye relief is.
Andy
The old thing about "exit pupil" size being the objective lens diameter divided by the mag seems to be about right.
Theoretically, optically, the exit pupil of the scope should match your eye pupil diameter for an optimum alignment. If the exit pupil is larger than your eye pupil, then you don't have to get your eye aligned with the centre of the lens, and consequently you may get a parallax error, regardless of whether or not you have a px adjusting scope, as you may not be aligned with the optical centre of the optical system.
However, it's quite difficult to get even a 3mm exit pupil lined up reliably in terms of eye relief, as any FT shooter will tell you.
By the way, your eye pupil diameter has much less to do with the ambient light than it has to do with the quality of your scope's optical transmission.
The maximum dilation that the (human) pupil will acieve is 7mm (and only 6mm over the age of 30). Some of the scopes out there whack on a massive objective lense in the effort to create an impression of superior light gathering qualities.
Unfortuanately the exit pupil size is but one yardstick, the quality of the coatings and lens grinding are of massive importance as well (the Leupold MkIII 4x40mm with a theoretical exit pupil of 4mm is probably brighter than most scopes with an exit pupil of 7mm plus).
The tube diameter is also a factor (30mm as opposed to an inch). The Leupold mentioned above has a 30mm tube which clearly helps.
Why should a 30mm tube make any difference? Surely you can't get more out of one end than you put in the other, if you get my drift?
The only thing that could make this possible is that the amount of light reflected or absorbed by the glass of the lenses is greater, percentage-wise, at higher luninosity. This, as as far as I am aware, isn't the case.
Certainly Barsness, in his coffe table experiments, could detect no difference between 1" and 30mm scope tubes.
Andy
Thanks Jono, I wish I could have put it that well. When my eyes were young, I didn't seem to notice optical quality too much, but I went to a Whitetail classic about two years ago - an improvement. Then to a (American) Weaver last year - pretty GOOD. Three months ago, I went for broke and got a s/h S&B, and its WONDERFUL. I've gone from 4x40, to 4x36, to 6x42. I'm still with a 25mm tube, but the difference in optical quality has given my eyes 20 years!
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