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 Post subject: Optics Primer
PostPosted: Sat Jan 26, 2008 12:57 pm 
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Location: Coalmont BC
I started to write this and then got carried away.... so I guess it's now going to be an Optics Primer.... :lol:

Any Telescope has two primary parts, an OBJECTIVE (Lens or Mirror) that gathers the light and forms the image, and an EYEPIECE that looks at the image and displays that image to your eye.... With Astronomical Telescopes, that's it.... the image is displayed upside down.... but for Terrestrial use, there is also an ERECTING SYSTEM, either prisms or lenses placed between the two, to turn the image over so that up is up.... :shock:

The brightness of a Scope (or binoculars) has one PRIMARY factor, and a whole bunch of secondary ones.... The one single biggest factor is the diameter of the EXIT PUPIL.... You can find the theoretical size of that by dividing the Objective lens diameter by the magnification.... ie 32 mm / 4X gives an 8mm exit pupil.... You can view the actual Exit Pupil of your Scope by holding it and arms length and looking through it at a bright sky (NEVER the Sun).... The human eye can open up to 7mm in complete darkness.... it will be around 5mm inside a house or at dusk and dawn.... and will close down to as small as 2mm or less in bright sunlight.... For reference, consider the following numbers:

4 x 20 gives a 5mm exit pupil
3-9 x 40 gives 13 - 4.4 mm
4-16 x 50 gives 12.5 - 3.1 mm
6-24 x 56 gives 9.3 - 2.3 mm
8-32 x 56 gives 7 - 1.75 mm
10-50 x 60 gives 6 - 1.2 mm

You will notice that at the lower powers, virtually ANY scope should provide as much light as you can physically cram into your eyeball.... especially if you're not shooting at night.... At the higher powers, however, you can quickly run into the situation that even with PERFECT optics there is simply not enough light coming out of the scope.... and of course no optics are perfect.... :?

The second most important factor in the brightness and contrast of the image is the quality of the COATING on the lenses and the OPTICAL QUALITY of the glass.... This effects the EFFICIENCY or percentage of light transmitted through the Scope.... Usually this is what separates the men from the boys, or at least the expensive from the cheap.... with one exception I have personally noted over the years.... IMO, Bushnell optics are (or at least were a few years ago, I haven't purchased one lately) far better than other inexpensive ones for brightness, equalling and sometimes surpassing such names as Leupold and Redfield.... There is NO substitute for actually LOOKING through your "short list" of Scopes AT NIGHT (or at least peering into a dark corner in a store) to assess the brightness of different Scopes.... note that you MUST have the power set the same to do this comparison.... Since most stores won't carry an assortment of FT suitable Scopes, then I would try and do this at a FT meet.... look through as many Scopes as you can.... under dim light conditions.... :wink:

The other things that can effect the brightness and contrast of the image are the diameter of the internal lenses and erecting system (restricted by the tube diameter), the number of elements (VARIABLE POWER Scopes have more elements, and hence more losses) and the size and placement of baffles inside the Scope.... Virtually any optics have baffles in various places inside.... they are there to sharpen the edge of the image field, or to cut down on unwanted internal reflections.... There is always a baffle (called the STAGE) at the primary focus of the Objective.... The Stage (which in a riflescope is basically a washer moved by the adjustments) contains the RETICULE, and the diameter of the Stage sets the FIELD OF VIEW.... This is where the Objective lens focuses the incoming light, and it is why you need an Adjustable Objective for shorter ranges.... so that the image is focused exactly at the same location as the Reticule to avoid PARALLAX.... The Eyepiece focus adjustment needs to compensate for your individual eye and provide sharp focus at the reticule as well.... basically, the Eyepiece is looking at an image formed by the Objective.... In between is the Erecting System, and the Variable Power System, if any.... and if the lenses here are too small, they can restrict the amount of light transmitted, reducing the Exit Pupil or otherwise degrading the image brightness, particularly around the edges.... In addressing the 1" vs. 30mm Scope tube question, consider the following.... in a larger tube, there is room for bigger Erecting Lenses and Variable Power Lenses.... and the Stage can be moved further, giving more adjustment range.... both good things.... Increased Zoom Ratio (a 3-9 has a ratio of 3:1, a 10-50 is 5:1) generally also degrades the image brightness, but I am unsure if that is due to Exit Pupil diameter or number of elements required (perhaps both?).... As an aside, if you are talking Binoculars (or Monoculars) which use Prisms to Erect the image, and the Prisms are too small, you will notice a squared off darkening around the edges.... Hold them at arms length and look at a bright sky.... if you see a squared off Exit Pupil, they skimped on the Prism size.... :roll:

So there you have it.... If you are looking at shooting a Grouse at dawn or dusk in dense cover.... or shooting a Rat at night.... you need at least a 7mm Exit Pupil to cram every bit of light possible into your eye.... That is why the old standby Scope for deer hunting is a 4 x 32.... 8mm Exit Pupil.... The same thing applies to Binoculars.... the BEST for night time use are the old 7 X 50.... while for most dawn to dusk use a 7 X 35 or 8 X 40 is just fine, with their 5mm Exit Pupil.... The "pocket" sizes (like a 12 X 25 with a tiny 2mm Exit Pupil) are just that.... fine for catching a glimpse of things, but don't expect too much when the light starts to fade.... :)

For FT use the higher powers are used so that the focus will "jump" in and out and you can use the Adjustable Objective as a Range Finder.... Just remember, that in Optics, you don't get something for nothing.... More power means a dimmer image PERIOD.... Considering how shaky I'm getting as my beard gets whiter.... I'd use the high power to find the range and then crank back until the image is nice and bright with lots of contrast to shoot.... :wink:

Bob

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Last edited by rsterne on Sat Jan 26, 2008 3:02 pm, edited 3 times in total.

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 Post subject:
PostPosted: Sat Jan 26, 2008 1:18 pm 
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Great Post! I certainly learned a thing or two.

Tim

Funny how in your last paragraph you stated how you would range at high power and shoot at low power. I know a few FT shooters that range in the 30-35X area and then turn it up and shoot at 50X. Looking at a .25" kill zone at 10-15 yards with 50X makes it seem like you are shooting at a good sized hole. Looking at it with low mag makes it seem as small as it is.

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 Post subject:
PostPosted: Sat Jan 26, 2008 1:59 pm 
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Good information, but the 30mm bightness issue has been glossed over a bit. It is not all about the amount of adjustment you get from the larger erector tubes. Also, optical glass and coatings play a tremendous part in light transmission. I have two top end 30mm scopes that do not fit the simple math presented as the glass is above average. One is just above average, the other is way above average. Ratio's of zoom also play a great part in the light loss in one scope to the next. A 4-1 ratio drops less light than a 5-1, and it is very seldom that a 3-1 zoom scope is very dark at all.

A fixed power scope, like the Leupold Competition Series, trashes the ratios and presents one of the brightest images around for the power of the scope. This becomes very clear when you put a top rated zoom scope up beside the Competition series and bob your head from scope to scope. The difference is remarkable. It is also a good benchmark to test your scope against. If your scope is close to the Leupold, you are doing very well indeed.

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 Post subject:
PostPosted: Sat Jan 26, 2008 2:50 pm 
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Keyrigger....

Thanks for the comments.... I edited the original post to include the losses due to the Variable Power System, Zoom Ratio, and Optical Glass quality.... Simply forgot to include them first time around.... :oops:

Bob

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Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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 Post subject:
PostPosted: Sat Jan 26, 2008 3:51 pm 
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Location: Ontario
rsterne wrote:
Keyrigger....

Thanks for the comments.... I edited the original post to include the losses due to the Variable Power System, Zoom Ratio, and Optical Glass quality.... Simply forgot to include them first time around.... :oops:

Bob


Very good writeup Bob. Thanks for taking the time to share that information.

Most are not as analytical as you are. Tracking how the end result happed is quite interesting.

Many seek out what the most common scopes for their shooting application, confident that the seasoned shooters have done their homework.

For FT, researching the National and World results are a good way to see what the shooters are using. Using the 2007 US Nationals as an example, the four most popular scopes for the PCP open class are:
1) Nikko Sterling 10-50x60/BSA 10-50x60
2) Leupold Competition 35x (some 40x)
3) Nightforce 36x
4) Bushnell Elite 4200 8-32x40

The 10-50x60's hold the strong majority of the four.

I own the Nikko, BSA, Leupold and Bushnell.

They rate:

Optical Clarity Bright Light:
1) Leupold Competition
2) Bushnell Elite 4200 8-32
3) Nikko/BSA

Low Light:
1) Nikko/BSA
2) Leupold
3) Bushnell Elite 4200 8-32

Range Finding:
1) Nikko/BSA
2) Leupold Competition
3) Bushnell Elite 4200 8-32

Take the Nikko and Bushnell out as darkness settles, and there is no comparison between the two. The Nikko/BSA wins hands down (both at 32x).

The Leupold and Bushnell have such good optics that ranging is a matter of:
- Good-better-best-better-good (it takes time to learn)
-Of these two, the Leupold has the ranging advantage due to the larger OD after market side wheels (127-152mm vs. 40mm) allowing for more space between yardage markings.

The Nikko/BSA is more like:
- clearly focused-out of focus (much easier to distinguish distances)

In FT clarity vs. ranging is common discussion. Most choose accurate ranging and a little more light for those dark lanes.

Hunter FT open up a whole new discussion about optics considering the maximum magnification of 15x and the inability of using your scope as a range finder.

Thanks again Bob!

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 Post subject:
PostPosted: Sun Jan 27, 2008 12:42 pm 
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CJN....

Just some comments....

Quote:
Take the Nikko and Bushnell out as darkness settles, and there is no comparison between the two. The Nikko/BSA wins hands down (both at 32x).


The Nikko has an exit pupil of 60 / 32 = 1.875 mm
The Bushnell has an exit pupil of 40 / 32 = 1.25 mm

I would expect they would perform exactly as you say.... You can think about it another way.... a 60mm lens has an area of 28.26 sq.cm..... a 40 mm lens has an area of 12.56 sq.cm.... That means the Nikko has 2.25 times the light gathering ability of the Bushnell.... You don't give an objective size on the Leupold, but I'm guessing it's smaller than 60mm.... since it doesn't perform as well in low light as the Nikko, and yet the Leupold is not variable.... The lack of extra elements in the Leupold is undoubtedly part of the reason for it superior performance in bright light.... fewer glass/air interfaces to lose/scatter the light.... :wink:

Quote:
The Leupold and Bushnell have such good optics that ranging is a matter of:
- Good-better-best-better-good (it takes time to learn)


Quote:
The Nikko/BSA is more like:
- clearly focused-out of focus (much easier to distinguish distances)


Assuming the ranging is done at maximum magnification.... I would expect the Nikko/BSA (at 50 X) to be MUCH more critical on focus than any of the others.... especially at FT ranges.... :)

Bob

_________________
Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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