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Joined: Aug 2007
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Originally Posted by Magnumdood
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You ain't gonna see schit with an objective that small.


Travis

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Ohhh hell noo! Dood breakin' out that 72mm schitt. grin
I secretly owned one for a short time. The view was incredible but the bulk I could not live with.
Had to add a cheekpiece to accomodate that monster.
Dont know what I was thinking.
Definitely a specialty piece of kit.

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I have a lowly VXIII 2.5X8 and it simply gets it done in low light.

This elk fell at the last inkling of legal light, last moments of the season this week.

I owe this one to the leupy and a lucky off-hand shot...

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Our pupil's maximum dilation is 7 mm. That means, an exit pupil for maximum light needs to be 7 mm. After that, it's a compromise you have to make regarding how much magnification you can handle/use, and how big a front objective you can tolerate in mounting.

exit pupil x magnification = max objective diameter in mm

7 mm x 4x = 28 mm (note: 1 inch=~25 mm)
7 mm x 6x = 42 mm
7 mm x 7x = 49 mm
7 mm x 8x = 56 mm
7 mm x 10x = 77 mm

physics rule:
"The intensity of light varies inversely to the square of the distance from the source."

Bob Bell wrote an excellent article in the 1989 Gun Digest (43rd edition, red cover, page 6) called "Why big scopes make a difference." It addresses why "big" scopes are brighter in darker conditions.

If you have a target at 500 yards away in dark or twilight conditions, you may just be able to make out the object. If you have a 6x42mm scope mounted (a pretty good practical low light scope due to the 7 mm exit pupil it gives and relatively low scope mounting), the effective distance through the scope is 500 yards/6x = 83 yards. Square the number (83x83) to get a value of ~6890. The inverse is 1/6890.

This means the light intensity reflected from the target (visible to the eye) is 1/6890 of what it would be at 1 yard. You would see the object at 1 yard, but the further away it is, it gets harder to see.

Compare the same object (same dark conditions and 500 yard distance) with two other "big" scopes".

An 8x56 scope would give an effective distance of 500/8 = 62.5 yards. Square this (62.5 x 62.5)to get a value of ~ 3900. Take the inverse, and you get 1/3900.

A 10x77 mm scope would have an effective distance of 500/10 = 50 yards. Squaring this yields 2500. The inverse value is 1/2500.

This means the light intensity of a 10x77 scope is 6890/2500 times more "bright" than the 6x42 scope, or 2.7+ times brighter. (6890/2500 =2.756)

Compare that to a 4x32 scope. The 500 yard target has an effective distance of 500/4=125 yards. Square that to get 15,625. The light intensity reflecting off the target is the inverse of the square, so the reading is 1/15,625 or, 2500/15,625 times LESS bright ("intense") for a value of 16% of the intensity ("brightness") of the object seen through a 10x77 scope.

How about that 2.5x scope that was marketed to have a high "twilight factor" due to a "large field of view." Same object/distance/light conditions gives you an effective distance of 500/2.5 = 200. Square this to get 40,000. The inverse of the squard is 1/40,000 or 2500/40,000 = 6.25% of the light intensity compared to the object seen through the 10x77 scope at the same distance and conditions.

It's not just magnification, it is exit pupil, or the "tube of light image" that gets to your eye's maximum "open iris condition" of 7 mm, which exits the rear objective of the scope into your eye. That's the rate limiting reaction on the receiving end-7 mm exit pupil.

You then have all the mechanical issues with quality of the glass and lens coatings for light transmission. You the have to make the decision of how big a scope you want on your rifle (how high to mount it). An 8x56mm scope has a front objective bell diameter over 2 inches wide (56 mm/25.4 mm per inch = 2.2 inches). A 10x77mm scope would have a 3 inch front objective diameter.

What's the BRIGHTEST scope? It's the scope with the highest magnification you can use, with the largest mountable front objective you can secure, that gives you a 7 mm exit pupil, that you can tolerate or use for your hunting conditions.

That's the physics behind it. The rest is up to you, and your wallet. Try to find Bob Bell's article. It is a classic.


"Behavior accepted is behavior repeated."

"Strive to be underestimated."
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As a rule of thumb, people up to age 30 or so have an average maximum 7mm pupil dilation. When you reach 40, on average again, your pupil can only dilate to 5mm.

I said average because I'm 52 and my maximum pupil dilation isn't in the "normal" range. I know of a person on this board who is older than me, and his pupil dilation falls way outside the normal range for his age.

IC B2

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The Zeiss and Nightforce scopes seem to me to be among the best for low light shooting. However, not everyone needs to pay $2,000 or more for another minute of shooting time. I use a Leupold 3.5-10x50 for most of my low light hunting and I'm not giving up too many hunting minutes with that. It has served me very well right down to the last minute of legal hunting time.

I recently aquired a Bushnell 4200 6-24x50 tactical for my long range coyote rifle and love the Firefly mildot reticle even though it doesn't provide quite as bright a view. You can shine a flashlight on the reticle and it remains illuminated for about 2 hours. No more worries about batteries failing just as the coyote moves into range. Although I like lots of light gathering, my biggest night time coyote hunting issue nowdays (since most scopes provide very good light) has been visibility of the reticle. I don't like illuminated reticles but the Firefly I like. I can see my dots perfectly and light is sufficient to see and target coyotes out to 500 yards or more. I even had to crank up to 24x last week to see one in the grass at 600 yards as light first broke and that worked fine (unfortunately, I didn't get the coyote because I was fiddling with my rest).

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Originally Posted by Mikey300
...No more worries about batteries failing...

Your flashlight batteries could fail. grin

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i realize this is a older post but i'll put my 2 cents in. i own zeiss,tasco,bushnell,weaver,redfield,leupold and several others over the yrs. it wasn't until i bought my first zeiss that i realized how much of a difference a quality scope makes. you can not go wrong with just about and german or austria made scope. most of them have been building optics longer then most gun manufactures. when i got my first zeiss i easily added 30-45 mins of combined hunting time (morning & dusk). one of the reason european optics are in a class of their own is their hunting hour are much different then here in the states. in some areas your hunting at 10pm and later. it's not in total darkness but twilight like in alaska. the zeiss conquest line is probably the best dollar for dollar line you can get. sure there are better scopes but your going to pay for them dearly. even zeiss,swarovski,khales bottom line models will out perform 98% optics made here in the states and asia. looking through them in your local sporting good store will not even come close to showing what they can do. it's not until your sitting in your stand in the woods and the sun is down that you'll SEE the difference. and they can take the pounding of the biggest magnum's and not even flinch. i am getting ready to buy my 3rd zeiss very soon. IMHO they CAN'T BE BEAT!

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Zeiss diavari 6-24x72
ZEISS HT 3-12x56
zeiss diavari 3-12x56


"Every record has been destroyed or falsified, every book rewritten, every picture has been repainted, every statue and street building has been renamed, every date has been altered."
― George Orwell, 1984
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