Pro's and Cons for fixed V variable power scopes please

[ogre6br]

Hi
I use Fixed power scopes on all my rifles except my air gun and that hardly every changes of 8X so it may as well be fixed.

I am getting a .338 Federal and I am wondering if I should scope it with a Fixed 2.5 scope
or something in a variable 1.5-6 or 2.5-10?

I have really no modern experience with variable scopes and have no idea what they are capable of in terms of accuracy of movement, movement of reticle when power changed, sturdyness, first or second plane reticle- which is better, side focus V AO, variation in eye relief, changing in twilight factor and exit pupil with power change.

all the variables I have ever owned were purchased years ago and were in the low to very low $$ end of the market and had all or some of these problems.

I am looking for something in the $500-$1000 range, probably with a 30mm tube and will consider a larger than 40mm objective but dont really know their advantage with a lower power variable. please explain if there is one.

Thanks

later
p

[kjd]

Mate I can only speak from my own experience and that is I use my variables! If I'm varminting I wind it back to glass a warren and when I find a target I wind it up to shoot! With my airgun same deal.

Shooting of a night I wind my scopes back to lower variables to see more easily and of a day I usually have them zoomed in. Maybe I am different in this regard but I'd find a fixed power limited when I wanted that little bit more magnification!

[Model70]

Mate I have a 4-16x40 on my 22-250 and the only time I use the variable
power is at the range where I wind it right up, otherwise it stays on 8. It is at the lower end of the scale as far as scopes go so it may be different if I had a 'good' one. My .308 has a 10x40 fixed and it is perfect for what I use it for.

[Rinso]

ogre6br,

You sure know how to ask a question

Re Focal plane .. basically the Front focal plane means that the cross hair increases or decreases in size with any corresponding change in magnafication ..ie wind it up the crosshair gets bigger with the image and vice a versa. Benfits no change in point of impact with this focal plane.

rear or esecond focal plane changes the image size with magnification chnages but the crosshairs remain a constant size. This can have a minor impact on POI when you change settings, this is why it is recommended to sight in variables on the highest setting.
I prefer 2nd focal plane reticles as i do not like to have the target obscured by the crosshairs when I wind up for a long shot on a small target.

Sturdyness I do not think is an issue between the fixed - variable but rather a quality issue between manufacturers.

Side focus IMHO is better than front AO as it is easily adjustable from the firing position .. front AO is not so easy.

Eye relief variation is not enough to be an issue either way.

Dusk to dawn / low light is about light gathering not fixed - variable a 30mm tube and a bigger objective wins.

I have many variables and fixed power scopes I find I have moved all my fixed powers to target rifles where ranges are known and no suprises.

All my hunting guns have variables as I like the flexibility it gives me, low power in tight country and wind up for a long shot. As an example I use a 6.5-20 Leupold on my 25.06 and generally its on 10 x but when in deer country its on 6.5 x. When varmiting I will use up to 20 x depending on the distance. I can also use the 20 x for target work like F classand short range BR.

Just buy something good...by that I mean you cant expect to get quality for peanuts .. a $150 scope will perform like a $150 scope.

The 2-7 would be a good choice for the 338 IMHO. A Vortex Diamondback series in 1.75 - 5 x 32 will set you back about $330. A Vortex Viper seriesin 2 - 7 x32 will be $500.

Check out the website http://www.vortexoptics.com.au
It should launch tomorrow 2/7/07
or use the US site http://www.vortexoptics.com

Sent you a pm

cheers
Rinso

[zzsstt]

Dusk to dawn / low light is about light gathering not fixed - variable a 30mm tube and a bigger objective wins.

Actually the 30mm tube makes no difference to light transmission. It allows the manufacturer to fit goodies like side focus and allows a larger range of adjustability for windage and elevation. It also, potentially, make the scope stiffer and tougher, but it will make no difference to the brightness or any other aspect of the image.

Bigger objective and better quality lenses transmit more light. Better quality lenses (better lense matching, coatings etc.) for any given objective size will give a brighter and higher contrast image.

Adjustable objective, or "side focus" systems are designed to prevent parallax issues. "Parallax" is where the reticle appears to move in relation to the target when you move your head from side to side (the gun staying still, obviously!). It's caused by the image of the reticle being overlayed with the image of the target, but the focal plane of the target image will move forward and backward as the target changes distance from the scope. When the two images are not exactly aligned, moving your eye causes them to shift relative to each other - line up your finger with the corner of the screen, then move your head to the side to see what I mean. Scopes without this adjustment are simply correct for parallax, hence the two images are perfectly aligned and the reticle stays still, with a target at one preset distance, normally about 100yds.

Do you need AO or side focus? Depends... If your cheek weld is good, and your eye always looks through the centre of the ocular (not off to one side) there will be no parallax effect anyway. How accurate do you want to be? For most hunting purposes parallax will have such a tiny effect that you will never miss a shot because of it. If you are shooting small targets at long range and your cheek weld is variable with your eye off centre on the scope, then AO or side focus will give you one less excuse for missing.

So, my view would be that for a varmint rifle, where you intend sitting in one spot taking your time, it's a good thing but not vital. For a hunting rifle where snap shots at big targets are required, it's not required at all. Having said that, leaving it set at 100yds or so would effectively make the scope "fixed" so you're not disadvantaged by having it, unless the target actually appears out of focus because it's too close!

It does, however, cost money. Potentially a better scope, or larger objective may be a better way to spend your money, depending on the usage. For a hunting rifle, a Khales (or similar) steel tubed, fixed power, fixed parallax scope is still hard to beat. However for about $700, forget parallax, forget 30mm tubes and import something like a Leupold 3.5-10x50mm Duplex VXIII from the States. You'll be very happy with it.

2.5-10 power is a four fold increase. Only very expensive scopes will achieve this without significant changes in point of impact. Stay with three fold (3.5-10)!

[El Toad Man]

I've been thinking about getting something like a 45/70 lever action and I too have been thinking a fixed 2.5x32 would be about right. Of course it depends on the calibre & application. With my 243 I like having a variable, even though most of the time it stays on 5x, it's nice to have the option to change when the terrain opens up or closes in.

I guess I could ask what would YOU prefer? And how much do you want to spend. A cheap variable is going to be more of a disappointment than a cheap fixed power. You get better quality for the money with a fixed.

As for objective size and light gathering, the size should be in proportion to the magnification. ie low magnification does not require a large objective. Focal length is the guiding dimension here. Objective diameter divided by the magnification (eg. 40mm / 4x) gives focal length (in this case = 10) As a guide this number should be greater than 4, give or take, to give sufficient light gathering. Being far greater than 4 will give only small benefit. Being much less will mean a dim scope in low light conditions. This clearly shows that for low magnification scopes, 32 or 40mm is usually plenty.

[zzsstt]

As for objective size and light gathering, the size should be in proportion to the magnification. ie low magnification does not require a large objective. Focal length is the guiding dimension here. Objective diameter divided by the magnification (eg. 40mm / 4x) gives focal length (in this case = 10) As a guide this number should be greater than 4, give or take, to give sufficient light gathering. Being far greater than 4 will give only small benefit. Being much less will mean a dim scope in low light conditions. This clearly shows that for low magnification scopes, 32 or 40mm is usually plenty.

What you're talking about is called "exit pupil". Focal length is something else entirely.

If we regard the output of the scope as a parallel beam of light, then the exit pupil (the objective diameter divided by magnification) is the diameter of that beam, and should be at least the pupil diameter of the eye for maximum brightness. The average human pupil is about 4mm during the day, and about 7mm at night (maximum dilation). A scope with an exit pupil lower than the dilation of the iris will give a dimmer image. So:

50x7 scope exit pupil = 7.1 which is fine at night
30x4 scope exit pupil = 7.5 which is fine at night
40x8 scope exit pupil = 5 which is fine during the day but not so good at night

Conversely a scope with an exit pupil much greater than the pupil of the eye will appear to have the same (no more) brightness than one that exactly matches the eye. Perhaps more importantly it may have cost more money than required, i.e. a 50x5 scope (if such a thing exists) will produce a 10mm circle of light which is bigger than the pupil of the eye, so you are paying to illuminate your iris.......

For any given exit pupil, the higher the quality of the lenses and coatings the more light will be transmitted, so a cheap scope (assuming poor quality) with the same exit pupil will still not produce as bright an image as the more expensive one.

[El Toad Man]

Oops, getting my terminology mixed up.

The average human pupil is about 4mm during the day, and about 7mm at night (maximum dilation). A scope with an exit pupil lower than the dilation of the iris will give a dimmer image.

Aah, now I see the big picture. So that's the relevance of the 4mm (& 7mm!) Thanks for that!

[El Toad Man]

And using that logic, would a good quality 4x32 be brighter than a cheap 4x40? As they both have an exit pupil greater than the human eye can make use of. Meaning it comes down to lens quality, not size.

[zzsstt]

And using that logic, would a good quality 4x32 be brighter than a cheap 4x40? As they both have an exit pupil greater than the human eye can make use of. Meaning it comes down to lens quality, not size.

Yes, once you get an exit pupil larger than the pupil of the eye. A lens system that transmits 90% of the light will be dimmer than one which transmits 95% for a given exit pupil or one bigger than the iris dilation.

Note that your pupils may dilate more or less than the average 7mm, so you may still gain from, say, an 8mm exit pupil.

The top flight lenses that transmit 99.8% (or whatever) used by Swarovski will perform far better than those ebay specials. They will also give a sharper image, truer colours, more contrast and better performance near the edges of the image.

HOWEVER, the same as TV's and Hifi's you must be prepared to test the scope AS YOU WILL USE IT. It's very hard to see $2500 difference between a Tasco and a Swarovski when looking through the dirty glass of a shop window at midday, with the scope wobbling around as you try to hold it steady. Make the same comparison at dusk with the scope firmly fixed to a rifle on a bipod and the difference will become much more obvious.

[7mmmag]

I like to have the capacity to zoom in and out with the variable when required, it definitely comes in handy when spotlighting and shooting in low light situations.

As for the adjustable objectives, i have been told to steer clear of the side focus scopes and stick with the A/O models.

[kjd]

As for the adjustable objectives, i have been told to steer clear of the side focus scopes and stick with the A/O models.

Who told you that mate I've never heard it before and only heard good things about good quality SF scopes!

[HiWall]

With the quality of top brand variables now available I think they are just as good as fixed power scopes optically etc, and much more verstatile.

Only fixed power scopes I run now are leupold FX-II 2.5x20 Ultralights - because they are on close range guns such as a .30/30, or on BG rifles where high magnification is not required but quick target acquistion is very important. No way I want to get stuck on the wrong magnification when I am among the piggies.

Leupold FX-II 2.5x20 Ultralight on CZ550 .458 Lott.


Another on Marlin 336SS .30/30.

[Rinso]

HiWall,

Good looking gear mate ... how big are the pigs up your way

Must be bloody big if you need a 458 Lott to drop them.

cheers
Rinso

[HiWall]

Dreams of Africa mate, but she's too much gun for me!