How do Adjustable Objectives work?

[kfrye]

I apologize for the ignorance, but aside from allowing more light in, could someone explain the technical end of it, and what is better, the AO or a fixed objective like a 50mm Leupold?

[green 788]

The adjustable objective works by moving the front lens in/out just a little bit. This allows the image to focus on the same focal plane as the crosshairs are on. This gets rid of parallax (when the target image and crosshairs sort of "side step" each other when you move your eye left to right. That causes inaccuracy.

With lower power scopes (up to about 10X) you can get by without an adjustable objective. The higher the magnification, the more you'll need the AO.

Side focus scopes work in much the same way, except the side focus moves a different lens in/out.

Dan

[kfrye]

Thanks Dan,

So a higher power scope with a fixed objective should have the parallax adjustment on the side along with turret knobs for windage and elevation?

Ken

[green 788]

I would say that any scope over 10X in power should have either the Adjustable Objective, or the side focus. I've seen some 4-12 scopes on the market without either and they have been hard to use for precise target work because of the parallax problem that would pop up at various ranges.

Fixed power scopes are adjusted for parallax at one yardage, usually about 150 yards. Once you get beyond that figure, the parallax gets worse and worse, but this is less noticeable/bothersome on lower power scopes.

Dan

[4ked Horn]

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but aside from allowing more light in,

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AOs dont let more or less light in. This is purely a function of Objective lens size (which is always fixed ie: 25mm 42mm 50mm) and the quality of the coatings on the lenses and the quality of the glass its self.

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could someone explain the technical end of it, and what is better,

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There is a great post at the top of this Long Range Optics section covering this exact thing. It is a good read. The answers above are good so I'll just add more in my own words. An adjustable objective is a way of placing the image coming through the scope (the deer you see) on the same focal plane as the cross hairs. Example: extend your hand and put your index finger pointing up. Now look past it at an object on the wall. Now move your head while leaving your finger still. Your finger will sppear to line up with another point on the object moving around relative to where your head is. Now the example of putting them on the same focal plane (or adjusting your objective). Go put your finger on the object on the wall. Move your head all you want and the finger is still lined up on the object. There is no movement of the image relative to your head.

Back to scopes. If you have a 4x scope that is adjusted for parallax (what I described above) at 150 yards and your target is at 100 yards the relative movement is going to be fairly insignificant. If you are shooting a deer at 650 yards the parallax will be more pronounced. Your head MUST be exactly in the correct place for every thing to be PRECISELY in line. But if you adjust the objective lens for 650 yards the effect is reduced to nearly zero and this error in sight allignment is again insignificant.

Which is better? It is dependant on your use of the scope is.

Lastly and quite simply the adjustment may be on the objective bell or on the port side of the scope with the other turrets. It will be called a side focus but it is not a focus. It is a focal plane adjustment. Your focus is on the Occular lens bell at the rear of the scope.

Now go read the long version written by smarter folks than me. I hope this helped.