Adjustable Objective and point of impact

[JonnyR]

Hi All,

 

Having recently acquired a .22 AA410K I was busy zeroing it in the garden with the wife

 

I set a target about 20 yards away and all was good so I moved back to 30 yds. With the Objective Adjustment ring set to 20 yds the 30yd target was out of focus and I needed to set the Obj Adj ring to more like 100 yds to get it really pin sharp.

 

It is a Bushnell Sportsman 4-12 X 40 by the way.

 

I have read lots about AO being used to compensate for parallax but that is not my issue. The question I have is "If you change the 'range' on the AO do you change the point of impact?

 

Presumably these types of scopes are calibrated for rifles, i.e. longer ranges. Do I just ignore the 'range' setting, zero the scope to say 20 yards, use the AO to get the focus right and then work out the hold over and under respectively?

 

thanks, Jon.

[John_R]

POI shift can occur with some scopes when you change the AO, but I have no idea if your scope is known for that.

 

It does sound odd that you have a clear image at 20 yards but need to have the ring near 100 to get a clear image at 30 yards, that does not sound right at all. Truth be told, the markings on most AO rings is only a guide, but that is a long way out.

 

Are you doing this at maximum magnification? That is the only way to be sure the parallax is correct because the depth of field is at the least, ie there is only a narrow band of focus.

 

If you are shifting the magnification back and forth and zeroing at all sorts of mag settings you might be adding another variable in to confuse matters. POI shift can also happen at different magnifications, so you do need to change only one thing at a time.

[Colster]

The question I have is "If you change the 'range' on the AO do you change the point of impact?

 

No, it doesn't although due to parallax it could change where you "think" you're aiming. It sounds to me like your eyepiece focus ring is not set up correctly, this will change at what distance you get sharp focus, if this is set up right then you can use the AO ring as a sort of rangefinder.

 

Try this:

 

Turn the focus ring as far anti clockwise as it will go, then point the rifle at something blank and a long way away (blue sky for example), now move your head away from the scope then quickly move it back to look down the scope. The reticle should be a bit fuzzy but after a second or so your eye will compensate and draw it into focus, this is bad and where parallax becomes an issue. Keep making small adjustments to the focus ring until your eye sees the reticle as sharp as soon as you look at it. If your focus ring has a lock, then lock it now and never touch it again :yp:

 

Now rest the rifle somewhere so it can't move about facing a target 25 yards away (or another distance that is marked on the AO ring of your scope) and look down the scope, whilst looking down the scope move your head slightly up and down and side to side, if you can see the target appear to move independant of the crosshair then adjust the parallax ring until there is no independant movement. Look at where you are on the AO setting and even if it says 50, that is your setting for 25 yards. The markings are only a guide and shouldn't be read as gospel, you need to find your own reference points.

 

tbh at the ranges achievable by an air rifle AO doesn't make that great a rangefinder but it does help a bit. Basically when you look at a target and it's only clear in focus with the AO turned out to say 50 yards then it gives you an indication of the distance, bear in mind this is based on your own 50 yrd reference point, not necessarily what it says on the scope's AO ring. You would still need to make your own adjustment for hold over though.

 

Hope this helps.

Edited July 13, 2009 by Colster

[Benellimelody]

Well explained Colster