Scopes and parallax

[meles meles]

How important is parallax correction, oomans? Ought we stick with a fixed parallax scope (we shoot at 25, 100 and 300 yards, with an occasional play at 600), or buy one of these scopes with a side wheel for parallax correction ?

[billgatese30]

Give me a few hours badger and I'll dig out a spreadsheet I have somewhere where you can calculate how much error can be had depending on distance and scope objective diameter

[billgatese30]

Right found the link eventually. You can either use the online version or save it to your hard drive and open in Excel.

Direct link
https://skydrive.live.com/?cid=be897297 ... &app=Excel

Link if you want to download direct (AO_Calc_Table 20110217.xlsm)
https://skydrive.live.com/?cid=be897297 ... 6281%21120

Basically out of a 40mm scope with parallax set to 100m if you shoot at 600m then you could have a maximum of 100mm of error (according to the spreadsheet). Its not exactly the huge disaster internet scaremongers out there would have most people believe, but if you are doing F class or Benchrest then you would be daft to ignore it as a 56mm scope would have over 250mm possible error at 1000 yards!

[1066]

How important is parallax correction, oomans? Ought we stick with a fixed parallax scope (we shoot at 25, 100 and 300 yards, with an occasional play at 600), or buy one of these scopes with a side wheel for parallax correction ?

Really think you will need adjustable parallax Badger - There's a good probability that a fixed parallax won't be in focus down to 25 yds, you might get away with it if it's a low mag scope but I would certainly try before you buy.

If you are going to get an adjustable parallax scope then I would recommend a side adjuster rather than front adjuster.

[meles meles]

Can a fixed focus scope be made to sufficiently precise tolerances that parallax errors don't occur?

[billgatese30]

Yep, you could adjust it to suit a particular distance. Normally by screwing the front most lens element in or out. But its not exactly field adjustable. You'd have to do it at the range you wanted and then leave it. Move from 300 to 600 yards on range between a morning and afternoon session and your back to square one. But handy if you say have a scope that has been parallaxed for an airgun/rimfire etc (25 or 50 yards or so) and want to adjust it to say 450 yards so you can make the best compromise between 300/600 yard shooting etc.

[ovenpaa]

The other problem is the users interpretation of parallax, eyes differ from person to person and the scale on the 'scope is not always accurate. I am quite happy with the scale on my PMII however my 42x Nightforce has my take on the parallax marked on the ring but then again I am not a fan of Nightforces.

[GazMorris]

Can a fixed focus scope be made to sufficiently precise tolerances that parallax errors don't occur?

As I understand it, it's not a question of tolerances or accuracy of manufacturing but one inherent to optics. I'm not an expert when it comes to scoped shooting, so if I've got it wrong someone who knows better please correct me!

When the reticle and the target aren't in the same focal plane your head moving and around causes the parellax inaccuracy effects; if you held your eye perfectly in the optical centre of the scope then there would be no error but as this isn't possible because you can't position your head that precisely and repeatably, you get an error.

Gaz

[TJC]

Here is a good explanation from US Optics:

OPTICS TECH: PARALLAX EXPLAINED (Analysis by U.S. Optics)
Whenever rifle scopes are discussed, a topic that frequently arises is parallax. There seems to be a great amount of misunderstanding and confusion concerning this subject. Parallax can be defined appropriately to rifle scopes as the apparent movement of objects within the field of view in relation to the reticle.

In a telescopic sight, parallax occurs when the “primary image” of the object is formed either in front of, or behind the reticle. If the eye is moved from the optical axis of the scope, this also creates parallax.

If the primary image is formed on the same focal plane as the reticle, or if the eye is positioned in the optical axis of the scope, then there is no parallax, regardless of the position of the primary image.

High magnification scopes, or scopes for long-range shooting, where even slight sighting errors would be serious, should be equipped with a parallax adjustment. This adjustment of the objective part of the optical system would ensure that the target can be brought in the exact focal plane of the reticle at any distance. Tactical style scopes are not usually supplied with parallax adjustment because the exact range of the target can never be anticipated. Scopes of lower magnification are not usually supplied with parallax adjustment either, because at lower powers the amount of parallax is so small as to have no importance for practical, fast target acquisition.



TWO FACTORS WHICH CAUSE AND DETERMINE THE AMOUNT OF PARALLAX IN A RIFLESCOPE:

1. The distance of the target to the objective--The objective lens forms a primary image of the subject being viewed and subsequent components invert the image, and there is no parallax. The actual position at which the image is formed is dependent on the distance the target is from the objective. Closer targets are formed farther away from the objective and farther targets are formed closer to the objective. Since the reticle is in a fixed position within the scope housing, the image is not always formed in the same plane as the reticle and, hence parallax.

2. The distance the eye can move from the optical axis of the scope, is determined by exit pupil size. There is no parallax, at any distance, as long as the eye is lined up exactly with the optical axis of the scope. An exit pupil small enough to do this would be impractical. It is important to know that in every scope, there is some parallax. It is also important to know that in every scope, there is some one shooting distance at which there is no parallax. In most rifle scopes this one point of zero parallax is usually placed at a suitable mid-range point in the scopes' focal range.

In lower-quality scopes, there are other sources of parallax. If the reticle is not precisely placed the correct distance from the objective, the distance of no parallax will be exaggerated. Reticles that are not securely mounted and allowed to move even a few thousandths of an inch, will always have changing amounts of parallax. Parallax is also caused by optical deficiencies in the objective, either by design or manufacture. If spherical or astigmatic aberrations have not been corrected, images will form a considerable distance from the reticle. If you see a scope in which the apparent movement of the reticle compared with the image viewed is different from when you move your eye up and down than when you mover your eye side to side, it is because of a bad objective. No adjustment of the scope will eliminate these faults or optical deficiencies.

You can check the parallax of any scope by sighting an object at normal shooting distance (not indoors), by moving your eye side to side (then up and down), as far as you can, keeping the sighted object within the field of view. The apparent movement of the reticle in relation the target is parallax.

[meles meles]

Hmmm, so it would appear that we would be best off going for a good quality, fixed, moderate magnification scope. Maybe a x 6 or x 10, as we shoot mainly at 300 and 600 mards?