This is not so simple as it first appears.

Some 'scopes have excellent depth of focus, i.e. what the previous posters mean when they talk about "parallax" but which actually describes the ability to maintain both near and far objects tolerably well focussed.

This is a useful quality, however such a 'scope will still suffer more or less severe parallax errors whilst still giving a sharp image. Unless the distant image and the crosshairs are in EXACTLY the same focal plane, then small off-centre movements of the eye will result in the crosshairs apparently moving compared with the target image. This utterly destroys any accuracy potential, unless the shooter can place his eye exactly in the optical centre of the light path from the eyepiece, every time

It is quite easy to make a 'scope which appears to have a great depth of field, by using simple tricks like inserting a small aperture inside the tube, a bit like stopping down a camera lens. It also sharpens up the image, but it also cuts out a lot of light, which will not be noticed in good daylight.

More honest 'scopes acknowlege such trickery and simply use small objective lenses and lowish magnification to give enormous depth of focus, e.g. 1.5-5x20 or 2-7x32 designs. They are also usually much better quality for the same price, but seemingly undesirable compared with bigger 44 and 50mm objective 'scopes using cheaper glass and coatings to no benefit.

You can buy "focus free" binoculars which use the same tricks, but as anyone who has used a good pair of e.g. 7x50 binoculars will agree, they actually have pretty limited depth of focus and have to be adjusted at almost every range, even well beyond 100 yards. The larger the objective lens, for a given magnification, the smaller the depth of focus, unless "stopping down" tricks are used which negate the light gathering benefits of the large objective. Basic laws of physics.

So to summarise:

For good depth of focus, choose small objectives and low magnification, but realise that true parallax error will be substantial, when used at anything other than the "parallaxed" range

For accuracy, choose larger objectives and higher magnification with an adjustable objective, and accept the need to re-focus the objective to suit the range and so eliminate parallax error.

For true simplicity, accuracy and optical quality, choose a fixed mag. fixed focus 'scope e.g. 6x40 or 7x50, parallaxed at your usual range, e.g. 30 yards for airguns, 100 plus for centrefire. Zoom and PA adjustments will always move the zero, by how much and how repeatably depends on the precision, tolerances and cost of the mechanisms involved. Zoom mechs. require extra lenses which must degrade quality and/or increase price.

How many zoom'scopes end up just being used at one magnification every time ?