BirdForum is the net's largest birding community dedicated to wild birds and birding, and is absolutely FREE!

Register for an account to take part in lively discussions in the forum, post your pictures in the gallery and more.

Reply to thread

Message: <blockquote data-quote="ronh" data-source="post: 3281903" data-attributes="member: 55514">I would like to distinguish longitudinal from lateral color error.  The traditional criterion for an achromat (non ED doublet objective) to have unintrusive longitudinal error is for the focal ratio to be about 3 times the aperture in inches.  For a 1.5 inch binocular, that would be f/4.5.  Binoculars don't meet that, usually being about f/4 for Porros down to f/3.5 to some roofs, but on the other hand, that criterion is good even at limiting magnification around 25 to 50 per inch of aperture, way beyond the optical demand made on a small binocular.  So, it seems to me, a non ED binocular should not have significant longitudinal error.  That's a good thing, since longitudinal error is the same all over the field of view, there is "no escape".Color fringing, or lateral error, is the problem in binoculars, and it is, in my experience and as claimed by advertisements, significantly reduced by ED objectives.  But unlike longitudinal error, lateral error varies with the distance off axis in the image, and falls to zero at the center. There is a caveat to this: the eye must be perfectly centered on the eyepiece, or the point of zero lateral error will also be off the center of the image.  Because the visual concentration is also confined to a similar small spot, ED glass is a luxury that can be gotten along without, if you can only locate that error free spot and do your critical viewing there.  Not to say the view will not be rife with color fringing, but if the finest detail, not a "lovely view" is what is sought, a lack of ED glass need not limit the optical performance.  The more experienced users, especially those who always use the same binocular, will be practiced at eye placement and the masters of this art. Ron</blockquote>

Verification