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Message: <blockquote data-quote="kabsetz" data-source="post: 3173340" data-attributes="member: 10167">Henry was talking about axial aberrations, which are the ones influencing centerfield resolution and MTF. Once we start examining things off-axis, or outside the central 10% or so of the image, the eyepiece design of course plays a significant role.I apologize if this comes off as sounding unintentionally arrogant or aloof, but at least for me it would have been very difficult to come to understand what influences what without having done both star-testing and resolution testing of telescopes, eyepieces and binoculars. Especially having had the opportunity to test back-to-back multiple specimen of the same telescope model and/or eyepiece (and prism) models and being able to compare their varying aberrations has been extremely educating. It is through these exercises I have reached the same conclusion that Henry mentioned earlier in this thread, namely that unless the eyepiece is glaringly faulty (which is exceedingly rare, thankfully) it plays very little part in the quality of the centerfield image. The obvious exception to this generalization is minuscule differences in image contrast and color balance coming from coating and glass type differences, but in the telescopes with exchangeable eyepieces that I have tested, even these differences from the eyepiece tend to be obscured by differences in the scope objective/prism.Another comment I'd like to make concerns the objections some make about the pertinence of prism spiking seen on bright light sources to image quality in daytime viewing. This is a subject not so much of this thread but some others recently. Here we need to understand that what we see the light doing in bright point sources is like an isolated and more clearly seen sample of what light passing through that optical system generally does. Thus a binocular or a telescope that images stars more or less as diffraction-limited Airy disks is going to have visibly better resolution and contrast rendition also when viewing a flock of sparrows in a field or mallards in a pond. To what extent the differences are visible will depend on how poor the reference binocular or scope would be, on viewing conditions, and on the visual acuity of the viewer. It was about a couple of years ago that Holger Merlitz posted how David had pointed out to him that human eyes (for individuals with very good eyesight) are essentially diffraction-limited at pupil sizes around 2-2.5mm. This means that the optics we use don't have much leeway for axial aberrations for a stopped-down entrance pupil. The DIN-ISO 14490-7 standard for resolving power for "Hochleistungferngläser" or high-resolution instruments states that it is enough if their full-aperture resolution is equal to or better than 55% of the diffraction limited maximum for that aperture. Holger said he'd talked to some optical designers within the industry about this, and had heard that they had not thought about it this way. Like Holger and David, I think this standard is far too lax, and this is the main reason why we have these endless discussions on whether or not alpha binoculars are as good as they should be.Kimmo</blockquote>

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