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Thanks Les, That is what I was trying to figure out.

--- les@... wrote:

From: "Les Niles" <les@...>
To: [email protected]
Subject: Re: [Losmandy_users_io] Calculating arc-seconds per pixel Question
Date: Fri, 1 Jan 2021 11:39:01 -0800

Do you mean “effective focal length”? That term is used in relation to SLR cameras and lenses, to indicate the field of view relative to a full 35mm frame. For example, a 200mm telephoto lens on a camera with an APS-C sensor has a field of view of about 6.5°. To get that field of view on a full-frame 35mm sensor would require a 320mm lens, so the 200mm is referred to as a “320mm effective focal length” on an APS-C camera. But the effective FL has nothing to do with image scale; it’s just a convenience for photographers who are used to thinking in terms of the view of various lenses on 35mm cameras. For calculating image scale and other astronomical purposes, you want the actual FL.?