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#11
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In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
writes If my calculations are correct, a 25mm tube will allow the 400 to focus from 8 to 22 feet. Someone posted a formula for the math below. But remember that internal focus lenses shorten the focal length at closer distances. So focal length plus extension at close up is always equal to focal length at infinity. What exactly does this mean? -- David Littlewood |
#12
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In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv
writes If my calculations are correct, a 25mm tube will allow the 400 to focus from 8 to 22 feet. Someone posted a formula for the math below. But remember that internal focus lenses shorten the focal length at closer distances. So focal length plus extension at close up is always equal to focal length at infinity. What exactly does this mean? -- David Littlewood |
#13
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If I remember the math correctly, (it's been a while since I worked this
out) at the near focus distance (about 11.5 feet) the 400mm canon lens has a focal length of about 360 mm. So in effect you actually have a 360mm lens with a 40mm extention tube. Hope this helps. Sorry due to spam I shall not provide a valid e-mail address. Please reply to group with questions or comments. "David Littlewood" wrote in message ... In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv 8 to 22 feet. Someone posted a formula for the math below. But remember that internal focus lenses shorten the focal length at closer distances. So focal length plus extension at close up is always equal to focal length at infinity. What exactly does this mean? |
#14
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If I remember the math correctly, (it's been a while since I worked this
out) at the near focus distance (about 11.5 feet) the 400mm canon lens has a focal length of about 360 mm. So in effect you actually have a 360mm lens with a 40mm extention tube. Hope this helps. Sorry due to spam I shall not provide a valid e-mail address. Please reply to group with questions or comments. "David Littlewood" wrote in message ... In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv 8 to 22 feet. Someone posted a formula for the math below. But remember that internal focus lenses shorten the focal length at closer distances. So focal length plus extension at close up is always equal to focal length at infinity. What exactly does this mean? |
#15
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If I remember the math correctly, (it's been a while since I worked this
out) at the near focus distance (about 11.5 feet) the 400mm canon lens has a focal length of about 360 mm. So in effect you actually have a 360mm lens with a 40mm extention tube. Hope this helps. Sorry due to spam I shall not provide a valid e-mail address. Please reply to group with questions or comments. "David Littlewood" wrote in message ... In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv 8 to 22 feet. Someone posted a formula for the math below. But remember that internal focus lenses shorten the focal length at closer distances. So focal length plus extension at close up is always equal to focal length at infinity. What exactly does this mean? |
#16
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In article BIRZc.1300$xr1.221@trnddc04, Roy G. Biv
writes If I remember the math correctly, (it's been a while since I worked this out) at the near focus distance (about 11.5 feet) the 400mm canon lens has a focal length of about 360 mm. So in effect you actually have a 360mm lens with a 40mm extention tube. Hope this helps. Sorry due to spam I shall not provide a valid e-mail address. Please reply to group with questions or comments. "David Littlewood" wrote in message .. . In article uiwZc.577$HY1.98@trnddc02, Roy G. Biv 8 to 22 feet. Someone posted a formula for the math below. But remember that internal focus lenses shorten the focal length at closer distances. So focal length plus extension at close up is always equal to focal length at infinity. What exactly does this mean? Well, I think I understand what you are saying. You seem to be asserting that the focal length of a lens with internal focussing is always equal to focal length at infinity minus the amount of extension form the helical focussing mechanism. This implies that the position of the rear nodal plane (the point from which image distance is measured) is unchanged relative to the position of the lens elements. However, I believe this is unlikely to be true in most cases. It also assumes the lens has internal focussing only, i.e. no helical focus mechanism to bodily move the lens elements away from the body. There are lenses of this type, but not all are. If there is a helical focus system, the position becomes more complex. In all, the rule-of-thumb you put forward may work when it works, but could not be taken as being applicable in all cases. David -- David Littlewood |
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