[quote name='Vieux loup' timestamp='1304885872' post='8136']
What does it mean in practical terms to loose focus to infinity.
You focus up to a certain distance I guess and then you loose the focusing ability?
Are there any rules about how far out you loose the focusing?
[/quote]
Well, the symptom is as follow:
1) The distance scale on the lens appears to be wrong.
2) When the lens shows "infinity", things at infinity aren't sharp ... instead something
(much) nearer is sharp.
3) The situation is completely equivalent as when using a lens with an extension tube.
If you have a look on how a lens focusses, you will see two methods:
- by extension ... the optical groups are shifted forward (away from the focal plane) to focus nearer.
- by internal focussing ... not all groups are moved ... sometimes only an internal group is
moved, so the length of the lens does not change.
If you compare "focusing by extension" with adapting FD to EF (or Nikon), you will find that
in principle the same thing happens ... in one case by moving the focus ring, in the other case
by mounting the lens in the "wrong" position. As you likely know from your own experience,
long focal lengths need more extension to focus to something near than short focal lengths
do ... so for a 20mm lens a few millimeters change focus from infinity to below one meter,
while for a 400mm lens these few millimeters only focus down to say 50mtr.
This means, the maximum distance you can focus on (with such an adapted lens that is not
correctedfor infinity focus) is further away the longer the focal lengths is. Short lenses it will
become more or less useless (exept macrolenses, where the effect might be quite welcome).
Some (but by far not all) FD-lenses can be modified to fit EF ... in some cases this modification
is reversible, in others it is not. But you really need a hand for this in order to get a proper result.
There are quite a few things that need modification for this.
Rainer
PS It's in german (which might or might not be overly helpful for you), but have a look on
the pages linked here ... [url="http://www.4photos.de/camera-diy/Foto-Selbstbau.html"]http://www.4photos.d...-Selbstbau.html[/url]
This guy has modified quite a few FD-lenses to fit EOS/EF.
What does it mean in practical terms to loose focus to infinity.
You focus up to a certain distance I guess and then you loose the focusing ability?
Are there any rules about how far out you loose the focusing?
[/quote]
Well, the symptom is as follow:
1) The distance scale on the lens appears to be wrong.
2) When the lens shows "infinity", things at infinity aren't sharp ... instead something
(much) nearer is sharp.
3) The situation is completely equivalent as when using a lens with an extension tube.
If you have a look on how a lens focusses, you will see two methods:
- by extension ... the optical groups are shifted forward (away from the focal plane) to focus nearer.
- by internal focussing ... not all groups are moved ... sometimes only an internal group is
moved, so the length of the lens does not change.
If you compare "focusing by extension" with adapting FD to EF (or Nikon), you will find that
in principle the same thing happens ... in one case by moving the focus ring, in the other case
by mounting the lens in the "wrong" position. As you likely know from your own experience,
long focal lengths need more extension to focus to something near than short focal lengths
do ... so for a 20mm lens a few millimeters change focus from infinity to below one meter,
while for a 400mm lens these few millimeters only focus down to say 50mtr.
This means, the maximum distance you can focus on (with such an adapted lens that is not
correctedfor infinity focus) is further away the longer the focal lengths is. Short lenses it will
become more or less useless (exept macrolenses, where the effect might be quite welcome).
Some (but by far not all) FD-lenses can be modified to fit EF ... in some cases this modification
is reversible, in others it is not. But you really need a hand for this in order to get a proper result.
There are quite a few things that need modification for this.
Rainer
PS It's in german (which might or might not be overly helpful for you), but have a look on
the pages linked here ... [url="http://www.4photos.de/camera-diy/Foto-Selbstbau.html"]http://www.4photos.d...-Selbstbau.html[/url]
This guy has modified quite a few FD-lenses to fit EOS/EF.
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