Jeff Keller wrote:
>----- Original Message -----
>From: "Fernando Gonzalez Gentile" <fgnzalez@xxxxxxxxxxxxx>
>Subject: [OM] Re: F-stops [Was:
>
>
>
>
>>Should I understand that the front nodal point is somewhere in front of the
>>
>>
>>lens? How does one find it? If so, I assume it is a virtual point, since
>>rays coming out of the front would be parallel. But if rays come out
>>parallel, does it make practical sense measuring the focal width at a
>>certain point in front of the lens?
>>
>>
>>
>
>My less than perfect understanding of the front nodal point is that it can
>be in front of the front element or within the lens depending on the lens
>design. It corresponds to the point where a simple lens could be placed to
>act as if it were the complex lens. For instance if the point source of
>light weren't exactly at the focal plane, the rays would either converge or
>diverge coming out the front of the lens. However they would act as if they
>were coming from a simple lens having the diameter of the f-stop positioned
>at the front nodal point.
>
Sounds like a pretty good description to me. I don't believe one can
find out where the front node is without optical test equipment or a
computer model of the lens.
It's easy enough to calculate where the rear nodal point is. The rear
node is defined just like the front one, except from the rear of the
lens. The 200/5 is 105 mm long from mounting flange to front of the
lens, a bit less to the front of the FE. Add the register distance of 46
mm from lens flange to film to get 151 mm. Since it acts to the film
like a single element lens 200 mm away, its rear node is about 50 mm in
front of the FE. This is where the term retro focus first came from.
Later, the same principles were applied to wide angle lenses to place
the rear nodal point inside the area swept by the mirror on SLRs. The
rear node of the 18/3.5 is 28 mm behind the mounting flange.
Moose
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