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-   -   Focal lengths ? (http://www.pixalo.com/community/general-photography-questions-answers/focal-lengths-26395.html)

samleo 07-08-2008 19:27

Focal lengths ?
 
Hi,
Can anybody answer a couple of questions; What parameters ars used for assessing the actual physical focal length of a lens, and from what point on the lens is the minimum focusing distance measured ?:confused:

silkstone 07-08-2008 20:17

Re: Focal lengths ?
 
For the focal length question, take a look here and see if that helps...

Measuring Focal Length

The focusing distance should be (and generally is) measured from the film plane, denoted on many cameras by a circle with a line through it.

AQ 07-08-2008 23:37

Re: Focal lengths ?
 
That is an interesting link. I will have too read through it more carefully that I have just done but I could not resist copying this sentence from it.

Assuming that the lens is surrounded by air or vacuum (refractive index 1.0), the focal length is the distance from the secondary principal point (which in this case is also the secondary nodal point) to the rear focal point of a lens.

Well we all knew that didn't we :D :confused-

samleo 08-08-2008 00:20

Re: Focal lengths ?
 
Well that certainly clarifies the situation, its so obvious I feel a bit stupid!

samleo 08-08-2008 00:33

Re: Focal lengths ?
 
Well I visited the link you suggested but now my head hurts and I wish I had,nt asked!:dizzy::duno:

Charlotte 08-08-2008 01:33

Re: Focal lengths ?
 
TBH I thought that about the link too, but didn't say anything in case it was just me being dim :whistle2:

Had a quick google + found some other similar horrendous links ... but found this one as well, which could not be simpler nor more concise, with a pleasant lack of text + an excellent diagram :thumb:

Edit: However, do not venture beyond this page, as the rest of the website is as confusing as Silky's link :D

silkstone 08-08-2008 02:16

Re: Focal lengths ?
 
Now you know why I am permanently confused. :p

I wouldn't worry too much about focal lengths except to know that the higher they are, the more magnification you get at a given distance from the subject. The more interesting bit is that the focus distance is not measured from the lens, but from the focal plane (aka the film plane or sensor plane, inside the camera).

So if the minimum focusing distance for a macro lens is, say, 300 mm, and the lens is, say, 150 mm long, and the focal plane is, say, 20 mm behind the lens mount, the front of the lens will be 130 mm from the subject. :)


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