Stupid Question
Jan 8, 2022 15:13:50 #
RichKenn
Loc: Merritt Island, FL
Yes, there are. Here's one. I always thought the focal point, where the two lines from the subject to the sensor cross, was within the lens or at least behind thee objective lens. Yet, from the tip of my 500 mm lens to the back of the camera is only about 330 mm. Does that mean the focal point is 170 mm out in front of the lens? Of course, with 19 pieces of glass I suppose they can do wonderous things.
Jan 8, 2022 15:17:59 #
RichKenn wrote:
Yes, there are. Here's one. I always thought the focal point, where the two lines from the subject to the sensor cross, was within the lens or at least behind thee objective lens. Yet, from the tip of my 500 mm lens to the back of the camera is only about 330 mm. Does that mean the focal point is 170 mm out in front of the lens? Of course, with 19 pieces of glass I suppose they can do wonderous things.
Yes the rear nodal plane of the lens is in front of the lens and that is why it's called a telephoto.
Jan 8, 2022 15:18:57 #
Ysarex
Loc: St. Louis
RichKenn wrote:
Yes, there are. Here's one. I always thought the focal point, where the two lines from the subject to the sensor cross, was within the lens or at least behind thee objective lens. Yet, from the tip of my 500 mm lens to the back of the camera is only about 330 mm. Does that mean the focal point is 170 mm out in front of the lens? Of course, with 19 pieces of glass I suppose they can do wonderous things.
Your telephoto lens is designed to be physically shorter than the actual focal length to make it more user friendly.
https://en.wikipedia.org/wiki/Telephoto_lens
Jan 8, 2022 17:25:36 #
RichKenn wrote:
Yes, there are. Here's one. I always thought the focal point, where the two lines from the subject to the sensor cross, was within the lens or at least behind thee objective lens. Yet, from the tip of my 500 mm lens to the back of the camera is only about 330 mm. Does that mean the focal point is 170 mm out in front of the lens? Of course, with 19 pieces of glass I suppose they can do wonderous things.
It's not simple optics when you have compound lenses.
Jan 9, 2022 00:02:57 #
Jan 9, 2022 06:17:53 #
Jan 9, 2022 06:27:12 #
tcthome wrote:
That the Captain Morgan Rum guy ? 
Captain Obvious . . .
Jan 9, 2022 09:03:45 #
RichKenn wrote:
Yes, there are. Here's one. I always thought the focal point, where the two lines from the subject to the sensor cross, was within the lens or at least behind thee objective lens. Yet, from the tip of my 500 mm lens to the back of the camera is only about 330 mm. Does that mean the focal point is 170 mm out in front of the lens? Of course, with 19 pieces of glass I suppose they can do wonderous things.
That is why there are all those glass elements.
look at old simple telephoto lenses or rifle scopes and their length answers your question.
Jan 9, 2022 09:29:16 #
RichKenn wrote:
Does that mean the focal point is 170 mm out in front of the lens? Of course, with 19 pieces of glass I suppose they can do wonderous things.
NO - and yes, the 19 elements does do wondrous things !
.
Jan 9, 2022 14:10:28 #
Jan 9, 2022 14:30:58 #
awesome14
Loc: UK
'Focal length' and 'focal point' distances are quoted from simple lens technology, because that is a bigger number. But it only corresponds to the relative size of a given image at a certain distance, as would be produced by a simple lens. So, there is no mathematical way for end users to calculate anything from the advertised focal length. But we always know the focal point is behind the rear element, or front element if one thinks of a telephoto as a reversed lens.
Jan 9, 2022 14:46:54 #
Alafoto
Loc: Montgomery, AL
Gene51 wrote:
Captain Obvious . . .
Captain Morgan is a swashbuckling piratical looking guy.
Jan 9, 2022 16:29:40 #
TimHGuitar
Loc: San Francisco, CA
The focal length of a lens is determined by the distance of the focal point to the recording plane of the image (the sensors in this case - or the film plane if shooting film) only when the lens is focused at infinity. So if you are focused on a far away scene (in your example) then yes, the lens has been designed to work in the manner you are describing. My guess is that this was done in order to balance the lens along with the body of the camera so that, when used on a tripod, the weight is shifted towards the back. In the old days, telephoto lens had a tripod socket on the lens itself in order to maintain this balance.
Jan 9, 2022 21:30:40 #
awesome14 wrote:
'Focal length' and 'focal point' distances are quoted from simple lens technology, because that is a bigger number. But it only corresponds to the relative size of a given image at a certain distance, as would be produced by a simple lens. So, there is no mathematical way for end users to calculate anything from the advertised focal length. But we always know the focal point is behind the rear element, or front element if one thinks of a telephoto as a reversed lens.
Well that’s wrong. The focal length is an optical characteristic of the lens and has nothing to do with the relative size of an image at a certain distance. It’s the distance between the focal plane, (the sensor), and the optical center of the lens. With a compound lens it’s possible, or even probable with longer lenses, for that optical center measurement is in front of the physical lens. Only whether that focal length is considered telephoto is based on the image size. My 85mm Z mount lens is called “short telephoto” but technically it’s not a telephoto because it’s longer then 85mm, (and even longer when you add the flange distance).
Jan 9, 2022 22:21:45 #
TimHGuitar
Loc: San Francisco, CA
SuperflyTNT wrote:
Well that’s wrong. The focal length is an optical... (show quote)
Yeah, you're probably right. My information is old... from view camera days. The new design of len's are beyond my knowledge. Thanks.
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