If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
The length of the focal length?
Just for fun, I measured a 200mm lens from the front objective, to the camera film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only 230mm?? What gives? Thanks! |
#2
|
|||
|
|||
The length of the focal length?
On Oct 4, 7:40 pm, wrote:
Just for fun, I measured a 200mm lens from the front objective, to the camera film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only 230mm?? What gives? Thanks! Telephoto designed lenses focus an image at a distance shorter than their focal length. That's their main virtue. Long lenses (such as your 200mm) for medium and small format cameras (including digital cameras) are typically of telephoto design. I'm surprised at the results you got with the 200mm. I suspect it's a large format lens adapted to a medium format camera. Can you give us the types and brands you were measuring? |
#3
|
|||
|
|||
The length of the focal length?
What gives?
Two properties of a lens: effective focal length versus back focal length. Not the same thing. |
#5
|
|||
|
|||
The length of the focal length?
Richard J Kinch wrote:
What gives? Two properties of a lens: effective focal length versus back focal length. Not the same thing. Quite. My undergrad optics textbook (1st published 1957) defines the ratio of the two (though it uses "equivalent focal length") as the "telephoto magnification". Mike. -- If reply address = connectfee, add an r because it is free not fee. |
#6
|
|||
|
|||
The length of the focal length?
On Oct 4, 10:34 pm, Richard J Kinch wrote:
What gives? Two properties of a lens: effective focal length versus back focal length. Not the same thing. That doesn't explain the OP's "discrepancies". Back focal length is measured from the rearmost element, focal length is measured from the nodal point and is given by the manufacturer, effective focal length is the focal length at a given focusing distance, as given by 1/F effective = 1/u + 1/v. You can see that F effective equals F when u (focusing distance) is at infinity. |
#7
|
|||
|
|||
The length of the focal length?
On Oct 5, 1:12 am, Paul Furman wrote:
wrote: Just for fun, I measured a 200mm lens from the front objective, to the camera film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only 230mm?? What gives? Pinhole 'lenses' will follow that rule... which makes it pretty much impossible to do a wide angle pinhole shot on an SLR with a swinging mirror that can't belocked away while shooting (I tried with tin foil). The focal length of a pinhole lens is equal to the distance from the pinhole to the film (or digital sensor, these days) plane. Paul is right, that WA pinhole photography is impossible with a fixed (as against a view) camera. I believe the distance between the film plane and the front mount on a typical SLR is of the order of 50mm, so that a pinhole lens is perforce 50mm or longer. |
#8
|
|||
|
|||
The length of the focal length?
On Oct 4, 8:40 pm, wrote:
Just for fun, I measured a 200mm lens from the front objective, to the camera film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only 230mm?? What gives? Thanks! They all use negative lenses in the optical train to effect a longer focal length that the total length would imply. A tele-converter is in nearly every telephoto lens. |
#9
|
|||
|
|||
The length of the focal length?
In article ,
writes Just for fun, I measured a 200mm lens from the front objective, to the camera film plane, and it was 200mm... nice... but I tried a 300mm lens and it was only 230mm?? What gives? Your 300mm is definitely a telephoto lens, your 200mm is just a long focal length lens - the two are not the same, no matter how popular that misconception is. http://en.wikipedia.org/wiki/Telephoto You can also get inverted telephoto lenses where the distance from the rear element to the focal plane is longer than the focal length. Almost every wide angle lens for use on an SLR camera is an inverted telephoto lens. -- Kennedy Yes, Socrates himself is particularly missed; A lovely little thinker, but a bugger when he's ****ed. Python Philosophers (replace 'nospam' with 'kennedym' when replying) |
#10
|
|||
|
|||
The length of the focal length?
Bob G writes:
That doesn't explain the OP's "discrepancies". It does explain, because it is the fundamental optic theory underlying why a lens system's physical length may be short than its focal length. Back focal length is measured from the rearmost element, focal length is measured from the nodal point and is given by the manufacturer, effective focal length is the focal length at a given focusing distance, as given by 1/F effective = 1/u + 1/v. You can see that F effective equals F when u (focusing distance) is at infinity. No. In optics, the "effective focal length" is the distance from the principal point to the focal point. Object distance has no bearing. The words "effective" and "focal length" may be used in photographic discussions as you have cited, but that is not the "effective focal length" of optical theory. |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Focal length manipulation | [email protected] | Digital Photography | 5 | April 14th 07 03:08 PM |
Focal length for APS sensors | Lars Forslin | Digital Photography | 3 | March 15th 07 12:45 AM |
Is there a formula to convert digital lens focal length to 35mm focal length ? | narke | 35mm Photo Equipment | 5 | March 1st 05 12:31 AM |
Focal length chart | LEICA | Digital Photography | 16 | January 20th 05 04:29 AM |
calculation of focal length | TS | Digital Photography | 21 | August 8th 04 06:48 PM |