 Ziad H. Dabash |
lens measurement when I read lens description I sow many Varity of lenses such as 50mm normal, 28 mm wide angle 28mm-80mm wide angle and zoom please what this number means ( 50mm,28mm,etc ) is it the radius of the lens or what
|
|
|
|
||
|
Tom Walker |
That's the focal length of the lens, and I believe it's the distance from the rear element of the lens to the film plane. Something to remember is that your f stop is in relation to this length,ie. f8 is 1/8th the focal length, for a 50mm lens this would be an opening of 6 and 1/4mm.
|
|
|
|
||
|
John P. Sandstedt |
I have to disagree with Tom. The focal length is the distance from the optical center of the lens to the focal plane. In a 50 mm lens, that about 2 inches [1 in. equals 25.4 mm.] Th f/number is the ratio of the focal lens to the diameter of the diaphragm of you lens. So, if you have a f/2 50 mm lens, the diagram has amximum opening of about 1 in. This is the reason that f/1.0 lens are so large and heavy. The filter size for a f/1.0 lens might be 80 mm. The diaphragm would be 50 mm [2 in.]
|
|
|
|
||
|
Tom Walker |
john isn't that what I said?
|
|
|
|
||
|
John P. Sandstedt |
Tom - Don't think so. Your response was, "..distance from rear element of the lens to film plane." In fact, the opticalcenter center of some lenses can be in fron of the "front lens." It a mathematical fact, not necessarily hardward [metal, glass, plastic] issue. Further, the speed of the lens assume the diaphragm is at its widest and that the lens is focused at infinity. Thus, f/8 does not equate to 1/8 of the lens's length.
|
|
|
|
||
|
Tom Walker |
john, I wasn't sure where the focal length was measured from exactly but f8 is 1/8 th that distance, you guys can nit pick a question to death and leave the asker more confused than when he started
|
|
|
|
||
|
John P. Sandstedt |
Tom, I think you're absilutely incorrect. The f/number starts with the aperture wide open. The as you close down, each stop reduces the light reaching the film plan by one-half. It's for this reason, coupled ith halving or doubling shutter speed, we have EV tables. Tell me how your theory works with a 400 mm lens, when the widest aperture may be one or one and one half inches. Come on, it's optics [calculations from Physics] that controls all of this.
|
|
|
|
||
|
Jon Close |
Ziad, As stated in the above responses, 50mm normal, 28mm wide angle, 28-80 wide angle zoom refer to the focal length of the lens. A lens with a single focal length is a fixed or prime lens. A lens that can vary the focal length (28-80) is a zoom lens. The terms "normal," "wide-angle," "telephoto" refer to the relative angle of view given by the lens. A normal lens gives a view comparable to human vision (exluding peripheral vision). Shorter focal lengths give a wider angle of view; telephotos give a narrower view and act like a binocular or telescope in seeming to bring distant objects closer. What constitutes normal, wide or tele depends on the size of the film frame or recording sensor. Typically, a normal lens will have focal length equal to the length of the film frame diagonal. In 35mm film, with a image size of 24mm x 36mm, the diagonal is 43.3mm (though 50mm has become the standard). For medium format (120/220 roll film) cameras, the normal lens focal length is between 75mm to 110mm, depending on whether the camera is 6x4.5, 6x6, 6x7, or 6x9 format. Most digital point & shoot cameras have very small sensors. A 1/2" sensor measures just 4.8mm x 6.4mm. A normal lens for it would have focal length of just 8mm. Because there is wide variance in digital sensor sizes, and because of the dominance of the 35mm film format, many digi-camera specs give the focal length as a "35mm film equivalent" so that consumers have an idea of the relative view given. Focal length is a theoretical concept based on a single lens element with no thickness. Actual camera lenses are very complex constuctions using as few as 3 and as many as 24 separate lens elements, and may be physically longer or shorter than the designated focal length. For example, on a 35mm SLR camera, lenses with focal lengths of 14mm to 35mm are common, even though the closest a lens element can be to the film plane is 40mm-45mm because of the space for the mirror. Lenses typically carry another measurement in mm. A single number with the Ø symbol, eg. "Ø58mm" refers to the diameter of the screw threads at the front of the lens for attaching filters or other accessories.
|
|
|
|
||
|
Ziad H. Dabash |
I get confused heir
|
|
|
|
||
|
Ziad H. Dabash |
so¦let¦say¦i¦take¦a¦shot¦using¦lense¦80-200mm¦i¦take¦photo¦shot¦on¦80¦and¦i¦take¦the¦same¦view¦on¦200m¦what¦will¦be¦the¦result?
|
|
|
|
||
|
Ziad H. Dabash |
so¦let¦say¦i¦take¦a¦shot¦using¦lense¦80-200mm¦i¦take¦photo¦shot¦on¦80¦and¦i¦take¦the¦same¦view¦on¦200m¦what¦will¦be¦the¦result?
|
|
|
|
||
|
Ziad H. Dabash |
so¦let¦say¦i¦take¦a¦shot¦using¦lense¦80-200mm¦i¦take¦photo¦shot¦on¦80¦and¦i¦take¦the¦same¦view¦on¦200m¦what¦will¦be¦the¦result?
|
|
|
|
||
|
Jon Close |
>>"So let say I take a shot using lense 80-200mm. I take a photo shot on 80mm and I take the same view on 200mm. What will be the result?"<< Assuming you took both pictures from the same spot, the one taken with 200mm lens would appear to be simply an enlargement of a portion of the photo taken at 80mm.
|
|
|
|
||
- Gregory LaGrange Contact Gregory LaGrange Gregory LaGrange's Gallery |
That's funny, what are those lines for? Anyway, small numbers mean a wide angle view and the subject looks farther away than it is.
|
|
|
|
||
|
This old forum is now archived. Use improved Forum here
Report this Thread |
||