I'm probably going to sound a littel dumg but i'm pretty new to photography and was wondering if someone could explain to me in simple terms what the number on a lense mean example 70-300mm?
That's the focal length of the lens. In this case, a zoom lens. Shorter lenses give wider fields of view, and longer lenses give narrower fields of view (more magnification when focused to the same distance).
A 300mm is a pretty significant magnification of whatever you are shooting. a 70mm lens a little longer than the "standard" 50mm lens. A standard lens is about an equivalent of what the human eye can see. So a 70-300 is a pretty commendable zoom although you are trading off some quality for the convenience.
thanks. i beging to understand. However if something is too close to the lense my camera will not take the picture do those numbers have anything to do with how close you can get (or how far away you need to be) from the object in order to take a picture?
Minimum focus distance is a different part of the equation. If you see the label "macro" on a lens it means it has very close minimum focus distance. High magnification lenses normaly have more problems focusing on close subjects though.
It's a bit more confusing because when we talk about longer focal lengths providing more magnification, stictly speaking we're assuming the lenses are at infinity focus. Exceptions to this "rule" happen all the time when we're focused closer.
For example, a 300mm lens will provide more magnification, when focused at infinity, than a 50mm lens. But if we focus on an object very close -- say, the minimum focus distance of each lens -- then it's possible that the 50mm lens will provide more magnification than the 300mm lens. This would happen if the minimum focus distance of the 50mm lens was substantially shorter than the minimum focus distance of the 300mm lens. In the extreme example, the 50mm lens could be a macro lens which provides 1:1 magnification. (1:1 magnification means the item being photographed will be the same size on film or on the camera sensor as it is in real life. i.e., a 1" object will take up 1" of film.) In that example, the 50mm lens is supplying substantially more magnification than the 300mm lens.
I understand the discussion above, but I have always wondered what the "mm" measurement in refering too. I understand that "mm" is just a specific measurement, I assume, as in millimeter, however I have been trying to understand how the number has any physical relation to the lens and why 50mm is about the same as a human eye.
Not sure if all that made sense, but basically looking for the background and history of using "mm" to describe the magnification of a lens. Any info would be appreciated.
mrchngsax wrote:
I understand the discussion above, but I have always wondered what the "mm" measurement in refering too. I understand that "mm" is just a specific measurement, I assume, as in millimeter, however I have been trying to understand how the number has any physical relation to the lens and why 50mm is about the same as a human eye.
It is millimeters, and it's the physical measurement of the focal length of the lens. The focal length is the distance behind a simple lens at which a distant object is rendered in sharp focus. By "simple lens," I'm referring to a single-element optic. Things are more complicated with multi-element lenses, but the basic principle applies.
I forgot to address the idea that 50mm is the same as a human eye.
First, that assumption is not based on the focal length of the human eye, which is actually much shorter than 50mm. When people say a 50mm lens matches the human eye, they're making two assumptions. First, that we're talking about a 35mm camera lens, and not a medium format, large format, or other format. The second assumption is that we're not talking about actual focal length, but the field of view covered by that focal length (50mm) on that particular format (35mm cameras).
Essentially what we're saying is that the field of view captured by a 50mm lens on a 35mm camera body is approximately the same as the field of view taken in by the human eye.
While this makes for a convenient rule of thumb, it's actually wrong on two counts. First, because the human eye doesn't take in a fixed field of view that's sharp across the entire field and then stops hard at a given width. It actually takes in only a very narrow field in truly sharp focus, a wider field in reasonable focus, and a very, very wide field which at the edges register no detail but can detect movement far outside the field of view of a 50mm lens.
The second reason it's wrong is more of a historical issue... the "normal" lens for the 35mm film format is actually about 43mm. Since 43mm is a relatively rare focal length for prime lenses, the nearest focal length that is common, 50mm, was adopted as the "normal" lens. It's an approximation, and it's reasonably close, but it's not really accurate.
