Focal Lengths - actual distances?


TPF Noob!
Jan 2, 2008
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Hi photolovers,

I have a noobie question regarding focal lengths.. I don't have any lenses to play with and compare. What can I get with a 18mm-70mm, or a prime 50mm or a 200mm ? I assume this tells me how far (or close) an object can be for focusing. What are the actual focal distances can I get with these lenses? How far away or how close do I need to be to use these focal lengths?


Ken :blushing:
Don't mean to sound like a real smart-a$$ but that all depends on the lens model, brand, quality level or if a special propose lens. It's all in the design and intent. The 50mm would approximate about your normal eyes field of view. (43mm is said to be closer, but 50mm is considered a normal lens in full frame 35mm) In full frame the 200 would approximate about a 4X magnification of your normal field of view. A 300 = 6X and so on.
the mm number is actually something to do with lens elements and the focal plane...but meh.

Focusing distance is determined not by that focal length... Some lenses are just better at it. Macro is your friend if that's what you're looking for.

Also, there's no real rule on "how far you can use" a lens. It depends on what effect you want.
Focal Length and focusing distant are two different things. I have a 100mm Macro lens (100mm being the focal length technically) that will focus really really close to things.
Maybe the best thing would be to go into a camera store and pick up a camera & lens. Look though the viewfinder and zoom the lens back and forth. This will instantly give you an idea of what each focal length will give you, in terms of Field of View (FOV).
The focal length is the point where the lense elements form an image or, in the case of photography the equivelenet length. It has nothing to do with the focusing distance.
So when they talk about telescopic lenses and zooming ability, this doesn't have to do with distance to the object and focusing ability? Hmmm..

I checked out the tamron site.. neat stuff. The focal length does what I thought it does.. it zooms! So if i want to be able to read someone's t-shirt from distance you need a higher focal length lens. But it depends on how far you are away from this t-shirt. That's what I'm trying to get at.

Sorry i realized that I may have the terms mixed up. Focal Length vs Focal Distance. Lenses, I assume are advertised with focal distance in mm, not the focal length...

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