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Easy Question For Amateurs!

Tumeg101

Tumeg101

macrumors 6502a
Original poster
Jun 30, 2007
523
0
Orange County, California
I was looking at lens'
and was wondering what the "20mm" is referring to, in the lens name..
Like maybe for a macro lens, "105mm" what does this mean, and what is it referring to?
 
Lovesong

Lovesong

macrumors 65816
Sep 15, 2006
1,137
1
Stuck beween a rock and a hard place
Focal length. You can think of it as how "zoom'ed" in you can be.

Normal vision in humans is considered to be 50-70mm. At 35mm you're talking about your entire field of vision, including peripheral. Anything more (over 70) is considered telephoto, and works like binoculars for the camera.

When you're looking at DSLR lenses, remember that unless you're using a high end Canon, or are purchasing the D3, you will need to take that focal length on the lens, and multiply it by either 1.6 (canon) or 1.5 (nikon, sony, etc) ((oh, and Olympus and Panasonic, you'd multiply by 2). So to clarify that, if you have a Nikon camera, which the 105 macro is used by, you'd be looking at a ~150mm macro lens.

I can explain to you where the focal length mm terms come from, but you probably don't care, and will likely confuse you.
 
A

andmill

macrumors member
Dec 10, 2006
45
0
basically the lower the number the farther out its going to be zoomed.
Example:
anything less then about 20 mm is fisheye
20-70 is for normal pictures
70+ is getting into macro or telephoto
this is an over simplification but i think thats what your looking for
 
S

seniorstinky

macrumors regular
Feb 22, 2007
121
1
Phoenix, AZ
andmill said:
basically the lower the number the farther out its going to be zoomed.
Example:
anything less then about 20 mm is fisheye
20-70 is for normal pictures
70+ is getting into macro or telephoto
this is an over simplification but i think thats what your looking for
Click to expand...

Just a clarification that macro is independent of focal length (although I don't remember seeing any macro lenses less than 50 mm)
 
epicwelshman

epicwelshman

macrumors 6502a
Apr 6, 2006
810
0
Nassau, Bahamas
andmill said:
basically the lower the number the farther out its going to be zoomed.
Example:
anything less then about 20 mm is fisheye
20-70 is for normal pictures
70+ is getting into macro or telephoto
this is an over simplification but i think thats what your looking for
Click to expand...

Yeah, macro capabilities doesn't determine focal length. And less than 20 isn't fisheye. Sure, a wide angle may become somewhat distorted, but a fisheye is an actual specialised lens.
 
Lovesong

Lovesong

macrumors 65816
Sep 15, 2006
1,137
1
Stuck beween a rock and a hard place
furious said:
Could you please do that. I would like to know?
Click to expand...

OK. But you asked for it.

The focal length of a lens is the distance between the central focal point on the medium that you are recording on (film, sensor), from the optical center of a lens, when that lens is focused to infinity.

Naturally, different media give you different focal lengths for a specific representation of an image (medium film is different from APS is different from 35mm), but mainly due to the popularity of 35mm film, most manifacturers have adopted the notion of representing the focal length as a 35mm-equivalent. So next time you go to Circuit City, pick up an ultra compact and look at the specs on it. Then look at the lens. It will say something like 4.5-5.6/ 6.33-19.6. This is a 3X optical zoom camera, which has a lens with focal length of 6.33mm to 19.6mm. Because the camera uses such a small sensor size, this ultra wide lens is equivalent to about 35mm to about 110mm or so in 35mm-equivalent.

Is that clear as mud?
 
epicwelshman

epicwelshman

macrumors 6502a
Apr 6, 2006
810
0
Nassau, Bahamas
Lovesong said:
OK. But you asked for it.

The focal length of a lens is the distance between the central focal point on the medium that you are recording on (film, sensor), from the optical center of a lens, when that lens is focused to infinity.

Naturally, different media give you different focal lengths for a specific representation of an image (medium film is different from APS is different from 35mm), but mainly due to the popularity of 35mm film, most manifacturers have adopted the notion of representing the focal length as a 35mm-equivalent. So next time you go to Circuit City, pick up an ultra compact and look at the specs on it. Then look at the lens. It will say something like 4.5-5.6/ 6.33-19.6. This is a 3X optical zoom camera, which has a lens with focal length of 6.33mm to 19.6mm. Because the camera uses such a small sensor size, this ultra wide lens is equivalent to about 35mm to about 110mm or so in 35mm-equivalent.

Is that clear as mud?
Click to expand...


I knew what this meant in terms of film/digital SLR's, but I always wondered why my P&S would give the exif as 8mm. I'd be thinking "that sure as hell is NOT 8mm!"

Nowwww I get it. Haha.
 
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