Lens - A crash course, please!

trillian · Oct 31, 2006

Hi All,

I just joined this forum because I have never been able to find the co-relation between mm and zoom. For example, I would like to buy the new Nikon D 80 which comes with 18-135mm AF-S DX Zoom lens. Now what type of zoom is this? 3x, 10x? How do I find this out? Is there any chart, that I can corelate too? Could someone give me crash course on lens?

Thanks,

Trillian

dsp921 · Oct 31, 2006

135/18 = 7.5, it's a 7.5x zoom.

trillian · Oct 31, 2006

Thanks for that Dave, but why did you divide 135 by 18? So, 28-70 mm would be, 70/18 = 3.8x or 70/28 = 2.5x ?

fmw · Oct 31, 2006

trillian said:
Thanks for that Dave, but why did you divide 135 by 18? So, 28-70 mm would be, 70/18 = 3.8x or 70/28 = 2.5x ?

It is the range of the focal lengths. Longest focal length/shortest focal length. The more extreme the zoom lens (higher X) the more compromises in design and the poorer the performance. 2X zooms are usually excellent. 10X zooms are usually (perhaps always) poor.

dsp921 · Oct 31, 2006

trillian said:
Thanks for that Dave, but why did you divide 135 by 18? So, 28-70 mm would be, 70/18 = 3.8x or 70/28 = 2.5x ?

The "x" zoom refers to the amount the lower number is multiplied to get to the the larger. The 28-70 would be 70/28 = 2.5, (28 x 2.5 = 70). That's where the 2.5x comes from.

Edit: Fred got in there while I was typing...He also makes a good point regarding zoom ranges.

trillian · Oct 31, 2006

Thanks Dave and Fred for your prompt replies!!

Trillian

mentos_007 · Oct 31, 2006

but....
what with for example 100-400 ? you have much bigger zoom with 400mm than with 100 and 400/100 = 4 ... only 4... so please do not think that 18-135mm will give you a bigger zoom than the 100-400.

xfloggingkylex · Oct 31, 2006

The most important thing to remember is that X zoom is all relative, and it is a measure of range more than what most people typically consider zoom (magnification).

Like the post about, the 100-400mm is a 4x zoom but will be able to take picture from much further away than the 18-135 7.5x zoom. I hope I explained that in laymans terms.

Also a 600mm prime lense (no zoom) will have a much tigher field of view (making things look closer) than any of the previously mentioned lenses.

dsp921 · Oct 31, 2006

mentos_007 said:
but....
what with for example 100-400 ? you have much bigger zoom with 400mm than with 100 and 400/100 = 4 ... only 4... so please do not think that 18-135mm will give you a bigger zoom than the 100-400.

Zoom and focal length are two different things. The 18-135 will give a bigger zoom (7.5 vs. 4). It has a shorter focal length (135 vs 400) and therefore won't provide the field of view (or reach) the 400mm will. Zoom (x) is not related to focal length (mm).

xfloggingkylex · Oct 31, 2006

reach, thats a good way to explain the focal length.

fightheheathens · Oct 31, 2006

on a full frame sensor or on film a 50 mm lens is equivalent to about what we see with the naked eye. so relative to your vision a 50 mm lens is about 1x. At 100 mm the magnification is 2x. at 150mm 3x, at 200mm 4x etc.
ive always assumed that when someone asks how much zoom does it have they were referring to how much it magnified an image relative to what you see with your eye.

digital sensors that are smaller have different magnification effects and to get the film equivalent you often have to multiply by 1.5 or 1.6 or something

xfloggingkylex · Oct 31, 2006

fightheheathens said:
on a full frame sensor or on film a 50 mm lens is equivalent to about what we see with the naked eye. so relative to your vision a 50 mm lens is about 1x. At 100 mm the magnification is 2x. at 150mm 3x, at 200mm 4x etc.
ive always assumed that when someone asks how much zoom does it have they were referring to how much it magnified an image relative to what you see with your eye.

digital sensors that are smaller have different magnification effects and to get the film equivalent you often have to multiply by 1.5 or 1.6 or something

while your idea is note worthy, when a lense is advertised as a 4x or anything like that it is more refering to the range the lense has, that is the difference between widest angle and the most telephoto angle.

for instance an 18-55mm lense has a zoom of 3x, because 55mm is about 3 times the magnification of 18mm.

markc · Oct 31, 2006

I think a lot of people will mistake "zoom" for telephoto. A zoom lens is just one that has a variable focal length (as opposed to a "prime", which has a single focal length). For 35mm film cameras, 45mm-55mm is considered "normal". Anything under 45mm is a wide-angle. Anything above 55mm is a telephoto. You can have a wide-angle zoom, like an 18mm-35mm.

This repeats what others have said, but the "x" just tells you how big a difference there is between the two ends of the zoom, which isn't terribly useful. The "mm" will tell you more about what kind of images you can get with the lens.