I use to think how many X zoom is my 18-55 mm lens or my 22-250 mm telephoto zoom. But I never bothered to search it until today. And finally spent some time to find answer for it.
I found these details from yahoo answers . Hope you will find it useful too.
If it is a zoom lens, you divide the long length by the short length and
you get the "x" value. 18-200 would be about an 11x zoom. 200 divided
by 18 equals 11.11.
Zoom Power
You don't really need a website to do this conversion, as it is 3rd
grade math. Just divide the little number into the big number and you
get your "X" factor.
An 18-55 zoom is a 3X zoom. A 70-210 zoom is also a 3X zoom. How can
this be? You know that a 200 mm lens wold give you a much larger image
than a 55 mm lens, right? The thing is, with a non-SLR camera, we tend
to almost never pay attention to the focal length of the lens. It would
be incredibly confusing if we tried to anyway! Image magnification is a
direct function of the sensor size. There are many different sensors
out there and without having some standard reference, comparing focal
lengths would also become meaningless. This is why everyone still
refers to the "35 mm equivalent."
It seems like most point and shoot cameras with a zoom lens start out in
the area of 28-or-35 mm. Let's just choose 35 mm for this example. If
the camera has a 4X zoom, it would be a 35-140 equivalent. 140 mm is a
moderate telephoto. Suppose it starts at 28 mm, though. This would be
a 28-112 equivalent. 112 mm is also a moderate telephoto, but hardly
impressive. It would seem that a 5X telephoto would be "better," but
the 5X zoom starting at 28 mm would yield exactly the same 140 mm as the
first example.
In other words... The "power" expressed in terms of [some number]X
doesn't really mean that much. This is especially true in digital
SLR's. What matters is the actual focal length of the lens.
Fortunately - at least for sake of comparison - the majority of DSLR's
have pretty much the same magnification factor and that is around 1.5 to
1.6. It makes direct comparisons a lot easier, but you have to develop
a sense of what constitutes a "long" vs. a "moderate" telephoto.
Unless and until we have a major paradigm shift, it will still be
helpful to think in terms of 35 mm equivalents when you are buying a
lens. This is the "language" that we speak, using 35 mm as the de facto
standard. Focal length comparisons make more sense that expressing the
magnification by "zoom power."
Here's a chart that I'm making up just to get you started. It is in 35
mm equivalents. If you are putting a lens of this focal length on most
DSLR's, you would have to do some math. Multiply the numbers I gave by
.67 if the camera has a 1.5 "lens factor" or by .625 if it has a 1.6
factor. For example, in my list, I say that 50 mm is the "normal" lens
for 35 mm cameras. If the DSLR has a factor of 1.5, this would mean
that a 33.5 mm lens would be "normal" for that DSLR.
Ultra Wide - 10-20
Wide Angle - 24-35
Normal - 45-55 (50 mm is the accepted "normal")
Medium Tele - 85-135
Telephoto - 150-300
Super Tele - 400-600
Okay, now we need to know what your camera is before really assigning a "power" to a 300mm lens. If you have a 35mm film camera
or a full format digital camera, a 50mm lens is roughly the same angle
of view as your regular vision, so we call that a "normal" or "1X" lens.
If you have a cropped sensor (APS-C, like Nikon D5000 or Canon T1i,
etc.), a 35mm lens is the "normal." This means that a fixed 300mm lens
would be about a 6X view on a full frame camera and about an 8.6X view
on an APS-C sensor.
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