Something I don't get about zoom lenses.

I've read that the magnification of a lens is calculated by
dividing it's larger zoom number by it's smaller zoom number.

If true, an 18 - 200mm lens has a magnification (not
considering crop factor) of 11.1...

But a 70 - 200mm lens would only have a maximum magnification
of less than 3 times?

Did I miss something?

If that's true, why would anyone buy a 70 - 200 in preference
to something that can better pull in distant objects?

Obviously, I haven't yet taken a photography course or this
would have been one of the first questions I'd have asked.

Any help is appreciated.

 

200/70 has nothing to do with magnification, it is the zoom range.
Magnification would be determined by the 200mm or the 70mm, the larger
the focal length number the greater the magnification. Some people
like a greater zoom range, it means fewer changes on lenses, fewer
lens purchases. Some people, like me, believe that lenses with very
great zoom range are not as good optically.

 

You're confusing zoom range with magnification. Zoom range is easy.
It's just the larger focal length / smaller focal length.

Magnification is a little more complex. For a telescope or binoculars,
it is the focal length / effective exit diameter, which is the
eyepiece focal length. That's all well and good because you are
looking right through the lenses at an object. With a camera, it's a
little different because it's projecting onto a sensor. You can take
the exact same lens and it will have different *effective*
magnification for cameras that have different sensor sizes. This is
true even though the actual magnification of the lens hasn't changed,
and is the focal length / effective exit diameter.

It gets a little more complicated for many zoom lenses with internal
focus that have a small min focus distance. Take the 18-200 as an
example. When you focus close, say less than 10', with the lens zoomed
out to 200mm, it isn't really a 200mm focal length. For close focus,
the lens compensates for the fact that it's length is restricted due
to internal focus by reducing the focal length. That lens can't get
much more than 140mm when the subject is close. At infinity focus, it
does go out to 200mm. So don't believe your zoom ring markings unless
the subject is far away.

I have a quick and dirty calculus for lens magnification which relates
to the "fact" that a 50mm lens on a 35mm camera is considered to be
about 1x magnification. You just take your focal length and divide by
50. Thus, a 200mm focal length would be a 4 power lens if you wanted
to relate it to something like binoculars magnification.

If you're shooting with an aps sensor that has a crop factor of 1.5,
just take whatever you get above and multiply it by 1.5. So on a Nikon
APS sensor, a 300mm lens would be a 1.5 * 300/50 = 9x lens and would
have the same magnification as 9x binocs.

There are more detailed analysis you can get into if you're really
interested. Things like the lens reproduction ratio, which is
dependent on subject distance and therefor ability to focus at
distance X. Just google reproduction ratio and lens focusing formulas
if you really want to get into it.

Steve

 

Meaning a picture taken at 200mm compared to one at 18mm is enlarged
11.1 times.
Think of it as zoom range.

You get the same magnification/view at 200mm. And lose the wide view
at 18mm.

Marketing concentrates on megapixels and zoom range (x10, x30 etc).
These matters least. Look at aperture before anything, sometimes
called the f number, the lower the better.
It means the lens will let a good amount of light in.

I guess you own a good camera with interchangeable lenses. Take some
shots in Av (aperture) mode at various settings.

After getting my camera with the kit 18-55mm lens I have rushed and
bought the 55-250mm. Then I got a 50mm f/1.8 prime (no zoom).
I use the 50mm most and 55-250mm least. I would suggest you get the
50mm prime first as it is the cheapest and a joy to use.


DanP

 

In 35mm terms, you can consider a 50mm lens to be essentially 1:1 and
on an APS sensor to be 1.5:1 or 1.5x magnification. On a 35mm sensor
for every 100mm, you are adding 2x. So a 200mm lens magnification
would be 4x a 300mm lens would be 6x. You simply multiply that
magnification by the sensor crop (1.5x, or whatever) and you have your
effective magnification. A 500mm lens on an APS (1.5 crop) sensor
would be: 500x2 = 10 x 1.5 (the crop) = 15x.
This is all more or less arbitrary, the only important thing being how
one focal length relates to another, 1000mm is 5 times the
magnification of 200mm, etc.

 

I suggest you stop reading that source then. The ratio of longest focal
length to shortest focal length is the zoom *range*.

The magnification of a lens depends on the format of the camera used.
But for a 35mm slide camera is approximately 50mm = 1x, and about 0.7x
that = 35mm for a typical digital SLR. For larger formats the lenses
have longer focal length for a given magnification and for small APS
point and shoots much shorter lengths and smaller sensors.

Not magnification zoom range.

Because there are extremely well optimised designs for zoom lenses that
over a 3x range work almost as well as a fixed length prime lens.

To get a 10x zoom range some design compromises on either vignetting,
edge/corner sharpeness, barrel distortion and lateral colour have to be
made. These days with software correction of certain aberrations on
digital cameras this compromise is not so bad as it was on film where
you were stuck with whatever image the lens put down.

People still use fixed focal length lenses too (zoom range = 1). Such
lenses can be optimised to be sharper than any zoom (although the
difference between zoom optics and fixed length lenses is much less now
than it was in the past). Zooms used to be pretty dire in the distant
past going soft at long lengths and curvy barrel effects at short.

If you are trekking then weight matters and one 10x zoom instead of a
rucksack full of glass might be quite tempting.

Regards,
Martin Brown

 

Thank you.

Now I'm gonna see if I can find the web page that tells people
that the magnification is the lenses' large number divided by it's
small number and thank the author for not knowing what he's talking
about.

I had to wonder, because it didn't make a lot of sense to me
to have various lenses ending at 200mm and a cornucopia of smaller
minimum numbers.

 

That's what I didn't get, because I did a search and some
idiot with access to the internet and web authoring software (actually
more than one) decided to tell everyone that a lenses' magnification
ability is calculated by dividing the lenses' maximum focal length by
it's minimum.

With an 18-200mm lens, it would work out to a bit more than
11x, if the camera has no "crop factor", and with a 70-200mm, it would
be less than 3 power. It struck me as idiocy that companies would
produce so many lenses with varying minimum focal distances, so I
decided to ask here... and am glad I did.

Oh yes. Once I found out about aperture settings, and checked
out the prices of F3.5 lenses vs. f2.8 lenses on ebay and other sites,
I became well aware of at least that difference.

Thanks for the explanation.

Now, to find the halfwit someone taught to use a computer...

 

If I start doing portraits a lot I'll get a prime lens with a
large aperture, but for now I want to be able to bring objects farther
away a bit closer and still have decent performance in low light.

Perhaps NASA will release some optics and electronics combos
that let us take shots in near total darkness with a 40x zoom in a
package you can carry around in your pocket...

 

[]

... likely you can already do something near to that, given a tripod and
long enough exposure.

David

 

Well, that's true to a degree.

The D3000 is noisy above 400 ISO, but I've taken a long
exposure at night in what I thought was very little light and it
showed a lot of things I was very surprised at.

I also managed to catch some lightning near the end of a 10
second exposure a while back and it was a nice shot.

 

I mean where you end up getting the image magnified 40
times...

 

Well I don't think I'll be buying one anytime soon if that's
the case.

So the "magnification" on the lens would not be 200mm divided
by 18 then, if I understand what you're saying.

Is there a formula then by which magnification can be
calculated?

And thanks for the info. This will help when I determine which
of the zooms I can afford will best suit my needs.

 

I think the term magnification is what is confusing things.
We could confuse this further by mentioning telescopes, binoculars and
microscopes

Then we could compare magnification with field of view too.

If the OP said what sort of things he wants to shoot then maybe some
examples
of what a specific lens would be best might more useful than these
definitions and calculations.

 

And just to make life arbitrarily more confusing some camera makers have
become totally inconsistent and describe digital camera lenses (crop
DSLRs and P&S cameras) with their lenses "notional" 35mm full frame
equivalent focal length. This makes comparisons much harder even for
people who know what they are looking at.

A workable heuristic definition for effective magnification is given by
focal length of lens divided by 1.5 x sensor diagonal length. It is
crude but close enough to be useful as a rough guide.

As SavageDuck says 50mm f.l. lens on a 35mm format is ~1x

Anything much longer than 300mm on a 35mm camera and you will also need
a decent tripod and/or image stabilisation.

Regards,
Martin Brown

 

I'm sure you don't really mean that either. Consider that an image of
a 6' man would be 240' high. What a camera![/QUOTE]


40X magnification would be 2000mm. Here is ~45X magnification (1500mm x
1.5 crop factor):

http://www.netaxs.com/~mhmyers/d80/DSC_1516w.jpg

 

That clears things up a bit. Thanks.

 

Whatever it is you have calculated, it is not magnification.[/QUOTE]


Ok then, what is it?

 

http://www.mystd.de/album/calculator/

http://www.dofmaster.com/digital_coc.html

 

I'm not quite sure. I don't understand what it is you think you are
doing.[/QUOTE]


If 50mm is 1x magnification then 1500mm is 30x and then add in the 1.5x
crop factor.

The lens states it is 1500mm.