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Something I don't get about zoom lenses.

 
 
no_one_cares@whatbusterthinks.com
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      06-03-2011
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.
 
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charles
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      06-03-2011
On Thu, 02 Jun 2011 22:31:49 -0400, http://www.velocityreviews.com/forums/(E-Mail Removed)
wrote:

> 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.
 
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TheRealSteve
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      06-03-2011

On Thu, 02 Jun 2011 22:31:49 -0400, (E-Mail Removed)
wrote:

> 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.


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
 
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DanP
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      06-03-2011
On Jun 3, 3:31*am, (E-Mail Removed) wrote:
> * * * * 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...


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

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


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

> * * * * 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?


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.

> * * * * 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.


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
 
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RichA
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      06-03-2011
On Jun 2, 10:31*pm, (E-Mail Removed) wrote:
> * * * * 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.


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.
 
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Martin Brown
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      06-04-2011
On 03/06/2011 03:31, (E-Mail Removed) wrote:

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


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.

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


Not magnification zoom range.
>
> 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?



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.
>
> 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.


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
 
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no_one_cares@whatbusterthinks.com
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Posts: n/a
 
      06-05-2011
On Thu, 02 Jun 2011 20:05:00 -0700, charles <(E-Mail Removed)>
wrote:

>On Thu, 02 Jun 2011 22:31:49 -0400, (E-Mail Removed)
>wrote:
>
>> 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.


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.

>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.

 
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no_one_cares@whatbusterthinks.com
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Posts: n/a
 
      06-05-2011
On Thu, 2 Jun 2011 20:14:20 -0700, Savageduck
<savageduck1@{REMOVESPAM}me.com> wrote:

>On 2011-06-02 19:31:49 -0700, (E-Mail Removed) said:
>
>> 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.

>
>You have to talk differences in the lenses beyond zoom range, and also
>clarify some of your misconceptions.
>
>First, the two types of lenses you have named in your question are an
>18-200mm, an a 70-200mm. Both lenses have a maximum FL of 200mm, both
>have the same telephoto capability to "pull in distant objects."


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.

>
>Most manufacturers versions of 18-200mm lenses are general purpose,
>walk-around, do-it-all lenses, usually with maximum aperture range of
>f/3.5-f/5.6. The larger zoom range is a compromise for image quality,
>usually at the long end of the range, and sometimes at both ends of the
>range. For many it is a compromise they can live with in order to have
>a reasonably wide (18mm) and some reach (200mm) in one lens. The same
>is also true for some of the lenses with even wider zoom ranges such as
>the Sigma 18-250mm, and the Tamron 18-270mm at 15X. All of these are
>usually considered consumer quality lenses which the users are prepared
>to accept image quality compromises due to reasonable cost.
>
>Most 70-200mm lenses are Pro grade lenses, usually with a maximum
>aperture over the entire zoom range of f/2.8, which makes them useful,
>fast lenses for use in difficult lighting situations, including sports,
>weddings, portraits, etc. They are also among the sharpest lenses the
>various manufacturers build since they do not have the compromise of
>the wider range of the 18-200mm.
>The 70-200mm f2.8 will also have a premium price over the 18-200mm.


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...
 
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no_one_cares@whatbusterthinks.com
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Posts: n/a
 
      06-05-2011
On Fri, 3 Jun 2011 01:57:44 -0700 (PDT), DanP <(E-Mail Removed)>
wrote:

>On Jun 3, 3:31*am, (E-Mail Removed) wrote:
>> * * * * 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...

>
>Meaning a picture taken at 200mm compared to one at 18mm is enlarged
>11.1 times.
>Think of it as zoom range.
>
>> * * * * But a 70 - 200mm lens would only have a maximum magnification
>> of less than 3 times?

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

>
>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.
>
>> * * * * 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.

>
>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


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...
 
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David J Taylor
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      06-06-2011
<(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
[]
> 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

 
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