Lens mm numbers

Discussion in 'Digital Photography' started by Povl H. Pedersen, Dec 22, 2003.

1. Povl H. PedersenGuest

I have what I hope is a simple question:

A 75-300mm zoom lens is listed as having 4x magnification.

But, I would imagine that the 300mm / 18mm = 16.7x the
magnification of the 300D kit lens in wide mode. Is this
correct ?

And the 75mm = 4x zoom.

Povl H. Pedersen, Dec 22, 2003

2. PhotoManGuest

"Povl H. Pedersen" <> wrote in message
news:...
> I have what I hope is a simple question:
>
> A 75-300mm zoom lens is listed as having 4x magnification.
>
> But, I would imagine that the 300mm / 18mm = 16.7x the
> magnification of the 300D kit lens in wide mode. Is this
> correct ?
>
> And the 75mm = 4x zoom.

A 75-300mm lens is said to have a 4X "Zoom range", not "magnification".
The 4X as related to focal length normally refers to the magnification
compared to a "Normal" 50mm lens. For example, your 75-300 would be said to
have 1.5X to 6X magnification.
Joe Arnold

PhotoMan, Dec 22, 2003

3. Joseph MeehanGuest

Those numbers get kicked around a lot. It is wise to find out exactly
what the writer means.

A zoom lens will have a max power which means objects will appear about
X times larger/closer at the max zoom extension than at "normal," "Normal"
is generally defined as the diagonal measurement of the recording media.
This number may be different for the same lens used on different cameras.

A zoom lens also will have a ratio the shortest to longest focal length.

The advertising guys could care less and use the facts as needed without
much regard for truth.

--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math

"Povl H. Pedersen" <> wrote in message
news:...
> I have what I hope is a simple question:
>
> A 75-300mm zoom lens is listed as having 4x magnification.
>
> But, I would imagine that the 300mm / 18mm = 16.7x the
> magnification of the 300D kit lens in wide mode. Is this
> correct ?
>
> And the 75mm = 4x zoom.

Joseph Meehan, Dec 22, 2003
4. Jim TownsendGuest

Povl H. Pedersen wrote:

> I have what I hope is a simple question:
>
> A 75-300mm zoom lens is listed as having 4x magnification.

No.. It's listed as having 4x zoom.. not 4x magnification. It's important to
differentiate between 'zoom' and 'magnification'

When they refer to zoom, they mean you can change the focal length (zoom) from
75mm to 300mm... In other words, 300 is 4X greater than 75. (4x75=300).
That's what the X is describing.. The difference between maximum and minimum
focal length.

This is NOT the magnification power. A 30mm to 300mm lens would have a 10X
zoom rating, but would not bring anything closer than the 75 to 300mm lens
with a 4X zoom rating since the maximum focal length in both is the *same*.

> But, I would imagine that the 300mm / 18mm = 16.7x the
> magnification of the 300D kit lens in wide mode. Is this
> correct ?
>
> And the 75mm = 4x zoom.

No.. It's generally accepted that for a 35mm film camera lens, a 50mm lens most
closely approximates what our eyes see. By dividing the focal length by 50,
we can get a ballpark estimate of the actual magnification.

300mm / 50mm = 6 So a 300mm lens would have a magnification factor of 6X

With the 300D's 1.6 crop factor, the 'effective' focal length is 480mm... So
you could say that a 300mm lens on a Digital Rebel is roughly the same as
using a 9.6X telescope.

Jim Townsend, Dec 22, 2003
5. Paolo PizziGuest

Povl H. Pedersen wrote:

> I have what I hope is a simple question:
>
> A 75-300mm zoom lens is listed as having 4x magnification.

You are mistaking zoom range for magnification.

A 75-300 zoom has a magnification factor ranging from
1.5x to 6x, 50mm being the de facto 1:1 standard (i.e.
what the human eye sees. In reality, it should be 42mm.)
This is, of course, assuming that your camera is a 35mm
format.

Paolo Pizzi, Dec 23, 2003
6. Povl H. PedersenGuest

On 2003-12-22, Jim Townsend <> wrote:
> Povl H. Pedersen wrote:
>
>> I have what I hope is a simple question:
>>
>> A 75-300mm zoom lens is listed as having 4x magnification.

>
> No.. It's listed as having 4x zoom.. not 4x magnification. It's important to
> differentiate between 'zoom' and 'magnification'
>
> When they refer to zoom, they mean you can change the focal length (zoom) from
> 75mm to 300mm... In other words, 300 is 4X greater than 75. (4x75=300).
> That's what the X is describing.. The difference between maximum and minimum
> focal length.
>
> This is NOT the magnification power. A 30mm to 300mm lens would have a 10X
> zoom rating, but would not bring anything closer than the 75 to 300mm lens
> with a 4X zoom rating since the maximum focal length in both is the *same*.
>
>> But, I would imagine that the 300mm / 18mm = 16.7x the
>> magnification of the 300D kit lens in wide mode. Is this
>> correct ?
>>
>> And the 75mm = 4x zoom.

>
> No.. It's generally accepted that for a 35mm film camera lens, a 50mm lens most
> closely approximates what our eyes see. By dividing the focal length by 50,
> we can get a ballpark estimate of the actual magnification.
>
> 300mm / 50mm = 6 So a 300mm lens would have a magnification factor of 6X
>
> With the 300D's 1.6 crop factor, the 'effective' focal length is 480mm... So
> you could say that a 300mm lens on a Digital Rebel is roughly the same as
> using a 9.6X telescope.

Thanks, this helped a lot. I always assumed 35mm = 1:1, and that
digicams with 3x zomm would actually magnify 3 times what you normally see.
I had a feeling this was not true, and now I have that confirmed.

My 1200 mm scope will thus always have a zoom factor of 1x and
a magnification of 1200/50 = 24x at f/13.3 (since the opening is
90mm in diamter). And on the EOS 300D it magnifies 1.6x more,
so giving 40x magnification.
Using it visually, I am using 25x - 75x zoom.

Povl H. Pedersen, Dec 23, 2003
7. Don StaufferGuest

Yes, the 4X is the zoom range. Ad copy writers are not optical
engineers.

"Povl H. Pedersen" wrote:
>
> I have what I hope is a simple question:
>
> A 75-300mm zoom lens is listed as having 4x magnification.
>
> But, I would imagine that the 300mm / 18mm = 16.7x the
> magnification of the 300D kit lens in wide mode. Is this
> correct ?
>
> And the 75mm = 4x zoom.

--
Don Stauffer in Minnesota

webpage- http://www.usfamily.net/web/stauffer

Don Stauffer, Dec 23, 2003
8. Paolo PizziGuest

Povl H. Pedersen wrote:

> My 1200 mm scope will thus always have a zoom factor of 1x and
> a magnification of 1200/50 = 24x at f/13.3 (since the opening is
> 90mm in diamter). And on the EOS 300D it magnifies 1.6x more,
> so giving 40x magnification.

Not exactly. The 1.6x is a CROP factor, not actual magnification.
It means the 300D will cover a (smaller) field of view, equal to 1.6
times that of a 35mm camera.

Paolo Pizzi, Dec 23, 2003
9. Bryan OlsonGuest

Povl H. Pedersen wrote:
[...]
> Thanks, this helped a lot. I always assumed 35mm = 1:1, and that
> digicams with 3x zomm would actually magnify 3 times what you

normally see.
> I had a feeling this was not true, and now I have that confirmed.
>
> My 1200 mm scope will thus always have a zoom factor of 1x and
> a magnification of 1200/50 = 24x at f/13.3 (since the opening is
> 90mm in diamter). And on the EOS 300D it magnifies 1.6x more,
> so giving 40x magnification.
> Using it visually, I am using 25x - 75x zoom.

Sort-of-but-not-really. Photographers rarely talk about the 'X'
or 'power' of lenses. The terms are commonly associated with
microscopes, telescopes and binoculars, in which case they
describe a property that photographic lenses do not have.

Microscopes, telescopes and binoculars create virtual images
viewable through the eyepiece. A magnification power of 10X
means that the virtual image viewed through the eyepiece
subtends an angle 10 times greater than does the object when
viewed directly.

The mapping between focal length and this usage of 'power' is
indirect at best. For example, the magnification power of a
telescope is the focal length of the objective divided by the
focal length of the eyepiece.

The words have more than one meaning. 'Magnification' is
important in macro work, and means the size of the image divided
by the size of the object. Magnification depends on not only
the focal length of the lens, but also how far away the object
is.

The optical 'power' of a lens is the reciprocal of the focal
length. The unit of power is the diopter, which is one divided
by a meter. Photographers do not commonly employ the measure,
and it's not the same meaning as the power of a scope.

--
--Bryan

Bryan Olson, Dec 24, 2003