Zoom Lens and Aperture

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to me,
so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 - 5.6 and
the other is the 24-70 2.8. I tested the lens and sure enough the
18-200 lens when set to an aperture of 3.5 changed to 5.6 at the other
end of the zoom, but any aperture of 5.6 or smaller stayed at the same
aperture through out the zoom. And the other lens stayed at the same
aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?

--
Sheila
http://swdalton.com

On Sun, 30 Nov 2008 22:18:48 -0500, Sheila
wrote:


In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to me,
so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 - 5.6 and
the other is the 24-70 2.8. I tested the lens and sure enough the
18-200 lens when set to an aperture of 3.5 changed to 5.6 at the other
end of the zoom, but any aperture of 5.6 or smaller stayed at the same
aperture through out the zoom. And the other lens stayed at the same
aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?


Engineering. The 18-200 is normal, with the 24-70 the designers made
changes to the lens that permits the aperture to change with the focal
length, thus presenting the same 1/f ratio.

"Sheila" wrote in message
...

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to me, so
I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 - 5.6 and the
other is the 24-70 2.8. I tested the lens and sure enough the 18-200 lens
when set to an aperture of 3.5 changed to 5.6 at the other end of the
zoom, but any aperture of 5.6 or smaller stayed at the same aperture
through out the zoom. And the other lens stayed at the same aperture
through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?

--
Sheila
http://swdalton.com

In order for a lens to have a specific speed (f-ratio) it requires that the
lens have a front element diameter of a minimum size. For instance if you
want 200mm at f4, you need a front element clear diameter of 50mm, 200/4 =
50mm. So, 200mm f3.5 would need a front element 57mm in diameter. However,
when a zoom is designed, they opt to make the speed drop as it is zoomed to
save size, weight and cost. If the optical designs have this freedom, the
lens costs less to make and sell. An 18-200mm zoom f2.8 zoom (if it is
possible at all with current lens technology) would be very expensive, in
the many thousands of dollars. This is because more than the rule of
f-ratio comes into play when such a long zoom range is implemented. In the
case of the 18-200mm zoom, front element may well be over 50mm in size, but
the zoom at the 200mm end might not reach a speed faster than f5.6 because
of internal design requirements. The need to control aberrations (and cost)
outweighs the need for maintaining a fixed speed for the full zoom range and
it does not permit the entire unobstructed front aperture to be used.

If you look at long prime lenses (300mm f4.0 for e.g.) you'll see the front
element generally matches the rule, being about 300/4 = 75mm across. The
larger the element(s) the higher the cost, it is exponential, so a 300mm
f2.8 costs $5000 and has a front element at least 102mm in size (along with
large internal elements) while a 300mm f4.0 costs only $1200.
If the price rule holds due to the complexity of long range zooms, you could
expect an 18-200mm zoom with a fixed f-ratio of 4.0 to cost at least
$3500.00, a jump of over $2900 for a modest 1 stop gain on the long end.
The zoom would also be larger and heavier.

Sheila wrote:

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to
me, so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 -
5.6 and the other is the 24-70 2.8. I tested the lens and sure
enough the 18-200 lens when set to an aperture of 3.5 changed to 5.6
at the other end of the zoom, but any aperture of 5.6 or smaller
stayed at the same aperture through out the zoom. And the other
lens stayed at the same aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?


It's cheaper to make zoom lenses with a varying f/number.

David

On Sun, 30 Nov 2008 22:18:48 -0500, Sheila wrote:

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to me,
so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 - 5.6 and
the other is the 24-70 2.8. I tested the lens and sure enough the
18-200 lens when set to an aperture of 3.5 changed to 5.6 at the other
end of the zoom, but any aperture of 5.6 or smaller stayed at the same
aperture through out the zoom. And the other lens stayed at the same
aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?

All you need to do is to think about it .

--
Neil
reverse ra and delete l
Linux user 335851

Sheila wrote:

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to me,
so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 - 5.6 and
the other is the 24-70 2.8. I tested the lens and sure enough the
18-200 lens when set to an aperture of 3.5 changed to 5.6 at the other
end of the zoom, but any aperture of 5.6 or smaller stayed at the same
aperture through out the zoom. And the other lens stayed at the same
aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?


The actual lens part that sets the aperture is usually deep inside the
lens. It is not the front nor the rear element, but rather usually one
that sits near the optical center of the lens. There may not even be a
piece of glass close to the point where the aperture diaphram is
located, but there will be a lens surface whose outer diameter optically
provides the limit.

In a zoom lens the lenses inside the camera are moving around a lot as
you zoom. It is very hard to control the element that sets the aperture
to both stay near the effective optical center, and yet fulfill all the
other things it must do. So it is actually quite hard to design one of
the lenses that does NOT change aperture with zooming. Only modern lens
design software makes it practical to do so.

Sheila wrote:

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to me,
so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 - 5.6 and
the other is the 24-70 2.8. I tested the lens and sure enough the
18-200 lens when set to an aperture of 3.5 changed to 5.6 at the other
end of the zoom, but any aperture of 5.6 or smaller stayed at the same
aperture through out the zoom. And the other lens stayed at the same
aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?




Thanks everyone for the explanations.

--
Sheila
http://swdalton.com

Sheila wrote:

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to me,
so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 - 5.6 and
the other is the 24-70 2.8. I tested the lens and sure enough the
18-200 lens when set to an aperture of 3.5 changed to 5.6 at the other
end of the zoom, but any aperture of 5.6 or smaller stayed at the same
aperture through out the zoom. And the other lens stayed at the same
aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?


Sheila, you have mis-identified the poster. It wasn't me.

Allen

Allen wrote:
Sheila wrote:

In another thread Allen pointed out that one zoom lens stays at 2.8
aperture and another changes throughout the zoom. This was news to
me, so I got out 2 of my Nikon lenses. One is the 18-200 zoom 3.5 -
5.6 and the other is the 24-70 2.8. I tested the lens and sure enough
the 18-200 lens when set to an aperture of 3.5 changed to 5.6 at the
other end of the zoom, but any aperture of 5.6 or smaller stayed at
the same aperture through out the zoom. And the other lens stayed at
the same aperture through the zoom.

This baffles me as to why this happens. Does anyone have a SIMPLE
explanation?


Sheila, you have mis-identified the poster. It wasn't me.

Allen


I'm sorry.

--
Sheila
http://swdalton.com