# Fixed Aperture Zoom Lens

Discussion in '35mm Cameras' started by Mike - EMAIL IGNORED, Nov 11, 2003.

1. ### Mike - EMAIL IGNOREDGuest

As I understand it, the minimum f-stop of a lens is equal
to the focal length divided by the diameter of the lens.
For a given diameter of the lens, the longer the focal
length, the larger the minimum f-stop. This is why most
zoom lenses have a range of minimum f-stops.

However, there are some zoom lenses for which the minimum
f-stop is constant. On the face of it, it would appear
that to accomplish this, the minimum f-stop would be
specified for the longest focal length, and for shorter
focal lengths, some potential lower f-stop capability is
wasted.

I imagine that there is something wrong with this
reasoning. Illumination would be much appreciated.

Mike.

Mike - EMAIL IGNORED, Nov 11, 2003

But that is exactly how it is done. At shorter focal lengths the aperture is
limited.

3. ### street shooterGuest

The f-stop is the calculation of the focal length divided by the
diaphram opening. For sake of simplicity lets condider a 50mm f/2
lens. The maximaum opening of the aperture diaphram is 25mm (50mm / 2
= 25mm). A variable aperture zoom can allow for the same physical
size of maximum aperture diaphram opening in terms of absolute
measurement, but the maximum f-stop will consequently decrease (move
to a higher f-stop number) as the focal length increases. A fixed
aperture zoom requires a more complex design whereby the physical
measurement of the diaphram opening must increase as the focal length
increases. For example, a theoretical 50-100mm f/2 zoom lens would
require the maximum physical aperture diaphram opening to change from
25mm at the minimum end of the focal length range to 50mm at the
maximum end of the focal length range, and also to change at a
constant rate throughout the entire zoom range. Now, if you take that
same theoretical lens and make it a 50-100 f/2-4 variable aperture
zoom, then you'd only need one maximum physical aperture diaphram
opening of 25mm.

Michael

street shooter, Nov 12, 2003
4. ### Mike - EMAIL IGNOREDGuest

If I understand this, you are agreeing that, by virtue of the
more complex design, glass is being wasted at the longer
focal lengths. Why is this good?

Mike.

Mike - EMAIL IGNORED, Nov 12, 2003
5. ### brianGuest

Fixed aperture zooms have been available for more than forty years,
are straightforward to design, and do not require variable diameter
diaphragms. Most early zooms were required to have fixed f/#'s
because of metering considerations. The classic P-N-(P or N)-P
configuration *does not* require a variable diaphragm opening to
maintain a constant f/#. All that is required is to keep the
diaphragm stationary with respect to a rear group which is not moved
during zooming. This basic design has been reworked literally
thousands of times for all manner of applications, ranging from huge
TV broadcast zooms to 35mm zooms to camcorder zooms.

To see what I'm talking about, consider a ray diagragm for the
50-135mm f/3.5 Nikkor:
http://caldwellphotographic.com/50135Layout.jpg

Brian
www.caldwellphotographic.com

brian, Nov 12, 2003

It is not being wasted. The lens design is optimized for the limited
aperture.

7. ### David LittlewoodGuest

I think some of the replies you have received have been misleading.
There is no "wastage" of usable aperture involved in fixed-aperture
zooms.

Whether a zoom lens has a fixed or variable aperture depends on the
design. Most modern designs achieve smallness and lightness by a system
known as "mechanical compensation", which moves various elements axially
using complex mechanical cams. This has the effect that the aperture
stays the same physical size, and hence the f-number varies as the focal
length varies.

Other designs (which AIUI included the earliest traditional designs) use
a system of optical compensation, and in these designs the aperture is
located in a different place; this varies in effective size as the lens
is zoomed, in proportion to the change of focal length, and the effect
is a constant f-number. These zooms tend to be larger and heavier. In
the days of external metering and manual flash metering they were much
preferred (for obvious reasons) but now the mechanical compensated types
are cheaper to make and the optical type seem to be mainly used in
top-level pro zooms where the size is more acceptable.

If you want more detail, try "Applied Photographic Optics" (Sydney F
Ray), though I must say I found the explanation in there rather lacking
in detail.

David Littlewood, Nov 12, 2003
8. ### Don StaufferGuest

A better definition is that the f/# is the focal length divided by the
aperture diameter, not neccesarily a 'lens' diameter. The actual
limiting aperture is generally INSIDE the lens, and depending on its
position may not be the actual physical size of an aperture. If the
physical aperture is not at a given point in the lens, the effective
value for the aperture may be a magnification (or minification) of the
actual diaphram diameter, and this value may change as lens is zoomed.
The actual value for the aperture is a complicated story.

Don Stauffer, Nov 12, 2003
9. ### brianGuest

Hi David:
Mechanical compensation in a zoom isn't directly related to whether it
has a fixed relative aperture or not. The 50-135mm Nikkor I mentioned
above is a good example of a simple mechanically compensated zoom
which has a constant f/#. All the mechanical compensation does is
maintain focus by moving a small compensator group without having to
move the entire lens.

Optically compensated zooms are pretty much completely obsolete, and
were used back in the days when designers had a mental block against
the use of cams. Part of this mental block was due to the belief that
mechanically compensated systems would inevitably result in "holes" in
the zoom range where certain focal lengths were unattainable due to
the compensator crashing into adjacent elements. Proper design
techniques have eliminated this problem, however.

Brian
www.caldwellphotographic.com

brian, Nov 13, 2003
10. ### David LittlewoodGuest

Hi Bryan,

In which case it seems Ray is out of date - this does not surprise me, I
did not find the explanation in there either clear or convincing.

I guess the point is that some techniques place the aperture in a
position where it gives a constant f-number, and some don't, which is