variable apeture

Can someone explain in idiot proof terms why in many zooms we have a change
in the aperture size (always getting larger) as we go from say, 75 to 300mm?
And yeah, I know its got something to do with optics and light. A little
more detail would be good.

regards

--
Don From Down Under

I am not sure of your question so I will give two answers.

First let's consider the fact that some zooms offer a larger f number at wide angle than at telephoto (like f3.5 - 4.0) I will suggest that all lenses do the same thing, but some limit the available aperture at the short end of the zoom.

I suspect that there may be three reasons. First you get to say you have a f4.0 lens and not explain the changing f-stop thing nor do your customers end up getting everything set only to find that they have to make exposure changes if they zoom, others would like to advertise the larger F3.5. Second I suspect that the quality of the image may suffer at wider settings if the full aperture was allowed. Last it may be the added cost to adjust the aperture as you zoom.

The other possibility is you are talking about the fact that at a fixed f-stop like f8.0 the size of the opening increases at longer zoom settings. The answer to that is it has to be bigger to allow the same amount of light to show on the film. Sort of like needing a higher wattage lamp in order to read the evening paper if you are setting across the room than if you are sitting right under the lamp. As you zoom to a longer setting the opening gets further away and needs to be larger to get the same amount of light to the film.


--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math


"Don" wrote in message ...
Can someone explain in idiot proof terms why in many zooms we have a change
in the aperture size (always getting larger) as we go from say, 75 to 300mm?
And yeah, I know its got something to do with optics and light. A little
more detail would be good.

regards

--
Don From Down Under

"Don" wrote in
:

Can someone explain in idiot proof terms why in many zooms we have a
change in the aperture size (always getting larger) as we go from say,
75 to 300mm? And yeah, I know its got something to do with optics and
light. A little more detail would be good.

It’s cheaper that way. The aperture is not a size of the opening, it is a
ratio of hole diameter divided by focal length. 25mm diameter hole at
100mm focal length would be f4 (Aperture of 1:4 or 1/4) whereas zooming to
300mm you would need an opening of 75mm to still have an aperture of f4.
The cost of f4 at 300mm is high due to the size of the front element and
the weight of glass involved.

For a good example you should check the price of the Sigma 120-300 f2.8
lens, that’s a heavy and expensive lens!




--
Mark Heyes (New Zealand)
See my pics at http://homepages.ihug.co.nz/~markh/
"There are 10 types of people, those that
understand binary and those that don't"

"Don" wrote in message
...
Can someone explain in idiot proof terms why in many zooms we have a
change
in the aperture size (always getting larger) as we go from say, 75 to
300mm?
And yeah, I know its got something to do with optics and light. A little
more detail would be good.
As you would be aware, as you zoom in, the image is magnified. The same
amount of light gets spread over a greater area of film, so each portion of
film is actually having less light hit it. Less light equals larger aperture
number, hence the lens's aperture size gets larger as you zoom in.

regards

--
Don From Down Under

In article ,
Don wrote:
Can someone explain in idiot proof terms why in many zooms we have a change
in the aperture size (always getting larger) as we go from say, 75 to 300mm?
And yeah, I know its got something to do with optics and light. A little
more detail would be good.

The aperture size doesn't change, the "f-number" does. When you see an
aperture refered to as "f/2", for example, the "f" is short for "focal
length", and the / means "divided by", so a 50mm f/2 lens has an aperture
diameter of 50mm divided by 2, which is 25mm.

If you use a zoom lens and zoom out, without changing the aperture size, the
f-number will change. An aperture that's 25mm wide is f/2 at 50mm, but the
same 25mm aperture is only f/4 at 100mm.

The reason we use f-numbers instead of real aperture sizes is that it
removes consideration of the focal length from exposure values. 1/60
seconds, f/8 and 100 ISO at 50mm is exactly the same exposure as 1/60
seconds, f/8 and 100 ISO at 100mm, for example, even though for the 100mm
case, the aperture opening is twice as wide.

"Don" wrote in message
...
Can someone explain in idiot proof terms why in many zooms we have a
change
in the aperture size (always getting larger) as we go from say, 75 to
300mm?
And yeah, I know its got something to do with optics and light. A little
more detail would be good.

Your question isn't entirely clear. Perhaps you mean the F-NUMBER gets
larger, not the aperture. Larger f-number = smaller relative aperture. Most
(not all) zoom lenses have their largest maximum relative aperture (lowest
f-number) at the short end which becomes somewhat smaller (higher f-number)
as the focal length approaches the long end.

However, for the same f-number the PHYSICAL aperture must increase in size
with the focal length, since that's what the number means: it's the ratio of
focal length to aperture size. So, for example, any given f-number at 300mm
means a PHYSICAL aperture four times larger than the same f-number at 75mm.
Perhaps this is what you mean.

It's the f-number, the RELATIVE aperture rather than the physical aperture
size, that determines the amount of light being effectively passed by the
lens.

Think of it this way: If you're reading a book while sitting near a window,
the window need not be very large to admit enough light for you. But the
farther away from the window you are, the larger the window would have to be
to deliver the same amount of light to where you are. The principle here is
similar. Going to a longer focal length effectively moves the "window"
farther away from the film (or CCD) plane.

"Don" writes:
Can someone explain in idiot proof terms why in many zooms we have a change
in the aperture size (always getting larger) as we go from say, 75 to 300mm?
And yeah, I know its got something to do with optics and light. A little
more detail would be good.

In order to have a constant f/number, the entrance pupil of the lens
would have to increase in size in proportion to the focal length change
during zooming. For example, f/2 requires a 37.5 mm entrance pupil at
75 mm focal length, but a 150 mm entrance pupil at 300 mm. For this to
happen while the actual aperture diaphragm remains the same size inside
the lens, you need a particular sort of optical design for the zoom, and
you need a front element that is at least 150 mm in diameter!

If the lens is allowed to change f/number during zooming, this gives
more freedom in lens design, and the resulting lens can be smaller and
lighter and cheaper. For still camera use, where the light metering is
done at the shooting focal length, this can work fine.

Constant-aperture zooms are still made, but they tend to be used for
more specialized purposes like movie cameras (where you will zoom with
the camera running).

Dave

Bingo! You win.

The question was clear, as was your answer.

Even more simply, the greater the lens is zoomed-out, the more light is
bounced around & lost. As a result, the maximum f/ will be less (Larger),
when at full zoom.

"Jimmy G" wrote in message
news:2LWDc.2914$151.1044@fed1read02...
Bingo! You win.

The question was clear, as was your answer.

Even more simply, the greater the lens is zoomed-out, the more light is
bounced around & lost. As a result, the maximum f/ will be less (Larger),
when at full zoom.

GUFFAW!

If he wasn't confused before, he ought to be by now.

"Dave Martindale" wrote in message
...
"Don" writes:
Can someone explain in idiot proof terms why in many zooms we have a
change
in the aperture size (always getting larger) as we go from say, 75 to
300mm?
And yeah, I know its got something to do with optics and light. A little
more detail would be good.

In order to have a constant f/number, the entrance pupil of the lens
would have to increase in size in proportion to the focal length change
during zooming. For example, f/2 requires a 37.5 mm entrance pupil at
75 mm focal length, but a 150 mm entrance pupil at 300 mm. For this to
happen while the actual aperture diaphragm remains the same size inside
the lens, you need a particular sort of optical design for the zoom, and
you need a front element that is at least 150 mm in diameter!

If the lens is allowed to change f/number during zooming, this gives
more freedom in lens design, and the resulting lens can be smaller and
lighter and cheaper. For still camera use, where the light metering is
done at the shooting focal length, this can work fine.

Constant-aperture zooms are still made, but they tend to be used for
more specialized purposes like movie cameras (where you will zoom with
the camera running).

Dave

Canon, Nikon, Tamron, Tokina and Sigma make a few constant aperture lenses,
not particularly specialized, but on the expensive side...

--
Skip Middleton
http://www.shadowcatcherimagery.com