f/stop on lenses

[rjpierrard]

<p>Hello,<br>

I understand the basics of f/stops and etc, but while looking at prospective lenses online the vast majority of those I come across are described as having "f/3.5-5.6" or "f.4.5-5.6."<br>

The thing the only pictures of lenses I`ve seen have them with ranges around f/2-11 or so, and most of the longer exposure photographs I`ve seen have f/stops between 11 and 22.<br>

So I`m wondering if the values listed in the product description describe the f/stop range they can produce, or if the number has to do with something else entirely.</p>

<p>Thanks for all your help!</p>

[john tonai]

<p>They are only listing the maximum aperture. The range is because they are zooms. The larger f/stop refers to the wider angle focal length. As the focal length gets longer, the maximum aperture gets smaller. Most zooms go down to f/16-32</p>

[mark_pierlot]

<p>The f/stop number of a lens refers to the smallest f/stop value, that is, the widest aperture, that the particular lens has. If there is a range of f-stops, that means that the lens is a zoom with its short end having the smallest value (widest aperture), the long end having the largest value (or narrowest aperture), and intermediate focal lengths having values in between. These are called "variable aperture" zooms; zooms that maintain one f/stop over their focal length ranges are better, albeit more expensive.</p>

[JAPster]

<p>When you see this on a lens, f/3.5-5.6" , that's telling you its a ZOOM lens, and that when the zoom is set to the widest view (wide angle), then the Max Aperture (largest opening) that lens can produce for you is f/3.5, and when you ZOOM on out to the maximum zoom point, then at that point the max aperture is f/5.6 which is a bit smaller. So this type label is telling you the <strong>Max Aperture spec</strong> for that particular zoom lens at each end of the available zoom range.</p>

<p>The lens will also have a <strong>working f-stop range</strong>, like f/3.5 out to f/22, something like that. The specifications of the lens should list that range of available apertures.</p>

[rjpierrard]

<p>aah, thank you very much :)</p>

[mart_e]

<p>AFAIK - and someone is sure to correct me if I'm wrong :)...</p>

<p>The physical aperture size does not change as you zoom. It is a physical feature of a particular lens.</p>

<p>But as the f number is a ratio of the aperture size to focal length (to keep the exposure calculations constant - ie longer focal length is gathering light from a smaller area => less light is hitting the sensor) as you zoom the ratio changes.</p>

<p>Aperture is focal length / diameter of lens (the optic component of the lens - not the number listed on the front rim which is the filter size) - so longer focal length = higher f-number.</p>

<p>Martin</p>

[mart_e]

<p>Meant to say - the physical <em>maximum (wide open) </em>aperture size does not change as you zoom.</p>

<p>Martin</p>

[jim_momary]

<p>Make the OK sign with your hand (thumb touching forefinger).<br>

Hold it close to your eye and look thru the circular opening you've formed.<br>

Extend your arm a bit. Note how your field of view gets smaller.<br>

Imagine what you are looking at is 'light' (This is a crude example at best, so don't bite my neck off guys.)<br>

So, in effect, you have just spread less 'light' over the same area (your retina).<br>

In a very crude way, you have gone from F 3.5 (close fingers) to F 5.6 (distant fingers).<br>

You've 'simulated' <strong>effective aperture</strong>, which is what the OP asked about.<br>

Silly Jim</p>

[mendel_leisk]

<p>Zooms, even ones that vary in length as they go through their zoom range, don't invariably have varying max aperture. Hmm, that's an "interesting" sentence...</p>

<p>Just thinking of some Canon "L" zooms for example, both the 24-70mm and 24-105mm varying in length when zooming (the 24-70 is actually longest at 24mm end), but both hold a constant max aperture, throughout the zoom range.</p>

<p>Max aperture is traditionally, and simplistically, the ratio between focal length and diameter of a lens, ie: if you have a 50mm lens that is 50mm diameter: it's f1.0. Next case: 50mm lens with 25mm diameter: it's f2.0. And so on. But, there is more to it then that, and physical dimensions and the relation to f-stop kind of goes out the window with zooms.</p>

<p>But anyway, that's the basic, and often overridden principal. I think.</p>

[leighb]

<p>In its simplest form, the f-number equals the diameter of the objective (the front element) divided by the focal length. That's why it's written as a quotient (f/2) or as a ratio (f:2).</p>

<p>Either of these calculations will yield the diameter of the objective. For example, a 100mm lens with a maximum aperture of f/4 will have a 25mm diameter objective.</p>

<p>For any given objective diameter, the f-number increases as the focal length increases, so on zoom lenses a variable maximum f-number is common.</p>

<p>There are tricks that can be used during the design of the lens that will make the maximum f-number the same at all focal lengths, at the expense of a smaller maximum number than could otherwise be achieved. The number thus obtained equals the objective diameter divided by the longest focal length.</p>

<p>- Leigh</p>

[dan_south]

Robert, good question.

 

Inside a camera lens is a device that works like the iris of they eye. It can open up wide or close down to a small

diameter (almost like a pinhole).

 

The opening in the lens' iris is called an aperture. We measure the size of the aperture by it's diameter. It would be

possible to state this diameter with a number. For example, we could set the aperture to 25 mm (approximately 1 inch) for

one shot and then to 50 mm (2 inches) for the next shot. But instead of using actual numbers, we find it useful to express

the size of the aperture (opening) as a ratio of the focal length. I'll explain why shortly, but first some examples.

 

Let's say that our lens has a focal length of 100 mm. The 25 mm aperture is 100/4 or f/4. The 50 mm aperture is 100/2 or

f/2.

 

But what if our friend has a 200 mm lens? How big would f/2 and f/4 be on his lens?

 

His f/4 = 200/4 = 50 mm or 2 inches

His f/2 = 200/2 = 100 mm or 4 inches

 

In other words, any particular f-stop on his lens is twice the diameter of ours. His f/4 is 50 mm while ours is only 25 mm.

Yet, the interesting thing is that for both lenses, f/4 sends the same amount of light to the film or sensor. His longer 200

mm lens spreads the light over a larger area. Imagine pointing a flashlight at a wall and then backing up slowly. The circle

of projected light gets larger and more dim as you back away from the wall. Longer lenses project a larger, dimmer image

circle, so we have to compensate by letting them pass the light through a larger opening to brighten it up.

 

That's why it is useful to express the aperture as ratio of the focal length. No matter what the focal length, a given f-stop

sends the same amount of light to the sensor. F/8 at 24 mm gives the same amount of light to the sensor as f/8 at 200 mm even though

the aperture size is vastly different (3 mm diameter versus 25 mm).

 

With zoom lenses, sometimes the maximum aperture has to change as the focal length extends. Let's say that we design

a 50-200 mm zoom lens. Let's also say that we want our maximum opening to be f/2. At 50 mm that's a 25 mm aperture,

or about an inch. But at 200 mm that would be a 100 mm aperture. That means that our lens would have to be well over

four inches in diameter to include the casing and electronics and motors and glass that extends beyond the four inch

maximum aperture. That's a big, heavy, expensive lens. But if we sacrifice some speed and accept f/4 at the long end, of

200/4 = 50 mm (2 inches) we can make a much smaller lens.

 

So our final design might be 50-200 f/2-4, which would be much smaller and lighter than 50-200 f/2.