What does it mean: f/n-n.n

I understand the relationship between Aperture size and DOF, and the larger is smaller thing, but I don't understand the f/ stop range on Zoom lenses.

Most lenses cannot maintain the maximum aperture throughout the zoom range. So the aperture range specified represents max aperture at minimum zoom, and max aperture at maximum zoom. They always have a smaller aperture at the long end in this case.

The nicer L zooms are constant aperture, so zooming won't change your exposure. But if you're at f/3.5 zoomed out on a non-L zoom, running manual settings, then zooming is going to pinch off some light and you might be underexposing.

The effective aperture rating is a ratio between the lens' focal length and the effective opening. So, obviously, if the focal length increases then the opening diameter must increase in order to maintain the same ratio.

As waterrockets mentions, many zoom lenses are not designed this way, so they have variable apertures based on the focal length. You will frequently see f/3.5-5.6 or the like indicating that the maximum aperture at the short end of the zoom range is f/3.5 while the max aperture at the long end of the zoom range is f/5.6.

'L' lenses are not the only Canon zooms that offer constant apertures (17-55) and, conversely, not all 'L' zooms do offer constant aperture (100-400, 28-300, 70-300 for example).

runninmann wrote in post #17084032
'L' lenses are not the only Canon zooms that offer constant apertures (17-55) and, conversely, not all 'L' zooms do offer constant aperture (100-400, 28-300, 70-300 for example).

The 100-400 is a good example of the advantages with a lens that doesn't have a constant aperture. If f/4.5 for the full range, it would have to be made much larger and heavier. And if instead making it f/5.6 for the full range, then we would lose lots of light at the wider end.

So while a constant aperture lens has the advantage that you can use M on the camera and keep your exposure even when zooming, the non-constant lenses instead gives the advantage that they try to get the most possible out from a given size lens with given size front element.

runninmann wrote in post #17084032
The effective aperture rating is a ratio between the lens' focal length and the effective opening. So, obviously, if the focal length increases then the opening diameter must increase in order to maintain the same ratio.

As waterrockets mentions, many zoom lenses are not designed this way, so they have variable apertures based on the focal length. You will frequently see f/3.5-5.6 or the like indicating that the maximum aperture at the short end of the zoom range is f/3.5 while the max aperture at the long end of the zoom range is f/5.6.

'L' lenses are not the only Canon zooms that offer constant apertures (17-55) and, conversely, not all 'L' zooms do offer constant aperture (100-400, 28-300, 70-300 for example).

Thanks, yeah. My intention was to illustrate many L zooms as constant aperture, not all, nor L exclusively.

Great answers; thanks, that helped

To put it in simple term, the physical size of the aperture (physical opening) is dependent on the focal length.

Ex : a 50mm lens at f/8 will have a physical opening of 50 divided by 8 = 6.25mm.

What happens with constant aperture lenses like the Canon EF 70-200mm f/2.8 L is that when you zoom out, there is a mechanism that opens up the aperture so at all focal lengths the aperture remains constant but the physical opening changes as you zoom.

Makes sense?

yogestee wrote in post #17086764
To put it in simple term, the physical size of the aperture (physical opening) is dependent on the focal length.

As in when you physically zoom in and out?

Ok, but I don't have a constant aperture lense so I have to be concerned about something I'm still not getting. So the question was about the range; what does it mean and what must I keep in mind when considering it? And while your explanation made it very clear; to a point

so far so good; and very well put

You lost me here
opens bur remains constant

Almost

mikecox wrote in post #17093257
As in when you physically zoom in and out?


Ok, but I don't have a constant aperture lense so I have to be concerned about something I'm still not getting. So the question was about the range; what does it mean and what must I keep in mind when considering it? And while your explanation made it very clear; to a point

so far so good; and very well put

You lost me here
opens bur remains constant

Almost

If the ratio stays the same (remains constant) then the physical opening must increase (opens) as the focal length increases.

In simple terms, a constant aperture zoom lens has a variable maximum opening size while a variable aperture lens has a constant maximum opening size.

Let me try this.

Let's say I'm making a zoom lens that goes from 100mm-400mm and the aperture will open 80mm. Now I can choose one of two options:

1) I let you open it up as far as you can. In this case at the close end you get 100/80=1.25 and at the far end I get 400/80=5. This would give you a lens with an F/1.25-5.

2) The other option is that I open it all the way up at the 400 end and then put in a mechanism that as you move toward the 100 end, the mechanism that moves the lenses in and out also closes the aperture some so that the ratio stay the same. In this case at 400 the opening would be 80mm and at 100 it would be 20mm. This would be a F/5 lens.

Most people don't deal with aperture or don't deal with it enough (cameras are really good about working around the issue) for companies to justify the extra expense for constant aperture. Professionals on the other hand tend to have some much other stuff they are dealing with (multiple flashes set for a specific amount of light, actually trying for a specific amount of background blur, etc.) that it is worth the extra money to them, which is why you see it more in higher end cameras than you do in lower end ones.

Aperture = hole in lens (entrance pupil) where light passes through, or the diameter of this hole. This diameter changes during zooming.

F-stop = ratio of lens focal length to diameter/aperture, This stays constant while zooming with a constant f-stop zoom.

mikecox wrote in post #17093257
As in when you physically zoom in and out?

Ok, but I don't have a constant aperture lense so I have to be concerned about something I'm still not getting. So the question was about the range; what does it mean and what must I keep in mind when considering it? And while your explanation made it very clear; to a point


so far so good; and very well put


You lost me here
opens bur remains constant

Almost

Maybe I didn't make it all that clear Mike. With constant f-stop zoom lens, the physical diameter of the lens aperture opening gets bigger as you zoom out and smaller as you zoom in. There is a mechanical/electrical mechanism within the lens that does this. Don't confuse this with f-stop number.

Eg the lens aperture diameter may be 6mm at 70mm (focal length) and 18mm diameter at 200mm (focal length) at f/8..Only playing with numbers here by the way.

Lenses like the EF-S 18-135mm STM f/3.5-5.6 don't have a mechanism which opens or reduces the diameter of the lens opening at different focal lengths. So in effect the physical diameter of the lens opening remains constant. I bet the last sentence confused you even further.

Just don't mix up the f-stop number eg f/2.8, f/4, f/5.6 etc with the physical diameter of the lens opening which is measured in millimetres.

Jurgen even in a Constant aperture zoom the actual physical size of the aperture hole doesn't change. The difference is caused by the way the lens elements are placed in relation to the actual diaphragm. The only time that the physical diameter of the diaphragm is guaranteed to actually be the same as would be calculated is for an ideal perfect single element lens. What is important is the apparent size of the diameter of the diaphragm while observing it through the front element of the lens. If this were not the case it would become impossible to have any lens with a physical aperture larger in diameter than the lens mount diameter. This is obviously not the case as for example the 800mm f/5.6 lens would need to be 142.86mm in diameter all the way down.

Depending on where in the system the physical diaphragm is along with the way the elements are put together you can make the aperture appear to remain the same diameter as you zoom. This would be the variable aperture lens, and is actually easier to do. You can also build the lens so that as you zoom the aperture, although a fixed size seems to zoom with the lens. This is the constant aperture lens, and is a more complicated, so more expensive design. One other thing to consider is that although you might have a Constant aperture value, where you have physical aperture that is smaller than that needed by the calculations, thanks to it being magnified by the lens system, the front element diameter still has to be large enough to accommodate the largest physical size for the calculated f/number. So a 400mm f/5.6 lens must have a front element size of 71.43mm.

Alan

It was pointed out that I used the word fixed, where I meant constant. I have edited my post in order to avoid potential confusion to anyone.

BigAl007, just so I'm clear, when you refer to "fixed" aperture, can I assume you mean a "constant" aperture?

I've always understood a fixed aperture lens to have only one possible f-stop instead of a range.

i'm not trying to be pedantic, but I want to be sure I understand your intent.

yogestee wrote in post #17093487
Just don't mix up the f-stop number eg f/2.8, f/4, f/5.6 etc with the physical diameter of the lens opening which is measured in millimetres.

Ah, yes; I think that's what I was doing; thanks for the clarification.

This question has turned into a "master class"; but with one student and several instructors. It's

Thanks to everyone who contributed; you guys are awesome