G15: Can't zoom with low f-number, why?

The G15's widest aperture is 1.8, but zooming makes it unavailable. I might want a shallow DoF for a scene I can't get close to. In Av mode, using optical zoom moves the left end of the available f-stop range to 2.0 and then 2.2 . . . up to 2.8. Is there a mechanical or optical reason that this limitation is necessary? Would 5x zoom together with f/1.8 or f/2.0 make bad shots in some way, so that the camera is designed to protect me from the consequences of a poor decision?

no its working correctly. the lens is variable aperture meaning its 1.8 at the wide end and 2.8 at the long end.

Your zoom lens is a variable aperture lens. Because the aperture is a ratio of the focal length f1.8 is a larger physical opening at 100mm than it would be at 35mm. Because the size of the physical aperture is limited by the diameter of the lens, they design the lens to allow for a larger aperture at the wide end of a lens which naturally is limited on the long end.

Tommydigi wrote in post #16395542
no its working correctly. the lens is variable aperture meaning its 1.8 at the wide end and 2.8 at the long end.

Thanks, but . . .

I know it's working as designed, because the manual says zoom will make some stops unavailable. What I'm asking is why. Is the restriction on aperture dictated by geometry?

The stops don't finish at 2.8. They go up to 8.0.

What do you guys mean by "wide end" and "long end"?

OhLook wrote in post #16395575
What do you guys mean by "wide end" and "long end"?

Wide, as in wide angle, the shortest focal length on the zoom. Long as in longer focal length is the zoom range.

OhLook wrote in post #16395571
Thanks, but . . .

I know it's working as designed, because the manual says zoom will make some stops unavailable. What I'm asking is why. Is the restriction on aperture dictated by geometry?

The stops don't finish at 2.8. They go up to 8.0.

Not sure I follow what your issue is. Just because the lens goes down to 2.8 does not mean you need to shoot at 2.8. You can stop it down to 5.6 or F8 or whatever.

The reason it varies as you zoom in is because of the limitations of the lens itself.


I don't understand what you mean about geometry

The numbers printed on the lens are the lowest f-stop number you can use throughout the zoom range. The highest f-stop number available to the lens varies and is not printed on the lens.

I don't know the exact specs of the G15, but say a lens was labeled 28-90 f2.8-4.5. That would mean at 28mm, the lowest f-stop number it can pick is f2.8 and at 90mm the it is f4.5 and anywhere in between in the middle of the zoom range. If it said 28-70 f2.8, then it would f2.8 at any focal length.

Why? That's how the lens was designed. lower f-stop numbers at longer focal lengths require very large aperture openings, so it is more expensive and makes larger lenses to keep it the same throughout the whole zoom range.

gonzogolf wrote in post #16395580
Wide, as in wide angle, the shortest focal length on the zoom. Long as in longer focal length is the zoom range.

So they're ends of the range. Thank you. I come to photography from the art side, not the technology side, and people use jargon without explaining it. In ordinary English, "wide" and "long" aren't opposites. The opposite of wide is narrow, and the opposite of long is short.

Tommydigi wrote in post #16395685
Not sure I follow what your issue is. Just because the lens goes down to 2.8 does not mean you need to shoot at 2.8. You can stop it down to 5.6 or F8 or whatever.

Right, and I often do, and I don't know what the 2.8 signifies. It clearly isn't the minimum aperture. The other end of "1.8–2.8," the left end, is the maximum aperture. EDIT: Taylor answered this part while I was writing.

Geometry in the sense that articles about how cameras work are illustrated with diagrams of their insides that show what happens. The light comes in here, the lens refracts it like this, the sensor records it upside down like this . . . there are lines and angles, and you can see how the geometric relations go. The geometry would be visible if I could see inside the camera.

I could understand a physical explanation of this kind: "When you change the _____ setting from _____ to _____ , the _____ moves [toward/away from] the _____ . This movement takes up all the space in the _____ , and therefore the _____ doesn't have room to _____ ."

I could also understand if someone said "There's always a tradeoff between zoom and large aperture because it's physically impossible to have both of them" or, alternatively, "It's possible to have zoom and large aperture at the same time, but the camera would have to be a lot bigger and more expensive."

OhLook wrote in post #16395873
So they're ends of the range. Thank you. I come to photography from the art side, not the technology side, and people use jargon without explaining it. In ordinary English, "wide" and "long" aren't opposites. The opposite of wide is narrow, and the opposite of long is short.


"

It couldnt be that hard to figure out if you gave it some effort.

gonzogolf wrote in post #16395924
It couldnt be that hard to figure out if you gave it some effort.

No, really, my background is different from yours. I was wondering: the wide end of what? the long end of what?

Look at wide angle or telephoto. When you zoom as far in as you can go its the "telephoto" end or long end. When you zoom out your at the "wide" end or wide angle.

Creating a fast ( lower number aperture ) telephoto lens is usually big and expensive. Look at a 200mm 2.0 lens as compared to a 24 1.4 lens.

OhLook wrote in post #16395571
... Is the restriction on aperture dictated by geometry?

...

Well, it's based on the physics of lens design. Your aperture value is a fraction based on the diameter of the opening that light comes through and the focal length of the lens being used. Decent writeup on what the 'f number' is, here

The usefulness of that number, though, once you move to setting your own exposure or deciphering why a particular shot came out the way it did.

F stops are broken out into full stops (f/1.8, f/2.0, f/2.8, f/4, f/5.6, etc) where an adjustment "down" (f/4 to f/5.6) cuts the amount of light coming through the lens in half; conversely, an adjustment "up" (f/4 to f/2.8) doubles the amount of light coming through the lens. The ratio is a little backwards when you first start looking at these, in that the higher the f number is, the smaller the actual aperture is (f/8 indicates a smaller hole than f/4, for example)

Virtually all zoom lenses in compact cameras ("point and shoots") are variable aperture lenses, meaning that the maximum aperture available will vary based on the focal length being used. So, when your camera lists f/1.8 - f/2.8, it means that when you are at your shortest focal length, your available maximum aperture will be f/1.8 and when you are at the longest focal length, your available maximum aperture will be f/2.8; and that maximum will vary at certain 'breaking points' throughout the focal ranges in between.

For example, if you have a zoom lens whose focal range is 18mm to 24mm and the aperture is listed as f/1.8 - f/2.8, that means that at 18mm your maximum, available aperture is f/1.8; but at 24mm will be f/2.8. In between things might look like 18-19mm = f/1.8, 20-22mm = f/2.0 and 23-24mm = f/2.8.

They're never quite that cleanly broken out, but I figure it makes an easier visual

I wish my G1X had an maximum aperture of f/2.8 at the long (narrow angle) end ... I'd be oh so happy

Heck, I just point, shoot, print and enjoy.