GoofyNewfie Loc: Kansas City

I was afraid that could be confusing.
Your lens' maximum aperture range is f/3.5 to f/6.3.
F/6.3 is the smallest maximum aperture.
If you set it to f/6.3, it shouldn't move from f/6.3 no matter what focal length you are using.
Keep in mind, using it at f/6.3 is wide open at the longest focal length and is probably not going to be the sharpest aperture.
All zooms are a compromise, some less, some more, and super-zooms, though convenient, can be noticeably less sharp.

GoofyNewfie Loc: Kansas City

I think you are correct, but that's too many brain cells ago for me.
There weren't that many good zooms back in the '60's.
I bought my first one in the mid-70's.
All of the current variable aperture lenses I have (I think I have only 3) stay where you set them.

MT native Loc: Big Sky Country — Montana

This is why I enjoy UHH, very good information shared by all.

MaryFran Loc: Front Royal, VA

Oh, okay. Thanks.

MaryFran Loc: Front Royal, VA

Okay, so when I have the lens not zoomed at all, the widest open the aperture can be is 3.5. I see that. And if the lens is zoomed out as far as possible, the widest open the aperture can be is 6.3. Okay. For what I am doing right now, it doesn't matter if the pictures are not the sharpest, though I will try to get in closer to my subjects for my own satisfaction. I am in charge of all the pictures for the yearbook of a small private school and it is close to time to take the individual student shots. When it warms up a bit more and I can take them outside for natural light. The pictures will not be large enough in the yearbook to make any lack of sharpness noticeable.

GoofyNewfie Loc: Kansas City

If that’s the biggest size they’ll be used, you won’t have any sharpness issues at all.

jerryc41 Loc: Catskill Mts of NY

[quote=GoofyNewfie][i]Many mistakenly call these "Fixed aperture" lenses, but that is not the correct term.[/quote]

traderjohn Loc: New York City

Good point. What kind of pasta are we making for Good Friday???

JMCPHD Loc: Maine

Maybe it will help to understand that the f number for aperture is based on the ratio of the size of the opening and the focal length of the lens. Thus in a zoom lens like the one you are using if you go from shorter focal lengths like 18 mm to 300mm the longer focal length divided into the size of the opening produces a smaller ratio and the amount of light reaching your sensor drops.

lamiaceae Loc: San Luis Obispo County, CA

You are getting fine explanations. I have two "similar" zoom lenses a "Kit" smc-Pentax DA 1:3.5-5.6 18-55mm AL Zoom Lens that came with a used camera I bought; and a "$$ high-end" smc-Pentax DA* 1:2.8 16-50mm ED AL IF SDM Zoom Lens. The widest aperture for the DA* 16-50mm is always f/2.8 for all focal lengths. For the DA 18-55mm the widest available at 18mm is f/3.5 and at 55mm f/5.6, with say f/4.6-5.5 in between. Luckily both lenses have high IQ, often "kit" lenses do not have great IQ. This way my wife on occasion can use a second camera with a similar lens.

Billybeau

The reason your f stop changes as you zoom is the light passing thru your lens has to travel a longer distance to reach your sensor in the camera, the f stop size is not changing. If you have ever seen say a 70-200 2.8 lens you would notice a very large front and rear elements in that lens which allows more light to pass thru to sensor or film.If you have ever seen a view camera. A 210 mm lens is a 8 inch lens, you put a 360mm lens on which is a 14 inch lens the light has to travel 6 inches longer thus creating a exposure difference. The same works with every camera lens combination ever made. To determine your focal length in inches use 25.5 mm to the inch. Their are conversion charts available to convert the light falloff to f stops.I know this is a little complex but that is very basically how it all works.Better yet try using a true macro lens on your camera, focus at infinity on a fixed light source that doesn’t change, put your camera on S (shutter priority) now set your shutter speed some where in the middle range of your light source, now focus in to your closest and look thru your camera while focusing and you will see your f stops changing as the light has to travel a longer distance.

camerapapi Loc: Miami, Fl.

Basic photography once again. You are using a variable aperture lens. At 18mm it will use f3.5 as the maximum lens opening (aperture) while at 300mm it will use f6.3. G is a designation from the manufacturer meaning that the lens apertures will be handled electronically. Old lenses have the apertures engraved in the lens barrel.
Variable aperture lenses, except when set at maximum lens opening at their extreme focal lengths can be set at another aperture and the aperture will be kept throughout all focal lengths. An example, if you set the lens to f8 that aperture will be kept at 18mm and through 300mm.

cdayton

Approximately, unless you assume no lens breathing.

sjb3

You said precisely what I was going to say. I salute you.

photoman022 Loc: Manchester CT USA

I need not add more