MaryFran Loc: Front Royal, VA

I have a Nikon 7100 with an 18-300 lens which says 1:3.5-63G (not sure what the "63G" means). If I set it for aperture mode, I can set it for anything between 3.5 and 22. Often I want the aperture to be 3.5 to blur the background for portraits. Sometimes the camera will do that and sometimes it will not allow me to set it anywhere below 6.3. Can someone please explain to me WHY it doesn't consistently let me use 3.5? Is there anything I can do to make it allow me to use the aperture I want? Thanks.

GoofyNewfie Loc: Kansas City

Because this is a variable aperture lens, the maximum aperture changes depending on the focal length you use.
At 18mm, the maximum aperture of the lens is f/3.6 and from 200 to 300mm, the widest aperture is f/6.3.

Constant aperture lenses, like the 70-200 f/2.8, maintain the same maximum aperture throughout the zoom range.
They come at a price.
Many mistakenly call these "Fixed aperture" lenses, but that is not the correct term.
Lenses with a "fixed" aperture have only one aperture that cannot be changed. They include most mirror lenses and some specialty lenses like the Portragon

Aperture chart for the 18-300 from Ken Rockwell:

Marilia Loc: Dallas, TX

That lens is a variable aperture lens, you would need a fixed aperture lens to maintain the aperture you want.

CHG_CANON Loc: the Windy City

The aperture varies between f/3.5 and f/6.3. As a variable aperture lens, the maximum aperture varies over the zoom focal length. At the minimum 18mm, the maximum available aperture is f/3.5. However, as you zoom, the maximum available aperture changes. You'd have to research online if someone has documented the focal lengths where the changes occur. By the point the lens is zoomed to the maximum focal length, the maximum aperture available is f/6.3

Your options are to work at a specific focal length that provides the aperture you desire. You can also work with a model with more distance between the camera and the model and a background that has some distance behind the model. Both these approaches help to create a blurred background behind a subject in perfect focus when working with a consumer-grade variable aperture lens. You can also replace this lens with a constant aperture zoom or a prime lens at your preferred focal length and desired aperture.

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

The "G" means it's a "gelded" lens, one that has no aperture ring. The 63 is actually F6.3 which is the F-number at the maximum focal length. Others have given you good explanations as to types of lenses.

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

Most, but no all zooms, lose light (becomes slower) as you zoom out from 18-300 mm. A fixed aperture zoom, like the Nikon 200-400 f4, does not lose light but stays at f4 through out the zoom range. If your lens had a fixed aperture, it would be a lot more expensive.
So, at 18 mm your lens has an aperture of f3.5 (this is your fastest part of the zoom and where you will get out of focus backgrounds if you get very close to your subject and the background is not right behind it, but further away.
As you zoom the lens out toward 300 mm your lens gets slower, and at 300 mm your lens becomes a f6.3. At 300 mm, you can get background blur, but again you must fill the frame with the subject, be close, and have the background further away from the subject, the longer the background is away from the subject, the more pronounced the blur, or the more out of focus it will become.
Between 18 and 300 mm your lens gradually goes from an aperture of f3.5 to f6.3. I am not aware exactly where each stop accrues but surface to say your lens gradually gets slower as you zoom out.
So, remember, if you want to get good out of focus backgrounds, get close you your subject and try to widen the gap between the subject and the background. And keep your lens as wide open as possible, that is, use the widest aperture possible if you want your backgrounds to blur out of focus.
Hope this helps and keep on shooting until the end.

MaryFran Loc: Front Royal, VA

Thank you SO MUCH. All of these replies help solve the mystery, especially the ones that went into detail. Now I understand and know what to do to get what I want to achieve. I had known about having lots of distance between the subject and the background, but had no idea what the lens was doing.

AP Loc: Massachusetts

Hi, Mary Fran! Your aperture will not stay at f3.5 because it is not a fixed aperture of f3.5 though its whole focal length of your zoom lens - 18 to 300mm. When you change the zoom range to your choice, the aperture f3.5 will change also, the best setting for f3.5 on your fine zoom, is 18mm up to about 50mm.

However, if you set "A" aperture priority to f6.3 and your outside photographing, that aperture will stay locked in at f6.3 no matter what range you zoomed, 18 to 300mm!

Once again, you can buy a FIXED aperture of f2.8 through out the zoom range you choose, will stay at f2.8! Buying a fixed aperture lens is quite expensive! Just look up the price of a 80-200mm f2.8 ED Nikkor, maybe $500.00 USED!

Study focal length lenses and fast apertures . . . and why their so expensive! Did you ever hear of LEITZ & LEICA check out their prices, you'll fall off your seat! CIAO, AP

GoofyNewfie Loc: Kansas City

Good point!
This is one thing people overlook when using a variable aperture zoom lens.
It only varies if you're shooting wider than the smallest maximum aperture.
(and you can do this in manual mode as well)

AP Loc: Massachusetts

Thank You for your fine response, Goofy Newfie! My photographs go back to Vietnam, 1966-68 - and, I'm still learning! Only pass judgement, if you are positive and knowledgeable of what you are talking about! Your response is welcome indeed, keep in touch! AP

MaryFran Loc: Front Royal, VA

[quote=AP]

However, if you set "A" aperture priority to f6.3 and your outside photographing, that aperture will stay locked in at f6.3 no matter what range you zoomed, 18 to 300mm!

Does the same thing apply if I set the "A" priority to 3.5, will that aperture stay locked in at f3.5 no matter what range I zoom to? I will have to test that out in the field.

MaryFran Loc: Front Royal, VA

I don't understand what you are saying here. "shooting wider than the smallest maximum aperture." Please give me something concrete---example.
Thanks.

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

Nope...[quote=MaryFran]

BebuLamar

But if you're talking about cameras back in the 60's (or even up to the late 80's) if you have a variable aperture zoom the aperture always changes as you zoom regardless of which aperture you set it at.

Screamin Scott Loc: Marshfield Wi, Baltimore Md, now Dallas Ga

Meaning any aperture opening larger than the smallest opening on the lens. If the smallest maximum aperture is F22, any aperture larger than that (F16, F11, F8, etc) is shooting wider than the smallest maximum aperture...