Dumb question

[derrick937]

If you have a fixed lens, is the aperture preset for fstop ? Can that be changed , like, can i get f22 if the lens is like a kit lens ? Like a 18 /105 , sure hope someone understands what im trying to ask cant think of a better way to explain

 

[jwbryson1]

If you have a fixed lens, is the aperture preset for fstop ? Can that be changed , like, can i get f22 if the lens is like a kit lens ? Like a 18 /105 , sure hope someone understands what im trying to ask cant think of a better way to explain

18-105 refers to the focal length of the lens. In your case, the lens is adjustable from 18mm to 105mm. If that's the case, it's not a "fixed lens" such as a 50mm fixed focal length lens which does not zoom but stays at a constant 50mm. A fixed focal length lens is also called a "prime" lens.

The f/stop is the size of the lens opening, not the length of the lens. Some lenses have a fixed f/stop such as f/2.8, and others have a variable f/stop such as f/3.5 to f/5.6, in which the f/stop will vary as the lens is zoomed in or out. A fixed f/stop lens is generally more expensive (especially one with a large aperture such as f/2.8) than a lens with a variable f/stop.

Most lenses can get down to f/22 (a very small lens opening).

 

[Infinite_Day]

By "fixed" jwbryson1 means that a zoom lens rated at f/2.8 will be able to retain a maximum 2.8 aperture throughout the zoom range. It's not always fixed at that aperture and may be stopped down as he said to f/22 or so depending on the lens. The number listed on a lens for the f/stop is always just the maximum f/stop (largest aperture) the lens can achieve. They can always be stopped down to smaller apertures. Don't mean to step on any toes - just thought some clarification might help.

 

[derrick937]

Ok, so my 18 105 has 1: 3.5 - 5.6 , i cannot go higher or lower on that lens ? Sorry for the dumb question,
And my 55 300 is 1:4.5 - 5.6 only ? But my 60mm is 1:2.8 biggest to ? Im reading understanding exposure and from what I'm getting a kit lens cant be changed ? Is it preset for for each focal length ?

 

[Gaerek]

All that means is that at it's shortest focal length, your maximum aperture is f/3.5. At the longest focal length, your maximum aperture is f/5.6. The f/number is a function of focal length divided by aperture diameter. So as you zoom out, your effective f/number goes up. Higher end lenses have a fixed maximum (as has been stated) that doesn't change as you zoom.

EDIT: Why not put both lenses on your camera and set the aperture to f/22? You'll see that you can use any number above your maximum aperture, up to usually about f/22. Though at the long end of your zoom, you might be able to get it to f/32, but I wouldn't recommend that (or f/22 for that matter) for most uses.

 

[o hey tyler]

Okay, when it says 3.5-5.6, that means that at the widest setting (18mm) your maximum aperture is f/3.5, you can't go any wider than that. When your focal length is at 105mm, your maximum aperture is f/5.6 meaning that you can't make it any larger, like opening it up to f/3.5. You can still go from f/5.6, to f/6.3, to f/7.1 to f/8, to f/11, and so on down to f/22. What the f/numbers on the lens represents is the WIDEST aperture settings that it is capable of. If you have a 70-200mm f/2.8, that means that you can be at f/2.8 at 70mm or at 200mm because it isn't a variable aperture. Therefore, it costs a ton more than a similar focal length lens with a variable aperture.

 

[derrick937]

I think i understand now , such a simple concept but i didn't understand . Thanks for the help.

 

[Infinite_Day]

What Gaerek said. I don't know where you got the notion that a kit lens' aperture can't be changed. You can change the aperture on most any lens. OK - let me try this one more time as well: You have the 18-105 lens - when at it's widest focal length (18mm) the maximum aperture you can use is f/3.5. You can make the aperture smaller if you want. IT IS NOT FIXED AT 3.5 at 18mm. When you zoom the lens all the way out to 105mm the largest aperture you can use is f/5.6. IT IS NOT FIXED AT 5.6 at 105mm either. You can make the aperture smaller if you want. With your prime 60mm lens the largest aperture you can use is f/2.8 but you can set the aperture smaller if you want. You need to understand the the f number actually represents a fraction so the smaller the number, the larger the aperture. f/1.4 is a larger opening than f/22.

EDIT: Somebody beat me to it. Glad you think you've caught on!

 

[derrick937]

Im not book smart, i was a master certified watch maker and diamond setter for years after an accident left me unable to walk,
I can do amazing things with my hands but when it comes to reading and learning , I'm dumb as a box of rocks ,
I will figure this out, its just gonna take time, its too fun to not learn, i haven't had this much fun in years,

 

[Infinite_Day]

Sorry to hear about your accident. Glad that you are enjoying photography. Good luck and keep asking questions. I apologize if I came across harsh at all - not my intention - sometimes I just don't explain things well. If anybody around here is off-putting don't take it personally.

 

[ph0enix]

Ok, so my 18 105 has 1: 3.5 - 5.6 , i cannot go higher or lower on that lens ? Sorry for the dumb question,
And my 55 300 is 1:4.5 - 5.6 only ? But my 60mm is 1:2.8 biggest to ? Im reading understanding exposure and from what I'm getting a kit lens cant be changed ? Is it preset for for each focal length ?

Your 18-105 kit lens can be set to f/36 when extended to 105mm and f/22 at 18mm. The 3.5 - 5.6 spec indicates wide open apertures of 3.5 at 18mm and 5.6 at 105mm. If you set the focal length anywhere between 18 and 105mm, your maximum (wide open) aperture will be somewhere between 3.5 and 5.6.

 

[jwbryson1]

i was a master certified watch maker and diamond setter for years ,

You should look up a guy on this forum called Bitter Jeweler. He's a hell of a nice guy, knows a LOT about photography and he also happens to make stunning jewelry.

You 2 may have something more than photography to discuss.

 

[jwbryson1]

By "fixed" jwbryson1 means that a zoom lens rated at f/2.8 will be able to retain a maximum 2.8 aperture throughout the zoom range. It's not always fixed at that aperture and may be stopped down as he said to f/22 or so depending on the lens.

Thanks for the clarification. I was a bit loosey goosey with my response.

 

[DiskoJoe]

Ok, so my 18 105 has 1: 3.5 - 5.6 , i cannot go higher or lower on that lens ? Sorry for the dumb question,
And my 55 300 is 1:4.5 - 5.6 only ? But my 60mm is 1:2.8 biggest to ? Im reading understanding exposure and from what I'm getting a kit lens cant be changed ? Is it preset for for each focal length ?

set you camera to a-mode and turn the dial until it reads f22. Then after that go read your manual. If you dont have one then download it online. READ YOUR MANUAL!!!! Seriously, there is lots of good info in the manual to get you started.

 

[KmH]

A fixed aperture lens has just one aperture that is not adjustable. Fixed aperture lenses are very inexpensive to design and build.
Most are catadioptic lenses, like mirror lenses, and dirt cheap - long focal length prime lenses that have a fixed aperture, usually somewhere in the range of f/5.6 to f/11.

There are constant aperture zoom that list just their widest (maximum) aperture (usually f/2.8, or f/4), but can be stopped down to smaller apertures. Constant aperture zoom lenses are the most costly type of lens to design and build.
In the case of a constant aperture zoom lens set to it's max aperture, the aperture f-number value does not change as the lens is zoomed. However, the aperture size does indeed change as the lens is zoomed, but it retains the same focal length to lens opening size ratio which is what the f-number describes.

For example assume a 100 mm to 400 mm focal length, f/2 constant aperture zoom lens.
When the lens is set to 100 mm and f/2 the lens aperture at 100 mm is (f)100 mm/2 = 50 mm wide but the lens aperture at 400 mm is (f)400 mm/2 = 200 mm wide. As the lens was zoomed the aperture opening diameter got 4 times larger. So though it's called a constant aperture zoom lens, constant refers to the ratio of lens opening size to the focal length setting, not the actual diameter of the lens opening.

Variable aperture zoom lenses are in the middle as far as design and cost to build.

Using the same 100 mm to 400 mm focal length as above, but with an f/4 to f/8 variable aperture range, which says the maximum (widest) aperture at 100 mm is f/4, while the widest (maximum) aperture at 400 mm is f/8. Doing the math again we see that the lens aperture at 100 mm is (f)100 mm/4= 25 mm wide and the lens aperture at 400 mm is (f)400 mm/8=50mm wide.
The variable aperture lens opening only got 2 times bigger, because the focal length to lens opening size ratio did not remain constant as the lens was zoomed.

Note that we can calculate the lens opening size at any focal length between 100 mm and 400 mm if we know the f-number. Assuming the f-number ratio of the above 100 mm to 400 mm lens changes linearly, half way between 100 mm and 400 mm (at 250 mm) the f-number would be f/6. The math shows the lens opening diameter would be (f)250 mm/6=41.67 mm wide.