Lens Aperture vs F-Stops on camera

[tesla]

Hi all,

another beginner's question that confused me alot.

So, here hopes some experts can demistify this fog.

I am confused with the relationship between the constant aperture number found in some luxury lens and the f-stop/f-number that is shown in the camera itself. I do not understand how can the f-stop inside the camera can be set to bigger than the max value of the lens' aperture. For instance, lens like EF 24-105mm f/4 L, my understanding is the aperture is constant at f/4 regardless whether it is zooming or out. so my expectation when I look at the info through my viewfinder I should see f/4 all the time, but this is not the case. by why??

Above picture I obtained from snapshot.canon-asia.com.  As you can see the author took the picture are f/9 while the lens itself has a max aperture of f/4. 

on my amature DSLR camera that has lens 18-55mm f1.5-5.6 when using aperture priority, I can set the f-stop to f/22 for instance. Why?  I thought the max value is f/5.6

I am missing a piece of jigsaw here ..... help??

thanks in advance.

glenn

[Waddizzle · Accepted solution]

Aperture describes a ratio of certain physical dimensions within the lens..  The aperture values that you see in the model number is the widest aperture setting that the can provide.  The smaller the number, the smaller the ratio, which means the wider the aperture.

Every lens should have its' maximum aperture printed on it somewhere.  Sometimes you will see the aperture described as a ratio,  Instead of seeing " f/4 ", you might see " 1:4 ".  Apertures can be set to narrower sizes than maximum, which is also known as "stopping down" the aperture.  

Sometimes you will see the aperture expressed on a zoom lens as two values like the EF-S 18-55mm f/3.5-5.6 IS STM.  This means that the maximum aperture changes as you zoom from 18mm to 55mm.  At 18mm, the maximum aperture can be set as wide as f/3.5.  When you are zoomed out to 55mm, the maximum setting is only f/5.6.

A constant aperture zoom does not change the maximum available aperture as you zoom from one end of the range to the other end of the zoom range.

[PLee · Accepted solution]

For Glenn, your EF 24-105mm f/4L IS USM has an f-stop range of f/4 to f/22. This entire range of f-stops is available at any focal length on that lens. The name of the lens only includes the smallest possible f-stop for the entire range of focal lengths the lens can cover. Since the EF 24-105mm f/4L IS USM can have an f-stop as small as f/4 throughout its entire focal length range, you only see one f-stop in its name.

Your less expensive EF-S 18-55mm f/3.5-5.6 lens is named that way because at 18mm, the smallest f-stop is f/3.5 and at 55mm, the smallest f-stop is f/5.6. At 18mm, that lens has an f-stop range of f/3.5 to f/22 and at 55mm, that lens has an f-stop range of f/5.6 to f/38. The first f-stop in the lens name corresponds to the minimum focal length of that zoom lens and the second f-stop in the lens name corresponds to the longest focal length for that lens. This is more commonly known as a variable aperture lens.

If you are using a shooting mode that allows the camera to select an f-stop for you, then the camera may select larger f-stops for your lens in brighter scenes like outdoors on a sunny day. This is why you may see a photograph with f/9 on the EF 24-105mm f/4L IS USM in your sample picture or an f-stop of f/22 on the EF-S 18-55mm.

[PLee · Accepted solution]

For Glenn, your EF 24-105mm f/4L IS USM has an f-stop range of f/4 to f/22. This entire range of f-stops is available at any focal length on that lens. The name of the lens only includes the smallest possible f-stop for the entire range of focal lengths the lens can cover. Since the EF 24-105mm f/4L IS USM can have an f-stop as small as f/4 throughout its entire focal length range, you only see one f-stop in its name.

Your less expensive EF-S 18-55mm f/3.5-5.6 lens is named that way because at 18mm, the smallest f-stop is f/3.5 and at 55mm, the smallest f-stop is f/5.6. At 18mm, that lens has an f-stop range of f/3.5 to f/22 and at 55mm, that lens has an f-stop range of f/5.6 to f/38. The first f-stop in the lens name corresponds to the minimum focal length of that zoom lens and the second f-stop in the lens name corresponds to the longest focal length for that lens. This is more commonly known as a variable aperture lens.

If you are using a shooting mode that allows the camera to select an f-stop for you, then the camera may select larger f-stops for your lens in brighter scenes like outdoors on a sunny day. This is why you may see a photograph with f/9 on the EF 24-105mm f/4L IS USM in your sample picture or an f-stop of f/22 on the EF-S 18-55mm.

[tesla]

Thanks @PLee - much appreciate your explanation here. Light and easy to understand. So, it means that my RF 24-105mm f/4L IS USM is not really a "constant" aperture lens. May be this is where my confusion is. I was expecting the aperture to be "constant" across the focal length (24-105mm) as they said it is.

[Waddizzle]

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@tesla wrote:

Thanks @PLee - much appreciate your explanation here. Light and easy to understand. So, it means that my RF 24-105mm f/4L IS USM is not really a "constant" aperture lens. May be this is where my confusion is. I was expecting the aperture to be "constant" across the focal length (24-105mm) as they said it is.

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Your 24-105mm f/4 lens is a constant aperture lens.  You have the same full range of aperture values available at any focal length.

[jrhoffman75]

tesla - since you seem to lack an understanding of the basics of exposure control I am not sure anything that is given to you here will make any sense to you.