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

[ebiggs1]

"I understand your wish to be precise and commend you for it but I believe that sometimes too much information can hinder people who are just starting out."

I agree it is best to do small chunks at first but 'clear' chunks, at the same time. I know you and I don't have a problem understanding what we are talking about.  A lens as a piece of glass has an f-ratio and f-stop.  But people don't think of a piece of glass as a lens. They think of a black round thing that goes on a camera. In this case the f-ratio and f-stop are two different things and/or, at least parts, of the black round thing.

[ebiggs1]

John Hoffman your photo shows exactly what I am referring to.  The f-stop mech and the "lens" are two separate parts of a camera lens. Once a physical lens (a piece of concave or convex glass) is ground, its physical properties can not change unless more physical grinding or machining occurs. That is why you need a separate system to change the amount of light that goes to the sensor. In this case f-ratio and f-stop are two seperate things.

[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.

[Waddizzle]

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@ebiggs1 wrote:

John Hoffman your photo shows exactly what I am referring to.  The f-stop mech and the "lens" are two separate parts of a camera lens. Once a physical lens (a piece of concave or convex glass) is ground, its physical properties can not change unless more physical grinding or machining occurs. That is why you need a separate system to change the amount of light that goes to the sensor. In this case f-ratio and f-stop are two seperate things.

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Your lesson was impeccable.  Except it did not answer the question.  

Look at the image in the original post.  The OP did not understand why there were two different aperture values, f/4 and f/9.

[ebiggs1]

"Except it did not answer the question.  ...    The OP did not understand why there were two different aperture values, f/4 and f/9."

Apparently you missed reading the correct answer, here it is again for your convenience.

"Cameras can shoot in a various levels of brightness and dark. Since we already established a lens can not change its f-ratio, how do we deal with that?  We add a system of reducing the light that the lens can transmit.  These are called f-stops.  And, long way around, this is what you see when you look at the LCD or editing software and it tells you it was shot at f9, in you example. Your lens never changed but it reduced the amount of light it let the camera see."

[tesla]

@ebiggs1 - thank you so much explanation. Honestly I only half understood it. 

Like what @waddizzle nailed my confusion, I do not understand why there are two apertures, f/4 and f9.

I do understand when you explained another "system" is added to control the light. But it raised another question to me. The lens says F4 constant aperture and I paid a lot for that (a lot because I do not have big budget, small for others perhaps), then I am expecting it to stay constant at f4 🙂

But like you said because such lens is zoom lens then the focal length is variable so the f-stop becomes variable too.

Then why the heck canon (and other brands actually) market it as "constant" aperture when in practicality I can never get constant. Knowing this is zoom lens, it will never constant like you said.

I feel a bit cheated here by the lens manufacturer 🙂

I am sorry that I am a bit thick here....

[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.

[Waddizzle]

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@tesla wrote:

Then why the heck canon (and other brands actually) market it as "constant" aperture when in practicality I can never get constant. Knowing this is zoom lens, it will never constant like you said.

I feel a bit cheated here by the lens manufacturer 🙂

I am sorry that I am a bit thick here....

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I thought I had explained that in my first post.  As you zoom the lens, the maximum available aperture changes on some lenses.  The maximum available aperture does not change on "constant aperture" zoom lenses as you zoom the lens.

[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.