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.

[kvbarkley]

Yes, the listed aperture is the "natural" aperture of the lens, the widest it can go. The lens also has a set of blades, called the aperture, that can "stop down" and artificially make the lens have a smaller aperture as required, to reduce the amount of light and increase the depth of field.

[ebiggs1]

" "natural" aperture"

??? 

[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.

[ebiggs1]

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

Yeah, you are and the answers so far are not a lot of help. So, let me give it a go.

Most people think of a round black tube that goes on a camera as a "lens".  OK, it is, but a lens is actually any piece of concave or convex piece of glass. No tube, no camera.

Let's say you have a "lens", a piece of glass,  25mm diameter and it had a focal length of 100mm then the focal ratio would be f4 because 100 ÷ 25 = 4 (a ratio). If you increase the focal length to 200mm but do not change the physical diameter size of the piece of glass "lens" size then it becomes 200 ÷ 4 = 8, now it's an f8 lens. This can never change.  However, in the case of a camera lens, there are two types. One is called a prime lens and the other a zoom lens. A prime lens can not change its focal length I.E., a 50mm f1.8 prime lens is always a 50mm f1.8 lens.  A zoom lens due to the addition of several more "lenses", pieces of glass, it can change its focal length. Unless special steps are taken the f-ratio will change as it zooms.

OK, now where you are losing it, is light.  Cameras can shoot in a various levels of brightness and dark. Since we already established a lens can not change it 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.

[Ray-uk]

A common beginners misunderstanding. Think of aperture as meaning an opening or hole (which is what it is), this can be bigger or smaller, it cannot be higher or lower.

An F stop or F number is calculated from the size of the hole in relation to to the focal length of the lens.

This means that a larger hole will have a small F number and a small hole will have a large F number. So for instance an f4.0 lens is description of it's largest opening and if you adjust this to smaller opening then the f number will get bigger.

I am sure you will get the hang of it eventually.

[ebiggs1]

"An F stop or F number is calculated from the size of the hole in relation to to the focal length of the lens."

This is not exactly correct.  There is a difference between the f-ratio (F number?) and a f-stop. The f-ratio (F number) is a ratio of the lens diameter and its focus point. An f-stop is a number dealing with light. If I read you correctly and understand what you were trying to say.

It becomes confusing because people think of a lens as a black round tube that goes on a camera.  However, the f-ratio is found on any piece of concave or convex piece of glass.  A lens, as a piece of glass, doesn't have f-stops.  This is usually the front element of a camera lens. To this other complicated groups of concave and/or convex "lenses" are added to create a camera lens.. Some way is needed to reduce the amount of light going to the film or sensor. This is f-stops and is another element in a camera lens. Natural f-stops are either half or double the one before or after it because they let in half or twice the amount of light to the sensor.

[Ray-uk]

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@ebiggs1 wrote:

"An F stop or F number is calculated from the size of the hole in relation to to the focal length of the lens."

This is not exactly correct. 

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As we are attempting to explain the concept to a beginner I was trying to keep things simple. I could have provided a whole screen full of information showing the mathematical concepts, even going on to T stops but I doubt that the OP would have read it or even needed to understand all the technicalities.

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.

[Waddizzle]

"As we are attempting to explain the concept to a beginner I was trying to keep things simple."

.

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"Aperture describes a ratio of certain physical dimensions within the lens."

Same here.  Kept it simple.

[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.