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.

[jrhoffman75]

Remember your math - 1/2 dollar is larger than 1/4 dollar, so f/4 is a larger value (opening) than f/9.

[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.

[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....

[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.

[ebiggs1]

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

Nah, I don't think so. It's just those one line snips don't offer or explain enough.  Nobody would understand them.

[ebiggs1 · Accepted solution]

"Honestly I only half understood it."

No problem let's try another approach. Do you have a binocular at home that you use? It is just a lens. Just like your camera lens. It has a constant f-ratio of, let's say f4.   However they do not have a means of reducing the amount of light coming into the binoculars and the amount that comes out for your eye to see. Now if your camera lens did not have that extra system, the aperture blades, it would work the same way.

"But it raised another question to me. The lens says F4 constant aperture and I paid a lot for that ... I am expecting it to stay constant at f4 :-)"

There are two different points here. Did you mention what lens you are talking about?  It really doesn't matter but let's assume it was a ef 24-105mm f4L (constant aperture) zoom.  When you zoom any lens, say, from 24mm to 105mm you change the focal length and therefore you change the f-ratio. Some lenses do not have special internal systems to cope with this so their f-ratio changes. An example might be the ef-s 55-250mm f4-5.6.  However, a lens like the ef 24-105mm f4L has internal lens groups to correct for this and its f-ratio remains constant.

Now where you seem to jump the track is when we have to add exposure to this equation.  All the pictures you take can not be shot at the constant f4.  So, how do we solve this dilemma?  We add an aperture system that has the ability to reduce the amount of light that hits the sensor. These are called f-stop numbers and although they sound like the same number as your  constant aperture f4 lens they are not.  Bottom line here is your constant f4 lens shot the scene at f4 just like Canon claimed but the camera told the lens aperture system to reduce the amount of light to f9 because that is what made a perfect exposure. If you removed the aperture system from your constant f4 lens, it would shoot all shots at f4 and some would be over exposed.

Consider this, as we wrap this up and I hope I made it more clear for you, all prime lenses are constant aperture lenses. Zoom lenses can be either constant or variable aperture but both still have the aperture system to adjust light coming in and going out. Two, there is nothing inherently better in a constant aperture zoom lens over a variable aperture zoom lens.  And, the down side is cost and weight because they need that extra lens grouping inside to keep the f-ratio constant. Plus they generally are faster lenses and that costs more.

[tesla]

Arrrrrggghhhhh..... I got it now. I was mixing the f label you see on the lens' barrel with the f-stop. The explanation on adjusting the amount of lights hitting the sensor through f-stop brings pieces together. I knew you mentioned this in your earlier post but I did not get it now I did.

In closing this thread, I would like to mention special thanks to:
- @ebiggs1 for your (i) patient in explaining this to me, and (ii) bring the technical details into the explanation, mixed with day-to-day example... made it easier to chew he material so to speak.
- @waddizzle to took the simplicity approach to explain this to me. Keeping me off from being more confused with all the explanations.
- @PLee who has no nicely took me to the beginning on understanding those numbers printed on the lens barrell, the max and min aperture as you zoom in and out. Combined with @ebiggs1 explanation on reducing the amount of light (otherwise you would have over exposure) it nailed the coffin.

Thank you folks! I owe you one.

[ebiggs1]