topic Re: Lens Aperture vs F-Stops on camera in EF & RF Lenses

Lens Aperture vs F-Stops on camera

tesla — Mon, 03 Aug 2020 12:24:53 GMT

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

Re: Lens Aperture vs F-Stops on camera

kvbarkley — Mon, 03 Aug 2020 13:06:48 GMT

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.

Re: Lens Aperture vs F-Stops on camera

Waddizzle — Mon, 03 Aug 2020 14:56:14 GMT

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.

Re: Lens Aperture vs F-Stops on camera

ebiggs1 — Mon, 03 Aug 2020 15:55:15 GMT

"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.

Re: Lens Aperture vs F-Stops on camera

ebiggs1 — Mon, 03 Aug 2020 15:57:11 GMT

" "natural" aperture"

???

Re: Lens Aperture vs F-Stops on camera

Ray-uk — Tue, 04 Aug 2020 14:28:54 GMT

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.

Re: Lens Aperture vs F-Stops on camera

ebiggs1 — Tue, 04 Aug 2020 15:06:22 GMT

"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.

Re: Lens Aperture vs F-Stops on camera

Ray-uk — Tue, 04 Aug 2020 16:25:48 GMT

@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.

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.

Re: Lens Aperture vs F-Stops on camera

Waddizzle — Tue, 04 Aug 2020 16:33:49 GMT

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

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

Same here. Kept it simple.

Re: Lens Aperture vs F-Stops on camera

jrhoffman75 — Tue, 04 Aug 2020 23:16:16 GMT

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

Re: Lens Aperture vs F-Stops on camera

ebiggs1 — Wed, 05 Aug 2020 14:32:25 GMT

"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.

Re: Lens Aperture vs F-Stops on camera

ebiggs1 — Wed, 05 Aug 2020 14:42:45 GMT

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.

Re: Lens Aperture vs F-Stops on camera

PLee — Wed, 05 Aug 2020 16:20:24 GMT

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.

Re: Lens Aperture vs F-Stops on camera

Waddizzle — Wed, 05 Aug 2020 16:40:08 GMT

@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.

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.

Re: Lens Aperture vs F-Stops on camera

ebiggs1 — Thu, 06 Aug 2020 15:01:35 GMT

"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."

Re: Lens Aperture vs F-Stops on camera

tesla — Fri, 07 Aug 2020 14:17:48 GMT

@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....

Re: Lens Aperture vs F-Stops on camera

jrhoffman75 — Fri, 07 Aug 2020 14:24:59 GMT

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.

Re: Lens Aperture vs F-Stops on camera

Waddizzle — Fri, 07 Aug 2020 14:29:33 GMT

@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....

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.

Re: Lens Aperture vs F-Stops on camera

tesla — Fri, 07 Aug 2020 14:32:17 GMT

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.

Re: Lens Aperture vs F-Stops on camera

Waddizzle — Fri, 07 Aug 2020 14:35:28 GMT

@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.

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.