topic Re: Question about variable zoom lenses and aperture in EF & RF Lenses

Question about variable zoom lenses and aperture

stevet1 — Thu, 04 Dec 2025 18:34:54 GMT

As I understand it, with a variable zoom lens, as you zoom in, your available aperture gets smaller..

As you zoom in, the amount of light hitting your sensor is reduced. If your aperture gets smaller, that reduces the amount of light hitting your sensor gets reduced even more.

That seems to be counter intuitive to me.

What am I missing, or don't know?

Steve Thomas

Re: Question about variable zoom lenses and aperture

SignifDigits — Thu, 04 Dec 2025 21:40:55 GMT

Now I'm curious about this too.

There are (I own two) fixed aperture zoom lenses, and I'm wondering how they achieve that, as well. I'm guessing that the (typically less expensive and lighter) zoom lenses really have a fixed aperture and the smaller field of view delivers less illuminance/lux/foot candles to the sensor, and that the f-numbers are really just the loss of illuminance (effectively smaller aperture) by that design. If so, then a fixed aperture lens would have to somehow have an expanding physical aperture relative to the narrowing field of view as you zoom in. That would explain the heavier and more expensive nature of them. I'm sure someone here will know and tell us.

Re: Question about variable zoom lenses and aperture

TomRamsey — Thu, 04 Dec 2025 22:25:07 GMT

Think about if you are standing in a tunnel, if you are close to the end you are hit by a lot of light, go deeper in the tunnel and there is less. In a lens it is about getting light to the sensor and it should fill the entire sensor. That gets more difficult as the lens get physically longer, which is why fixed aperture zooms are bigger around. This is the very simple version, the lens elements are also involved in this.

Re: Question about variable zoom lenses and aperture

normadel — Thu, 04 Dec 2025 23:18:33 GMT

What do you mean by "As you zoom in, the amount of light hitting your sensor is reduced"? This is what happens when the maximum aperture is reduced at long focal length. This is what "f/4.5-5.6" in a lens name means. This is what a variable aperture lens means. In order for a lens to keep its maximum aperture (f/number) throughout its zoom range, it becomes a heavier, more expensive lens to design and build.

So YES, the amount of light hitting the sensor at max aperture gets reduced by zooming in....because the maximum aperture gets smaller. Your autoexposure system takes care of this, by slowing the shutter, increasing ISO, etc.

Re: Question about variable zoom lenses and aperture

stevet1 — Fri, 05 Dec 2025 02:37:14 GMT

What do you mean by "As you zoom in, the amount of light hitting your sensor is reduced"?

As you zoom in, your field of view gets smaller. Ergo, less light is coming in.

It's got nothing to do with your aperture per se.

Steve Thomas

Re: Question about variable zoom lenses and aperture

AtticusLake — Fri, 05 Dec 2025 09:55:21 GMT

It's important to understand that YOUR F-NUMBER IS NOT YOUR APERTURE. Yes, I know, many people will tell you that it is, but many people are wrong. If you look at the definition of F-number, for example on Wikipedia, you will see that F-number is FOCAL LENGTH DIVIDED BY APERTURE (where aperture is the diameter of the entrance pupil).

In other words, the F-number depends on BOTH focal length AND aperture.

The F-number is actually a measure of the amount of light coming in through the lens. As you reduce the aperture, this is obviously reduced; but as you zoom in, it is also reduced. Zooming in means you're selecting a smaller part of the scene, so obviously you're getting less light. The intensity of light in a given part of the scene isn't changing, but you're choosing less of it.

This is why, in general, zoom lenses have a variable maximum aperture.

Some zoom lenses have clever optical engineering to compensate, so that they can keep a constant maximum aperture. If "clever optical engineering" sounds like "big, heavy, and expensive", then you're dead on. In other words you can have a constant aperture zoom if you're willing to pay for it (in money, size, and weight), though generally this only works on lenses with smaller zoom ranges, like a 3:1 zoom. A 24-70 can do this, for example.

Hope this helps.

BTW, F-numbers are dumb. They go the wrong way because they are focal length divided by aperture; it should be the other way round. And they go in stupid steps because they are based on aperture diameter, when the amount of light you get actually depends on the area.

Re: Question about variable zoom lenses and aperture

SignifDigits — Fri, 05 Dec 2025 14:26:36 GMT

And all these years I thought I was dumb because the f-numbers were so counterintuitive! I'm glad to hear the f-stops were really the dumb ones! Seriously, I'm glad to hear someone else thinks it. Who thought "lets make this really confusing and use these numbers that go the opposite way of what the person using the camera"? Probably someone before cameras decided this was the convention for optics and it got carried over to cameras.

So my first guess was correct in that most lenses have a fixed aperture and f-numbers rise zooming due to the smaller FOV/light. I will have to find an article on the "clever optical engineering". I assume the geometry of the optical elements move around and the aperture really just is still physically the same. That is just basic optics/physics, but still beyond my abilities.

And there ARE a LOT of sources that say they are the same!

Re: Question about variable zoom lenses and aperture

ebiggs1 — Fri, 05 Dec 2025 15:36:57 GMT

Some good info here and some not so good. A single lens, just the piece of glass, has a focal length and that FL is used to calculate its f-number. The FL is the focus point where the parallel lines of light converge. The very same method is used for a complex camera lens. The same principle of f-number is consistent. A way to measure the light gathering ability.

In a complex camera lens, the elements in front of the actual diaphragm magnify or unmagnify the arbitrary size of the lens and f-number. Two main things come into play here. First, and most important, the cost and will anybody buy the lens at that cost. And secondly, how heavy will it be as magical elements are heavy. If the designers had carte blanche there isn't anything they couldn't design and build but would it be practical and/or would anyone buy it. Much more would they be able to carry and use it.

Re: Question about variable zoom lenses and aperture

AtticusLake — Fri, 05 Dec 2025 15:46:16 GMT

@SignifDigits wrote:

Probably someone before cameras decided this was the convention for optics and it got carried over to cameras.

I think that's more or less it... I think it was from when you would set the aperture by measuring it with a ruler, so the formula probably made sense then. But today, in modern photography, it's really just not helpful.

Re: Question about variable zoom lenses and aperture

AtticusLake — Fri, 05 Dec 2025 15:50:42 GMT

@AtticusLake wrote:

This is why, in general, zoom lenses have a variable maximum aperture.

Ugh, I meant "variable maximum f-number". See, even I'm doing it.

... so that they can keep a constant maximum aperture.

F-NUMBER!!!! <smacks forehead>

Re: Question about variable zoom lenses and aperture

shadowsports — Fri, 05 Dec 2025 16:03:21 GMT

Greetings,

I think relevant points have been made. Some use the terms fixed and variable aperture. Others (myself included) use constant and variable to describe the differences between the two lens types. Ian and Ernie both described it well. Its all in the lens design.

In most lenses the diaphragm sits at or near the middle of the lens's focal length.

Variable aperture: Requires a less complex lens design. As focal length increases less emphasis is made to maintain the amount of light capable of entering the lens pupil as focal length increases.

Constant aperture: Requires a more complex lens design for lenses with longer focal lengths. It's not a hard and fast rule. In some longer focal length constant aperture lens designs, the diaphragm sits in between the lens groups. In variable aperture lenses, the diaphragm is behind the lens group. The former is a more complex and more expensive design, and as the guys said, equates to "bigger, heavier and more expensive".

Great thread Steve. 😊

Re: Question about variable zoom lenses and aperture

ebiggs1 — Fri, 05 Dec 2025 16:27:23 GMT

Always fun to think and discuss this stuff. Also to see what others believe to be so and what perhaps is not so, well, so.

Re: Question about variable zoom lenses and aperture

normadel — Fri, 05 Dec 2025 18:03:27 GMT

There is so much confusion going on here I can't even follow it all.

There's nothing at all "backwards" about expressing lens aperture as an f/number. F/number simply means the aperture DIAMETER is equal to the focal length divided by the bottom number. When you go from full stop to full stop (f/2, 2.8, 4, 5.6, 8,11,16,22etc. you are doubling the AREA of the aperture opening. Thus, doubling light gathering ABILITY. How much light hits the sensor is a combined result of aperture size and light brightness.

YES, light hitting the film is increased/decreased with changes in focal length (zoom level). When you zoom, the distance from the lens nodal point changes. Light falls off the further it has to go to the sensor.

AS to the original question, changing angle of view (by zooming) changes brightness of light hitting because the lens is seeing different area of a scene that is more or less bright. That makes perfect sense. But if you are viewing, say, a large white wall, or a cloudless sky, all of it is the same brightness, so what the lens sees is the same at different angles of view.

Re: Question about variable zoom lenses and aperture

AtticusLake — Sat, 06 Dec 2025 09:15:52 GMT

> There's nothing at all "backwards" about expressing lens aperture as an f/number.

Well, it's a matter of opinion, I guess. But I can't count how many times I've seen people explaining to newbies that a LARGER f/ number means LESS light, and a SMALLER number means MORE light. Give how often this needs to be explained, it seems fair to assume that some people see it as backwards.

> F/number simply means the aperture DIAMETER ... you are doubling the AREA of the aperture opening....

Yes, this is how f/ numbers work. But that doesn't make it intuitive or easy, particularly for people wanting to get into photography. If you find the number sequence 2, 2.8, 4, 5.6, 8, 11, etc. natural and intuitive, that's fine, but I think some people at least would see it differently. And completely inconsistent with how shutter speed works.

The point I'm making is not that the f/ number system doesn't work - it absolutely works, and can be and is used to calculate exposure perfectly well. The point is that it's weird and counter-intuitive, and this creates a barrier for newcomers wanting to get into photography.

> When you zoom, the distance from the lens nodal point changes. Light falls off the further it has to go to the sensor.

I really don't see what you're getting at here. Zooming doesn't change the distance between the subject and the sensor. Only moving the camera or subject can do that.

> But if you are viewing, say, a large white wall, or a cloudless sky, all of it is the same brightness, so what the lens sees is the same at different angles of view.

Well... the point is that when you zoom in, you see LESS of the white wall, or cloudy sky. And hence get less light. Which is why the f/number - brightness - changes.

Re: Question about variable zoom lenses and aperture

ebiggs1 — Sat, 06 Dec 2025 15:03:18 GMT

" I can't count how many times I've seen people explaining to newbies that a LARGER f/ number means LESS light, and a SMALLER number means MORE light."

Then these folks probably slept through middle school math class the day they learned about ratios.

"...all of it is the same brightness, so what the lens sees is the same at different angles of view."

AtticusLake that is correct. What you are missing and confusing is brightness with total amount of light. The brightness remains the same. That does not change but the total amount of light is different since you are seeing less of the wall and/or sky.

Re: Question about variable zoom lenses and aperture

AtticusLake — Sat, 06 Dec 2025 15:15:44 GMT

> AtticusLake that is correct. What you are missing and confusing is brightness with total amount of light. The brightness remains the same. That does not change but the total amount of light is different since you are seeing less of the wall and/or sky.

Sorry, I didn't make myself clear. When I said "brightness", in terms of F-number, I mean the brightness of the lens, i.e. the total amount of light coming through the lens. The brightness of the scene itself, of course, does not change.

Re: Question about variable zoom lenses and aperture

justadude — Sat, 06 Dec 2025 15:31:02 GMT

"BTW, F-numbers are dumb. They go the wrong way because they are focal length divided by aperture; it should be the other way round."

This is the exact reason why so many people that are new to photography struggle to understand how aperture works. Explaining the exposure triangle to someone new, there have been many times I've been told by the new photographer "I think you are explaining part of that backwards" or "Doesn't a bigger number mean more?" or something similar. I'll typically reply "Think of it as a golf score where a lower number is better... in this case meaning more light." Sometimes I get a blank stare after saying this. lol

Re: Question about variable zoom lenses and aperture

SignifDigits — Sat, 06 Dec 2025 16:13:07 GMT

I was an engineer, managed PhDs in physics, had enough hours to minor in math. For a while worked on optical simulations and ray tracing algorithms. I understood the math, luminance candellas per meter squared, lux, etc. I understand 1/r2 degradation in electromagnetic radiation in atmosphere and the full electromagnetic spectrum including light. And linear versus logarithmic scaling. I think my first post demonstrates that I'm smart enough to intuitively understand what's happening. I have still struggled with the the f-stops application as it was counter-intuitive to me personally. I still don't like them. Enough shooting is the only thing that has lodged it into my brain and I still haven't shot enough that I don't have to think about it every time I take a shot and release the shutter. I'm sure they are elegant in a mathematical way. Maybe I am just a clod in that respect.

Re: Question about variable zoom lenses and aperture

ebiggs1 — Sun, 07 Dec 2025 15:17:08 GMT

"I was an engineer, managed PhDs in physics,..."

I have no degree in physics or engineering but I don't find it difficult at all. "... folks probably slept through middle school math class the day they learned about ratios."

Re: Question about variable zoom lenses and aperture

SignifDigits — Sun, 07 Dec 2025 22:47:02 GMT

Of course you are correct that degrees have nothing to do with it. I think it's great for you that you never struggled with a number which increases and represents a logarithmic scale of a decreasing - halving - of the luminance exposing the film and/or sensor at each increment and each increment also indicating an approximate linear doubling of the depth-of field . This relationship has, it seems, not proven intuitive to everyone - even to those who have some grasp of the physics and mathematics behind what I just said.

I felt it was important for me to speak up for those of us who have struggled and to offer a word of encouragement to them. It is an elegant system knowing that each stop you move you are halving (or doubling, depending upon direction) exposure. It has been harder for some of to catch on that higher is lower and vice-versa when it comes to exposure than you. It doesn't help that, higher is up for depth of field, even though I understand the cone of focus. I, for one, would appreciate your patience and compassion in the struggle to overcome this weakness and I learn to become more like you and others on this list who haven't struggled with this issue. For me, it got conflated in my brain early and is still not as intuitive as I wish it were, sadly.

I'm VERY thankful for mirrorless cameras that allow me to visually learn (as I am a visual and tactile learner) and pray over time that I will erase this "bug" in my brain.

I have top-of-the-chart spacial reasoning skills, and can't remember names worth anything. I often am amazed at people struggling with a map, as I assume everyone can read one like I can, or visualize spaces in a building in their heads like I can. Of course, with age all of that ability has declined. I have learned that we all have gifts and differences and I need to work to celebrate differences and become more compassionate - with others and with myself as I cannot do some of the things that I used to be able to do and that trend is likely to continue.