Question about variable zoom lenses and aperture

[stevet1]

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

[shadowsports]

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.  😊

[ebiggs1]

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.

[normadel]

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.

[AtticusLake]

> 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.

[ebiggs1]

" 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. 

[AtticusLake]

> 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.

[justadude]

"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

[SignifDigits]

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.

[ebiggs1]

"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."

[SignifDigits]

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.