Musing about fixed max aperture zooms . . .

Bear with me, as I talk this out. Innocent daydreaming about focal length and aperture ratios has led me to a question that must have a simple answer . . .

Take a 24-105 f4 for example, for ease of mathematics. For f4 at 105mm, the physical aperture must be about 26mm in diameter.

But for f4 at 24mm, the physical aperture is about 6mm (to satisfy the ratio).

Given that it is the same aperture apparatus, why is it that you can't open the aperture to 26mm when the focal length is 24mm, and therefore have a ratio of 1 (give or take)?

I'm assuming that there is a simple explanation. Before I outline any hare-brained ideas that occurred to me, can someone with real knowledge explain this?

Think about it.

I'm surprised you cant figure it out for yourself.

Neither can I though...!

dodgyexposure wrote in post #16688271
Bear with me, as I talk this out. Innocent daydreaming about focal length and aperture ratios has led me to a question that must have a simple answer . . .

Take a 24-105 f4 for example, for ease of mathematics. For f4 at 105mm, the physical aperture must be about 26mm in diameter.

But for f4 at 24mm, the physical aperture is about 6mm (to satisfy the ratio).

Given that it is the same aperture apparatus, why is it that you can't open the aperture to 26mm when the focal length is 24mm, and therefore have a ratio of 1 (give or take)?

I'm assuming that there is a simple explanation. Before I outline any hare-brained ideas that occurred to me, can someone with real knowledge explain this?

Heya,

I imagine it's because the lens is physically in a different spot, it moves forward and backwards inside, right? So it's not as simple as just considering it's ratio the same and going with aperture/length. At 26mm aperture opening, with the lens at 24mm, I think the internal lens moves forward, further away from the rear element. And at 105mm, it's closest to the back, near the element.

Hell I don't know. This is why I don't make these for a living.

Very best,

MalVeauX wrote in post #16688277
Heya,

I imagine it's because the lens is physically in a different spot, it moves forward and backwards inside, right? So it's not as simple as just considering it's ratio the same and going with aperture/length. At 26mm aperture opening, with the lens at 24mm, I think the internal lens moves forward, further away from the rear element. And at 105mm, it's closest to the back, near the element.

Hell I don't know. This is why I don't make these for a living.

Very best,

^ This is along with my thinking.

Have you ever looked down the front element and fiddled with a lens? Zoom in and out, play with the DOF preview button to close the iris and see what it looks like and what it does? If you do, you will notice that it physically looks smaller when zoomed out and larger when zoomed in.

My only guess why this is would be because lenses are basically a group of magnifying glasses working together to create a clear image. And when positioned to magnify as weakly as possible or maybe demagnify (zoomed out) they make things appear smaller, such as the aperture iris.

I've wondered the same thing... was thinking about it just a few days ago.
Good question, i look forward to the answer(s).

The f/number isn't actually determined by the physical size of the aperture blade opening, but by the optical image of the aperture as seen through the front of the lens. As the zoom range changes the perceived size will also change.

http://en.wikipedia.or​g/wiki/F-number

http://en.wikipedia.or​g/wiki/Entrance_pupil

Hopefully this will help confuse things even more.

Thanks! I thought that the answer would be optical, but didn't expect it to be definitional!