Technical question about a zoom lens

Just trying to understand whats happening with a zoom lens regarding the variable lens speed. Lets take for instance my Tamron 18-270 (f/3.5-6.3). So as I understand it, the lens speed at 18 mm is 3.5 and at 270mm, 6.3. Now lets do some math:

At 18 mm and f/3.5, the aperture would be 5.14 mm (18/3.5=5.14)

At 270 mm and f/6.3, the aperture would be 42.9 mm. (270/6.3=42.9)

I assume these are effective apertures and not actual correct?? I would assume that the aperture is the exact same opening (its max) whether it is at 18 or 270 mm at the fastest f value for that focal length, Correct??

Any insight or corrections to my logic would be appreciated.

Aperture measurement, or "F-stop", is dimensionless (i.e. you don't express it in millimeters) and it a relative expression of the amount of light that passes through the lens at a focal length. So while it's technically read f/N, the 'N' is really the only thing you need to be concerned with as it is meant to serve as a relative constant across all focal lengths. In other words, f/6.3 represents the same relative amount of light reaching the sensor (or film) regardless of focal length or actual aperture size (i.e. f6.3 at 270mm is wide open, at 18mm it is slightly smaller) allowing for consistent exposures. It's a relative measure of light in vs. light out, so a f/1 would mean that essentially all of the light going into the front element came out the rear element at maximum aperture. The size of the opening at the end of the lens does stay the same, it's just the amount of light that reaches that opening is reduced as you zoom.

So with your 18-270mm f/3.5-6.3, what these numbers represent are the minimum available aperture at the endpoints of the zoom. At 270mm your widest available aperture is 6.3, while at 18mm it is 3.5. If you were to put the camera in Aperture Priority mode, zoom all the way out to 18mm and set the Aperture value to f3.5 and then slowly zoom in you would see the Aperture value change automatically throughout the zoom range, raising eventually to f6.3 when zoomed all the way in to 270mm. This is because as the lens elements move within the body of the lens, light is lost as a part of the shaping process, so while the elements are more compressed (as they are at 18mm) less light is lost, while at maximum expansion (i.e. when you zoom the lens generally gets longer, moving elements apart from each other) light is lost in the re-shaping process - think of it as the light "missing" the element and being absorbed by the lens wall between elements.

It is possible to have a fixed aperture throughout the focal length, like the 70-200mm f/2.8, but you will notice that these lenses have a lot more glass in them - more elements and bigger elements, which preserve the light throughout the zoom range. You'll also notice that most fixed aperture zooms also tend to be fixed length, with all movements occurring between the front and rear elements and not with the barrel getting longer.

Revet said:

I assume these are effective apertures and not actual correct?? I would assume that the aperture is the exact same opening (its max) whether it is at 18 or 270 mm at the fastest f value for that focal length, Correct??

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Probably is same physical diameter. The effective or working aperture diameter computed is not the physical diameter, but instead is as seen by the subject, from the front outside, through the magnification of the front elements. The physical diameter is not necessarily even at a focal node.

The picture at page top Understanding Photographic Aperture f-stop, Shutter Speed, ISO might help. In a zoom, the internal glass lens element positions zoom somewhat (front output angle increases dramatically at wide angle) relative to that physical diameter, which appears as if it sort of zooms back toward sensor, appearing smaller from the front (watch it as you zoom).

Just trust it, f/8 is f/8.

The mm I was depicting was not the f stop but the actual mm of the aperture opening. Why even go there though since I believe this only would work if we had a single lens at a fixed focal length like we did in physics lab. With a compound lens system, what you and Wayne say makes so much more sense. f/1 = 100% light at the sensor, f/1.4 is half of that and so on.

Wayne, if I read you correctly, I am right in that at the 18 and 270 mm focal lengths, the actual aperture size in mm is pretty much the same (maximum opening capable by the lens blades controlling the aperture) but the amount of light reaching the sensor is less at the 270mm (f/6.3)

Revet said:

Wayne, if I read you correctly, I am right in that at the 18 and 270 mm focal lengths, the actual aperture size in mm is pretty much the same (maximum opening capable by the lens blades controlling the aperture) but the amount of light reaching the sensor is less at the 270mm (f/6.3)

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That is my belief, but it is more just an assumption. The diameter change we see looking in the front during zoom is clearly just an optical magnification view.

There are zooms that do offer constant max aperture regardless of zoom, which is much harder. The design of the variable max aperture lenses is less expensive, they don't have to worry with the aperture diameter.

This is a related question so I will keep it here and not start a new thread. I was shooting with my Tamron 60mm macro f/2.0 lens last night getting a picture for this months challenge. I had it on Manual setting and the lowest f stop I could achieve was 2.4. I was taking some more pictures this morning and I got it down to 2.2. I am pretty sure I have seen it at 2.0 before. Since this is a fixed focal length lens, why is the lowest f stop attainable variable???

The aperture you select will vary depending on the focus distance, not the zoom range like a typical variable aperture lens. I had the same confusion when I first used the lens.
I assume it's set up this way because it's the only way to achieve focus up close.

Focal length actually changes as the focus mechanism moves the elements. You can notice this, especially on a macro as you focus and the blur seems to enlarge/reduce. As focal length changes the f/stop changes since f/stop is a product of aperture diameter and focal length.

Eyelight said:

Focal length actually changes as the focus mechanism moves the elements. You can notice this, especially on a macro as you focus and the blur seems to enlarge/reduce. As focal length changes the f/stop changes since f/stop is a product of aperture diameter and focal length.

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So does that mean my focal length is not actually fixed at 60 mm on this macro lens?? I think you were responding to that thread I just put on.

Revet said:

So does that mean my focal length is not actually fixed at 60 mm on this macro lens?? I think you were responding to that thread I just put on.

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Correct. The designated focal length of a lens is when it is focused at infinity.

Revet said:

So does that mean my focal length is not actually fixed at 60 mm on this macro lens?? I think you were responding to that thread I just put on.

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The marked focal length is at infinity focus. The focal length increases as the lens is extended to focus closer (lens is farther from sensor plane). This is not much issue at mild close ups (up to say 1:10, but it is at f/1.1). Generally at 1:1, the focal length becomes 2x the focal length at infinity. Also note that 1:1 means subject distance is equal to sensor distance (focal length). Since f/stop number = focal length / aperture diameter, then 2x focal length means 2x focal length number, which is as if two f/stops stopped down (at same maximum diameter, so f/2.8 1:1 becomes f.5.6... but internal focus lenses might be slightly less, maybe f/5). Macro lenses today know to keep up with the correct f/number, and they report the actual adjusted number, so at closer distances, it appears not to open to f/2.8. It is fully open, it is just not f/2.8 at extreme closeup.

Of course, we would not use f/2.8 at 1;1, but the f/16 we do use computes f/8 at infinity. f/32 is the previous f/16.

Near page bottom at Field of View (FOV) Calculator is the "thin lens equation" which is a simple explanation of this 2x fact.