Variable Aperture zooms. How do they work?

J

JaredSMark

Active member
As far as I understand it, when you zoom in the F-number changes because the entrance pupil's apparent size is changing due to how some of the glass on the inside of the lens is magnifying it. Since I think the aperture diameter itself isn't changing that means you should still get the same depth of field. Is that right? I wonder if the amount if light is changing though.

Also, I'm guessing calling it a Variable Aperture zoom lens is somewhat of a misnomer since it's the entrance pupil that's changing and not the aperture size itself. Variable F-Stop is maybe more accurate.

I've read about constant F-Stop zoom lenses but I'm unsure how those would work. The entrance pupil somehow stays the same size and 'keeps up' with the focal length, so I've read. Does someone know more about this? I'm not sure why this would even be necessary since I don't think the depth of field changes when you zoom, so maybe it's more about the light that's coming into the camera or something.
 
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The F-stop is not a measurement of the size of the aperture ring, but instead a relationship between that ring size and the entrance pupil of the lens. The limitation of the f-stop happens when zooming in and the entire size of the front entrance pupil is no longer being utilized for the gathering of light, generally because a front lens group is moveing forward and only allowing light to enter from the center of the front element. So even though the front element is physically the same size, the "virtual" size is reduced because the extra area is ignored by the rest of the lens.

Does that help?
 
I thought the F-Stop number was a relationship between the entrance pupil and the focal length?

It's looking like the there actually IS a change of light when zooming in, since that's kind of what the F-Stop number is telling us, hence why it's changing. So that should be pretty much the only difference when zooming.. That's what I get from all that I'm reading but correct me if I'm wrong.
 
Constant aperture lenses I'm aware of tend to have quite short zoom ranges. In fact 17-50mm, 70-200mm, 28-85mm...looking at these they are all pretty close to 3x. Maybe that has something to do with it. Perhaps there is less variation in the range in say a 10x zoom. If there is a non linear relationship of say a logarithmic or exponential nature, then over a short range it could be quite minimal yet which could become significant quite quickly beyond that. It may be that constant aperture isn't really constant but that the variation is sufficiently minimal that it might not make a subjective difference or easily corrected. I'm making this up as I go along, just speculating. I have no idea.
 
Ignore everything I said. I just realised the lens I just bought for my BMPCC is a parfocal 10x constant aperture zoom.
 
The f-stop of a lens is the ratio of its focal length to the diameter of its iris aperture. If the iris remains the same size as the lens is zoomed to a longer focal length, then that ratio changes and the lens is referred to as being "variable aperture" (although it would be more accurately termed "variable f-stop").

Conversely, "constant aperture" zooms like my Canon FD 28-85mm f4 change the physical diameter of their aperture as the lens zooms, so that the f-stop stays constant.
 
Lenses have physical dimensions. You could take a ruler to it and measure its physical length, say, 50mm, and its iris at a given f-stop setting, say, 25mm. Exposure is measured in f-stops, which is the length divided by the iris, which in this case would be 50 / 25, so f-2. But I lied.

Lens optics is voodoo. A lens's focal length is not its physical length, and its aperture is not necessarily the diameter of its physical iris. You can try reading the Wikipedia articles on it, but my brain locks up: https://en.wikipedia.org/wiki/Focal_length, https://en.wikipedia.org/wiki/Aperture.

However, after spending 20 seconds googling "how do constant aperture zoom lenses work" I came across this quote:

There are a couple of related misconceptions about lens design, and once you understand them the whole thing about constant vs. variable aperture zooms makes more sense. The first is that the size of the aperture in question is the actual size of the physical aperture in the lens. The related misunderstanding about constant aperture zooms is that they have to change the physical aperture size as they zoom in order to maintain their constant f-number.

Actually, the f-number depends on the size of the "virtual aperture", that is the apparent size of the physical aperture as it's seen through the lens. If the lens has a positive front group*, like most normal and longer lenses, the positive group works as a magnifier, so the virtual aperture is larger than the physical aperture. If the front group is negative, like retrofocus lenses, the negative group makes the virtual aperture smaller than the physical aperture.

When you zoom a zoom lens, it works by changing the magnification of the front group, the rear group, or both. If it changes only the front group, the change in the focal length of the lens is exactly the same as the change in the magnification to the aperture, so the f-number remains constant as you zoom. If you change the magnification of the rear group, the change to the lens as a whole will be smaller than the change to the front group, so the f-number will change as the lens zooms. If you made a lens that worked by changing only the rear group, the change in aperture would be as big as the change in focal length. Some lenses actually do that; AFAIK it's restricted to very long telephoto zooms. In practice, most lenses do the majority of their zooming with the front group.

As I understand it, there are a few constant aperture lenses that do change the physical aperture. The designers wanted to make a constant aperture lens for practical or marketing reasons, but the best optical choice involved a design that had some zooming with the rear group. In that case, it's possible to cheat by mechanically linking the physical aperture to the zoom mechanism, but it's very rare to do so.
Click to expand...
 
Thanks, that clears up the fact my 10x doesn't appear to alter the iris size throughout it's range (something I checked by eye after reading Graham's post). I guess it can be done either cheap and compact with fine control of the iris in live lenses like Graham's or more expensively through careful arrangement of optics like my weighty manual zoom.
 
I forgot about this whole thing.

Mitch said that the entrance pupil changes when you zoom and my lens does appear to do that, so that's one thing. The other thing is that the focal length is changing, and since the F-number is the division between the focal length and the entrance pupil, I can see how the F-number would change. It seems like the focal length has more to do with the changing F-number than the entrance pupil though, but both probably have something to do with it.

I've seen videos and read a lot about how focal length itself has nothing to do with depth of field, and I agree. However what I was mostly wondering about is whether or not there's a change in the amount of light and depth of field when zooming, with focal length taken out of the equation. If the entrance pupil is changing, and it does seem like it is, then that means the light it can capture and the depth of field should be different when zoomed in too. Maybe only very slightly though, since the entrance pupil doesn't change much. The focal length doesn't seem to have anything to do with those changes (only the F-number).

I think that's right anyway. My brain can't take too much more of this :tongue:
 
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