Zoom lens apertures

[David Lyga]

One thing that I always wondered about was zoom lenses with FIXED maximum apertures. In other words, zooms, say 70 - 200, can have a maximum aperture of, for example, f4 to f5.6; i.e., the actual maximum changes with focal length.

However, there are some zooms that retain the SAME aperture throughout the focal length range. How can this be achieved? - David Lyga

 

[DWThomas]

I suspect it's mostly a combination of where in the pile of optics the aperture is positioned and the amount of glass used, but maybe also some mechanical wizardry. I own a couple of f/2.8 zooms for my Canon EOS gear, one is the classic original 24-70mm L series. That sucker has something like 16 elements in 13 groups, is huge and heavy, and there is a lot of mysterious stuff going on inside one can feel a bit as one cranks it in and out. Presumably the elements have to be large enough in diameter to support the maximum aperture f-number at the longest focal length setting. that takes more glass, more weight, and yeah, more $$$, which would be why the variable stop versions are cheaper.

 

[AgX]

The front lens is big enough to warrant the F-value for the long FL, and too big for the short FL, but in the latter case the surplus light is not used.

 

[Chan Tran]

One thing that I always wondered about was zoom lenses with FIXED maximum apertures. In other words, zooms, say 70 - 200, can have a maximum aperture of, for example, f4 to f5.6; i.e., the actual maximum changes with focal length.

However, there are some zooms that retain the SAME aperture throughout the focal length range. How can this be achieved? - David Lyga

I don't know but even with lens with variable aperture and if the opening is the same diameter then most zoom still change the aperture very little. For example my 70-300 is f/4-f/5.6 that's only 1.4 times yet the focal length changes by more than 4 times.

 

[BrianVS]

Most zoom lenses made before 1980 retained constant aperture. Variable aperture zooms are slow to use with manual focus cameras. Once Auto-Exposure became the norm, so did variable aperture zooms. Less glass in them, as the optics do not have to be large enough to maintain a constant aperture at the maximum setting. There are still constant aperture zoom lenses being made- the more expensive ones.

Think of a zoom lens as a fixed lens with an Auxiliary lenses built into it. The F-Stop is maintained up to a point. Put a 1.6x Aux telephoto onto the front of a 135/3.5, get a 180/3.5. Put the same 1.6x aux tele onto a 135/2, you might get a 180/3.2. The diameter of the Aux lens is not large enough for F2, but is good enough for f3.5.

 

[David Lyga]

I think that I now have a clearer understanding. I had assumed, correctly, that maintenance of a fixed aperture involved a bit of 'waste', in that the front element was, at some focal lengths, bigger than it had to be. Thank you. - David Lyga

 

[bernard_L]

David, you might perform a simple experiment. Zoom lens mounted on a body (some lenses, e.g. FD, will not achieve max f-number if not mounted). Shutter open (Bulb). Look from the front as you zoom. You can see an image of the open diaphragm through a number of elements; technically called the input pupil. That is the effective aperture, not the front lens, nor the physical diaphragm. As you zoom, the magnification of the above-mentioned image changes; If designed for constant f-number, its diameter will be in proportion to the focal length. And, before declaring that parts of the front lens are "wasted" at the shorter focal lengths, think that the lens works not only on-axis, but over the angular extent of the field of view... which is larger at shorter focal lengths.

 

[AgX]

The angle of view of a lens has nothing to do with its front lens diameter.

 

[ic-racer]

If you ask me, I'd say modern zoom lenses are the most sophisticated and highly engineered optical devices ever created. How or why the aperture changes through the zoom range likely depends on a myriad of design factors.

 

[bernard_L]

The angle of view of a lens has nothing to do with its front lens diameter.

Please let me restate. Sorry if my explanation is longer than your remark.

I was referring to the statement by David in his post #6:

I had assumed, correctly, that maintenance of a fixed aperture involved a bit of 'waste', in that the front element was, at some focal lengths, bigger than it had to be.

and assuming that, looking into the zoom, especially at the short FL setting, he would see the input pupil (image of the diaphragm) to be significantly smaller than the diameter of the front lens. In that respect, there seems to be "wasted area" in the front lens. But the same input pupil, at that short FL setting, has to collect light not only from bore-sight, but also from off-axis points of the scene. Still staring into the lens as I had suggested, but moving your eye to the vantage point of a scene point at the edge of the FOV, you will see that the input pupil appears to move close to the edge of the front element. In that respect, the front lens area is not "wasted" as much as one might think at first sight.

 

[Nodda Duma]

The answers here are correct: Control of the pupil magnification during the design of the zoom will maintain a fixed f/# over the entire zoom range. The trade-offs are larger optics or greater number of elements (increase of cost to maintain the f/#). Photographic zooms have a relatively small zoom range (I've designed 20x and 100x infrared zooms), so that particular trade-off isn't a big one in your world...I could see it being taken into consideration within the details of the design requirements (specific focal length range, size/weight, and price point).

I think that I now have a clearer understanding. I had assumed, correctly, that maintenance of a fixed aperture involved a bit of 'waste', in that the front element was, at some focal lengths, bigger than it had to be. Thank you. - David Lyga

It's not always wasted, and it's not a function of maintaining a fixed f/#. I've seen all cases. What actually happens often in a zoom design is that at some zoom settings (typically the shorter focal length), the light rays for different field angles intercept the front element at different distances from center. At other zoom settings (typically the longest focal length), the rays for all field angles come close to filling the front element. It's all in the details of the design. This is most clearly seen in a cutaway ray trace. For example, consider the cutaway layout below of a classic 100-200mm f/5 zoom for 35mm format. This shows optical elements and light ray paths for four different zoom settings ranging from the shortest focal length at top to longest focal length at bottom. The different colors represent different field angles. Blue is 0 degrees and red and green are increasing field angles (the actual numbers aren't important). In this particular case, the maximum f/# is fixed at 5 throughout the zoom range. As you can see, pretty much the entire diameter of the lens elements are used throughout the zoom range. This is advantageous in some cases from a stray light perspective, but it often depends on what can be reasonably traded off.

So bottom line, fixed maximum f/# is fairly easy to design for and is often specified in the design requirements, but sometimes the customer doesn't care or other things are important (like cost and size) and so it isn't. To be honest it doesn't really impact the overall design process for a zoom system or even compared to a fixed focal length lens except for added mechanical complexity.

One other thought: If I needed to reduce the size and weight of this design, I could reduce the diameter of the front two elements by a couple of millimeters. This would trade off vignetting at longer focal lengths for reduced size and weight..

 

[Nodda Duma]

If you ask me, I'd say modern zoom lenses are the most sophisticated and highly engineered optical devices ever created. How or why the aperture changes through the zoom range likely depends on a myriad of design factors.

They are complex, but not nearly the most complex.

 

[AgX]

But "wasted" in the sense that at short FL setting there are rays that hit the front element but not the film though entering in parallel to rays that are well accepted.

 

[Nodda Duma]

But "wasted" in the sense that at short FL setting there are rays that hit the front element but not the film though entering in parallel to rays that are well accepted.

I wouldn't talk about "wasted" glass... the semantics imply that the optics could be smaller. It's like you're saying the blue response layer of color film is "wasted" because it doesn't respond to red. It doesn't make sense...of course the blue layer isn't "wasted". Very simply if the optics were smaller to remove this incorrectly perceived "waste," you would have vignetting.

It's better to say the entire clear aperture of the front element is not filled by rays from all field angles. Even better, say what you mean: The aperture stop isn't on the front element. That's not just a zoom lens characteristic. It applies to *any* optical system where the aperture is not on the front element. Imagine the ray paths through a prime focus double gauss (i.e. a Planar) or a Cooke Triplet.


Anyways, to answer David's original question:

One thing that I always wondered about was zoom lenses with FIXED maximum apertures. In other words, zooms, say 70 - 200, can have a maximum aperture of, for example, f4 to f5.6; i.e., the actual maximum changes with focal length.

However, there are some zooms that retain the SAME aperture throughout the focal length range. How can this be achieved? - David Lyga

It's just a feature to optimize for during the design if the requirements ask for it, or to include if it doesn't cause some other parameter to fail to meet a requirement (like size or price point). Not a big deal, really... The more I think of it, I'm pretty sure zoom designs naturally tend to gravitate towards an approximately fixed max aperture (it's been a few years). To really nail it down requires a bit of control.

 

[bernard_L]

They are complex, but not nearly the most complex.

+1 !
Two examples:

• Adaptative optics used in Vis-NIR astronomy. Deformable mirror with hundreds of actuators updated approx every millisecond. CPU-starved.
  https://www.eso.org/sci/facilities/develop/ao/what_ao.html
• Microlithographic lenses for deep UV exposure. Used to make your smartphone.
  http://www.lithoguru.com/scientist/...lens_history_and_evolution_Matsuyama_2006.pdf
  (jump to page 6)

 

[Nodda Duma]

Yes, microlithographic lenses and modern observatory optical systems are definitely top contenders for most complex!

Another one is probably (deceptively simple looking) designs using new gradiant index materials in plastic and chalcogenide glasses just making their way out of the research labs ... I can count on one hand the number of optical designers in the world that can successfully put together a design using those materials.

 

[cooltouch]

I feel that it is worth pointing out here that not all constant aperture zooms are indeed constant aperture. I own one that lies. It is a Kalimar 28-200mm f/4. But when I mount it to a camera and I watch the meter's activity, as I zoom from 28mm to 200mm, the meter clearly indicates the loss of one stop of light. So in reality that Kalimar is a 28-200mm f/4-5.6. Which is not unusual for a lens with that focal range. But I had bought it only because of its alleged constant aperture. Kalimar had misrepresented their product. I guess I shouldn't have been surprised with it being an off-brand Korean-made zoom and all.

 

[Nodda Duma]

Yeah I could envision a slight change in f/# for a fixed aperture *design* -- perhaps +/- 1% if it wasn't clamped during optimization -- but definitely not a full stop difference. Sounds like a shady labeling.

 

[DWThomas]

I feel that it is worth pointing out here that not all constant aperture zooms are indeed constant aperture. I own one that lies. It is a Kalimar 28-200mm f/4. But when I mount it to a camera and I watch the meter's activity, as I zoom from 28mm to 200mm, the meter clearly indicates the loss of one stop of light. So in reality that Kalimar is a 28-200mm f/4-5.6. Which is not unusual for a lens with that focal range. But I had bought it only because of its alleged constant aperture. Kalimar had misrepresented their product. I guess I shouldn't have been surprised with it being an off-brand Korean-made zoom and all.

Was that while metering off a large monocolor evenly lit surface? That is a heck of a zoom range, and on a conventional scene such a large change in range might result in some contrasting object becoming a larger percentage of the field of view. But it could indeed be a shady setup. Can't say I've actually tried such a test myself.

 

[dynachrome]

An early lens with varying focal length and a constant f/2.8 aperture was the 35-100 Konica Hexanon. It had decent sharpness with a little distortion but was a varifocal and not, strictly speaking, a zoom.

 

[BrianVS]

Now I need to look closely at Vignetting at each end of the zoom range on a constant aperture lens. I suspect the "wasted glass" on the short end of the range reduces vignetting. Most fixed-focal length lenses have front elements "larger than required" to reduce vignetting. The Leica Summitar and Summar are both F2 lenses, the Summitar has a much larger front element with the stated purpose being to reduce vignetting.

 

[David Lyga]

I have a Takumar A 70 - 200 f4 and I like it a lot. The outpouring of information on what I had initially considered rather pedestrian is rather amazing and, thank you, especially nodda duma and bernard l. Of course these particular optics are complex and if there had been no computers to figure out design, we would not have them. I think that I uncovered some latent interest here. - David Lyga

 

[E. von Hoegh]

I have a Takumar A 70 - 200 f4 and I like it a lot. The outpouring of information on what I had initially considered rather pedestrian is rather amazing and, thank you, especially nodda duma and bernard l. Of course these particular optics are complex and if there had been no computers to figure out design, we would not have them. I think that I uncovered some latent interest here. - David Lyga

Au contraire.
Zoom lenses for cinema go back at least to the early 1930s. While there were no electronic computers then, there were rooms full of people skilled in performing rapid and accurate computations, these people were called "computers". There were also slide rules, mechanical calculators, and abaci. All were used in lens design as well as many other fields.

 

[David Lyga]

OK, this is interesting and, even though I knew that cinema lenses were available, it is also true that resolution standards are much more intense with still shots (as is grain). But, yes, von Hoegh, you are correct with asserting this fact. Even I remember when 'figure clerks' could always find employment (I was one). - David Lyga

 

[Nodda Duma]

Performance of zooms benefited significantly from computer aided design. Prior to the 80s, the design process was just too complex to directly optimize using the traditional fixed-focus hand-calculated design methodology which minimized the ray trace calculations to keep design time down to a few months. Now it's not a big deal, and the designer has been able to focus on deviating from the first order approximation zoom layouts of the pre-computer era.

The underlying theme is that optics are almost always designed to be "good enough" and not beyond because of the signifucant design effort required. I've always found that aspect to be fascinating.