The executive summary opened by writing, “due to the convergence of trends in technology and the evolution of the battlefield, [directed-energy] military capabilities have reached or passed a tipping point in their criticality” for military operations by the U.S., its allies, and their potential rivals. They say prospects for 2060 do remain limited for effects at ‘vast ranges,’ similar to the thousands of kilometers that the Strategic Defense Initiative of the 1980s envisioned for space-based nuclear missile defenses. But they are more optimistic about shorter ranges.

Please talk to me like I'm stupid. :) I am hoping for a completely lay explanation. In particular, I'd like to understand in what situations I'd use a telephoto lens, and in what (other) situations I'd use a zoom lens.

Why bother with specific telephoto lenses? Because as opposed to generic zoom lenses with wide and long reach, they will have fewer image defects in their core range and will usually be considerably faster in that range. Their default long reach without too much detrimental optical trickery will also render the telephoto lenses the longest lenses to lug around, and being fast comes at the cost of a large entrance pupil and large front lens diameter. They are usually the largest and heaviest of all lenses. If you have an f=300mm lens with an aperture of f:1.4, its entrance pupil has a diameter of 214mm, and if it is not to suffer from significant vignetting, its front lens should be larger than that. Of course, you don't get them quite that fast. But there is little point in making them into a zoom lens also good at wide angle when a separate fast wide angle lens with good quality will add less weight and cost into your gear box.

Most of the time people say 'Telephoto' they just mean 'zoomed in' or in other words 'high mm' or 'long focal length' As has been mentioned, Nikon seems to say that 85mm is the shortest focal length that they will call telephoto. I have seen people call anything above 50mm telephoto.

Those tests will show what range the 50 kW lasers can achieve and how well they can perform on the field. The allure of scaling weapon lasers to 300 kW is the hope of stretching their tactical range to tens of kilometers, which would make them more valuable defense systems. Even if they can’t reach the ideal range, the higher power should make them more potent over shorter ranges. Getting to 300 kW is a challenge and reaching that power in the laboratory is a long way from having one ready for deployment. Laser weapons are far from a sure bet, but the giggle factor of the “Star Wars” era is gone.

Other answers have stated the same thing in the same technical terms which might be meaningless in this instance. There is also a practical approach.

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Spectral beam combination was the first fiber laser technology to produce high beam quality at tens of kilowatts. Its early lead came from the fact that the fiber laser beams being combined are at different wavelengths in evenly spaced spectral slots, so the beams do not have to be in phase to blend into a single high-power beam without destructive interference. (In fact, the input beams can’t be coherent because their wavelengths differ.) Lockheed Martin and its Aculight subsidiary demonstrated a series of spectral beam combination lasers—first the 30 kW, then 60 kW, and reaching 100 kW in October 2019. That put Lockheed on track to scale to 300 kW.6 In November that year, they received a HELSI contract to build and demonstrate a standalone 300 kW laser.7

'Zoom' just means that it has a range of focal lengths. That range could be telephoto, it could be below telephoto, or it could range from below telephoto to above telephoto.

So "zoom" = focal length you can change, and "telephoto" = long focal length. A lens can be one, or the other, or neither, or both.

The distributed-gain laser is a descendant of the High Energy Liquid Laser Area Defense System (HELLADS) developed over a dozen years ago by General Atomics. Details remain classified, but the original concept centered on flowing a liquid coolant with its refractive index matched to that of a solid-state laser material through channels in a laser made of that material. That index-matching would prevent refraction when light passed between the two materials, improving heat removal.

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Meanwhile, the High-Energy Laser Scaling Initiative (HELSI), managed by the Office of the Secretary of Defense, has researchers trying to scale four laser technologies to 300 kW-class power for future military weapon systems. Reaching that target within the next 12 months is “ambitious but achievable,” says Mark Neice, executive director of the Directed Energy Professional Society (Albuquerque, NM). Reaching that power range would stretch the range of tactical laser weapons to tens of kilometers, an important step up from the current 50 kW systems now in testing.

There's another flip to that which is what you see in SLR photography a lot - the retrofocus design. With a Nikon F mount, the flange distance is 46.5mm. This allows the mirror to clear the back of the lens when it flips up (this is a major issue in SLR design). So the closest you can put a lens would be about 47mm away from the focal plane. But yet, there are lenses such as a 24mm lens which have a focal length that is shorter than this distance. (Note: this is part of why interchangeable lenses on rangefinders and mirrorless systems can made more cheaply - they can use simpler designs for their shorter flange distances).

Telephoto lenses can be either zoom or prime. Zoom means that they can change how far they are looking at or prime means they have a fixed amount of magnification and can't be altered. (Like an old instant camera with no zoom, but with much better picture quality.) The reason for using a prime is that they are a) cheaper and b) produce much, much higher quality images for the price.

In 2018, Griffin told Congress that fiber lasers could be scaled to 300 kW to defend Air Force tankers from attack. Initially, power-scaling plans called for the Missile Defense Agency (MDA) to test a low-power laser demonstrator—an odd name for an effort to generate 300 kW. It was based on the successful demonstration of spectral beam combination in fiber lasers; essentially a very high-power version of wavelength-division multiplexing, which Lockheed Martin (Bothell, WA) had used to combine the output of 96 300 W fiber lasers into a 30 kW beam.4 High-level Pentagon discussions shifted plans to focus on demonstrating the feasibility of 300 kW output for tactical missions spanning tens of kilometers in what became the HELSI project, managed by the Office of the Secretary of Defense with the Army, Navy, and Air Force. Extending laser capabilities to strategic ranges of hundreds of kilometers will come later, Neice says.

The required beam quality and power were demonstrated in 2019.9 Scaling of the technology then was transferred to Boeing (Huntington Beach, CA) under the HELSI program.10 nLight is also involved under a separate HELSI contract.

I've heard it said that 50mm is the standard as far as lens focal length goes, meaning that at 50mm your camera will take a photo as close to how the human eye sees it, i.e. a similar zoom and angle of view. So, anything over 50mm is considered a telephoto (zoomed in from how you see it) and anything under is considered wide angle (zoomed out from how you see it). A fixed focal length lens will also be called a Prime lens, a variable focal length lens is a zoom. So to sum up, a lens with- 50mm is considered standard. 24mm would be a wide angle prime (only one focal length). 18-35mm would be a wide angle zoom. 200mm would be a telephoto prime. 70-200mm would be a telephoto zoom. 18- 200mm would be both a wide angle (below 50mm) and telephoto (above 50mm) zoom. Basically if you're focal length is below 50mm you're in wide angle mode and if you're above 50mm you're in telephoto mode. Hope this helps.

Telephoto, roughly, means that the lens has a relatively narrow field of view, thus it can be used to look at things further away.

A zoom lens means the lens can change focal length via zooming, i.e. it is not a prime lens. A telephoto lens has a long focal length (I do not know if there is an official threshold to call a lens telephoto, but Nikon seems to start it at 85mm.

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The MDA considers DPALs the most promising laser for boost-phase missile defense because it offers an attractive SWaP combination, near-infrared wavelength, and its potential to retain high beam quality when power is scaled.13 Advocates claim DPALs should be able to generate megawatt-class power, a level some think is beyond the reach of solid-state or fiber lasers.

The term "telephoto lens" has a particular technical meaning in terms of lens design, but in common usage it refers to a lens with a long focal length.

The simple definition of a telephoto lens is a lens that has a focal length longer than the physical lens. Many lenses that are 'long' (as opposed to 'normal' or 'wide') are telephoto in design. This is because it sometimes is impractical to put that much of a barrel on the lens. In the SLR world, one often uses 'long lens' and 'telephoto lens' synonymously.

The Pentagon is looking further into the future. Two years ago, the Office of the U.S. Air Force Chief Scientist for Directed Energy began assessing the potential importance of directed-energy weapons in 2060, a century after the military spent its first million dollars on laser research. Titled Directed Energy Futures 2060, its report was released at the end of June 2021.1 Its assessment of weapons based on beams of electromagnetic energy, including lasers, radio-frequency devices, high-power microwaves, millimeter waves, and particle beams, was optimistic but realistic.

So far, Neice says, the Armed Forces have commissioned construction of seven 50 kW solid-state lasers for installation in weapon systems on military platforms. All are spectrally beam-combined fiber lasers to be installed by contractors or the services themselves in standard military equipment.

HELSI has a broad remit of trying to scale four distinct laser technologies to the 300 kW range in parallel. Three of the technologies have already demonstrated powers in the 50–100 kW range: spectral beam combination in fiber lasers, coherent beam combination in fibers, and “distributed-gain” solid-state lasers developed by General Atomics (San Diego, CA). The fourth is diode-pumped alkali vapor lasers (DPALs), which may be able to reach even higher powers. Details are scarce on these projects because they fall under the Defense Counterintelligence and Security Agency’s new rules on “critical unclassified information.”5

The Army has already installed and successfully tested one in its M-SHORAD (Mobile Short-Range Air Defense) system (see Fig. 1), and it will receive three more 50 kW laser systems in the next 18 months. And the Air Force, Air Force Special Operations Command, and Navy all are integrating 50 kW systems into their own equipment for similar tests of laser weapon performance in real-world tactical situations.

Laser weapons have come closer to the battlefield since our last report. The U.S. Armed Services are already testing 50 kW-class solid-state lasers in full-scale military weapon systems.

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The focal length is normally measured in millimeters (mm). A zoom lens will have two measurements, for example "18-200 mm" (a wide-angle to telephoto zoom). It zooms from a short focal length of 18 mm to a long focal length of 200 mm. A non-zoom lens, also called a "prime" lens, will have a single focal length, for example "135 mm" (a moderate telephoto).

An 18mm lens is not a telephoto lens and it is also not a zoom lens because it has low mm and there is only one focal length. You'd call it a wide angle, prime lens.

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Some delays were inevitable. It’s logical for the Air Force to want to test a laser for their top guns, but squeezing a 50 kW laser into a pod on a fighter jet is a much more difficult task than putting one into a ground vehicle or on a battleship. What’s remarkable is how fast high-energy fiber lasers have matured to a point where so many of them are being tested in combat scenarios.

A 300mm lens is a telephoto but is not a zoom because 300mm is high mm (in other words, 'long focal length' or 'zoomed in') but it does not cover a range of focal lengths. (You can only use that lens at 300mm, not 299mm or 472674mm) Insead, we call these lenses prime lenses. A prime lens does not cover a range of focal lengths, just one.

The maximum extension for this camera is 315mm. You can't move the front of the camera more than that distance out. But what if you want to use a longer lens? The Schneider 400mm f/5.6 Apo-Tele-Xenar (that's a mouthful - the 'apo' means it is an apochromat lens and then there's that 'tele' there...) has a focal length of 400mm... but its flange focal distance is 285.1mm. It could fit on that camera (well, in theory - it also has a #3 shutter and that lens board can only fit #1 and #0 shutters... but other than that).

Similarly, a zoom lens can be in any part of the focal length spectrum. You can have a 17-40mm zoom, which is basically wide angle (and not at all telephoto) or you can have a 100-400 zoom, which is entirely telephoto. You can also have any variety of different combinations including things like 24-250mm lenses that span from wide angle all the way to telephoto in one lens.

The focal length of a lens determines its field of view on your camera. If it has a long focal length, it has a narrow field of view, making the things in front of you appear large in the photograph. If it has a short focal length, it has a large field of view--it's a "wide angle" lens that takes in a large area, making objects appear small.

The Air Force is working to integrate a 50 kW-class laser weapon into a flight pod for fighter jets to protect them from incoming missiles. Called SHiELD (Self-protect High Energy Laser Demonstrator), the project has been in the works at the Air Force Research Lab for several years. Lockheed is building the laser, with Northrop Grumman supplying beam control equipment and Boeing supplying the pod. In 2020, the Air Force pushed planned flight tests to 2023. The tests are intended to determine the maturity of the laser technology for use on fighters.18

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The Air Force Special Operations Command is working on its own demonstration of a 50 kW-class laser in an AC 130J aircraft, an advanced version of the venerable C-130 gunship used for close air support. The laser equipment is being integrated at the Naval Surface Warfare Center (Dahlgren, VA), with ground testing to start this year and integration into the aircraft and further testing next year.19

Aside from being optically pumped by diode lasers, diode-pumped alkali lasers (DPALs) are quite different than the three other types being scaled up under HELSI. The diodes in DPALs pump alkali metal vapors on transitions that have exceptionally small quantum defects; a rubidium DPAL emits 98% of the energy it absorbs from a 780 nm pump photon, which is tremendously efficient by laser standards. However, overall efficiency is lower because it must account for losses in converting electrical input into the pump diode and into light for pumping the alkali vapor.

Technically 'Telephoto' means that the focal length [the mm] is longer than the lens is. In my experience, people in the photography world usually don't talk about it under that definition.

Coherent beam combination requires matching phases of multiple input beams at the same wavelength. The MIT Lincoln Laboratory (Lexington, MA) has been developing coherent beam combination for over a decade for diode lasers and waveguide amplifiers as well as for fiber lasers.8 In 2016, Lincoln demonstrated record brightness in what they called a “fiber combining laser,” for which they also achieved near-ideal power and high beam efficiency based on coherent beam combination. A key advance was extremely precise phasing of the light in the input fibers at their connection points, so constructive interference combined them to a single beam.

So, if you want the sharpest images possible, you are looking at using a prime (not a zoom) lens. If it is longer than 85mm or so, it is likely a telephoto lens of some design. Glance at Canon's Forgotten 400 which shows a comparison between a 400mm f/5.6 telephoto prime and a 100-400mm f/3.5-5.6 zoom lens.... though do realize that with real life one typically isn't looking at brick walls at the edge of the frame.

So, instead of making a lens that has a focal length that is longer than the distance it is at, you make a lens that has a focal length that is shorter than the distance it is focused at. Retrofocus lenses often have big front elements.

Unless you are dealing with a large format system and looking at the rail on your camera, you aren't going to care about how the lens is designed and if the actual focal length is the distance between the lens and the focal plane or not. You've got a lens, you use it.

A 10-20mm lens is not a telephoto lens but is a zoom lens. It is not zoomed in at all. It has a short focal length, low mm. It's called wide angle. If you felt like it, you could shoot at 15mm when you feel jumpy and 16mm when you feel bumpy. You could not, however shoot at a high focal length as you could with the 300mm lens.

The rapid development of solid-state and fiber lasers over the past 40 years made the panel optimistic about the next 40. They expect military goals to benefit from progress by civilian industry, especially in reductions of the size, weight, and power (SWaP) of laser equipment “that would make airborne, space-based, and all-domain [directed-energy] missions more viable by 2060.” Increasing laser power to megawatt-class could reduce the duration of laser engagements at tactical range to less than a second. Optimistic estimates have laser powers reaching hundreds of megawatts by 2060.

A telephoto lens is optimized for long reach (good magnification at long distances). A zoom lens is one with variable focal length, usually ranging from wide to long. While a telephoto lens may have either fixed reach or variable reach, its focusing distance will contain ∞ and will usually start at a distance of more than a few meters (macro lenses also have good magnification but focus on near distances).

Field-testing 50 kW lasers So far, Neice says, the Armed Forces have commissioned construction of seven 50 kW solid-state lasers for installation in weapon systems on military platforms. All are spectrally beam-combined fiber lasers to be installed by contractors or the services themselves in standard military equipment. The Army has successfully tested one 50 kW laser built and integrated into a Stryker eight-wheeled armored combat vehicle (shown in Fig. 1) by Raytheon (Waltham, MA) and Dynetics (Huntsville, AL). The vehicle includes the M-SHORAD weapon system now entering deployment, which integrates existing sensors, guns, missiles, rockets, and now lasers to defend maneuvering troops against attack by drones and aircraft.15 The laser weapon is a follow-on to the Army’s HELMTT program. Northrop Grumman (Falls Church, VA) had been in the running to build the weapon lasers for Strykers, but problems in the power and cooling system left the company unable to meet Army contract criteria.16 Three more 50 kW laser systems will be delivered over the next 18 months. All four lasers will be put through their paces while installed in standard battlefield vehicles and weapon systems to test how effectively they perform under realistic combat conditions. The Navy now is integrating its first 50 kW laser from Lockheed with other combat equipment to give it full-up weapon capability. By year-end, it will be delivered to a San Diego shipyard for installation in a combat ship. Called HELIOS, for High Energy Laser with Integrated Optical dazzler and Surveillance, the weapon system includes surveillance equipment, a dazzler to deter attack, and a high-energy laser to zap attackers.17 The Air Force is working to integrate a 50 kW-class laser weapon into a flight pod for fighter jets to protect them from incoming missiles. Called SHiELD (Self-protect High Energy Laser Demonstrator), the project has been in the works at the Air Force Research Lab for several years. Lockheed is building the laser, with Northrop Grumman supplying beam control equipment and Boeing supplying the pod. In 2020, the Air Force pushed planned flight tests to 2023. The tests are intended to determine the maturity of the laser technology for use on fighters.18 The Air Force Special Operations Command is working on its own demonstration of a 50 kW-class laser in an AC 130J aircraft, an advanced version of the venerable C-130 gunship used for close air support. The laser equipment is being integrated at the Naval Surface Warfare Center (Dahlgren, VA), with ground testing to start this year and integration into the aircraft and further testing next year.19 Looking into the future Some delays were inevitable. It’s logical for the Air Force to want to test a laser for their top guns, but squeezing a 50 kW laser into a pod on a fighter jet is a much more difficult task than putting one into a ground vehicle or on a battleship. What’s remarkable is how fast high-energy fiber lasers have matured to a point where so many of them are being tested in combat scenarios. Those tests will show what range the 50 kW lasers can achieve and how well they can perform on the field. The allure of scaling weapon lasers to 300 kW is the hope of stretching their tactical range to tens of kilometers, which would make them more valuable defense systems. Even if they can’t reach the ideal range, the higher power should make them more potent over shorter ranges. Getting to 300 kW is a challenge and reaching that power in the laboratory is a long way from having one ready for deployment. Laser weapons are far from a sure bet, but the giggle factor of the “Star Wars” era is gone. REFERENCES For the full list of references, please see https://bit.ly/Hecht-Nov21-References.

The Army has successfully tested one 50 kW laser built and integrated into a Stryker eight-wheeled armored combat vehicle (shown in Fig. 1) by Raytheon (Waltham, MA) and Dynetics (Huntsville, AL). The vehicle includes the M-SHORAD weapon system now entering deployment, which integrates existing sensors, guns, missiles, rockets, and now lasers to defend maneuvering troops against attack by drones and aircraft.15 The laser weapon is a follow-on to the Army’s HELMTT program. Northrop Grumman (Falls Church, VA) had been in the running to build the weapon lasers for Strykers, but problems in the power and cooling system left the company unable to meet Army contract criteria.16 Three more 50 kW laser systems will be delivered over the next 18 months. All four lasers will be put through their paces while installed in standard battlefield vehicles and weapon systems to test how effectively they perform under realistic combat conditions. The Navy now is integrating its first 50 kW laser from Lockheed with other combat equipment to give it full-up weapon capability. By year-end, it will be delivered to a San Diego shipyard for installation in a combat ship. Called HELIOS, for High Energy Laser with Integrated Optical dazzler and Surveillance, the weapon system includes surveillance equipment, a dazzler to deter attack, and a high-energy laser to zap attackers.17

A zoom lens could "zoom" from a short (wide-angle) to long ("telephoto") focal length, making things look bigger and closer as you zoom in. Or it could zoom from an extreme wide-angle to a moderate wide-angle, never coming close to a "telephoto" focal length. Or any other range of focal lengths.

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Make it simple: zoom is a lens of multiple focal lengths of all focal ranges; telephoto is a prime from 85mm-300mm range and super telephoto is over 300mm+. (Info from Greengo).

The team focuses on two “militarily relevant use cases”—ground- or sea-based defense and operations above 30,000 feet, where the standoff range is up to hundreds of kilometers. Solid-state lasers on the ground or at sea have shot down short-range rockets, artillery, and drones with powers to tens of kilowatts—and more tests are planned with the 50 kW beams. A decade ago, the Airborne Laser (ABL) shot down guided missiles from the air, but its lethal range was said to be limited to 135 km, insufficient for stopping boost-phase missiles.2 The chemical oxygen-iodine (COIL) laser used in ABL has largely been written off as a weapon.

“I’m extremely skeptical that we can put a large laser on an aircraft and use it to shoot down an adversary missile, even from fairly close,” Mike Griffin, then undersecretary of defense for research and engineering, said in 2020.3

Telephoto and zoom are two completely different things. Zoom simply means that the focal length (apparent magnification) of the lens can be changed, ie, it looks like it can look at things either closer up or further away by adjusting it.

An 18-200mm lens is a strange beast. It is a zoom lens but it can be considered both a wide angle (at 18mm) and a telephoto lens (at 200mm) because it's zoom range is so huge. You could shoot at 200mm and it's telephoto or you could shoot at 18mm and it's wide angle. In my experience if it is capable of telephoto, then you call it a telephoto. You'd then call it a telephoto zoom

A zoom lens is useful when you are unable to move to get the crop of the scene that you want. Sometimes you can't step back further to get a wider view. Other times you can't go half way to the middle of the river to photograph the other side of the shore. You could pack a lot of lenses and select the one you want for the situation you want, or you could carry a zoom lens.

When you go to other formats, the distinction between a telephoto lens and a non-telephoto lens becomes important. Consider a nice 4x5 field camera:

A "zoom lens" is a lens whose focal length can change. You twist the barrel, or push a switch on the camera, and it takes in a narrower or wider field of view, making objects appear bigger or smaller.

Now to zoom lenses... and the reason I mentioned that bit about retrofocus design. Typically a zoom lens is made of a prime lens group in the rear, a middle group, and then a retrofocus group in the front. That's the 'ideal' design, though often they are more complicated to deal with aberrations and distortions that inevitably come with more complex lens designs. You can get a hint of how this works in the What Is Inside a Zoom Lens? article from Tamron. Though, it is probably more accurate to say that zoom lenses take design elements from telephoto design, retrofocus design, prime lens design, and a bit of other to make a very complex system.

In tests sponsored by the MDA, General Atomics demonstrated 100 kW output in October 2019. That pushed distributed-gain lasers across the threshold for scaling, and in April 2020, General Atomics received a HELSI contract for developing a prototype 300 kW class laser.12

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Jeff Hecht is a regular contributing editor to Laser Focus World and has been covering the laser industry for 35 years. A prolific book author, Jeff's published works include “Understanding Fiber Optics,” “Understanding Lasers,” “The Laser Guidebook,” and “Beam Weapons: The Next Arms Race.” He also has written books on the histories of lasers and fiber optics, including “City of Light: The Story of Fiber Optics,” and “Beam: The Race to Make the Laser.” Find out more at jeffhecht.com.

Improvements in supporting technologies also could enhance laser propagation distance and accuracy from fast-moving platforms. Adaptive optics now can compensate for moderate levels of turbulence; even pessimistic estimates envision focusing megawatts of continuous power on targets. Better beam stabilization might improve pointing accuracy to hundreds of nanoradians.

What is the difference between the two? Why would I want one over the other? I wiki'd telephoto lens but remain confused about this distinction.

However, it is now possible to build 100 kW-class fiber or solid-state laser that can destroy targets at “tactically relevant” distances to a few kilometers. Live-fire testing and simulations have demonstrated that a suitably powerful beam can disable or destroy targets within several seconds depending on their resistance to laser energy, their distance from the laser, and how tightly the power is focused onto the target. Little has been said about the lethal ranges that are actually achieved on specific targets, but according to the report, laser weapons can cause “destructive and damaging effects at militarily relevant ranges” to “small and relatively soft targets.”

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On the other hand, recall that bit about the complex lens design? The wider the range of the zoom, the more compromises in the design to give you that range. It comes at the expense of aberrations, less light getting through the lens (requiring longer exposures), and other distortions. The 'super zooms' of 20-200 or 15-300 have much more compromises to the design than one that has a narrower range (the classic 100-300). Some photographers try to avoid zoom ranges of more than 3x or 5x except when necessary (glass weighs a lot - if you go hiking, it might be easier to carry a zoom than 50 lbs of glass). While the zoom lenses of today are better than those from a decade or two ago, there is still some truth to that.