eli5: Why do cameras and other devices use more lenses and not just one more versatile lens?
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Different lenses focus light in different ways. If you want a single lens that can both focus for suuuuuuuper up close pictures, and also focus out to the horizon for landscapes, then it’s going to be enormous and expensive. Turns out it’s *so much easier* to just add a couple different lenses for a couple different purposes.
If you want to focus on things close up you want a lens that short and round. If you want to focus on things far away a more elongated lens is better.
Our eyes solve this problem by having a squishy lens with muscles to change it shape. Cameras solve it by using multiple fixed lens.
There are versatile lenses – but they become prohibitively expensive. It’s cheaper and more efficient to have specific lenses for specific purposes.
Some types of photography require certain lenses – and some types of photography cannot be obtained with certain types of lenses.
A close up macro lens is built very differently than a traditional wide angle because they require light to enter the lens and hit the sensor in different ways and the glass and mechanics need to be placed in the lens in certain ways.
Then you have aperture – which is the setting of how much light you let into the camera. Cheaper lenses have simpler mechanics, so as you zoom in, the aperture might need to close up (so the image gets darker as you zoom in).
More expensive lenses can have a constant aperture as you zoom in – but that requires more complicated mechanics – so the aperture is kept open using different mechanical offsets as the lens zooms in.
I’m on a video shoot right now where we have cinema-quality zooms (Angenieux brand) – each one is about $13,000 retail value. They’re about a foot and a half long each. It’s not very practical to carry around.
Because making a product for a specific job is usually better than trying to make something that ends up doing multiple jobs poorly.
There used to be a woodworking/shop tool called the Shopmate. It was a tablesaw, drill press, sander, lathe, and various other tools all in one. It was insanely expensive and all it ended up doing was 5 jobs poorly. And if the motor ever broke then all your tools didn’t work.
With lenses you have to choose the lens that’s best for the job you are doing. There are standard lenses for casual everyday pictures. But you can’t use a telephoto lens to take pictures of ants, and you can’t take pictures of mountains with a macro lens. And there is really no way to combine them.
In broader terms, a camera lens is always a compromise. Something like a macro lens is hyper specialized to take very close up photos of small things. It’s so good at this because it sacrifices usability in other settings. You won’t be able to take a landscape with it, for example. Making a single lens that’s versatile requires that you balance these compromises. So that lens won’t be as good at any particular thing as a specialized lens can be.
Think about race cars. An F1 car is hyper specialized for track driving, but if you put it on an off-road course it’s basically unusable. A rally car by comparison is a lot more versatile and can be fast over all sorts of terrain, but it won’t match the F1 car on the track.
Yes, you can use a single lens in a camera. A single lens will bend light exactly one way, every time. And it will work perfectly well as long as you’re shooting the same thing at the same distance in similar conditions at all times, and you design that lens for those conditions. A jeweler’s loop is a good example: It’s great at what it does, but to see the jewel in focus you have to move the jewel into the focal point.
A compound lens, on the other hand, can be manipulated to bend light many ways, allowing you to zoom, take pictures at different focal lengths and correct the many optical aberrations that are inherent to lenses in general. Adjustable binoculars are a good example: Rather than making sure the object is in the focal point, you can turn the dial and adjust the focal point to where the object is.
The main reason is to fight chromatic aberrations.
You can prevent this problem by “balancing the lenses” to cancel the aberration one with another. But this works at a fix focal lenght.
A movable set of lenses will eventually have aberrations on each end. The more variable lenght, the more aberration you will get.
So, for a perfect photo you need a good lens with fix lenght. Aka fix 35, fix 50, fix 200. But you have to carry all the different sets and stop in a clean place to swap them. If you are in a studio, you can easily have 30kg bags of lenses to swap, to always have the one that fits the picture.
for practicality is doable to carry short ranged zooms, like 35-60, 70-150. That’s the most commonly carried.
A extreme zoom like a 25-200 is gonna totally suck at both ends. Will suck at 25, be decent between 45 and 100, and suck again toward 200. But it’s a thing I used to have, if I have to stay in a dirty place, I don’t want to open the camera to change lenses. I’ll take slightly worse pictures, in favor of being able to zoom onto unexpected things. Like if I hike, I want to take a picture of a landscape but if a deer appears, I want to have a zoom. That’s where the broad range sets are good. You can still do pictures without aberration at the very center of the zoom range.
As you see it’s a trade off, between practicality and quality.
Google Chromatic aberration to see what type of distortion is.
A more versatile lens is too deep to fit in a skinny phone. Making it skinny enough is more expensive than using multiple lenses.
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