If you haven't heard of anamorphic, or don't really get what the hype is about, read this.
Anamorphic adapters were originally designed for projectors. Let's differentiate between an anamorphic adapter lens and an anamorphic lens. The adapters were the second step in achieving widescreen or cinemascope proportions in cinemas and film. The first step was to film using an anamorphic lens on the camera, which squeezed the image horizontally so that a wider field of view could be achieved in the filmstrip where each frame was of limited size and fixed proportion.
Now back to the cinema where the proud producer is watching their finished film for the first time. If the projectionist forgets to put an anamorphic adapter on the projector then you would still see all that squeezing. It would make me look taller (I'm the lead role obviously), but I wouldn't really be. I would look very wrong (disclaimer: sometimes I am very wrong). So an anamorphic adapter lens (or scope lens), specifically designed for going in front of the projector's lens is used to correct for the 'squeeze' and restore the correct proportions from a rectangular frame which now looks much wider than it actually is deep inside the projector. Voila, you now have proper widescreen.
So the concept of anamorphic was developed precisely to make the final image on the screen much wider when other technologies weren't available, but in going through this process the visuals gain a certain look which is very appealing, have far more filmic character than that from using spherical (normal) lenses, and so invoke the look of cinema as that's where the concept has always found its use.
You could get the wide view by cropping normal footage. Those who don't really get anamorphic are quick to point that out. It's not the same though. See the same links above. Anamorphic looks beautiful.
So the fundamental takeaway here is that anamorphic adapter lenses that we are talking about here were designed for projectors. Projectors don't move, screens stay where they are and you only have to focus everything once before you sell the popcorn. Two lenses here sandwiched together means two lots of focusing, but that's a process that at least doesn't have to be repeated often in the projection room.
So if real filmmakers want (and many do) the anamorphic look, they wisely use original 'proper' anamorphic lenses on set, as they focus using one focus ring easily and accurately, and besides, someone else is paying for them. They are extremely expensive. An indie filmmaker could maybe rent one, but that's not cheap either and we all know that living permanently with a lens is fun and it takes time before we uncover the magic that it's capable of, right?
So we want our own anamorphic solution and we're prepared to compromise. Luckily real anamorphic lenses are still so prohibitively expensive that despite the demand for, we can, for much less money, buy an old anamorphic adapter lens, designed for projectors, and then use it on our camera. It fits in front of a taking lens (like a prime lens, maybe 50mm, or 85mm or similar) and causes our image to be squeezed because it optically compresses the view in one direction but not the other. And these days we don't generally project our results in a cinema, we create films on our devices. So we can easily de-squeeze the image in post (without having to use another adapter later). And magically, beautifully, enchantingly, all the features of the anamorphic look are retained in our footage.
But then we want to focus our lenses. You know, like when we want to follow some action, change visual emphasis, or rack focus just because we can. But generally with an anamorphic adapter, we can't.
Sure, we can focus our rig if we have time. Getting both lenses to be focused on the same point isn't beyond us, but to smoothly change that to another position whilst filming, that's near impossible. You sort of can if you have a whole lot of alignment marks that correspond on each lens, but it won't be reliable and due to the sloppy setup of most rigs the image jumps around whilst trying. And the distortion inherent in anamorphic footage when the camera tilts off axis is distinct, not very pretty, and (I think) impossible to stabilize or correct in post. It gets used for effect, sometimes. A few of those times it was actually intended and looks a bit good.
And before we got to that point we needed to make some stuff. Clamps to join the two lenses. A support so the weight of the adapter doesn't rip our rig apart. And even then we ended up with an unsharp adapter despite someone's review of how sharp it is. Luckily, that can be fixed. You can tune an adapter for sharpness.
I think the above is a realistic overview of anamorphic adapters. The visual look is absolutely worth it, the focus difficulties perhaps not. So the holy grail of anamorphic adapters is creating a single focus solution.
I have created a solution. Read this. I'm not trying to sell something to you. I want you to build your own.
Others have solutions too. I don't think they are nearly as good but then remember my disclaimer above. I can be wrong.
(1) They use a variable dioptre lens in front of their lens combination. Let's say you have one of those. You set everything to a point of focus (like you would in the projector room) and then focus only using the variable dioptre lens. It works! But it's heavy (in the wrong place), pricey, and provides an extra set of lens surfaces to detract from sharpness and introduce optical degradation. By all accounts they aren't particularly sharp and they can also flare fairly synthetic looking blue lines, but I don't actually own one so that's all just secondhand knowledge. And I suspect focusing by gripping and turning the very front of a long setup is a recipe for creating unwanted camera movements.
(2) No, I think that's it. Just the one other way to single-focus at the moment.
But what about mobile phone anamorphic lenses? They are single focus right?
Yes, and no. You don't need to focus the anamorphic lens but that's because (a) the camera lens is such a short focal length that its own depth of field compensates for the need to focus the adapter, and (B) the adapter is usually only a 1.33x squeeze. The less pronounced the squeeze, the less critical the focus. Think of it as a sliding scale from looking through a clear lens to looking through an anamorphic lens. The clear lens preserves focus of the taking lens. Anamorphic squeeze max's out at 2x squeeze, so 1.33x is only a third of the way there. Cheap DSLR/mirrorless solutions are similarly only 1.33x. Easier to build, easier to achieve single focus characteristics, but where 1.33x misses out is on the visual beauty of anamorphic. The effect is nowhere near as pronounced. Subject separation from the background is harder to achieve, and overall the best thing about anamorphic, the distinctive bokeh, is underutilized.