OK, slightly vague query, but stabbing in the dark; this may offer some insight;
Ultra-Wide-Angle vs Kit & Stitch, featuring a fish!... Ultra-Wides are all about perspective; with a very wide angle of view, you can fill the frame from a very close vantage point, but, as the subject to camera distance increases, so does the perspective 'shrinkage' , and very much more dramatically. So it's commonly said that they are best for getting close to get added depth and open up smaller spaces, rather than packing lots of real-estate into a land-scape.
Fish? Well they are a different kettle of.... fish-eye lenses offer a distorted view of the world.... everything bows at the edges... it's actually not a 'distortion', as such, this is how lenses work; it's to do with arcs and chords and scale, and is a bit like the flat-earth idea. Over a very small angle of radius, the small arc of circumference you get is practically 'flat'... but as you increase the view angle and length of the arc of the 'subject plane', the ends are nearer on the perpendicular; So, a for a normal or telephoto lens, with a relatively small angle of view, 'flattening' that curved 'subject plane' onto the flat 'focal-plane' (of the film/sensor in the camera), doesn't stretch or squash it very much..... but, as you increase the view angle, you do. 'Non' Fish-Eye or 'Rectilinear' Wide & Ultra-Wide-Angle lenses, would then show the same distortion as a Fish, only they have some 'correction' elements to control it; they actually distort the angularly 'correct' focus and perspective, to make the scene look more 'normal'.. Fish don't! They ought to offer true angular perspective, rather than equated rectilinear perspective.. which is merely a little semantics TBH.... however...
As the focal length of a lens is reduced, so normally is it's near-focus distance.
Depth of Field is a % of the focus Distance, inversely proportional to the focal length, and proportional to the f-no. Ie... the shorter your lens, the more DoF you will tend to get for the same camera to subject focus distance, and the same aperture setting, while higher f-no's give more DoF.
So, reducing lens-length to increase angle of view, will tend to pull closest focus distances and infinity focus nearer the camera, and effectively increase the Depth of Field... remember the arcs; this means that the discrepancy between perpendicular focus distance and angular focus distance will tend to be less 'significant' as it's more often 'covered' by the Depth of Focus zone.
Oh-Kay... "Getting close"; I have the Sigma 4.5 Fish and the 8-16 UWA; UWA has critical focus between 24cm & tends to infinity at just 80cm; For the Fish, it's 13.5cm to 20! The UWA doesn't have particularly fast apertures, its f4.5/5.6... so the DoF is never going to be particularly shallow; Fish is reasonably quick at f2.8, but with even smaller critical focus range; anything over arms length away, at almost any aperture is going to be in the DoF... neither really 'need' a critical focus control, let alone 'auto-focus'! & in fact, my old 12mm 'fish' for my film cameras doesn't!
Now, I mention this because of your comment about 'Macro'; do not confuse 'Close Focus' features with 'Macro' features. Technically, Macro photography taking pictures of small objects, and a 'true' macro lens is one that will reproduce a image on the focal plane, film or sensor at a size larger than the object is in life..... a British 2p coin, is actually a little larger in diameter than will fit in the 24x36mm 'Full-Frame' of a 35mm film-trap or Full-Frame digi-sensor.... An APSC sized 'crop' sensor camera is about half that at 16x24mm ish.... you might just about get a 5p coin to fit on one.... but if you want to photo butterfly's? Yeah... you aren't going to need a 'true' macro lens unless you are using almost post-card sized flm-trap 'medium-format' cameras. Most lenses sold as 'Macro' these days then, aren't.. which is more semantics, and to explain with a little more, lenses usually 'shrink' large subjects into a small image, so a long-lens that makes far subjects seem closer, or makes a subject larger in the frame is't actually making it larger.... just less small.... so most modern 'Macro' lenses are lower-reduction lenses, to give that greater subject enlargement with an extended critical close-focus range, in order to allow you to get close to smaller subjects and fill the frame with them, they don't actually 'enlarge' the image on the sensor over real-life proportions, that's done in 'enlargement' when the captured image is reproduced in print or on screen.
THAT lot of semantics taken on board though, point is, that close-focusing wide-angle lenses, that have so much 'reduction' to shrink a very wide angle of view down to sensor size, tend not to be particularly useful for 'Macro' photography of small subjects; the narrower angle of view and lesser 'shrinkage' of a 50mm or 80mm or more, are more usual choices for that sort of subject, with close focus extended by reversal ring or extension tubes, or similar mechanism 'internal' to the lens, when they are sold as 'Macro-Focusing'.
Oh-Kay... see the linked article; UWA's are tricky lenses to get to grips with; Fish likewise, and as I sort of tried to make obvious in tutorial; packing a lot of land into a land-scape with a 'wide' lens will often simply make for more boring and less impact. People photo's can tend to be rather less than flattering, with the perspective making people take on alien insect proportions, heads or feet or arms stretched out of proportion to their body, from a small shift in viewing angle! Slightly more at home for interior shots or party type scenes, but they can still be a bit perverse and you have to beware how they can make images very busy or cluttered, and you can easily loose the 'subject' in the mele, or so get much distortion, it's hard to interpret. They are not easy lenses to get good results with, unlike a telephoto, that fills the frame with much less subject, which is easy to imagine or predict, and gives that added 'impact' from concentrating the viewers attention on such a small section of the scene, a wide doesn't do so much 'work' for you, but begs you do so much more work to make it deliver....
Actual lens options, then.... lets start with fish.I mentioned I have the Sigma 4.5mm 'Full-round'. I believe that this is still the only 'full-round' Fishe-eye lens with 180 Degree Field of View on both vertical and horizontal axis for Crop-Sensor cameras. If you want the 'full-fish', and don't want to go Full-Frame to get it, it is pretty much the only option.At about £500 though it's not a particularly 'cheap' one, and while it's a very sophisticated fully Auto-Focus lens, that is possibly a little redundant for a fish, as mentioned. When used, I frequently switch AF 'off', and rely on the DoF it offers to save it 'focus-hunting' with so much scene for the red-dot to get distracted by! Never had any of that malarkey on my manual-focus film cameras, and my film-camera's fish didn't even have manual focus!
Which makes the lower spec, manual focus or fixed focus 'Fish' worth more thought, the features they lack, are not all that important on one, which takes us to the popular Samya 8mm, usually around £250 ish, and far better value. At 8mm, though the angle of view is reduced quite a lot. Claims to offer 180 Deg FoV on a full-frame camera but that's only on the diagonal, and on a Crop-Sensor camera, even that s down to only about 165-170 degrees. Side to side, it' only about 150 or so.
Lets talk about vignetting....
About the only convenient 'full-round' from the article, the full-fish makes a 'full-round' image in the centre of the frame, a circle in a square, what's not picture is 'black', or vignette.. and as show, that has been cropped! There's even more black boarder in the original from camera!
Circle diameter is just a smidge smaller than the height of the frame, and its a circle, so its as wide as it is tall; frame has 2x3 proportions, so 1/3 the frame has been lopped off the edges to chop this square around the circle, and that is only covering about 1/2 the original frame area..... my camera has a 24Million pixel sensor, but the Fish-eye is only using about 12Million of them, and if you crop into that much more to get a conventional rectangular picture? Well you can be chucking quite a lot away, which can limit some of your possibilities for post-processing manipulations.. but before that is an issue, remember lenses 'shrink' the scene into the camera, and wider lenses are shrinking more... use a fsh-eye that is shrinking about as much as you possibly can, and THEN only filling half the view-finder with it? Composition is tricky.... the viewfinder image is tiny, its difficult to discern all that detail the lens is packing into it.
You also have problems with exposure; capturing such a wide field of view, you will frequently have a very large contrast range, and it's almost impossible to minimise that trying to exclude hot-spots in framing; I find 'in-camera' metering very unreliable with a fish, trying to cope with the range of lighting its given; frequently you get an awful lot of bright sky filling a large part of the frame that tends to make the meter want to under-expose the scene; Trying to work out how much compensation to use, or what exposure scheme is best? I find its often easier to take an incident reading with a separate meter, or just 'wing it' with an F-16-sunny guestmate and a bit of chimping & bracketing.. but either way, you are still likely to have to put up with areas of dark shaddow and blown high-lights, unless you have incredibly flat lighting to start with.
But back to the circle mask... the 4.5 Full-Round shows how much is masked when you have the full-round 180Degree FoV on both axis. At 8mm, the FoV is shrunk as mentioned; if that had bee shot with an 8mm Fish, the lens would have filled more of the frame, with less of the subject; side to side, it would probably lop the edges off at or close to the edge of the doorway, so not too much would be lost, but it would take a full 1/3 off the vertical.... but you would still have black corers, extending almost if not all the way up the sides.`
Packing less scene into the frame, and lopping off what is usually a lot of bright sky or uninteresting foreground, an 8mm fish is, I think, a lot easier to get to grips with, and for a toe in the water beginner, the better start, as well as the cheaper. but it does lack that 'full-fish' effect, for the very few occasions that may be important, or limit how far you can go exploring the fishiness.
So, back to actual lenses, and after the Samyang 8mm, which is only a full fish on full-frame, you have Nikon's own 10.5 Fish, and then third party 9's 12's and even 14's which as focal length increases, offer less 'Fishiness', even on full-frame....... but are increasingly more user friendly.
While I should mention the 'Fish Eye Adaptors' for kit lenses... at 1/10th the price of even a budget fish-eye lens, suggestion you get what you pay for ought to be mentioned. Attached to the filter ring of another lens, they introduce that as a link in the chain. Kit 18-55 I have is not a fantastic lens to start with, so adding a cheap extra element to the front, to distort its view and cast that over fewer of the available pixels? Its not likely to deliver anything particularly amazing, its more a novelty filter, like the prismatic ones that give ghost's and rainbows, its not really a fish-eye lens, and unlikely to give you any real feel for one.
So, the big question is really do you really need, or want a fish? Chances are you wont need one, so it's something to have a go at. If you want to give the a chance then you probably ought to dismiss the fish-eye adaptors, which starts the experiment with the £250 Samyang 8mm, or similar manual focus derivatives. Brand-New possibly a large expense for a lens likely to have little more than novelty value, and likely to not get extensively used, but lens is well enough regarded it doe give you a chance to explore the genre.
Which brings us to Ultra-Wides. Siggy 8-16 I have is I believe currently the widest UWA available for crop-sensor cameras, and again, thee's a premium price attached to that, its approx a £500 lens, but it gets down into the realms of fish, below 10-12mm, and delivers pictures that don't have black corners... it's a 'rectilinear' lens, so it has some elements in it that distort the scene to 'correct' perspective and make flattening the orange skin like a world atlas a little more 'natural'... doesn't completely get rid of the fishiness at 8mm, but, it's not so obvious! Sigma's 10-20, doesn't quite go so wide, but is half the price and doesn't go so low that the correction distortion is anywhere near as apparent. Tokina 11-20, is one of the most vaunted lenses in the UWA arena, again not going so far down the wide side it suffers less from distortion/correction problems, but at around £400 it's not a budget UWA for a beginner. There is/was a Tameron rival to the Sigma 10-20 that was almost identical for price and specs.. of the offerings out there, the Sigma 10-20 is probably the best VFM starting place. The 'old' f4.5 version has been superceded by a faster f3.5 version, so f4.5 versions that are often vaunted as the better lens, ca be something of a bargain... BUT...
As with the fish, its a question of whether you need or want one, and a lot of the 'niggles' with a Fish remain with a UWA. You can still suffer not so pleasant distortion at the edges; perspective effects can make them more challenging to achieve pleasing composition, and shrinking so much more into the view-finder, that can be even more challenging to achieve, whilst the increased area of scene will so often be accompanied by an increasd range in contrast, metering will also often be triky or hit and miss.
They are not 'easy' lenses to get great results with; they demand a lot more effort fro you as the photographer, they will not do it for you like a 'zoom' seems to so often, and they are likely to not be 'as' often used as you might suppose. Whilst neither is a 'Macro' lens or particularly suitable for macro type photography, despite the often very close focus distances they have.
So... answering a question with a question... what is your iterest in them? What do you hope to do with either?
