I have a Nikon D3400 and a limited budget for lenses. I'm using second-hand lenses, but also the second-hand can be quite expensive. I saw that there are quite a lot of extension tubes, especially for macro photography. I would like to know if it could be worth it to purchase one or two different extensions instead of a lens. Of course, having a lens is always better, but not always the best option available. I found discordant opinions on the use of these extension tubes. I saw also of reverse lenses, but, from what I understood, they are the worst option.
Tubes are not quite as good as the other methods; slightly inferior to reversing the lens & a lot inferior to a dedicated macro lens, but they're straightforward, cheap & fun to experiment with.
There is very little in photography that can be done cheaply.
What are you hoping to photograph?
Because tubes reduce the amount of light coming into the lens & also confuse the auto-focus mechanism, it makes it harder to focus on anything that might fly off before you're ready.
If your subjects will stay still, if you have additional light & if you don't mind a bit of trial & error, get a set of tubes. $£€ 35 or so will get you something quite acceptable. You want the ones with electronic contacts which will confuse your camera less. You can use them in any combination, depending on the length of your lens & how near you need to be.
There are some over 100 bucks - I cannot see why, all they are is a tube full of air.
If you're doing this in a controlled environment you might also invest in a macro rail for the top of your tripod [again, cheap on eBay, 20 bucks], so you can work on your multi-exposure focus stacking.
Because maths: see elsewhere for proper scientific explanations
I'm pulling some comments into an answer, just by way of explanation
Examples using a Tamron 70-300 with Macro function, vs Nikon 18-300 without any macro capability, vs a Nikon 50mm 1.4 and 36mm of extension tube [the largest from the pic above].
These are all rough, hand-held at the closest I could focus each time. No attempt at colour accuracy or any real consistency, just quick snaps by a partly sunny window for extra light.
This is a European 1 cent piece, btw, 16mm diameter, for those who don't recognise it.
Nikon 50mm can get closer/larger - but not if you're trying to photograph insects or anything that would spook. I was at about 5cm for the third shot, using my hand spanned between camera & floor to try keep still.
Note that the Tamron isn't a 'true' macro lens, it's a kind of compromise, but it does function reasonably well. I don't have a true macro lens for comparison.
For additional comparison, this is one I spent some time on, posted here before, a 2mm allium bud. This was done with the 50mm [idk exactly which tubes, with a 50mm lens the full 68mm of tubes focuses 'virtually' inside the lens, so you can't use all of them]
Additional answer. If you shoot in nature, tubes (and close-up add-on lenses) have an inconvenient: they make your lens myopic, because it can only focus to close things. So you can't back off to have a wide field, spot/frame your subject and then get closer because when you back off everything is blurry. So you have this sort of blind zone to cross without losing your aim. This is manageable but it will slow you down and possibly let the subject fly away.
With true macro lenses you can stay in focus all the way and it makes things a lot easier.
A macro lens is best if you are doing close-up photography. A typical camera lens is optimized to work subjects that are far and slightly compromised when tasked to work close. A macro lens is optimized to work close and slightly compromised when working far.
To work in close, the focusing mechanism must allow the lens to rack quite far forward thus elongating the distance lens-to-sensor. When working at magnification 1 (life-size sometimes called unity) the lens will be one complete focal length forward as compared with its position when imaging at infinity. Many standard lenses stop forward movement at about 2/3 meters = about 2 feet.
Additionally, as you focus on close-up objects, the amount of light that plays on the sensor is reduced. At unity, the light fall-off is 2 f-stops (4X reduction) in other words, the f-number engraved on the lens barrel become invalid and you must open up the iris to compensate. This is actually not a big problem because the cameras exposure meter will report the light loss and force you to open up or slow down the shutter speed to compensate.
A macro lens has a trick up its sleeve. The front lens group of the macro magnifies the actual diameter of the iris as you focus closer and close. This action compensates for the light loss. If you are using tubes on a standard lens, you are one doing the compensation. In other words, under exposure is likely unless you manually open up the iris.
Because the typical camera lens is optimized to work subjects at different distances, it is not optimized to work a flat field like imaging a postage stamp. The macro lens is optimized to work a fat field. We often reverse a standard lens when working close. This is because the rear of the lens is optimized to project an image on the flat surface of the image chip. By reversing the standard lens, its performance working a flat subject is usually improved. Additionally, a reversed lens usually images with slightly more magnification.
By the way, using tubes with or without lens reversal, with practice yield quite good close-up images.
I have 10mm and 21mm extension tubes for my Canon M6, an APS-C mirrorless camera. I use them mostly for static subjects indoors. The light reduction is then no problem, the autoexposure takes care of it. I mount the camera on a tripod, so a seconds long exposure works fine. I have seen advice not to use them with zoom lenses but I do so. I find the zoom is a great coarse focus approach. Set the camera and the subject in position, then zoom until the focus looks good. My autofocus then usually can get a fix, but I often switch over to manual focus. Coupling them to my 55-200mm zoom gets a reasonable working distance.