Interesting that when googling the retroadapter ring, most of what comes up is from Germany. Even on American eBay I only saw these rings, by themselves, sold from Germany. Further, virtually all of the videos explaining the concept that result from a search are in German. Not that it matters, just curious if American sellers do not support this type of ring or is there some other reason.
Anyway, I watched a German video where the guy compared this retroadapter ring with the raynox dcr-250. I understood most of it although I'm not familiar with most of the "camera-specific" German words. If I understood correctly, the raynox dcr-250 just snaps onto the front of the lens and does not mess with the contacts, so I can continue to set the aperture as I do now. The retro adapter requires the lens to be reversed, which seems strange, but I guess also means that I will have an issue with the aperture since my 50 mm lens does not have an aperture ring. What's your opinion of the raynox dcr-250? Below is a link to the video I watched.
https://www.youtube.com/watch?v=x6s3MlzwdZg
https://www.youtube.com/watch?v=x6s3MlzwdZg
I also had trouble to find anything according to Retrofit-Adapters for Lenses in english, but knew about them for Years. Seems to be a german thing, maybe the orgin is Novoflex, a german company, that still exists to this day, they came up with a ton of strange and obscure Camera Adapters and such, aswell as all the other German Optics Companys, which had been and still are making some Special Stuff i never seen anywhere else.
So i just linked to the German Ebay.
.
I generally am not strictly against diopters as the one you mentioned (but never heard about it, and also havnt watched the Videos about it), but they do lower your Image Quality. I will try to explain it like this: A optical system is generally optimized and built for one particular use, in the Case of something like the 50mm 1.8 AF-S, its a Rather sharp Normal short-Focus Gauss-Type Lens for 43 to 28mm Image Circle, meaning it will perform best on DX-Format Nikon Cameras and FX-Format Cameras with Focus from Infinity to around 50cm Distance. but since its image Quality can only truely optimized for one particular distance, its mosly somewhere Between infinity and 0.5 Meters, at wich it will perform BEST. It will, for most situations still perform "good enough" at infinity distance and close Focus distance 0.5m.
Now if you go really close, like 0.1m or closer, on some Lenses Image Quality will significantly drop, because the Lens is just not meant to do that. You CAN build lenses, which are not affected as much by focus distance, or just are better optimized for closer focusing distances, such are actuall macro lenses (beware of lenses with "MACRO" in the name or ones that just state they are macro, most are technically not, Macro starts at 1:1 Magnification, and a lot of lenses which Have Macro in the name dont come Close to that, so always look up, what kind of Magnification the Lens is built for), but that usually makes the Lens more expensive, and since by far not everyone needs a lens that can do this kind of stuff, its usually left out or worked around in a way to work somewhat acceptable for general use. They are either Calculated in their designs to have about the Same image Quality independent of Focus distance or will be optimized for a Rather Close Focusing point, at which they will perform best, those may not give best image Quality at Infinity Focus for example.
Now, every optical System (For this matter read "Lens") is optimized in it self to produce a image as good as possible as given By the Price Point and other criteria considered by the Designer, such as Zoom Range, available Glass-Types, Weight, Sharpness, Lens Speed and quite a few more. If you now add an Additional Optical Element to the Front, you kind of mess up the Optical System. Those Diopters are usualy made to fit a large range of Lenses, and therfor will never be a perfect fit to any of the Lenses and lower image Quality. There are a few Diopters especially designed for certain lenses, but most are universal. That beeing said, some Lens-Diopter Combinations can still work well, and i actually use a pair of really Old russian Diopters with good coatings of to-me-unknown-brand which give superb Image Quality in combination with my 32mm 1.2 1Nikkor, which is mostly used for Product-Shots. The diopters cover the Magnification ratio nicely, which i cant really can get with the Lenses Natural Close-Focus distance and for which the Macro tubes are too high Magnification allready. Those Two diopters (+1 +2) really nicely fill in that Gap and give me a Magnification Range from Infinity down to the Point where i can fill the Frame with an Ant.
These Old russian Diopters give good enough results for me, and you cant really tell any drawbacks in image Quality when using them on this lens, but (the only other ones i ever tested) i also bought a bunch of Cheap (~30 or 40€ for a Set of 5) China diopters New, those are just bad, no matter what lens they are used on. As Far as i can tell, they are not even Coated.
So in generall, Diopters are Worth a try if you really need to, but it will always be a compromise in image Q.
.
Now about the "Strange" Idea of just flipping a lens inside out to get higher Magnification ratios. As i explained in the beginning of this Reply, a lens is optimized for a certain Magnification Ratio, which could, for a lens like the AF-S 50/1.8, be something like Waist-Head Portrait-Distance on DX or full-Body Images on Full-Frame, which should be about the Same distance. Now Lets assume Horizontal Field of view for this would be about 80cm and Horizontal Sensor Width would be 23,5mm (Nikon DX), then your Magnification would be something around 1:30. That means the Lens is Optimized to render an big image onto a 30x Smaller Area. now since the Image in the Sensor Plane is calculated to be color-Corect and otherwise intact, the Lens can witout any problem be inverted, meaning it will now work best for rendering something small 30 times bigger onto the Sensor. This is actually really extreme, and i dont know, if the 50/1.8 will still perform that well at that Magnification, but the Principle works about like this. Lenses Dont really care, which direction the Light goes, thats why you can use most Projector lenses for image Capturing as well!
Its not that Strange, if you think about it twice and understand, whats going on. Most lenses Are just usualy built to project something Rather Large onto a Sensor, which is usually a lot smaller than the Object its Trying to capture, and the Lens is made to perform best for those situations. If you try to capture things of the Size of your Sensor or smaller, you then either need special optics made for that purpose, calculated to perform best in this kind of scenario, or flip a lens and make it project the other way around.
.
Hope this was kind of understanable of an Explaination about the Retro-Adapter-Thing.
