Luko
Contributor
Looks like you don't have the full understanding on what supermacro and necessary diopters are...I'm taking your exemple of a inon UCL165 +6 Diopter on 2 elements, you're not gonna go very far with subjects less than 2cm buddy, not even speaking of chromatic aberrations.Only €500? Sounds like a bargain.
Let's see:
Above water: Canon 58mm 250D (+4) diopter: $89, ~€80
Below water: Inon UCL-165 M67 (+6) diopter: $165 ~ €150
Ratio: 1/1.9
Above water: Cheap +5 magnifier: ~$10, ~€9
Below water: "Diving Magnifying Glass" (Maybe +4??): €119, ~$130
Ratio: 1/11
Hmmm...
This Inon lens you name is an old conception: I had one like that 16 years ago when i was using a Oly compact camera and the first housing for digital. The construction of these lenses was before Reefnet launched the Subsee and discovered you had better magnification and aberration when leaving air gaps between the glasses (the optics are like in a "case" that's why the lenses ate much thicker and more heftily priced .
Nowadays few companies still produce this kind of unefficient lens as the one you name, you should compare to Inon current lenses which will bring you to 330USD... .
I would hate to poop in your party but in terms of photography you probably lost 165USD buying such lens that has low magnification power, not more than 1,5 factor.
As for me, I'm using a SMC1 +20 diopter and as you know (or maybe not) pygmy seahorses are 5-10mm, with a 100mm lens that's a *3 enlargement factor or so, hence filling the frame. Ask yourself why this lens is the most popular with UW photographers.
But what do I know in 20 years of UW photography, sometimes published ?... Hmmh..
Apart from that, i'm not sure what's your point if there is any on your price ratio. Things UW are expensive? yeah it's a niche market with specific requirements... also why don't you take a chinese plastic lens you will find at elcheapo.com for 10USD rather than the best Canon addon. you'd then come up to a 1/9 ratio.
Here's another nonsense ratio : one bottle of supermarket plonk 5€, one bottle of Brézé Clos Rougeard 2010 150€, ratio is 1/30, if you still feel so, may I leave the plonk for you while I'll drink the Brézé.
These are comparisons of what cannot be compared, one thing you have to understand is that diopters are not comparable UW especially when the glass is "cased" or not.
FYIHere's what you get with +20 diopters and a pygmy seahorse.