A6500 + Case + Strobe - What to upgrade

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I doubt that the differences measured by DxO under ideal conditions in a studio account for much after you put several feet of murky water between the camera and the subject. The big improvement of 10-18 over 16-50 is that it lets you get closer and reduce the thickness of that image-degrading water layer - something that simply isn't a factor in land-based comparisons.
How come? Google gives me the same minimum focal distance for both lenses, 0.82-1 ft (25-30 cm).
 
The 25cm in specs for 10-18mm is very conservative, I measured it to be closer to 17cm from the sensor. More importantly, field of view at 10mm is much wider than at 16mm, allowing you to get considerably closer while framing the same subject. Plugging the numbers into calculator, at 10mm focal length, you will fit a 70cm wide subject into the frame at 30cm distance - but at 16mm, you need to be 48cm away for the same shot. A 3m-wide sea fan will fill the frame from 2 meters away at 16mm, but only 1.25m away at 10mm - 2 meters is ambient-light territory, while 1.25 meters can still be lit by strobes.
 
This may work for sea fans but underwater most often you are not the one who decides how close you can get. I was shooting dolphins the other day and every time they passed by it was them who chose how close I shoot, not me. If I had 10-18, they'd looked like ants on most shots. Ahough I took couple of shots at 16 mm to fit large pods in, most shots were in the 24-40 mm range by EXIF. Stationary objects are most common in macro photography, where magnification factor plays a major role. So unless you are shooting a whale or a whale shark point blank, what's the use of wide angle underwater?
 
There are plenty of non-macro subjects that are either stationary, or slow enough and willing to let you approach to your desired distance. Wrecks, corals, gorgonians, morays, pufferfish, lionfish, stonefish, scorpionfish, turtles, octopuses, cuttlefish, lobsters, crabs, bottom-dwelling rays, batfish (sometimes), groupers, large schools of fish in general, etc. Yes, some subjects such as sharks and dolphins are fast-moving and generally won't come closer than a few meters - but at these ranges, shooting through that much water, minute differences in image quality between lenses amount to very little, as water degrades the image quality anyway, and sides and corners, where most lenses suffer most, end up being mostly empty water anyway. If wide-angle photography was limited to sea fans, why are people buying all those huge expensive domes and $4k WACPs?
 
Interesting comments. I agree on the 10-18 points which I use, it's shockingly bad for the price it costs, slightly sharper than the 16-50 but much more vignetting (on land). However like Barmaglot said, underwater I doubt it will make such a big difference (for quality), while I see how 10mm vs 16mm can make a difference.
 
People use wide angle lenses underwater so they can get spectacular shots like this one sometimes. But most often they just get one or maybe two worthy shots per dive cause not all groupers are that diver-friendly, and a 1" nudi will span just about 1/10th of your frame at 18 mm and 15 cm from your lens (assuming what you said about focal distance of 10-18 is true). If I owned one for my underwater camera, I'd limit its use to some specific occasions, like snorkeling with whale sharks off Isla Mujeres; otherwise I'd stick to the regular zoom (which is not as bad at 25-40 mm as at 16 mm, BTW).

PS: As for the idea of getting a sharper picture by getting closer to the subject, this won't help against fringing. Which will ruin result in any case.
 
My last trip was to the Red Sea; I used my 10-18mm on 11 out of 14 dives, and to be honest, I haven't noticed any of that fringing that you dread so much. Didn't see a single small nudibranch, and the three Spanish dancers that I found were plenty big enough for 18mm. I did see plenty of nudis while diving off Koh Tao in Thailand, but trying to shoot them with 16-50mm without a close-up lens didn't produce any impressive results. I will have a 90mm macro lens in a couple weeks - that one should be good for the little stuff.
 
My last trip was to the Red Sea; I used my 10-18mm on 11 out of 14 dives, and to be honest, I haven't noticed any of that fringing that you dread so much. Didn't see a single small nudibranch, and the three Spanish dancers that I found were plenty big enough for 18mm. I did see plenty of nudis while diving off Koh Tao in Thailand, but trying to shoot them with 16-50mm without a close-up lens didn't produce any impressive results. I will have a 90mm macro lens in a couple weeks - that one should be good for the little stuff.
Which 90 mm is this?
 

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