ND filters for black backgrounds?

Hi all,

I've added the question in another, very specific thread, but maybe a separate thread is more appropriate. 

I have trouble creating black backgrounds when shooting macro in very bright conditions: Sunny weather, clear water, bright seafloor in only 3m of water.

Used the Sony A6500/90mm, set to F16 - F22, shutter at 1/160, ISO 100. 1/160 is the sync speed, so I cannot shorten this. F22 is the minimum aperture, below F16 focusing becomes increasingly problematic. But still, with these settings, even the not-strobe-lit areas are quite bright - to bright to black them out in post processing. As compared: taking shots with the same settings/strobe positioning under a pier in the shade with slighly darker seafloor worked perfectly fine and gave nice black backgrounds. So I assume, it really is the ambient light and not my strobe positioning (well, the pros could surley do it better, but at least I tried crossed strobes/inward lighting). Hence, I somehow have to reduce ambient light, which is not feasible with settings only.

Now my idea was to reduce total light by an ND filter (ND4 oder ND8) and crank up my strobes. My port has a M67 thread, so I could add/remove the filter underwater comfortably. Would be cheaper than buying a strobe trigger for the small step to 1/250...

Any thoughts on this approach? 

Thanks,

Fabian

An ND filter is probably your only solution, going from 1/160 to 1/250 is only 2/3 stop and you are talking about a 4 stop ND filter.  The TG-6 compact has an inbuilt ND filter to change from f2.8 to f8 so it obviously works.  You would want to minimize the strength of your ND filter so you are not pushing your strobes too much, I would be tempted to try a 2 stop ND filter first?

F22 is the smallest aperture you can use?

ive used an ND filter underwater for black background macro. It certainly helps. That said, I'm not aware of any that are water corrected so using one on the outside of the port could potentially add in some wacky optical issues. You could thread one directly onto the front of the lens inside the housing. Thats what I always did. It worked quite well, just make sure you get a high quality one with good glass

and, have you tried to use a snoot?  

I have one DIY and i got some good results

I don't believe there are optical issues with using a filter outside the housing.  The main concern I would have is the water resistance of the filter itself, the materials used probably are not ideal for salt water immersion and some filters are sandwiches of a gel in between glass which may not last well.  Through soaking of the filter after dives would help I think.

Hi all, thanks for your input!! Really appreciate it!

Regarding the two questions:

F22 is indeed the maximum (or rather, minimum) of the lens, the Sony 90mm doesn't go further. 

I've tried a snoot, the not-lit area still was to bright.

Has anyone tried adding the ND filter outside of the housing? If it works I'd prefer it as it is more flexible for a dive (my dives on this spot are easily 3h, so I like to have options for what I might encounter). If outside is problematic, well then it is and I'm going to mount one inside. If ND2 or 4 I honestly still have to decide. 

The answer is yes it works threaded to the Sony 90mm macro port. While my Sony A1 and A7R V test cameras have much faster sync speeds up to 1/500th I have still found the ND filters useful when shooting wide open at F/2.8 for shallow DOF in clear bright water conditions. At F/22 you need to consider that shooting through an HD-4 filter will add issues to the AF ability of the A6500  and the brightness of the EVF and LCD. 

9 hours ago, Phil Rudin said:

The answer is yes it works threaded to the Sony 90mm macro port. While my Sony A1 and A7R V test cameras have much faster sync speeds up to 1/500th I have still found the ND filters useful when shooting wide open at F/2.8 for shallow DOF in clear bright water conditions. At F/22 you need to consider that shooting through an HD-4 filter will add issues to the AF ability of the A6500  and the brightness of the EVF and LCD. 

This is another reason to use the minimum ND filter you can get away with and why I suggested maybe an ND-2.