CPF for Ringflash

[martycon]

I am considering the use of CPF with Ringflash for macro to reduce reflections on curved shiny subjects. Small curved surfaces like dimples result in numerous tiny reflections which some may call hotspots.
How would I adjust the CPF to optimum without using the light of the ringflash? If adjusted by trial and error, would the chosen setting suit most subjects. Plan B is to buy a cheap CPF and "suck it and see", as the great Mo Macacie would say.

[ricktas]

I would say you need to change your shooting settings and not even consider a CPL.

If you are getting blown highlights, then you are over-exposing your shots. Use a faster shutter speed, lessen the output from the flash, use a smaller aperture etc to ensure you do not blow the highlights.

If all that fails, consider an ND filter rather than a polariser. The ND filter will darken the entire scene for you and you will not need to rotate it like you do with a polariser.

[martycon]

Thanks for replying Rick. I do not think you fully appreciate the problem as stated, or I have stated it poorly. I have underexposed one and two stops, both intentionally, and otherwise. However tiny bright reflections remain. Consider the catchlight in an eye, which has the same cause. How would one avoid this by underexposing. I avoid normal blown highlights caused by the use of flash with macro, by using the camera's inbuilt highlight/shadow control which reduces highlight intensity on the RAW image. Some would say that the image is not truly RAW, but that is beside the point.
I have dificulty in defining my problem. It is like stopping sand sized grains of broken glass from appearing to glitter.
regards marty

[Tannin]

G'day Marty.

I reckon it's an idea worth exploring. The key issue to resolve is the nature of the hotspots. Will they respond to a CPL? If they are all parallel to one another, then quite likely yes. (Think of the example of reflections off glass or water: these respond very well to a CPL.) But I suspect that they won't. For starters, your ringflash is sending light from many different directions.

How diffuse is a ringflash anyway? I've never used one, but the first thing I'd do with a standard flash (or my twin head one, which I seldom use) is bounce it, or failing that use a diffuser. Anything to drop the contrast a bit and get a more even-all-over light.

Don't buy a cheap CPL. Cheap CPLs will rot your teeth, make your hair fall out, ingrow your toenails, and triple your insurance premiums. Oh, and wreck your pictures. Borrow a CPL from someone and try it out.

Normally, you adjust a CPL by rotating it and looking through the viewfinder until you like what you see, but it won't work if you are using flash unless you use it as a modelling light - very hard on the batteries, and I don't expect you'd be able to see much difference anyway, given that you'd have natural light as well to confuse the issue. But you can set a CPL blindfold simply by taking three shots with one-eighth turns between them - one of the three has to be pretty close to right no matter what angle you define as "right" and what setting you start with. Certainly close enough to work out if the technique has promise. If it does, then you can figure out a more reliable technique later. If not, you can return the CPL and try something else.

Oh, one last thing: generally speaking you'll get more headroom at lower ISOs (as you up the ISO, dynamic range shrinks). But you are probably doing that already.

[martycon]

Tony you have given me a glimmer of hope. Three shots with 1/8th turn increments is not too onerous. I have a slightly too big CPF which I should be able to tape in place. Now to find a suitable static subject.
The ringflash has a good inbuilt diffuser. I have experimented with extra diffusing layers, and see no benefit. I think the overbright spots are due to tiny shiny concave mirrors, or the glisten on a facet.
regards marty

[Tannin]

Cheers Marty. If your experiment works, you could get a step-up converter ring for about $10 or maybe $20.