As we know the Sony RX100 cameras, we will refer to both Mark I and II as the lens is the same, do not offer the best out of the box super close up performance.

To be clear no compact camera really does macro, as no compact can capture an area 36×24 mm in size without being on top of the subject and having a shadow cast on it.

The RX100 however are particularly unexciting as the capture area is pretty large at 76x51mm at the minimum focus distance of 5cm. This means a reproduction factor of less than 1:2 so things are half life size in traditional terms.

In water the minimum focus distance increases and so does magnification so performance is all in all the same.

This means that a typical small subject like a medium size nudibranch measuring 4cm will fill a bit more than half the frame, not great.

The problem can be addressed by close up lenses that have a set power that determines the focal length and working distance where we can use the full camera zoom.

So the close up lens sets the working distance whilst the camera zoom sets the magnification.

A typical close up lens will have a power of 5 or 6 diopters with a working distance in water of 200 or 165mm, what does it mean for the RX100?

I have done some testing in water using a ruler and an Inon UCL165 this is the result:

Our capture width is now 46mm so our 4cm nudibranch will nearly fill the frame. The working distance of this lens is 165mm so this is a very versatile solution as most critters have no problem being approached so close.

What happens with a +10 close up lens with a focal length of 100mm, this is another test

A +10 diopter like the Subsee or Inon UCL100 achieves exactly 35mm so life size macro. However a lens like this will not focus at longer focal distances without zooming out, furthermore there are plenty of subject that do not need this magnification.

Close up lenses can be stacked so this is the result of two + 6 diopters

The width is now 32mm which is 1.1:1 so more than macro. This is adequate even for pygmy seahorse and with the incredible resolution of the RX100 cropping is not a big issue. Working distance is around 8.5cm which is still tolerated by small critters.

For completeness I have also tested a +16 combination

At around 26mm this is 1.4:1 so real super macro, the working distance is however only 6.25cm which is really close and will scare most critters away.

Conclusions:

1. Without any close up lens the RX100 can only capture 5cm objects without extensive cropping
2. A close up lens with a working distance of 165mm allows for most of the critters we consider small but without super macro effects unless cropped
3. Two stacked +6 diopters offer super macro and can also be cropped for more suggestive effect
4. A single +10 close up lens achieves real macro however it is not versatile enough for every day usage
5. Stacking lenses does introduce chromatic aberration but this can be eliminated in photos and is barely noticed in video

Pay attention when you select your close up lenses that the focal distance that is in the specification is measured in water otherwise you will find yourself with a useless purchase