Flash
Fill-in Flash
When using flash on very overcast days the idea is to introduce enough light to boost the subject without overwhelming the natural light. Using a slow sync. setting and reducing flash output by 1.3 to 2.0 stops ensures that natural light provides the main illumination for the background and subject, while the flash adds a little extra to only the subject. The use of a diffuser also helps to blend the flash with the natural light by making the artificial light source larger relative to the subject. This is really only necessary with larger insects such as butterflies and dragonflies as the flashgun head is already relatively large compared to small insects such as beetles.
Whether homemade or commerially made the diffuser needs to be large relative to the flash head and placed well clear of it to ensure the light will spread out and not just create a hotspot on the diffuser.
The first image was exposed using only available light. The second was exposed using available light plus flash, with the flashgun set to slow sync and the output to -2.0 stops.
More Fill-in Flash
Flash can also be used to deal with situations where the background is too bright. Manual exposure mode is used with the exposure set to underexposure the background by 1 to 1.5 stops. Spot metering off the subject a TTL flashgun can be used to correctly expose the subject.
The first image was exposed with camera set to aperture priority. For the second image the camera was switched to manual and the exposure set to under-expose the background by one stop. Flash was then used to correctly expose the butterflies.