As usual with photography getting the shot right is about compromising on one factor to gain another and that is certainly true of macro exposures. This post is a brief exploration on my experiences of juggling shutter speed, aperture value, ISO and flash type and level.
Bear in mind I am a Nikon user and some of the detail may be different on other camera systems, though the principles will be the same. I always use manual exposure mode for macro and close-up shooting with flash so I know what shutter speed and aperture are locked into the camera, as any unexpected alteration of them can seriously compromise the shot.
I tend to find that when shooting at 1:1 and greater magnification then I will use TTL flash which meters directly off the subject and lights it, taking no account of the ambient background exposure. This makes sense as the animal will often fill a large area of the frame. I am generally not a fan of fully dark backgrounds caused by massive underexposure of the ambient conditions, so tend to select an aperture and shutter speed combo that allows some ambient light to creep in and give a colour to the background, or I make sure it gets some of the light from the flash. I tend to dial in a varying amount of negative flash compensation to make the light more subtle and reduce harsh reflections. Usually -0.7 to -1.
When shooting bigger animals at considerably less than 1:1, or including more of the animal's environment, clearly the ambient light has a much greater effect on exposure, which is a good thing as it adds to the environmental feel of the image. I still like to add some flash in many cases to fill shadows and to use its very short duration to help to freeze camera or subject motion. In this case I find that TTL flash can often overexpose and completely burn out the subject. Therefore for this type of environmental image I tend to select TTL-BL, which is balanced fill flash. This meters the whole frame, including the subject and background and uses the flash to balance the illumination of the subject with the background and therefore the ambient light conditions.
Being limited to a flash sync speed of 1/250th sec influences choice of aperture to achieve an ideal exposure value between 0EV and approximately -1EV relative to ambient light. Clearly if ambient light is very bright you cannot raise the shutter speed to reduce EV, you have to close down the aperture a bit and select a low ISO value instead. If ambient light is weak and you struggle to avoid serious underexposure you can drop the shutter speed a bit to say 1/200th, open the aperture up and raise the ISO. Always bear in mind that opening the aperture will reduce depth of field and raising ISO will reduce image quality...as always its a trade off to get the exposure you want, so alter things within a range you know works.
This shot of the mating damselflies is a good example of the issues and techniques discussed above. I wanted to use a balance of ambient and flash light while retaining a light, colourful and out of focus background. I also needed an aperture that would enable me to get the eyes of both animals in focus...very tricky owing to the small depth of field at close distances. The animals cover a relatively small area of the frame compared to a real close-in image, so it is more of an environmental shot.
As there was a slight but intermittent breeze and I was using a 150mm lens (225mm equiv. on APS sensor camera) I wanted to use the max shutter speed available of 1/250th. I set an aperture of F9 to give me enough DOF, but this combination resulted in a rather low exposure value relative to the ambient light. To get around this I selected ISO 400, which is usually the max I will go to for macro where detail is essential. I therefore gained a stop of exposure without altering shutter or aperture settings. Another benefit of raising ISO is that the flash output is reduced. This makes recycling faster to the next shot and makes the light more subtle, with less chance of nasty reflections on shiny insect cuticles and eyes.
Its all about practising with your kit and finding what works for you. I guess the holy grail of macro shots are those where it isn't obvious that you used flash as they have nice balanced lighting, no harsh shadows and no harsh highlights or reflections. Using a balance of ambient light and flash is one way in which this can be achieved with care.
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