A couple of days ago, whilst out looking for dragonflies, we came upon a clump of bistort growing wild alongside a stream, not far from home. I stood and looked closely at the flowers to see what insect activity was there and it wasn’t long before I spotted a number of fly carcasses beneath some flower heads. This indicated some predator behaviour, but there was no sign of spider webs. However, it wasn’t long before I was able to spot my first robber fly, hanging on a grass stem waiting for prey.
The robber fly (family asilidae), in this case between 10mm and 15mm in length, is also sometimes known as an assassin fly. I think the latter name is very apt because that’s exactly what the fly is – an assassin. The robber fly is an opportunistic predator. It waits near a food plant for its prey, which may be anything from bees, wasps, damsel and other flies to small moths and butterflies. It will attack arthropods much larger than itself, which it then injects with saliva containing toxins that immobolise the prey and liquefy its contents, allowing the assassin to feed from a liquid meal, sucking its prey empty and then leaving the carcass to decompose in situ. Yuk!
I only saw one robber fly that evening but returned the next day, about the same time, to see if I could find any more. This time I found a small number of them feeding on the bistort rather than waiting for prey and they were much more skittish – add to that poorer light and a fairly strong breeze and the photography was much more difficult, but I managed a few shots that were reasonably sharp. Here is a selection. I think the chunky pointy ones are females and the long thin fluffy ones are males. The first image is a fly carcass below the flower.
Canon 70D with Sigma 180mm f3.5 EX DG HSM Macro lens supported on a monopod and the camera’s inbuilt pop-up flash balanced at -1 stop. I bought the macro lens second hand last year to add to my Canon 100mm IS Macro (an older version, the Sigma f/3.5 was replaced in 2010 by the f/2.8 model). It does not have image stabilisation, so I find it quite difficult to hand hold (unlike my 100mm IS macro which I can hand hold under most circumstances) but it is pin sharp when used with care. The 180mm has a longer working distance than the 100mm, so I am also less likely to disturb the more skittish subject. It also has a narrower field of view, which means that background distractions are less troublesome.
Depth of Field
As magnification at the sensor plane increases, depth of field (DOF) reduces and that is regardless of what macro setup you use. In macro photography, DOF depends on just two variables: aperture value and magnification. At any given aperture value, the higher the magnification ratio, the smaller the DOF.
You will find a depth of field calculator at this link.
Increased depth of field can be achieved by stacking images, but that’s another story!
The sharpness of the image is also affected by diffraction limitation…as the lens is stopped down, light waves passing through the aperture begin to interfere with each other, causing small detail to become less sharp. Diffraction is affected by pixel size and aperture size…so is less noticeable with a large sensor at a given aperture. (Mobile phones are inherently affected by diffraction blur.)
You will find a technical article on diffraction limitation on my husband’s site at this link and another here. In practice you can decide on the practicality of stopping down more to gain more depth of field at the expense of increasing diffraction blur. That’s where experience counts…you decide yourself if it’s sharp enough!