Wednesday, September 18, 2013

Mirror lock-up

I tried a little experiment this evening. Today was quite dull and cool, but the sun came out as I was coming home from work and I noticed quite a collection of flies sunbathing on the shed roof in the garden (Sarcophaga - Flesh Flies, Pollenia - the cluster fly, Calliphora - Blue-bottles and Dasyphora type shiny green muscids). Because it was still coolish, they were quite sluggish and not inclined to move. So I decided to have a go at some piccies. I set up my Canon 100mm Macro on the Canon EOS 60D mounted on a old Olympus focus rail on the Benbo tripod. Here is the same setup (taken on another occasion):

What I wanted to try was comparing shots with and without mirror lock-up to see if it made any noticeable difference. Since everything was setup on a sturdy tripod, the flies were sitting on a solid shed roof, and generally keeping quite still, this seemed a good opportunity to give it a go.

If you haven't come across the mirror lock-up setting before: In an SLR camera, when you press the shutter release, the mirror (which is used to direct light coming in through the lens to the viewfinder) has to flip up out of the way of the shutter before making the exposure. This is what makes most of the noise you hear when you press the release button. When you are using a lot of magnification (as in macro photography or shooting through a long telephoto lens) the vibrations caused by the "mirror slap" as it flips up and hits the roof of the mirror box can cause a degree of what is effectively camera shake and so degrade the image. Enabling the mirror lock-up setting separates the two actions of raising the mirror and releasing the shutter. You have to press the shutter release button (preferably via a remote release!) twice: The first press raises the mirror; then you wait a sufficient time for vibrations from the mirror slap to have damped down before you press the button for the second time to actually release the shutter and take the shot. A couple of seconds is generally reckoned to be enough, although I have seen as long as 8 seconds suggested. On the 60D the mirror lock-up Enable/Disable function is the fifth option on the  "C.Fn III: Autofocus/Drive" menu. (Hint: remember to disable it again afterwards or it can be very disconcerting the next time you use the camera!.)

Here is a blue-bottle (Calliphora erythrocephala) busily engaged in one of its favoutite activities - expelling and re-ingesting droplets of saliva (often described as "blowing bubbles"). Exposure: 100 ISO, F16, 1/5 second. It was on the roofing felt of the shed which makes for rather a dark background, so with -1EV exposure compensation. The lens was set to a magnification of 1.5:1 and to Manual focus. I then focussed by moving the camera backwards and forwards using the focus rail. This image is somewhat cropped round the fly, but otherwise straight from the RAW file using UFRaw's default settings and with no additional processing (no sharpening).

I took five shots on these settings with mirror lock-up enabled and another five with it disabled. All shots were taken using a wired remote control to release the shutter. In the case of the mirror lock-up shots, I waited approx 4 seconds (estimated by counting) between the first press to raise the mirror and the second press to release the shutter.

Here are 100% crops of the head and thorax from what I considered by visual inspection to be the sharpest of each set of five.

With mirror lock-up enabled

With mirror lock-up disabled
I reckon there is a discernible difference with the facets of the eye and the fine hairs on the margins of the thorax being slightly clearer in the image taken with mirror lock-up enabled and I think this is pretty typical across all the shots I took.