Monday, October 21, 2013

Lighting specimens for focus stacking

This is the third of a series of posts about taking focus stacked images of insect specimens to illustrate identification guides. The first part was a general overview, and the second a description of the setup I use to acquire the stack. In this post I will cover lighting the specimen. The stacks that were shot for Britain's Hoverflies were lit using flash but, more recently, I have been using continuous lighting produced by three Yongnuo YN1410 LED panels. I will compare the results and discuss the pros and cons.

Yongnuo YN1410 LED video light with plain diffuser
Yongnuo produce quite a range of LED panels with anything up to 300 bright white LEDs. The more recent models have tended to be a bit thinner and lighter and to have rather more sophisticated brightness controls. Part of the reason I went for the YN1410 (which is an older, and probably superceded, model by now) is that it has a DC power socket. Many of the more recent units don't have this. They can be powered using 6 AA batteries or battery packs made by Pansonic or Sony for video cameras. The battery packs are expensive (quite a lot more than an LED panel!) and I didn't want to have to keep 18 AA batteries recharged! Instead, I bought a 60W, variable voltage DC power supply (panels need 7.2 - 9V) from Maplin and a 4-way splitter cable (I could only find 2-way or 4-way splitters!). This happily powers all three units for however long a session I want and means I don't need to cope with all that extra weight of batteries - which makes attaching and positioning them easier.

The LED panels have very simple controls: an on/off switch and a pair of buttons to increase or decrease the light output over 16 steps. They are quite well daylight balanced, but tends to be a little towards the blue side at max output (5800-6000K). I find that, with the camera set to auto white balance, I get very acceptable colour reproduction.The max light output of each panel is rated at 960 lumens. What is probably a more useful way to describe it is that, for the MP-E 65 macro at 1:1 reproduction ratio and f8, the exposure is around 1/60-1/80 second at ISO100.

Specimen lit with 3 x Yongnuo YN1410 panels
Hilara matrona (Empididae, Diptera) male. LED lighting.
100% crop

In the past I have used three flash guns: the two heads of my Canon MT-24 EX and the Yongnuo YN565EX as the third. These require some diffusion, so in the picture below, the MT-24 heads are fitted with plastic diffusers which roughly double the effective area whilst the 565 is fitted with a large, home made soft box. The flash exposure is manual and with the 565 set up as a slave controlled by the MT-24. For this particular shot, the MT-24 heads were both set to 1/16 power and the 565 to 1/64 power. This is best judged by taking test shots and assessing the image and histogram on the camera's rear LCD panel. (The articulated LCD screen of the Canon 60D really scores here. It makes it easy to view the screen without having to go through contortions to get down and directly behind the camera! Being young and bendy would probably be equally effective ...) For this sort of shot in the 1.5-2:1 range, the flash power require  is generally low. This has the advantage that the flashes recharge very quickly. At higher reproduction ratios, say 4-5:1 for shots parts of the animal, more flash power may be required. For things like tarsi details at 5:1 you may even need full power. In these cases the StackShot's programming will need adjusting to allow a longer pause between shots to allow the flashes time to recharge.

Three flash heads with diffusers. The YN565 has a large, home made diffuser head fitted.
Stronger specular highlights. Note those on the hind femur.
100% crop
Comparing the two images, the greater contrast and more pronounced specular highlights in the flash lit shot are obvious differences. More diffuse flash can be obtained by building a soft box around the specimen. This is especially necessary for more shiny and metallic specimens with strong specular highlights. One way I have often done this is to use plastazote sheets (ideally 6mm thick - the ones shown are thicker than I usually use!) to build a soft box around the specimen (note that, in this case, I didn't also use diffusers on the flash heads):

Plastazote softbox built around the specimen
Another idea is to use an expanded polystyrene vending machine cup (the sort of thing soup is sometimes sold in). Cut the bottom off, so you get a cone, narrowing outwards, and pin that to the backing sheet so that it surrounds the specimen.

Note that the MT-24 head is mounted on a small ball & socket head. I find these extremely useful for mounting lights and flashes. They can be bought quite cheaply off eBay.

Small ball & socket head, metal flash "cold-shoe" mount and standard 1/4 inch photo screw, all bought off eBay, are very useful for mounting lights.

Whilst all the stacks for Britain's Hoverflies were shot using flash, I have more recently almost entirely switched to using continuous lighting provided by the Yongnuo LED panels. I find that this approach solves a few niggles I had with using flash:
  • With flash, you need another light, such as desktop halogen lamp or an anglepoise lamp, to provide the light to to view the specimen for focussing and setting the start and end points. You then need to move that out of the way and move the flash heads into the correct position before you shoot.
  • You generally need several test shots to get the right flash exposure and to adjust the diffusion and flash head positions to manage specular highlights.
  • The exposure of successive flash shots can sometimes be a bit variable (probably an indication that the flashes did not have sufficient time to fully recharge between shots). This can impact on the quality of the finished image.
With continuous lighting, these are avoided. The main lighting for the shot is also used to focus and setup and, whilst you are focussing and setting up, you see shadows, highlights, etc. So everything is done in one operation and it is unusual to need more than one test shot to confirm the exposure. I have not found any detectable exposure variation between shots using the LED panels. All in all, faster and more convenient!

In the next post I will talk about software for processing the stacks.