Shooting bats at dusk

I've started this discussion in the Nature forum too so apologies if you've stumbled on it twice.

I've found a place nearby where there are lots of small bats (20cm across the wings) flying around at dusk. They look really interesting silhouetted against the evening sky and I'd like to capture that.
I've a digital SLR, a Pentax K10D with standard 18-55mm lens, and not quite sure of the best set-up for shooting fast-moving, dark, small objects against a lighter background in a low-light setting.
Here are some of my thoughts, questions and assumptions:

1) It's low-light (just after sunset) so I assume I'll need a higher ISO setting.
2) I'll use a tripod but maybe I should use the image stabilising, shake-reduction option instead.
3) I need to freeze the motion so a flash must be a requirement. Is it possible to adjust the flash speed incase it's not fast enough.
4) What distance is the flash effective to? The bats will be about 3 metres away.
5) The bats move too fast for the auto-focus to lock-on so I'll need to do that manually in advance.
6) If I allow in the most light with a low F-stop and large aperture it'll reduce the depth-of-field making it even harder to catch them in focus.
7) What white level setting would be preferable?
8) Should I take a spot meter reading from a tree branch which is also in the frame or is this irrelevant if I use the built-in flash?

Any suggestions or experience in similar scenarios would be appreciated. I know I can go out and test all these options myself but it would be nice to go out armed with a bit of received wisdom.

Tags: bats, birds, dusk, evening, low-light

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Permalink Reply by Tom Wilson on June 26, 2008 at 4:22pm

What camera and flash are you using? Be specific. This is a terrific opportunity to get some really neat shots. You will need a tripod, an open shutter, and a flash with some reach. Let me know what gear you are using and I will tell you how to set it up.

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Permalink Reply by Alan Edney on June 27, 2008 at 1:17am

Unfortunately I've nothing so exciting as "gear"! All I've got is my Pentax K10D with the built in flash and a tripod. Hmmm.... I've a feeling that it won't be up to the task.

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Permalink Reply by Tom Wilson on June 27, 2008 at 2:31pm

I was not familiar with the K10D so I looked it up on line. It looks to be a very capable camera. The built-in flash will probably not have enough umph to to what you have in mind. Moreover, this is a "Smart" camera. It will attempt to out smart you every step of the way. When you pop up the flash, my guess is that the program functions will take over to prevent "User-error" from screwing up the shot. The same override would occur if you mounted a Pentax TTL flash to your camera. Let's assume for the moment that you can reach into your bag and pull out an old manual flash. The objective here is to mount the camera to a tripod, set your focus manually, as well as your aperture. Shutter speed is one of your variables. The bulb setting is your best option but trial & error with the digital preview will determine what you like best. 15-30 seconds would be doable. The aperture & shutter speed selections will expose your background only. Now to the flash. The best way to do this is not to synch your flash at all. Fire it manually at the bats while your shutter is open exposing for the background. You may want to lie on the ground under or in front of your camera. What ever works for you. An old Vivitar 283 packs a good punch. The 285 has variable power which is terrific in this setup. At 1/8 power, for example, you will be able to trigger the flash sever times in rapid succession before it needs to take a breather to recharge the capacitor. Some of the newer and fancier flashes offer adjustable strobe effects. Some test shots with an old sock or brown mitten will give you a very good idea of the distance your flash needs to be from the bats. I did this shot 20 years ago with a 4x5 camera, pool side at my parents' house. It worked very well. Good luck!

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Permalink Reply by Alan Edney on June 27, 2008 at 3:21pm

That's great, thanks for your suggestions. I think that what you've described is the way I'll go. That is, using the bulb setting for 30 seconds or so to get a nicely exposed dusky landscape and then firing the flash manually with a hand-held gun. Unfortunately that piece of equipment is missing. I'll have to have a look at some prices....