Saturday, 9 February 2008

Using gels to correct light from flash and match ambient

For once this isn't a product related entry, but I hope it will still be of some interest to read.

I was shooting a charity burns supper the other night in a local hall. Quite a large location, probably about the size of a basketball court in total. About a quarter of the hall has a second level with seating, which was unused for the event, so I took advantage of a quiet location to shoot from, not to mention a more interesting angle than down below.

As soon as I arrived, I metered the hall and found it to be a typically dull 1/40s @ f/4, ISO 800. Worse than that, it was an even spread of very high up lamps leading to a lot of nasty downward light giving pretty unflattering shadows on everyone in the hall.

I grabbed the organiser and the band together and checked flash would be ok, and as soon as they confirmed they didn't mind, I disappeared upstairs to set things up. The first step was to get some control on the light. I found a bit of white painted wood and took a quick shot of it. At a glance I could see it was a very orange photo, which confirmed what I already thought - these lamps were producing light which was relatively close to tungsten balance. A very slightly fluorescent green was also noticable, but so little I didn't feel the need to balance for it. I'm guessing these were the large mercury vapour style lamps you often get in large buildings. The green cast is easily sorted when processing the raw files (pick a tint between which takes an average of the slight difference), and saves the flash needing to fire much harder through two filters than one. Had it been like some mercury vapour lamps I've come across which give a very green image when set to tungsten white balance, I'd have had no choice but to add a fluorescent gel.

I instantly deleted the test image out of habit, but I managed to find a wide angle shot I'd taken with available light, and cropped in on a white table setting. You can see how orange it appears with a daylight white balance setting.

Once set to tungsten, the colour casts all but disappear, at least to an extent which is good enough for what I needed.

Having established that I only needed a CTO gel to balance for tungsten light, I stuck it on the flash and hooked the flash up on the wall. This little decorative thing proved a useful place to stick the flash. You can see the gel attached on the front of the flash if you look carefully. I also zoomed the flash fully in to 105mm to give the best throw of light given the size of the hall.

Here's a shot which shows the CTO gelled flash firing with the camera set on tungsten and daylight balance. Lets you see how important balancing the flash is for decent looking images.

With the flash balanced to tungsten, and my camera balanced to tungsten, I was free to shoot from anywhere in the hall and have plenty of clean "white" light. I was now shooting at 1/250s @ f/5.6, ISO 1250. A much safer exposure given I was including ambient light, but as you can see, I'm now underexposing the ambient a bit from my original metering when I arrived. Adding up the numbers, you can see my shutter speed is 2 1/3 stops darker, my aperture 1 stop darker but my ISO 2/3 brighter. This means I'm underexposing the ambient light by 2 2/3 stops making that part of the exposure pretty dark, and therefore it won't make a huge difference to my images if I get any camera shake when using a longer lens (I was getting some close in shots on the singer/band etc.). The flash exposure, even with a fairly high power, will be faster than 1/700s which is plenty fast enough to give sharp images. The ambient light is providing some subtle fill though which is what I wanted. Why have I opted for a higher ISO speed? Well, it saves the flash having to work so hard, and a well lit image always hides noise that much more than a poorly lit one. Also gave me a touch extra depth of field to work with given I was able to stop down a little.

You can see the flash is easily powerful enough at this high ISO speed to fire right across the hall and light the far wall a good 50-60 feet away. It is also giving a subtle fall off in the curtain behind which helps when I'm taking shots of the band. Gives a nice vignette look to the image.

Here's a shot I took from the left of the balcony where it extends out across the hall a bit. You can see where I was at the left border of the previous image. From here, the light is close to side lighting the band, which gives a nice contrasty image, with my ambient exposure combined with reflections from the wall opposite the flash serving to control the contrast.

Here's a shot where I fired off a second shot too quickly and the flash didn't keep up. It gives you a feel of how I've underexposed the ambient and I'm really only using the flash to give my exposure.


Anonymous said...

Nice post, good example of how a small strobe is enough for a large venue. Good info on mixing light temperatures too. Cheers.


n506 said...

Thanks Dave

Blaise - Photographe Mariages said...

This is really an excellent post - I love it how you make me live the experience with you.

I'm curious though - did you only photograph the band? Or were you also taking table photos? If so then did you only use your balcony flash?

Blaise - Photographe Mariages said...

Oh - another question. That clamp you use looks very useful! Where did you get that? In a hardware store or in a photo store? Did it ever let you down?

n506 said...


I was photographing the band mainly, although I did go down and take photographs below. As I was using E-TTL, I had full control of the remote flash from anywhere in the hall, although when I went beneath the upper deck, I lost line of sight and firing the remote was pretty much a lost cause. I still had ambient and an on-camera 580EX though, which is good enough for a lot of shots. Incidentally, just because you're using E-TTL, doesn't mean you're letting the camera take control. A lot of people seem to think this. The E-TTL communication also lets you set the manual power of remote flashes, which is the really cool bit! Manual power control of a flash on the other side of the room? Priceless! :)

I bought the clamp from an electronics store we have in the UK called Maplin. Was about £10 if I remember right - I bought a bunch of them! It's pretty similar to the Manfrotto MN275 Mini Spring Clamp. It has never let me down, but you've got to be careful it gets enough grip, or you could be looking at a very expensive piece of plastic that used to be a flashgun. That said, it has really good rubber edges which get a surprising amount of grip, without the clamp spring being too hard to open.

If I'm putting a flash up in a very vulnerable location where it could get knocked, I'll either get a bit of string or tape to hold it up, or where that's not possible, I'll use a Caddy Speed Link LD which allows me to securely hook the flash to anything I can loop it over or through. This is especially important where the flash is sitting up high and people are walking below...last thing you want is a flash and clamp landing on someone!