Friday, 1 January 2010

Shooting on White

I don't shoot much against a pure white background, but it's definitely something I should re-familiarise myself with as it gives another dimension for your subjects whether its for high key portraits where high key == happy and youthful so I understand, or for product lighting.

Someone commented that some of my stuff looked like it was lit inside a cave, so lets see how we can get outside.

First up, the best resource anywhere I've seen is from Zack Arias and he breaks down the principles for shooting on white in the 2nd of his 4 part blog post here, although its worth reading the lot for an understanding/background.

Zack's other relevent blog entries are:There's only a few things that need to be taken into consideration when lighting on white:

Subject vs Background Ratio

To acheive a white background, the background must be lit 1.5 to 2 stops above the subject exposure and this should be verified using your incident flash meter or your histogram. So, for a standard portrait, we may want f/2.8 on our subject which means we need to be around f/5.6 (lets go with 2 stops it makes maths easier)

However once we have the correct ratios for the background lights we need to position the subject and this needs attention. Since we have essentially created a large softbox out of lighting the background, we need to make sure that the subject is not enveloped by the light coming off the background.

To do this, we need to ensure that the light reaching the BACK of the subject (meter facing the wall from the subject position) is metered to be the same as the subject exposure - ie we're 1.5-2 stops down from the light at the background.

An example:
subject @ f/2.8, background @ f/5.6, back of subject facing wall @ (roughly) f/2.8
Keeping the numbers as in the example above we ensure that there is no wrap/bleeding of light from the background onto the subject. However, if there is a need/desire for the wrap light from the bg, simple allow the metered reading from the back of the subject to be greater than that of the main exposure.

This point brings us back to the original 1.5 - 2 stops above subject reading for the bg. If we are lighting the bg up by 2 stops, then we will need to have the subject further away from the bg which may not be possible given the space we are shooting in, given that we also need room in front of the subject too. Therefore, 1.5 stops above main exposure can also be used for the bg light.

NOTE: Whilst Zarias' tutorial (and the examples above) are based on a +1.5 stop exposure above subject, many other recommendations come in stating that a 2/3 to 1 stop background exposure - when lighting a white background (white wall/white seamless etc) - will be sufficient for pure white

Even White Background Light

Ok, so we know that we need to light the bg but we need to make sure that the bg is consistently (or close enough) white. To do this, we need to meter from edge to edge and ensuring that all spots are within 1/10 of a stop of the intended power.

The very few times I've done this, I've still got it wrong. I've done this in studios with AC powered lights which allow me to get the positions of the lights wrong more but its still worth while to know how to get this right, particularlly if we're burning away the AA batteries in speedlights.

What's important is getting an enen spead so having a large light source in the first instance is helpful (an umbrella or softbox). For a largish white background we've looking at 2 lights at the sides of the background area we are lighting. With the 2 lights, we're shooting across the background so that the light is pointed at the other side (left light aimed at right side of background) and 1/2 way up on the vertical. Because we're shooting from the sides at an angel (say 45degrees), the light should be even as it hits without no hotspots but metering will confirm this.

When using bare (SB) lights as we do not have sufficient modifiers, we need to pull these back a little and use them at a mid zoom (35mm has been good for me). As a note, I normally start off on lowest power on the SB units and then get a consistent exposure. Then I simply up the power on the SB units such that I bring them to the required exposure - this will save us eating too much power from the AA batteries during setup.

For lighting a smaller bg (ie for single subject) usually one SB unit is enough.

I sometimes find that I've mis-metered and not got the intensity on the bg high enough for pure 100% white (eye dropper in photoshop shows 255-255-255) but if its close enough I can normally recover. This was the situation with the image below:

The wall was a beautiful peach colour and out of camera I found that there were some peaks at that were showing through as not 100% but this was easily filled in post.

The other thing to note about the image above is that exposure was at f/5.6 and I recall the bg being @ f/11. If we look closely at the edge of the subjects we see a little too much wrap coming off the wall - this was a limitation on my part for not metering again during the change for this sitting.

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