White Balance: how to use a colour chart to get tones perfect
When you photograph most subjects, getting the colours 100% accurate isn’t usually critical, and if you shoot on Auto White Balance, nine times out 10 your digital will do a pretty good job of getting the white balance roughly right, so that whites actually look white, blacks look black, and all the colours in between look how you’d expect.
But sometimes getting colours 100% accurate is critical – when you need to photograph a painting to be reproduced in a book, say. The only way to ensure accurate colours is to get your white balance spot on, and the best way to do this is to use a colour chart.
These charts are calibrated to ensure that the white square is pure white, the black one is pure black and so on. By using these colours as a reference, you can eliminate colour casts caused by different light sources.
Of course, most of us don’t photograph priceless artworks, but the principles apply to any subject. Once you’ve got your head round the basics, understanding white balance will transform the way you edit images.
Step 1: It’s hip to be square
The most important consideration when photographing a painting is to get square on to it, so that the front element of the lens is parallel to the plane of the painting – otherwise, some corners of the frame will be closer to the lens than others, resulting in converging horizontals or verticals, or both.
Step 2: Get the right length
You need to use the right focal length. Wide-angle lenses produce barrelling, with horizontals and verticals bowing outwards at the edge of the frame, while longer lenses produce pincushion distortion, with horizontals and verticals bowing inwards. A focal length of around 50-70mm is a suitable starting point.
Step 3: Bracket by a stop each way
Bracket your shots to maximise your chances of getting the correct exposure. Correcting the exposure in RAW is fine up to a point but it can introduce unwanted noise and colour artefacts, so is best avoided for colour-critical work.
Step 4: Aim for the sweet spot
To ensure that the painting’s details are as sharp as possible, it’s important to use your lens’s ‘sweet spot’. Lenses are sharper at some apertures than others, and for most the sweet spot is between f/8 and f/13 – any higher or lower runs the risk of elements near the edge of the frame looking slightly soft.
Step 5: Check the histogram
To learn how colour charts are used to ensure accurate colours, launch Photoshop and go to File>Open to open up chart_start.cr2. Looking at the histogram, which stretches all the way from the left to the right, you can see that the exposure is pretty spot on, but it looks slightly under-exposed.
Step 6: Highlight the highlights
Holding down the Alt key, drag the exposure slider right until red patches appear around the frame; +0.50 should be about right. Because the brightest edges in the frame are highly reflective, they’re supposed to be highlights, so we’re tweaking the exposure until these edges are only just blowing out.
Step 7: Get the balance white
To correct the White Balance, click on the pipette icon at the top-left of the screen then click on the white square at the far right of the colour chart. By telling the Camera Raw editor that this square should be white, you’re removing the yellow colour cast not only from this square, but from the whole image.
Step 8: Reset the white balance
We set Auto White Balance in camera, and it’s done such a good job of ensuring the white square is actually white that you might not even notice any difference. However, the Temperature has in fact changed from 4200 to 4150 and the tint has changed from +53 to +45; this indicates that the original shot had a slight warm – or yellow – cast.
Step 9: Take a bow
Once you’ve finished adjusting exposure and white balance, click Open Image to open the shot up in the main Photoshop Elements window. The next thing we need to do is correct the slightly bowed edges of the painting’s frame caused by lens distortion; to do this go to Filter>CorrectCameraDistortion and drag the Remove Distortion slider to -3.00 and then click OK.
Step 10: Play it straight
To check that your painting is square, go to View>Rulers. Click on the top ruler and drag it down to create a horizontal blue line. You can then use this to check that your painting’s frame is horizontal. Do the same for the sides. If your frame is tilted, you can rotate it by going to Image>Rotate>Custom. To finish, grab the Crop tool and click-and-drag to crop out the wall and the chart.
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on Monday, May 21st, 2012 at 5:20 pm under Photography Tutorials, Tutorials.
Tags: camera tips, photo editing, white balance