Posted by: David Vernon | April 28, 2009

Highlights

Fighting the CrowdsThe Intro to Digital Photography class at the Art Guild took a trip up to Matthiessen State Park last weekend. The weather was wonderful, the water was high, and the crowds were plentiful. It was a mostly overcast day – which meant low contrast images that were pretty easy to meter for. But every once-in-a-while the sun would peek out and contrast would skyrocket. What was once just a few stops of difference in a frame was now beyond the ability of the camera to capture. The result? Overexposed highlights or underexposed shadows – and details that were there – lost forever. There’s an old photography rule that says “Expose for the highlights, let the shadows fall where they may”. Simply put – your eye is drawn to bright things in images. For the most part, you want to keep your highlights in check so as to not draw the eye to the wrong place. How do you know you’ve done that? How do you know your highlights are okay? This is where your Image Review screens on your camera come in handy. In case you’ve been wondering “What’s all this information on my LCD?” when you view your image – well here are some answers. When I’m looking at the image I just shot on the back of my camera – I can scroll in the opposite direction of the way I go to move from picture to picture. This brings up things like what shutter speed, aperture, ISO, etc. – it tells you the story about the image you just took from a “state of the image” perspective. But it also has two other things that will save you from over-exposing your shot: histograms and flashing highlights. The histogram. A histogram is just a graphic of a series of counts. Each pixel in your image has a value – let’s say from 0 (completely black) to 255 (completely white). Let’s not focus on why 255 right now; that’s another post. But each pixel has a value. A histogram is just a visual representation of how many pixels are at 0, 1, 2, …, 254, 255. It looks like this for an earlier rendition of the image above:  

An overexposed histogram

A histogram with a spiked highlight

Okay – what are we seeing here? The three vertical lines that separate the histogram into four columns are very important. The one in the middle represents the pixels that are exposed right at middle grey. That is they render a value of 128 – right in the middle – for this exposure. The line to the right represents +1 stop overexposed. The line to the left represents -1 stop underexposed. So a histogram covers four stops – two over middle and two under middle. Remember – a histogram is a series of counts. It reads from 0 to 255 on the X (horizontal) axis. There is a line going straight up at each value representing the number of pixels in the image with that value. For example, there are about twice as many pixels at -1 stop as there are at 0. There are more pixels at 0 then at +1. The actual numbers aren’t super important, but we can see toward the left side of the histogram that a few values go all the way to the top. Those values are about -1.5 stops underexposed. This just means there are a lot of darker pixels in this image. Again – nothing wrong with that – there’s a lot of water and shadows in the image. In reality, I’m looking only at one vertical line – the one that goes up about 3/4 of the way on the far right side. That’s the danger zone and that’s where your histogram can really help. That line is a spiked highlight – all the values in the image that exceed 255 are represented by it. Those are all the pixels that are more then +2 stops overexposed. We don’t know if they’re +2 stops or +10 stops but we do know they’ll be rendered in the image as pure white. Pure white with NO detail. I can look at that and know I’ve got a lot of blown-out highlights. I’m not sure where they are – but I know they’re there. I can choose to take another image – underexposing a little – or I can live with it. It would be helpful however to know what I’m living with. Well – actually there’s a way to know. It’s called the highlights screen. The highlights. You have one other screen on the back of your camera (in some cameras it may be the same screen as the histogram). It shows the image and it may show a number of areas that are flashing. Here’s an example from an image I took a few minutes earlier – that showed a piece of the sky:  

The Blinkies - detail lost in the over-exposed highlights

The Flashing Highlights - More then a good band name

The black areas here (they’ll flash on your LCD) show you what the histogram can only hint at. These are the parts of the image that are completely blown out. Ok cool. Now at least I know. And now I can ask the one question that matters. DO I CARE? That’s right – do I care that these areas are blown out? There was no detail in the sky in this image – while blown out – it is still essentially going to look the same whether I pull it back or not. The water highlights – perhaps a bit distracting but it is what it is – a spectral highlight. A spectral highlight is a pure white spot from a shiny or reflective surface.   So I asked myself the same question for our original image and I decided to pull it back just a little. I felt the water highlights may be too bright for comfort. So I took a few more shots, with faster shutter speeds so I didn’t compromise my depth of field. Here’s how the one I used looked:  

A histogram with NO spiked highlights

A histogram with NO spiked highlights

There’s now NO vertical line at the +2 overexposed mark. I know now that I’ve lost no detail and my highlights screen will not blink anywhere. It can be a subtle difference. The first shot in this example was +0.7 stops over the second one. I actually shot a few different shots – in -0.3 stop increments – until I got the spike out of the histogram. So expose for the highlights – and consider whether or not to let them hang out in +2 land. There are some instances where it just doesn’t matter. There are some instances where you want a lot of pixels overexposed (high key shots and genunine spectral hightlights e.g.). But if it matters, use your histogram and your highlights screens to reign it in. It’ll save your picture.

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Responses

  1. Great article, David!
    I will say that I use the flashing highlights as a quick frame of reference much more than I use the histogram. It isn’t that I don’t care about the histogram, just that if I am shooting quickly – especially with people – the flashing areas tell me much more information and faster than the histogram would. It tells me if anything important is overexposed (like skin tones, wedding dresses, etc) and that is all I need to know. I can adjust quickly and move on.

    In fact, I beleive my camera settings are set up so show me the flashing highlights immediately on playback without having to change the screen. (I need to double check this statement though when I have the camera in front of me).


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