Your Friend the Histogram

For a guy that likes to photograph flowing water, this is the time of the year to be out in the woods. The melting snow and the spring rains send water blasting down ravines and fills gorges from wall to wall. The speed, the deafening roar, the gusts of spray pelting your face; it engulfs and invigorates.

I wish I could be out everyday. Doomed by the speed that creates it, all too soon it will wash itself away and the slower quieter rhythm of summer will ease in behind it. I’ll be left looking forward to next year. Until then I shoot whenever I can.

Oddly enough this brings me around to the subject of histograms. For water shots the histogram is your best friend. I love this feature of digital photography. See the problem is that the water is bright white and shiny (especially when the sun is out). On the other hand the stones, the plants, the trees, all the background stuff, is dark sometimes even black.

The difference between the dark and the light elements is too great for the average digital sensor or roll of film to handle. Most often either the white water gets “blown out” to a blank white hole and loses all detail or the dark background becomes just a blank black hole. The trick is to play the light – shoot at the right moment, from the right angle, and especially at the right exposure to create a balance between the opposing forces.

There are a couple of technical ways to fix this problem by avoiding it all together. There are digital cameras with much wider exposure latitude than the Nikon D200 that I use. They can handle a broader range of dark to light without losing detail at either end. Of course you have to pay for this kind of thing. I really can’t afford anything like that.

A cheaper solution is to use high dynamic range (HDR) software. The idea here is to shoot a scene several times at different exposures from really dark to really light. The software takes all the images, extracts the correctly exposed details from each one, and melds them into a single evenly balanced image. I haven’t played with HDR much yet – I’d like to, but I just haven’t had the time. One obvious drawback is that you need to be shooting things that don’t move between shots.

So for now let’s say that we are stuck with x number of stops of exposure latitude in our camera and our scene has x plus stops of exposure.

With a film camera the traditional solution is to bracket. Take multiple shots at various exposures and then pick whatever one looks the best after development. Burning costly film and development services when you know you will be tossing most of the images is a bit of a wrench. You are working blind here too. What if you take three shots and find out later that none of them are the right exposure? Experience and shooting more shots for a wider range can help ensure that one of them is right, but it isn’t always foolproof. What about when you are shooting on a partly cloudy day and the sun is in and out of the clouds between shots or maybe even during a shot if the exposure time is a long one? Hard to meter that one.

This is where the histogram comes to the rescue. My typical M.O. is to meter up each scene (I just use the in camera meter) and take an educated guess at the exposure. I take a pic and then immediately view the histogram on the camera’s preview screen. If the graph goes off the right end of the scale, I know that I have blown out the highlight detail. If it goes off the left end, I know that I have lost shadow detail. Next I take a series of shots (I never use any of the camera’s automatic features) adjusting the aperture, ISO, and shutter speed to balance the histogram as best I can.

I don’t rely on how the image “looks” on the preview screen. The preview screen is nice, but the ambient lighting is too variable, the screen is too small, and the in camera conversion from raw sensor data to the display is too crude to make serious exposure judgements.

Typically the goal is to push the graph as far to the right without going off the end. This makes the bright areas as bright as possible without losing detail. This also pulls the dark left end along as far as possible so that as little as possible is lost in the shadows.

This is the typical way to go and there is a school of thought that says you should in fact shoot every digital image as far to the right of the histogram as you can even if the image seems too bright overall. The idea being that this way you record as much digital info as possible and you can turn the final exposure down later in software during processing.

Having a science type educational background I can see the logic in this. It probably is the way to go. Unfortunately when I’m shooting photos I’m using the other side of my brain. What the image looks like in the immediate now while the scene is all around me is when I want to do my “work”. Hours or days later in my office far removed from the action is not the best time for me personally to try and pull something inspired out of cold logically produced sensor data.

I don’t push every image to the right edge. I tend to like more black in my images that most people so sometimes I purposely let things drop off the left edge.  I also have found that even when pushing the highlights I like to leave just a bit of room on the right side. I do this so that I can tweak things like brightness and contrast. If the graph is hard up against the edge, sometimes adding contrast and always adding brightness will push it over. On rare occasions I will even take an image over the right edge on purpose to flare some areas (a la the stick in the flowing water image shown here).

So my summary would be don’t rely on the preview screen to eyeball your exposures. Use the histogram all the time. Use it to set boundaries within which you can play with the light. On the other hand, once you get the hang of it, feel free to break the boundaries once in a while.

MDW

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7 Responses to “Your Friend the Histogram”

  1. Bernie Kasper Says:

    Great post Mark, my problem is I never believe my historgram and under expose everything !!

  2. forestrat Says:

    I actually checked my camera histograms against the PC I use for processing. I downloaded a bunch of images, but left them on the camera. Then I went through a bunch of the images comparing the histograms. It turns out that the two are close enough that I couldn’t tell any difference.

    So sometimes it is hard to disbelive your eyes when viewing the little screen on the camera, but the histogram really is a better indicator of exposure.

    MDW

  3. lookingforbeauty Says:

    I read this through a few times. I really will have to get out the manual for my camera one of these days and find some of the features that will help improve my photos. I hadn’t realized what that histogram was for. There are always new things to learn!
    That’s a nice bit of sunshine in your first photo.
    K

  4. forestrat Says:

    K,

    I’m not sure if all digital cameras will give you a histogram, but it seems like most would have it tucked away somewhere. My DSLR of course has it just one button away. The family point and shoot Canon will show it if you know which button combination to push. The display is pretty small though.

    MDW

  5. fencer Says:

    Thanks for the good histogram advice… I will try that when I next go out shooting.

    Regards

  6. Mark Says:

    I might add that you can’t always trust the in-camera histogram for overexposure blinkies either. I usually find myself pushing the image to the point of slight blinkies because it is amazing how much highlight detail you can pull back in a raw converter.

  7. ~donna~ Says:

    great post. very education for someone like me who has yet to figure out this aspect of digital photography. On my next photo outing, I am going to give it a try. Thanks!

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