Using the Histogram to Ensure you have covered the Dynamic Range of a Scene

As I discussed in this article: How many Exposures are enough the most important part of the -How many exposures do I shoot – is the fact that you need to cover the entire dynamic range of the scene. As the article pointed out the spacing between exposures was not AS important as covering the entire range.

In this article  Measure & Exposing for HDR I told you how to meter different areas of the scene to know the range of shutter speeds you would need to shoot to cover the dyanmic range. But even though it’s a good way to get you close, there still can be some margin of error because of course we know…sometimes the meter gets fooled.

So what is a better way even once we have a basic idea of what we will need to shoot by metering of ensuring that we actually have covered the full range we need to. And why do we really need to cover the full range.

We’ll answer that question first, and mostly is has to do with Noise. If you don’t shoot enough exposures the HDR software will compensate for that or WE will push the software to compensate for that by using too much strength or compression and bringing up the exposure of shadow and mid-tone information too much. When we do that we increase noise by a good amount. So if we have the right amount of exposures we can mostly eliminate that.

So what the best way to check? With our usual best friend – The Histogram.

Here is a scene that I shot of a Setting sun over a Grassy Field. The sun was just above the horizon and I was shooting with a very tight aperture -f/22- in order to get a good star-burst effect on the sun. Because I knew I was doing that I also knew I wanted to get a very tight ball on the sun. Because the sun was just above the horizon  I would be able to accomplish that and also have it look natural , where as that may not look natural earlier in the day because even with our eyes we can not see a clear ball of the sun because it is simply too bright.

So using the spot metering on my camera I measured my beginning shutter speed exposure on the sun  itself . NOTE NEVER do this any other time of the day since you can permanently damage your eyes and your camera. Instead measure off to the side of the sun and add a step. Or use this as your settings and you will be close ISO 100 f/22 1/1000..you can interpolate from there to if you desire other Apertures or ISOs

In this instance I measured ISO100 f/22 1/640 for the sun and for my shadowed foreground area I measure 1/4th of a second so I knew I had to go from 1/640 to 1/4 in 1 stop intervals. But would that be enough? I would soon find out.

Bear in mind this scene has a very high dynamic range; 18,000:1 contrast ratio so more exposures will be needed for this then most HDR scenes people shoot

We’ll run through all the images shot and comment on what they mean and where we would and should start and stop withnour range of exposures

 Exposures and Histograms

For most HDRs I would not start this dark of an exposure but as I said I was looking for a Sun Ball. If this image did not contain the sun ball this is an image you would NOT want in your series because it really contains no information and will only cause problems with noise and more importantly alignment

ISO 100 f/22 1/640

Probably not here either but I am going to shoot it, At this point I’m not really concerned with the histogram, I’m shooting from what I metered. I’m more worried about where to end my shooting

ISO 100 f/22 1/320

This is where I probably would start most HDRs, ones without a sun ball. It has  most of the information bunched to the left and a Pure black . Note that big spike you see is not shadow information, that’s not even on the screen, It’s the Highlights from the sun

ISO 100 f/22 1/160

Now we are just starting to see some of the Shadow information coming into the histogram from the left

ISO 100 f/22 1/80

We’re starting to see the Highlight tones move into mid-tone territory

ISO 100 f/22 1/40

This is probably what you would see if you just metered and shot the whole scene without HDR or this would be a middle image in a 3 exp bracket. The bright sun has caused the meter to feel the sun is a mid-tone but as you can see the shadows are all blocked up

ISO 100 f/22 1/20

This is what you would get if you biased you meter to the ground for a single shot a full histogram but as you can see the sun is blowing out. Look at this histogram closely because it contains a tell tale sign that the scene is beyond the dynamic range of the camera. large peaks at both ends and a huge valley in the center

ISO 100 f/22 1/8

Now this is where you may stop shooting, in fact metering the scene, this was where I was going to stop shooting. Look at the histogram the Highlights are clipping, we should stop… Actually we shouldn’t. The reason why is; We want the highlights to clip. At this point in our exposures we are not concerned with the highligh, now we are actually getting images for our shadows and mid-tones and we want our shadows to be close to a midtone value. So keep shooting

ISO 100 f/22 1/4

Our shadows are starting to move, that’s the big hump on the left but we might want a little more

ISO 100 f/22 1/2

Almost there, I think one more shot

SO 100 f/22 1 Sec

And that’s all we need we’ve moved the shadows off the shadow end into the mid-tones and don’t worry we still have shadows from other exposures

ISO 100 f/22 2 Sec

Now I wouldn’t shoot any more exposures because they will serve no purpose, we don’t need to make shadows highlights and if we go longer we will just get an image with very little detail to it and that will hurt alignment AND actually sharpness

So this was a total of 11 Images, 1 EV apart, Pretty extreme and not very ordinary in most HDRs but having the sun in the image adds to the range which like I said is 18,000:1. But the important thing is we covered the entire dynamic range of the scene. Now what we could have done is shot a 2 EV spread between exposures and got an image very similar. That really doesn’t hurt an image. It would be most apparent in an image with a clear blue sky where you may see slight banding but usually not. The important thing is that we cover the entire dynamic range.

 Final Image

Here is the final image, I merged the image  in Photomatix Pro because it does a great job of Alignment and de-ghosting, I saved the Image as a 32 floating bit Tiff but did  not Tone-map in Photomatix  instead choosing to go a different route and used Mini Bridge in Photoshop  to open the 32 Floating bit Tiff into Adobe Camera Raw where it was tone mapped  using ACR (you could use Lightroom 4.1 or later) and then I opened the image and down-converted to 16 bit and just added a bit of finishing work in Photoshop

And with the Histogram for good measure

Don’t forget you can get 15% off Photomatix Pro 4.2 with discount Code THEHDRIMAGE

Hope that helps…

PT