Metering Exposures for HDR

Metering Exposures

I talk a lot about metering my exposure for an HDR but I haven’t talked much about how I actually do that, so I thought I would give a quick run through. 

Now of course a lot of times I just do a 3 exposure auto bracket and in that case I only have to make sure that my middle exposure is correct (by Metering and locking exposure on a Mid Tone as I explained here) But what if I need more exposures to cover a larger range? Here’s how I do it. 

First thing I do is set my camera for spot metering, if you don’t have spot metering use center weighted. If you use Evaluative (Canon) or Matrix (Nikon) that samples the whole scene which is great in the case of a single exposure but not what we really want here since we want to only know a specific area. 

I place my center focus point over the area I want to sample and I really just want to know two areas; the brightest part of the scene, and the darkest. Sampling any more than that is a waste of time since we know we will be covering them anyway in our various exposures. We just need to get from one end to the other.

So here is my scene

With my camera in manual exposure mode, I first metered the brightest part of the sky and got a centered meter reading of 1/60 shutter speed (aperture and ISO were constant at f/16 and ISO 100). Just be aware of something when water is involved. In cases with water, the sky may not always be the brightest part of the scene. If you have Specular highlights – reflection of the sun in the water- even though the sun is not actually in your scene. THOSE may be your brightest area of your shot. 

Next I metered the darkest area, a hole in the rocks to my left and got a reading of 6 seconds. So I have a full 10 stops of range to cover to get this shot right. 

So now my next choice was how to I get from one end to the other, in other words how many stop intervals. In this case I chose 1 stop intervals because I didn’t want to shoot the scene twice. But I did have another thing in mind because I knew I was shooting for this article. To answer the question: Are 1 stop or 2 stop intervals best? 

So I started shooting my sequence and I started at 6 Seconds. Now I could have done the math and  to get the next exposure just divide the time in half to get 1 whole stop, or in half and then in half again for two full stops. But I hate doing even simple math, so it much easier to do a simple counting the clicks. 

 My camera and most camera are set up from the factory to change exposure in 1/3 stop increments, you can change that to ½ stop increments in your camera’s menu. Mine is set for 1/3 stops. So if I want to change my exposure 1 full stop, I simply count 3 clicks of the dial, shoot, count 3 clicks and so on (If I am doing 2 full stops I count 6 clicks) and then I simply watch my shutter speed until I get to my end reading that I wanted of 1/60. 

But you do it the way that suits you best. 

So here is my 10 exposure shot processed in Photomatix Pro 4.1(No other processing was done)

 

 BUT, here is my 5 Exposure shot of the same scene

 

 I really don’t see much difference; in fact there was a little more confusion in the water area of the 10 exposure shot because of the moving water. 

I’m still not convinced that 1 stop increments are at all necessary (although I did use it for my Shooting Interiors post) because of course, each single image covers a range and in the end may make some other things worse (alignment etc.) But I of course leave that decision to you. 

If you would like to see what the 5 exposures look like along with each images Histogram, here they are.

 

This shot and histogram really shows you how wide the dynamics were for this scene and how they were really biased at each end

Hope that helps,

 

PT

3 Comments

  1. Miguel Palaviccini November 26, 2011 at 12:11 pm #

    Really informative post! Thanks.

  2. JBowman November 28, 2011 at 12:29 pm #

    Informative article, Peter. Thanks!

    When you were processing the 10 stop and 5 stop files, did you notice any difference in the amount of noise between each image?

    I was just thinking that with the full 2 stop range, there might be more noise if Photomatix was trying to extract the info from the image.

    Of course, I’m generally wrong when I start thinking, as my wife will gladly attest to!

    • Peter November 28, 2011 at 12:33 pm #

      Thanks !
      Not at first glance but let me go back and look. I don’t think so because even with 5 images no one image needed to be boosted that much but let me look close and maybe post some 100% crops. Great question!!!

2 Trackbacks

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    […] said this before when we talk about measuring the dynamic range of our scene. ‘If you don’t need HDR don’t use […]

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