Understanding Exposure Meters and Histograms

by David Magahy

 

Is the digital Histogram the Holy Grail of the Perfect Exposure. I would like to show you some examples where I think the histogram is misleading.

We've been told to Expose to the Right. (Luminous-landscape). On many occasions this will render a great image. But what if you take a pic like the one below and end up with a histogram as shown. What would you do? If I guess that you would increase the exposure compensation in your camera by maybe one or more stops, would I be getting close.

 

The exposure details below the histogram show the meter reading as indicated by my incident light meter. I was using a polarising filter so I used a second ISO reading of 64 (not recorded in the EXIF data — one and two thirds stops difference... a polariser makes about a 2 stop difference to the non-polarised reading) to arrive at the appropriate shutter speed for my f18 aperture. In hindsight I probably could have made it ISO 50 for a full two stop difference giving a shutter speed of one thirtieth and a slighly brighter image owing to this longer exposure.

As you can see the Lilly Pilly flower is in bright early morning sunshine, as are some of the nearby leaves, while the rest of the tree is in deep shade. before I progress any further, let's look at what Lightroom came up with when the Auto button is pressed in the Develop module.
 

On the day, I didn't take a automatic or semi-automatic meter reading from my camera, as I felt that the many different cameras owned by the members may see light slightly differently and perhaps more or less efficiently than others. So I am using the above AUTO selection to illustrate what your camera will probably produce (no compensation adjustments made). This must in fact be the case as I see many images in our club competitions which have this appearance.

So which is the correct exposure?

The answer is not a simple first or second. The photographer has some creative input to apply before the answer can be given.

Let's look at the photo making process.

Did you want to see as much detail in the whole tree as possible? If so, the second image is right for you.

Did you want to make the image as bright as possible without blowing out any highlights, either in the white flower or the surrounding leaves which exhibit typical bright sun specular highlights. If this is your choice, then the first pic is the one for you.

More likely, and probably what a judge will look for, is something in between the two images. Slightly brighter that the first but not as blown as the second. (I am ignoring the Photoshop possibilities of extending the dynamic range by combining the best features of two or more exposures or two or more virtual exposures from Lightroom)

So what it boils down to is this: You make the creative decision on where you want the White Point (the brightest highlight) to be in your image, and hence in your histogram.

If you agree with me so far, I hope you can see that P for professional isn't going to cut it. This article isn't about whether to shoot Av or Tv or Manual (Nikon and other users can apply their respective dial letters). Nor is it about how to apply exposure compensation on your camera. Please refer to your camera manual or google the topic for a myriad of online sources for that information.
 

Using Lightroom on this RAW image (you could apply similar settings in Adobe Camera Raw or whatever RAW processor you might choose). The default Black point is 5 and I have used the Blacks slider to reduce this setting to Zero. This has the effect of lightening up the background a little. The low-res web image above may not clearly show this change on your monitor.
 
In this image, I have adjusted only the Exposure slider by opening it up by 1.19. This is just over one stop in camera terms and brings the histogram over to the right. The shadow is still dark but the flower has brightened noticeably. We appear to be possibly losing some detail in the leaves at flower left as we look at them.
 
In this pic I have gone back to the original but this time have applied some adjustments to the Tone Curve as shown. This has helped to brighten the flower but still appears to have kept the detail in the leaves left.
 

This is what I did to create my optimum image (remember, it is where I wanted the highlights to be). On the right below is a further tweak in Photoshop where I made a slight Levels adjustment. Below left is a 100% view of the leaves to the left of the flower, as we look at them. Some have bright areas but they do appear to have retained sufficient detail to please the Judges (well some of them anyway!!)

For those of you not familiar with Lightroom or Adobe Camera Raw (ACR), the default setting downards from Exposure would read 0,0,0,5, then 50 and 25 and finally 0,0,0.

 
Author: David Magahy    
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