Metering – Lock that Exposure!

If you have read my tutorials you will have seen that I do recommend using Aperture priority mode and Exposure bracketing to get a 3 exposure HDR image quickly and simply.

But how you meter for this or even in manual mode doing your own bracketing can make a big difference in the quality of your final HDR image especially when it comes to noise.

Now first off, we should understand how the metering system works on our cameras and also how to lock the exposure.

Canon and Nikon cameras work similarly although they may use different terminology. ( The following assumes the factory Custom Functions setting, varying custom functions can change your camera’s Auto Focus  and Exposure  functions, please consult your camera manual for more information)

For metering modes, Canon’s Evaluative metering and Nikon’s Matrix metering work very similarly: They use the selected  focus point as the main source of their readings and then take in the surrounding data to arrive at a correct exposure. a half press of the shutter will lock both the focus AND the exposure.

In other metering modes; Point, Center weighted, Partial, Use the center of the viewfinder for exposure ( in varying sizes) and  a half push of the shutter button (or the back focus button) will lock ONLY focus. The exposure will vary with re-composition unless you press the Exposure Lock Button on the back of your camera (consult your manual)

So knowing the mode you are in and how it locks exposure is critical

So let’s see how in practice this can affect our HDR images

In the examples I am showing it is an extreme example but one that really shows how important proper metering and exposure lock is. These images contain the sun in the frame and as you may have seen in my tutorials I most recommend more than 3 exposures if the sun is in the frame because of the huge amount of dynamic range that needs to be covered. But we don’t always have time or a tripod to do the extra exposures needed. So let’s look at this extreme but pongient example of something that you may run across.

In this first set of images, the exposure was metered and locked on the sun itself. So the camera chose that as the first median exposure for the three exposures. (click on images to enlarge) (Excuse the Vine wires in the image, I wanted them there for a quick tip in my next post)

 

 

 

 

 

 

 

 

 

 

We can clearly see that while our exposure did a good job exposing the ball of the sun, our +2EV image that we would hope to have a good exposure for the shadow area, is extremely under-exposed. So what will that mean to our final tone mapped HDR? It will mean that our HDR program will try to make that a midtone area and will bring upo the exposure at least 2 stops. Bringing that area up two stops in luminance results in a huge amount of noise as it tries to bring out information that just isn’t there in our underexposed image.

Here is the tone mapped result for those images, you can clearly see the color noise in the flower area

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In the next set of images, I meter and locked the exposure on the sky to the right of the ball of the sun, This resulted in our 0EV image being exposed 3 stops lighter, than in the first set ( as a rule of thumb, the sun itself is 3 – 4 stops brighter than the sky,depending on time of day)

 

We can see that the -2EV does a pretty good job of exposing the ball of the sun and our +2EV shot does  a much better job of exposing the shadow area in the forground

 

 

 

 

 

 

 

 

 

 

 

 

The resulting tone mapped image shows a similar total exposure to the first image but with MUCH less noise in the shadow area of the flowers since the program did not have to raise the exposure of that area and bring up the noise floor of the color area

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Now in a perfect scenario, I actually would have probably shot 5 frames to get the ball of the sun perfect and the shadow area even better exposed. But I think this shows you how much of a difference where you meter and how you lock it can make in your final image. As a general rule, we want to meter on a good mid-Toned area for our 0EV exposure, this can be blue sky, green grass or a gray cloud. Generally we do not want our 0EV meter to be a shadow area or a highlight area. Keep this and how your exposure locks in mind, especially if you focus and recompose your image,because what you focus on may NOT be what you want to expose for. 

Hope that helps! 

PT

6 Comments

  1. Duane July 12, 2011 at 1:29 am #

    This is a great explanation and tip. I have seen this in some of my recent shots and was wondering what the cause was.

  2. Duane July 13, 2011 at 2:23 am #

    Hey Moose!

    With Low sitting sun and clouds. I tried this tonight and it worked fantastic.
    I first did a 3 shot bracket metering off the blue sky and my (0) bracket was much darker then it should have been. So i ended up with 2 dark shots and 1 light. So then I metered off some stone on a building and the (0) bracket was perfect, so I had three good exposed shots then to work with.

    Thanks again for this tip!!!!!!!

    • Peter July 13, 2011 at 2:46 am #

      Thanks so much Duane.

      Yeah, it really came to light ( pun instended) when I was working with some images and they just didn’t work out at all like I thought. Then I looked at what happened and could see it just in the RAW shots. Glad it helped.

      Keep shootin, You are doing some great stuff!

  3. Alex October 3, 2012 at 7:34 am #

    Anything that reduces noise and encourages proper exposures is great in my books. Thanks for this article.

  4. ducsu July 26, 2013 at 7:42 am #

    I am trying to learn metering photos for hdr. If we meter a scene to get the correct exposure for the median (0EV), the histogram for that captured image is not truly correct. Am I correct on this? We need to look at the image itself to see if the image is properly or closely expose to where we want it.

    • Peter July 26, 2013 at 8:32 am #

      Well, it depends on how you are metering and what you mean by “Correct”

      If we metered the entire scene for our 0 EV point and then took an exposure of that, AND our scene was TRULY A HIGH DYNAMIC RANGE SCENE,we would have a “Correct” histogram but also a very odd looking one. We would not see the normal bell curve histogram but one with very large peaks at each end of the histogram and a big valley in between. This is indicative of a scene that is beyond what our camera can capture in oneimage (Hence the need for HDR)

      Also as the article shows, it depends on what you decide you WANT to be the center exposure and if you are using in camera bracketing. In that case as you state, if you want an area just to the right of the sun to be your 0 EV, you may not get a normal looking histogram for that exposure, it most likely would be pushed down to the lower end of the scale in say a 3 Exposure Bracket.

      Otherwise, in an HDR scenario, it’s not always important even that we have normal histogram on any of the exposures. What is more important is that we capture the entire range of Luminance. So if you did say 7 exposures over the entire range, You would see a tight band near the bottom for your first exposure and then that band move twoards center and broaden as you move towards the 0 point and then bunch up again as you get to the other end of your exposures and histogram.

      In the end, where you do want to see the most correct histogram, is when you are done processing the image to assure you have not under or OVER PROCESSED the image into less than the dynamic range of the medium, Screen Print etc.

      Good Question!!!

12 Trackbacks

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    […] the right number of exposures so in the HDR processing, the noise is not exacerbated as I showed in this blog post This entry was posted in HDR Lesson, HDR Quick Tip, Uncategorized and tagged When to boost your […]

  2. […] For more information on metering for low noise and better exposures, see the article here.  […]

  3. By Follow up on ” What to Focus on” on October 6, 2011 at 11:55 am

    […] ensure that those exposures that are bracketed around that o point are correct as I talked about in this post. But the truth is these are two separate steps because when we use Hyperfocal Distance. It is best […]

  4. By Metering Exposures on November 26, 2011 at 10:54 am

    […] 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 […]

  5. […] The other thing you need to watch for when the sun is in the frame. Is how you meter. The sun being bright can cause all three of your exposures to be underexposed which could have added more problems too. Not being able to see your three exposures I’m not sure if that happened but it is something to look for as I explained in this  article […]

  6. By I should have known this…but I didn’t on January 11, 2012 at 3:16 pm

    […] any other purpose, you are fully in control and  your exposure is consistent. As I discussed in this article. The problem with using Aperture priority is that you have to lock the exposure using the exposure […]

  7. […] 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 […]

  8. […] any other purpose, you are fully in control and  your exposure is consistent. As I discussed in this article. The problem with using Aperture priority is that you have to lock the exposure using the exposure […]

  9. […] ensure that those exposures that are bracketed around that o point are correct as I talked about in this post. But the truth is these are two separate steps because when we use Hyperfocal Distance. It is best […]

  10. […] the right number of exposures so in the HDR processing, the noise is not exacerbated as I showed in this blog post Posted in: HDR Shooting, High Dynamic Range (HDR), Landscapes   Tags: ISO, When to boost […]

  11. By Hand Held Meters and HDR on August 5, 2012 at 1:38 pm

    […]  I will say taking an incident reading may be a great way to determine your center exposure in an automated 3 Exp +-2EV series as it may not be affected by mis-metering our subject as I talked about in this article.  […]

  12. […]  I will say taking an incident reading may be a great way to determine your center exposure in an automated 3 Exp +-2EV series as it may not be affected by mis-metering our subject as I talked about in this article.  […]

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