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Tuesday, November 30, 2004

Understanding Metering

Well, I'm back after a full 3 months. No I had not given up on Photography or G5! Let's just say moving and sattling into a new house and getting broadband to start working again can be very hectic. Anyways.

Here is some explation on metering and why you need to understand it.

What difference does knowing about metering make anyways ?
You mighthave noticed symbols like below on your camera's buttons or dial.

[ o ]
[ ]

They indicate various metering modes supported by your camera. Why is metering important? Well take a look at the two pictures below.

Evaluative Metering Mode

Spot Metering Mode

As you can see metering makes a whole lot of difference. If you want to stop doing Point and Shoot and stop taking pictures with built-in flash, you must understand metering modes to make sure that you don't end up with under exposed or over exposed pictures.

The Problem
Let's say you are doing Portrait Photography, the subject is still, you want to increase the Depth of Field and you don't really care for the shutter speed. So you will put your Camera in Aperture priority mode, so that you can playaround with DOF while Camera keeps sdjusting the shutter speed adjusting such that your subject looks well lit. Now, how does the camera know what is "well lit" or "well exposed" ?

The Solution
The simple thing that your camera trys to do is to achieve exposure level that is referred to as 18% Grey. What this means is, if the amount of light coming in through each spot in the frame was same (i.e. a simple plain frame) then the resulting picture should be a certain shade of grey marked as "18% grey" (let's not go into "why 18% ?"). Such an exposure would look like the pic below.

18% Grey

Fortunately, none of the scenes are boring like the grey screen above :-). Usually you get a mix of dark and light areas in any scene. The camera "mixes-up" all the brightness and comes up with a calculation for a shutter speed or aperture opening such that total exposure is equivalent to the 18% grey pic above, no matter what the scene is. This calculation is called the Metering. In aperture priority mode, it is used to select the shutter speed; and in Shutter priority mode, it is used to select the aperture.

Going back to smart stove analogy (see "Cooking and Photography"), metering is equivalent to deciding how much heat is required to cook any particular food. Different ways of doing this are referred to as different metering modes.

In metering process, Your camera "looks" at the scene, checks how much various areas of the frame are lit and decides how much total exposure (total light) is required to create the picture such that it is equivalent to 18% grey shade.

For example, G5 supports following three Meterting Modes

Evaluative Metering [(0)]
Camera divides the scene into several zones and does calculations to come up with the right exposure. In Other words, Camera gives equal importance to the entire scene to make sure that over all best exposure is achived in the entire scene.

Evaluative Metering Mode

Take a second look at the elephant image with Evaluative Metering above. You can see that as most of the background is bright, to achive 18% grey level, the camera reduces the exposure to such an extent that we lose the fine artwork details in the elephant body as they are underexposed.

Center Weighted Metering [ ]
This is similar to the Evaluative Metering, except for the fact that instead of giving equal impiortance to all areas, Camera gives higher importance to center of the image. In other words, Camera trys to make sure that the center is properly exposed and may make compromise (underexpose or overexpose) the rest of the image.

Below is the same scene with center weighted Metering.

Center Weighted Metering Mode

This time, the camera is giving higher importance to the middle of the scene i.e. makes sure they dont get underexposed. Even after giving less importance to surrounding areas (i.e. risk over exposure as they are brighter) We are getting the artwork details on the elephant a little bit underexposed.

Spot Metering [o]
In this mode, the camera only considers a very small area of the screen to decide exposure level. In G5 this area can be selected as a small part in the Center or a small area around the adjustable Auto Focus Point. The Camera makes sure that 18% grey exposure is achived in this spot area, and doesn't care for over/under exposure in rest of the frame.

Spot Metering Mode

Take a second look at the Elephant image with Spot Metering above. As the center spot was selected, which happens to be covering mainly the back and side of the elephant body, the camera tried to achive 18% grey exposure in this area. So all the artwork details have come out very well. But the camera completely disregarded the exposure level in rest of the frame. To achive the required exposure in spot area, which has a lot of black, Camera reduced the shutter speed so much that the rest of the frame is completely overexposed ! But hey, we did get our artwork details !
Important things to remember about Metering Modes
  • Metering is camera's way of deciding how much exposure is required.
  • Evaluative metering mode trys to make the whole picture properly exposed.
  • Center Weighted metering mode trys to make sure that objects towards the center are properly exposed, objects on the edges may be under or over exposed.
  • Spot metering mode trys to calculates exposure based only on what is in the selected spot area. Rest of the frame can be over or under exposed.
  • The only time you are NOT using Camera's metering is when you go in full "Manual" mode, when you have to select both aperture and shutter speed.This makes it very important that you understand metering modes properly.

    [UPDATE : In all the other exposure modes like full auto, other programmed modes, or priority modes like aperture priority or shutter priority, the camera is selecting at least one parameter for you. So it has to use its metering algorithms based on the metering mode you have selected.

    In manual mode, you are selecting everything, so you are not really using camera's metering capabilities inherently. However that does not mean it becomes unavailable. Modern digital cameras show some form of indicator regarding how you are doing based on your selected parameters, compared to cameras metering. i.e. It may show you, how many stops above or below you are with respect to cameras metering. You don't have to use it, but it's there for reference. ]
  • Pay attention to subjects in your scene and make sure that you are not losing details in the shadows. Play around with various metering modes and try to get the details right.

The Backlit Subject
Above the elephant is heavily backlit subject. And we played around with spot metering to get the details out correctly, but at the cost of washing out rest of the frame in white over exposure! Well, there is a better way to take this picture and get oth the background scene as well as the main subject (a mainly black body) without losing the fine details. We can achive this by illuminating the elephant... I am talking about firing the flash!

Evaluative metering with Fill in Flash
As we saw, we were either losing details in the elephant or in the background. But if we fire the flash, then due to the extra light, the artwork details come out and our background scene which is very bright also comes out nice! This technique is commonly referred to as "Fill in Flash" as the Flash is used to fill in the details. Compare this image with the Evaluative Metering image and you can see how the flash is filling inthe details.
This is a very simple but very effective techniques while taking pictures at Sunset or in other backlit conditions where you can't use spot metering.

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12/07/2004 03:53:00 PM  
Blogger DigitalCamera said...

Hey saanga, I was searching blogs, and came onto yours, and I like it. I kinda landed here by accident while searching for something else, but nice blog..

If you got time , come visit my site, http://www.cool-digital-cameras.com. It pretty much covers cool digital cameras and similar stuff.

10/26/2005 09:40:00 AM  
Anonymous Anonymous said...

Hi, your explanation of metering is quite easily understandable by a beginner to digital photography.

It's good that you shown what you have mentioned in terms of pictures. Some sites just explain the different metering terms in words only. Hell, I don't really understand what they mean until I see the pictures.


6/07/2006 08:15:00 PM  
Anonymous Anonymous said...

found your site linked thru wikipedia, had to say you give some excellent advice :o)

6/12/2006 12:34:00 PM  
Anonymous Anonymous said...

Excellent expanation of the metering concept..,
As a beginner i understood it much better than other websites.
The pics were definitely a good idea.

8/21/2006 05:35:00 AM  
Anonymous Anonymous said...


Thanks for this explanatiuon by example. I have battled with this metering mode and like previous people said, words just don't make sense until you see it. Great, great, great.


1/23/2007 10:07:00 PM  
Anonymous Anonymous said...

Thanks for valuable informations..Also you can Visit ==> www.mycameraworld.blogspot.com

1/27/2007 09:28:00 AM  
Anonymous Anonymous said...

Great explanation and as the others have mentioned, the photos show it all very well - something about a 1000 words??
Thanks - very helpful

1/28/2007 12:43:00 PM  
Blogger Shakib said...

This explained far better than the otherwise popular sites such as kenrockwell or anything else! Thanks to you and whoever added this in wiki!

4/24/2007 11:25:00 PM  
Anonymous Anonymous said...

Thanks for the info...I found your link through wiki after someone referenced me their at dpreview.

I totally could not get what metering was about, and with the pics...it makes sense.




5/10/2007 05:55:00 AM  
Anonymous Anonymous said...

Wow - thanks for the info! HELPED A LOT!!!

7/16/2007 10:58:00 PM  
Anonymous Anonymous said...

Hey it was a really useful post but some of the pictures are missing. Could you please add those & email me to let me know if possible?

You have an excellent blog and i have opened a directory in my comp to save ur articles for reference. Nice going.

asrambik at cisco dot com

10/23/2007 01:59:00 AM  
Blogger MJar said...

Very nicely stated. Your explinations were well written with complimenting pictures. I look forward to reading your other posts.

11/28/2007 04:30:00 PM  
Blogger José David said...


Excellent blog! I love it, it's clear and very usefull. One of the best article commenting metering I have ever read, congratulation!

12/24/2007 11:16:00 AM  
Anonymous Anonymous said...

The manual on my Canon 40D does a very poor job of outlining what you have said in your post. Thanks for clarifying this for me...hopefully now my pictures will turn out much better!

1/28/2008 08:44:00 AM  
Anonymous Anonymous said...

Just wondering; which metering setting was used in the Fill-in Flash photo? I'm guessing it was Matrix/Average

3/18/2008 06:06:00 PM  
Blogger Dan said...

Thank you,man!
An illustrated
GREAT explanation.

6/21/2008 04:52:00 PM  
Anonymous Anonymous said...

Great blog! Thanks for explaining Metering for newbies like myself.
I just bought a Canon XSI/450D (My first DSLR) and I have noticed that I have been getting some bad bright spots in my photos as in, bright light areas of the photo have been "washing out". I just thought that was one of the weaker qualities of my camera because I read a review that one of the faults of the Canon XSI/450D was Metering has tendency to overexpose in very bright, contrasty conditions.
I have been using "Spot metering" every since I started using the camera because I read somewhere that was a great feature to have on a camera. I mean, How am I suppose to know what all that stuff means! LOL!
And don't tell me to read a book. I just don't have the attention span to read an entire book just about photography. haha!
Anywho, I hope switching my metering mode to "evaluate metering" helps with the light contrasts in my photos.
Thanks again for explaining what metering is in simple terms.

7/23/2008 03:08:00 AM  
Anonymous Anonymous said...

Wonderful information...I would call it 'Metering for Dummies (like me)'


8/04/2008 05:15:00 AM  
Anonymous Anonymous said...

Very Good useful information presented with good examples

9/18/2008 08:53:00 AM  
Blogger Adam Parker said...

Great post!!! I loved the sample shots, and that last bit about fill flash was great. I think this is a great illustration.

Anyone who can control their metering mode should learn how if they are going to get the best shots. I posted some more metering mode examples and descriptions here that may further explain metering modes and Exposure Values.

Thanks for the great read.

12/17/2008 03:50:00 PM  
Blogger Ashwini Kumar said...

Simple and Very Effective explaation of the Metering modes.

12/22/2008 07:01:00 AM  
Anonymous Anonymous said...

Wow.. i should've seen your site before I bought a handful of books. Some of the books were good, but I found the same info in your blog too, in less technical explanation. I'll be returning books w/in 30 days of purchase for sure... i'm a beginner, probably aware of only 10% of my D90, but your info came across quite effectively. KUDOS :)

2/05/2009 10:31:00 PM  
Anonymous Anonymous said...

why are the comments pointing about gaming? anyways....

this is a very nice article about metering. Many thanks to the author for giving clarity!

4/25/2009 01:01:00 AM  
Blogger Sanu K Sivan said...


A simple and informative explanation on metering mode..Ever since I took digital photography as my hobby,I didn't care about the metering modes,just b'cause i was unaware of what it does to the pictures i take and the explanations given by other sources were complicated.The way u explained with images made the whole thing interesting as well as understandable.Thanks a lot.

5/20/2009 04:23:00 AM  

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