Canon XSi Exposures & Metering
Obtaining usable exposures is perhaps one of the hardest fundamentals of photography. There are many books on the subject and some provide photos and an explanation of how the photo was taken.
I will focus on explaining how to understand the exposures on your XSi, how to use the different metering modes, and a few examples of when a specific metering mode may be appropriate. Remember your camera will save the metering mode you choose even when turned off, so always check what metering mode you are in to prevent lost moments.
Exposure in general: Exposures are hard to master, when you switch to a fully manual mode, it becomes as important as focus in terms of acquiring a usable image. When shooting in aperture or shutter priority, exposure remains extremely important in the outcome of the photo.
Exposure in Av or Tv modes: When shooting in either of these modes the camera will use the type of metering mode selected by the end user. The XSi has spot metering, evaluative metering, center weighted average, and partial metering. The camera will use the metering mode selected, review the metering data and calculate a suggested exposure setting to match either the aperture or shutter speed the user has manually entered. The cameras metering system, while convenient is far from perfect. The main benefits of digital slr’s come into play here, the instant feedback and ability to shoot many photos. Using the evaluative metering mode, and an exposure compensation of +- 0, meter a scene and take a photo. Review the photo on your XSI lcd. What do you see? do you see a perfect photo? perhaps, but more then likely you see the bright sunlight outside a window “blown out”; that area appears totally white, or some part of your image is too dark and you can’t see any details. Either way the exposure was not usable, and the user will have to make some adjustments to correct the camera’s best guess.
There are a variety of ways to do this, but we will begin with the most simple of forms. Look at your image, figure out whats wrong – is there a blown out highlight? is the sky too bright and the ground perfect? is the ground too dark and the sky perfect? If the highlight is too bright, you will need to lower the exposure compensation. Hold the AV button on the back of your XSi and using the same dial you control aperture or shutter speed with, dial to the left the exposure. You will see a -2 -1 0 1 2 bar with an arrow begin to slide towards the -1. Bring the arrow to -1, meter the scene again exactly as before and take the shot. The shot should now have the blown out area under control, if not reduce down to -2. Shoot again, how are the results? Do the opposite if your intended image is too dark, bring the exposure compensation to 1 or 2.
These are the basic exposure compensation adjustments that can be made to help achieve better results with your camera. You most likely already knew this, but I had to touch on it just in case.
Take your exposure to the next level: You have exposure compensation adjustment mastered, but your results still arent’ quite what you expect. Why? For starters evaluative metering is the cameras best guess, in the XSi the scene is divided into 35 squares which are then reviewed and averaged to find approximately “18%” grey. Yep, 18% grey, the camera is attempting to find the middle point between Black and White. Based on what it determines is the middle point, it then calculates the suggested exposure based on that 18% grey metering. Sounds great, so whats the problem? Not every scene contains 18% grey, and not every scene can be averaged to find 18% grey, so the camera is “fooled” into believing it found 18% grey by taking whatever it can. This results in whites being too bright, or darks being too dark. Certain scenes such as a dark room with a big window letting sun in will cause the cameras metering system to be tricked into believing there’s too much light or not enough.
To start with more advanced exposures and metering, hit your up arrow and select “spot metering”. Now, when your camera takes a meter reading it will only use the area precisely in the center focus zone. This is the most precises metering mode your XSi offers. It instructs the camera to ignore the rest of the scene, and only meter for the item in the center focus area. If we go back to our dark room with a sun lit window, we can point at the window and our camera will meter the scene realize there is a very bright amount of white and reduce the exposure appropriately so that the view outside the window will be visible and no longer washed out. As a result, our dark room will remain dark, but the window will be visible. Exposure compensations that you previously mastered can then be applied to find a better balance between the window and the dark interior. Of course you can also use advanced post processing techniques to lighten up the interior, while keeping the window view as exposed. Remember its easier to lighten in post processing, then it is to darken. When you darken an object, more detail is lost then if you were to lighten a subject.
Try this with a variety of subject matter and environments, nothing will help you understand exposure and metering with your XSI other then really getting a hands on feel for the results. Using Av or Tv mode will allow you to build a foundation for your understanding of metering and its effect on exposures. Don’t forget to set your metering mode back to the one you are most comfortable with at the end of any practice exercises.
Manual mode: By now you’ve taken a lot of images, you learned to focus your camera sharply, how to control depth of field, and how to make minor adjustments to your exposures. Manual mode is the next step to mastering your XSi. Its the hardest mode by far, and will instill stress, failed shots, blown highlights, black exposures, all while you attempt to run the gauntlet of manual exposure mode. Rest assured, this is why you like dSLR’s over film, you can learn at no additional cost!
Manual mode is a different animal then aperture or shutter priority. The camera no longer selects the shutter speed to match your aperture and vice versa. Now you control both the shutter speed, the aperture size, and the ISO speed. You need to combine your knowledge and skills of these three fundamentals to create the artistic image you want to capture. In order to achieve the results you want, you will also need to factor in the exposure meter once again. As in the aperture or shutter priority modes you can select the type of metering from evaluative, center weighted, partial, and spot metering. When you press the shutter button half way, the camera will use the selected meter mode and meter the scene or whatever part of the screen you are allowing it to meter. It will then calculate a suggested exposure level, and by adjusting your aperture and shutter speeds you will be able to bring the arrow located under the
-2 -1 0 1 2 bar too the 0. Why would you want to do this over just using aperture or shutter priority? Well, what if you want to meter on one scene and then photograph another? What if you want complete control and consistency in your images, such that the shutter speed and aperture size do not change from shot to shot? When you control these functions, the camera can not adjust them on you, only make suggestions. You can now choose to over exposure or under expose at will, and leave the aperture and shutter speed in your preferred settings. You may want a dark sky to be the middle point of your photo, or you may want a bright sky to be the middle point of your photo. You will evaluate your preferred lighting scene, press the shutter half way and adjust the aperture and shutter speed until you are satisfied they are exposing correctly as suggested by the camera (or under/over depending on your preference). Then move the camera so you are now framing the scene you want to capture, focus and press the shutter. Inside the view finder you will see flashing warnings of over or under exposure, ignore these you know the camera is metering for a different scene then you based your settings on. Take the photo and review it, make adjustments for over and under exposure by changing the shutter speed or aperture size based on the effect and outcome you desire. For more DOF, decrease the aperture width but increase the shutter speed, for less DOF, increase the aperture width but decrease the shutter speed, for less light or more light without affecting DOF, increase or decrease the shutter speed.
Implement the metering mode choices we have previously discussed to further refine the exposure meterings you are carrying out.
Metering Mode: If you are shooting in Av, Tv, P, or M, you can control the type of metering the camera will be using. The XSi again offers:
Evaluative: scene divided into 35 squares, compared to find 18% grey
Partial Metering: the center focus point and a bit more of the area immediately around it.
Center Weighted Average: a feathered zone, smaller than the evaluative, but larger then partial.
Spot Metering: the smallest metering zone available, only the area within the center focus circle.
Which ever mode you use, you need to meter appropriately. These modes will help you achieve better exposures, when the evaluative just isn’t cutting it. Evaluative on the whole, is pretty good. But you would want to use more concise metering modes if your camera is being fooled by evaluating the entire scene. Based on what you want to be exposed properly, you will want to choose a metering mode which more appropriately covers the area you want properly exposed. I normally stick with evaluative or spot metering and rarely use partial. Sometimes I will use center weighted if the background is lit from the sun, and the subject is not.
Use a grey card: Your camera is trying to calculate grey, why not help it out by buying a grey metering card, usually an 8.5×11″ piece of cardboard which is 18% grey in color. By using the spot meter and metering on the grey card, you know you have achieved the perfect exposure for your environment. Of course you will adjust your exposure based on preferred outcome, but this ensures a higher level of precision in your metering.
and as always…. shoot shoot shoot, review and shoot some more.