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August 18, 2012

Exposure

D7000 mode dial.jpg
I am pretty sure it's been mentioned before, but if we want to talk about exposure, we have to start from basics. So what is exposure and how does it works? Exposure is amount of light reaching camera's sensor, or if you like, reaching the film in your camera. There are three main elements which control the exposure - aperture, shutter speed and ISO. That means, in whatever light situation you find yourself, those three elements must be in right ratio to allow exact amount of light reach your sensor and make correctly exposed image.


If for some reason the ratio wasn't right, you'll end up either with overexposed, or underexposed image. This may be good use if that was what you were aiming for, but if you want to manipulate pictures in that way, first you have to understand how to get it right. We are lucky now days with digital cameras and big LCD screens, where you can check the result instantly and make necessary corrections, but it wasn't that easy in film era. But even LCD is not always good reference point, especially in bright sunny day, so if you're not sure, use histogram. It is that scary looking graph of pixels, at the left side you have black and dark tones, right side is assigned to the light tones and white. That means in the middle, there are mid tones of course. The higher the peak in particular area is, the more tones are in that area. Simple as that.

correst exposure.JPG
correct exposure - histogram with mostly mid-tones

overexposed.JPG
overexposed image - histogram with mostly light tones

underexposed.JPG
underexposed image - histogram with mostly dark tones


Don't be scared to look at the histogram, it is good little helper. From images above you should see that correctly exposed image has got histogram with most tones in the middle. Overexposed image is lacking shadows, or dark tones, and of course underexposed image is lacking highlights and white tones. You are aiming for more most tones in the middle, without clipping shadows or highlights.

Controls

On the mode dial of your camera, you have the wide choice of exposure modes. Some are automatic (AUTO, scene modes), some are semi-automatic (A - aperture priority, S - shutter priority, P - program)  and there are few witch allows you to take full control over your exposure (M - manual, U - user defined settings)

AUTO AND SCENE MODES
In AUTO and scene modes, camera does all light measuring by itself. How it works? Clever sensors in camera calculate light situation and decide what aperture, shutter speed and ISO would be the best of use. Basically, metering system assume that area being metered is plain 18% grey, so average of all tones in the picture will correspond to that. Even better explained, if you'll put all dark, light and mid-tones from your histogram into the shaker and shake well, what you'll pour into the glass will be 18% grey color. 

It is similar story with SCENE MODES, but camera tries to adjust settings more suitably to scene, or shall we say type of the scene, you trying to shoot. For example if you set the dial to "PORTRAIT", camera sets small aperture number to create shallow depth of field to blur background. In the other hand, in "LANDSCAPE" mode camera sets aperture to higher value to achieve biggest sharpness through the field. In "SPORT" and "NIGHT" mode, more attention is  given to shutter speed for following reasons. If shooting "SPORT", camera is guessing you don't want to end up with blurry images, so it sets shutter speed as fast as possible and adjust other values accordingly. Of course, with some sports there is panning technique you'd like to use, but for this effect you can't use automatic scene modes. You can find out how to do that in this article. For same reason to avoid blurred images caused by shaky hands, camera sets fast shutter if you tell it you want to do some "NIGHT" shooting, or if you like low light scene, increase the ISO value and sets aperture accordingly.

As you can see, you can't set anything by yourself, so you had better forget your camera has an automatic modes.


SEMI-AUTOMATIC MODES
Even if you are complete beginner and you've just purchase your first DSLR, you should start shooting on semi-automatic modes. This gives you more control over your work, but more importantly you'll understand faster what you doing.

A - Aperture priority mode
This mode is good starting point for everybody and even professionals shoot many pictures using aperture priority mode. Basically, in this mode, you set the aperture you think is most suitable for the job (portrait, nature, macro - low aperture number, landscape, architecture - high aperture number) and camera will do the rest. But don't worry, you have more control. You can also set your ISO and exposure override. That means you have full control how good or badly exposed image you'll end up with. You can choose it on scale mostly from -2 for underexposure going by 1/3 of the step up to +2 for overexposure. 

Now you probably asking, why the hell should I choose underexposed image if I am trying to make correct exposure? Well, because light conditions are rarely perfect, so you need to mess up with the settings to make them as perfect as possible, that's what photographers do. Example: if your background is sky with white clouds, exposure you want is detailed shadows and dark tones at your object on the ground, but you don't want bleached out highlights. So if your exposure override is set to zero - correct exposure, camera depending on metering mode (I will explain in next article) will calculate the exposure and set the shutter speed to achieve 18% average through the tones. It probably will have nice shadows, but clouds will be plain white with clipped highlights. To avoid this, you can adjust exposure override to -1/3 or -2/3 and you'll still have nice shadows, but this time your clouds will have details in them as well.

S - Shutter priority mode
Works the same as A mode, but this time you are choosing your shutter speed. You can adjust your ISO and exposure override, but camera will decide what aperture will be the best. This mode is good in sport photography, or low light conditions, where you'd like to avoid blurry images caused by motion of your camera.

P - Program mode
It is simply more advanced AUTO mode, but here you have control over the ISO and also you can adjust exposure override. This mode is quite good to use if you haven't got time to think about aperture - shutter speed relationship, but you want to control the outcome.

MANUAL AND USER DEFINED MODES

M - manual mode
Most advanced settings, you control everything. You set the aperture, shutter speed and ISO (yes, there is still option for AUTO ISO in every mode, if you'd like to use it). You can check accuracy of you decision on exposure override scale in your viewfinder, where pointer on the scale shows how correct your exposure is. And also you can check your result on LCD and adjust values accordingly.

U - User defined modes
If you shoot particular scene very often, let's say you shoot landscapes in nice sunny day, where exposure settings are often f11, 1/160s, ISO 100, you can set this as user defined setting and every time you turn dial to "U", camera will be set for this exposure, so you don't always need to change it. There are usually two or three of those, so you can preset your camera for various conditions.

Next time we'll talk about metering modes, so we'll understand better how camera does the exposure readings.

Posted by pkuzmin at August 18, 2012 6:40 PM

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