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Bracketing & Exposure

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    Peter_West
    3 Oct 2012 - 9:03 AM

    Hi All. Please can someone explian in layman terms to me whats the differance between bracketing and exposure compensation, and when would you use one over the other, just didnt grasp it last night at night school. Kind Regards Pete

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    mikehit
    mikehit e2 Member 45761 forum postsmikehit vcard United Kingdom9 Constructive Critique Points
    3 Oct 2012 - 9:28 AM

    Exposure compensation is used to overcome issues when the sensor is confused by the amount of light coming into the camera.
    In its simplest form, the sensor assumes that the overall tone of an image is mid-grey (this is nothing to do with colour). If the amount of light coming in is higher than mid-grey the camera reduces the amount of light by shortening shutter speed or narrowing the aperture.
    If you are taking a picture of a peron standing on snow, then the average light is very bright so the camera reduces the exposure. The snow comes out greyish and the person is horrendously underexposed. When using exposure compensation you are telling the camera 'I don't agree with you, I want you to give it 2 stops more light'. The person is now correctly exposed but the snow is a burnt-out white background.
    The same happens in reverse if you have a spotlit subject against a dark background.

    Bracketing is where you can take that same picture at two or more different exposure levels, one for the person and one for the snow. The intention is that in post processing you combine the two elements - the correctly exposed snow and the correctly exposed person - into one image.
    A development of bracketing (pun intended) is HDR where you take two or more images at different exposure levels and a program blends the images in a much more complex fashion. But that is a whole different story

    mikehit
    mikehit e2 Member 45761 forum postsmikehit vcard United Kingdom9 Constructive Critique Points
    3 Oct 2012 - 9:34 AM

    I should have added that even in a snowscape, it is useful to add exposure compensation to avoid the snow coming out grey.
    Nowadays camera meters are much more sophisticated. They have in their memory a database of several thousand images and on seeing that the scene is not an overall mid-grey it compares the image it detects with this catalogue and once it finds the closest match it applies the settings it thinks is most appropriate (I still think it is amazing that it can do this in a split second). So you may find that the snow scene comes out pretty close to ideal even without your intervention. And with face detection, if it detects a face it will expose for the face in preference to the background. Cool, eh?

    The beginner's bible on this topic is 'Understanding Exposure' by Bryan Peterson.

    sherlob
    sherlob e2 Member 82202 forum postssherlob vcard United Kingdom119 Constructive Critique Points
    3 Oct 2012 - 9:38 AM

    The difference is perhaps best described in the purpose behind the techniques. For example, exposure compensation is where you adjust the compensation in response to a known (or likely) problem with your camera's metering. E.g. a camera's light meter is set to give a correct exposure for 18% grey and therefore if you take an image of a snowy scene the resulting image will show the snow as being grey and not white. In other words the meter has been fooled by the snow and has underexposed the scene - hence adding between 1-2 stops of + exposure compensation can help override the problem. The point being that the photographer recognises the problem and adjusts the exposure accordingly.

    In bracketing the exact exposure is unknown. i.e. we can't be completely clear as to whether the meter is going to be correct, or whether a degree of exposure compensation is needed - or if it is how much. Bracketing an exposure allows us to hedge our bets - we take multiple images of the same scene at several different exposures (i.e. using different levels of exposure compensation) to see which is best at a later time. Bracketing is often an automated option on cameras - you can customise the degree of exposure compensation applied either side to the exposure set, and in some models the number of images taken. A modern use of bracketing is to blend exposures in post processing - e.g. taking the sky from one image and adding it to the foreground on another. HDR takes this a step further.

    Hope this helps,

    Adam

    lemmy
    lemmy  61672 forum posts United Kingdom
    3 Oct 2012 - 10:01 AM

    Bracketing means taking several photographs of the same subject with different exposures. Say your camera is giving you an exposure of 125th@f8 You shoot one at that exposure, another at 125th@f5.6 and a third at 125th@f11. This gives you the possibility of choosing which of the 3 exposures you prefer. Most modern cameras can be set to bracket 3 or 5 frames automatically.

    If you take one picture, you might find when you view it on your computer screen it looks different from on your camera's LCD. If you have bracketed your exposure as described above, you have the option of picking a frame that looks better exposed to your eye.

    Exposure compensation is like bracketing in that you change the exposure to suit the subject but implies that you choose the amount of correction yourself at the time of taking the picture. Say you are taking a portrait of someone with the sun behind them, a back-lit subject. The camera's exposure meter will tend to read the bright light around the subject and expose for that, making the person's face too dark. If you switch on your camera's exposure compensation and add + correction, the face will be lighter. You have compensated for the camera's tendency to underexpose in back-lit conditions.

    The best analogy I can think of is of someone shooting at a target. Bracketing is firing, say, 5 shots at the target and picking the one closest to the bullseye. Exposure compensation is judging the range, holding a finger up to feel the wind speed, taking careful aim and loosing off one shot that is as accurate as you can make it.

    Both methods have the same aim, getting an optimum exposure. Exposure compensation involves making decisions at the time of taking the picture, bracketing allows you to defer it until you get home.

    You wouldn't want to use bracketing for any subject where the moment in time you take the picture is important. If you are taking pictures of your cat chasing a ball of wool, the good exposure will be unlikely to be the one where the action is best. If your cat is black, compensate the exposure by + 1 stop and shoot away with your eye on the action.

    But, if you are taking a landscape picture with your camera tripod mounted, bracketing would be the way to go. Shoot 5 frames at half stop intervals and pick the best frame later. It would even give you the possibility of combining one or more of those pictures to get exactly the effect you want.

    You said you are a novice, though, so first things first!

    Peter_West
    3 Oct 2012 - 10:48 AM

    thanks all, I think, more reading is called for and out with camera this weekend and just shoot away trying different settings then, will check the book out Mike, just more thing, I understand the bit about bracketing taking 3 or 5 frames, but once you set say 3 frame bracket, do you actually press shutter three times or does it automatically take 3 separate pictures, btw I have a 600D, only asking because a mate uses the Panasonic FZ100 and when set his he just pressed shutter once but it fired three shots off, but mine just shot once, Pete

    mikehit
    mikehit e2 Member 45761 forum postsmikehit vcard United Kingdom9 Constructive Critique Points
    3 Oct 2012 - 10:53 AM

    If I recall correctly, the 600D will take one shot with one press.
    Remember to reset the AEB - it can be very confusing later on to wonder why the exposure keeps on changing in a 'weird' way when you are sure you had the right settings (BTDT) Tongue

    lemmy
    lemmy  61672 forum posts United Kingdom
    3 Oct 2012 - 12:11 PM


    Quote: , do you actually press shutter three times or does it automatically take 3 separate pictures

    Take a look through your menus. On my camera you can choose to rattle then off automatically or fire them yourself. Yours may offer the same choice.

    KenTaylor
    KenTaylor e2 Member 92967 forum postsKenTaylor vcard United Kingdom2 Constructive Critique Points
    3 Oct 2012 - 3:21 PM

    Digital has many advantages over film. With film exposure is crucial where to ensure a frame is well exposed bracketing could be relied on by taking a meter read exposure then taking two more, one stop over and one under. it can practiced easily from home outside or inside. Your camera will, or should, have bracketing on a menu on your camera. It will fire your choice of exposures over and under.

    This might help

    Compensation is similar by setting the exposure below or above the speed rating the camera is set on.

    Both options are useful in unusual or tricky lighting which is mainly bright and dark areas in the same shot.

    It will pay dividends spending an hour so at home, no need to travel miles, going through exposure options on manual. You can be efficient with all the automatic settings letting the camera tell you what the exposure is while I prefer to tell the camera. You can then have that knowledge fixed in your memory ready to be used with any camera, digital or film.

    cheddar-caveman

    A classic example of where you'd use exposure compensation is taking a picture of a bird flying (against the sky). Almost always the camera will expose for the sky and the bird will just be a black outline. To compensate for this you would increase the exposure so that the bird was correctly exposed but by so doing the sky would be over exposed.

    Bracketing, I use this for such techniques as HDR photography, deliberately setting the camera to take three identical shots, one exposed, one over exposed and one under exposed. The three images are then merged thereby taking advantage of the full colour and lighting capability of the sensor.

    Get out there and play..it's free!

    petebfrance
    3 Oct 2012 - 9:34 PM

    try this link for how to do 'bracketing' on the 600D:

    600d bracketing

    Last Modified By petebfrance at 3 Oct 2012 - 9:37 PM
    Peter_West
    4 Oct 2012 - 9:27 AM

    Hi All. Really appreciate you all taking the trouble to explain things to me, do feel a bit better now, did try it out last night at home, and can see the different results you can get, would have loved to go outdoors, but no chance unless I wanted a soaking, but weather looks ok for weekend round here though, little night school assignment to do Autumn colours, berries, fungi, reflection, so Iíll attempt to it all into practice and incorporate Bracketing & Exposure, like you said cheddar its free, ordered the book last night Mike
    Understanding Exposure' by Bryan Peterson.

    Kind Regards Pete

    lemmy
    lemmy  61672 forum posts United Kingdom
    4 Oct 2012 - 11:16 AM

    Do remember also that 'correct' exposure is what you like. No need to be slave to the meter.

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