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05/21/2005 02:18:46 AM · #1 |
whats the purpose of exposure compensation? couldnt i just over/under expose my shot? i was reading up on exposure compensation and its kinda confusing the only thing that i got was that it meters your subject as 18% middle gray. help me figure this one out guys. heres the likn to the site.
exposure compensation |
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05/21/2005 02:39:09 AM · #2 |
It is similar to changing/over-riding the f-stop setting. Check out this thread.
Message edited by author 2005-05-21 02:39:43. |
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05/21/2005 02:46:47 AM · #3 |
If you're not sure bracket :)
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05/21/2005 03:01:42 AM · #4 |
When your camera is on an automatic mode, be it full auto, aperture priority, or shutter priority, the "exposure solution" it delivers to the image is based on the assumption that the areas metered average out to 18% gray value. While this assumption gives a decent working exposure in most situations, it fails to deliver realistic results in very light or very dark scenes, and is particularly prone to blowing out highlights in a scene with a small, bright subject and a large, dark surround.
Since you'd have to leave auto mode to alter the relationship between f:stop and shutter speed, the manufacturer gives you what amounts to an exposure override, where you can instruct the camera to give more or less exposure than the automatic system considers to be "correct"; for example, in shooting a silhouette in the currently-running challenge you'd most likely be adjusting the EV compensation to minus-1 or even more so as to preserve details in the bright areas while allowing the silhouetted portion to go completely black. In those silhouettes that involve a figure against a sunset, for example, EV correction might well be as much as -1.7 or -2.0.
The proof of the pudding will be in your exposure review mode: shoot the shot and then go into playback and take a look at what you got. Identical results can be obtained by going into full manual mode (if you have one) and adjusting for "underexposure", in the above case of a backlit silhouette.
The same holds for images where the subject is relatively bright and the background darker; the indicated exposure will lighten the background unnaturally and overexpose the subject, so minus values are called for in the EV compensation.
It's relatively rare to want to use positive EV compensation (additional exposure beyond the indicated), but this would be called for when you are shooting a high key image, as (for example) in the recent "white on white" challenge. The problem with added exposure is that it runs the risk of blowing out the brightest areas of the image beyond redemption. It's generally much easier to work with an underexposed image than an overexposed one.
I keep the EV compensation on my camera set to -0.3, and often dial it to -0.7 as a matter of course.
Robt.
Message edited by author 2005-05-21 03:04:26.
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05/21/2005 04:24:12 AM · #5 |
Thanks Robt., I was waiting for the elaboration : )
I find the under-exposed shots are noticably noisier than usual, but at least there's data to work with. |
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05/21/2005 06:49:28 AM · #6 |
Originally posted by GeneralE:
Thanks Robt., I was waiting for the elaboration : )
I find the under-exposed shots are noticably noisier than usual, but at least there's data to work with. |
This is true if they are really underexposed, but not always true if you're underexposing relative to the camera's idea of correct exposure, which is often actually an overexposure in the first place.
Robt.
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05/21/2005 07:16:01 AM · #7 |
As a further elaboration; exposure compensation changes the value of grey the camera exposes for. That is, with 0-EV compensation the camera exposes as if the average area metered is 18% grey, but with a +EV value the camera exposes with the assumption the area metered has a tonality of a lighter shade of grey -- and likewise, a negative EV tells the camera to assume the average tonality of the area metered is a darker shade of grey. How much lighter or darker shades of grey depends on the EV value; a +1 EV will indicate the scene metered is to be exposed to a shade of grey that is twice as bright (one stop greater) than 18% grey, and so on with other EV values.
This, in conjunction with a spot meter, allows for very precise exposure of specific areas of the scene. For example, in portraits the face is often the focus of the image and it is vital it be exposed properly -- however, there are different shades of skin. If the portrait is metered off the face with no EV bias every image will come out with about the same tonality. For the image to show the tonality of each subject, setting the EV bias is great. For my moderately tanned skin a -0.5 EV works well, but for my red-headed and pale wife a +1.5 EV would be better. This ensures both portraits of both my wife and I are the same tonality we are and not the arbitrary 18% grey tone of some generic scene.
This, coupled with a knowledge of the Zone system, become a very valuable tool for exposing the scene with the desired tonality -- and becomes vital on many hard to expose shots. For example, a lone tree against a blue sky with white clouds -- the tree is the subject, but if it is exposed properly the sky will likely be blown with all detail lost Likewise, evaluative metering the scene with an arbitrary -EV bias may produce an image that works or may not -- and, since more noise is added the more it is underexposed, ensuring the tree is exposed as close to its natural tone as possible is desired. This takes a lot of practice to be able to judge a scene well enough to know what EV to set an evaluative meter on. However, if the brightest cloud is spot metered and exposed with a +2 to +2.5 EV (or similar) the clouds will be exposed at a very high tonality, but not high enough to blow out the detail. This also ensures the tree is not unduely underexposed and can produce a much cleaner image.
My view is different in this regard from Robert's but for the exact same reason. Since the highlights are so easily blown if care is not taken I find it useful to place the lightest areas of the scene at the tone I want them, and thus ensure highlights are blown only when I want them to be. Adjusting the lower tones afterward is then possible in the same manner. I find the use of a positive EV far more useful than negative.
David
Message edited by author 2005-05-21 07:17:07.
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05/21/2005 07:26:13 AM · #8 |
Originally posted by Britannica:
My view is different in this regard from Robert's but for the exact same reason. Since the highlights are so easily blown if care is not taken I find it useful to place the lightest areas of the scene at the tone I want them, and thus ensure highlights are blown only when I want them to be. Adjusting the lower tones afterward is then possible in the same manner. I find the use of a positive EV far more useful than negative.
David |
That's an extremely useful elaboration, David. Actually you and I are in the same page of the book here, with one caveat: I restricted my discussion to the use of EV compensation when set in an averaging meter mode, because it seems that this is where the OP is coming from. When we move up to taking selective readings with a spot meter mode, then your approach is precisely what is required, and indeed this is exactly what I do when I have the time to do it. But that's a pretty contemplative approach, and most people use their cameras in an auto mode with some form of scene averaging meter modality. So the issue in that situation is "How do I use the EV compensation to improve my exposures on the fly?" And the answer is, you can sort of generally prejudge the lighting you're working with and dial in a good amount of compensation to give you "ballpark correctness" in your exposure.
Speaking for myself, when I'm working with a tripod doing setups or scenics, I go over to full manual mode and spot meter several areas, then calculate my best exposure from those spot readings. But then, I have enough experience in Zone System to do this practically unconsciously. Yours is an excellent approach to take as well.
Robt.
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05/21/2005 12:46:51 PM · #9 |
| i understand that now (thanks guys) but i still have another question: since i have a digital slr that i use in manual mode, couldn't i just leave the ev control alone and just open up or close down my lense or did i still miss something? |
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05/21/2005 12:57:40 PM · #10 |
just keep it in manual mode all the time, and you won't have to rely on exposure comp.
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05/21/2005 01:57:23 PM · #11 |
Originally posted by bear_music:
It's relatively rare to want to use positive EV compensation (additional exposure beyond the indicated), but this would be called for when you are shooting a high key image, as (for example) in the recent "white on white" challenge. The problem with added exposure is that it runs the risk of blowing out the brightest areas of the image beyond redemption. It's generally much easier to work with an underexposed image than an overexposed one.
I keep the EV compensation on my camera set to -0.3, and often dial it to -0.7 as a matter of course. |
I'll dispute this, slightly. It's certainly true if you don't want to do post-processing, and is true if you need faster shutter speed, but a lot of images have headroom before highlights get blown, and most cameras store more than three times as much RAW data in the upper two light zones than they do in the lower three combined. I regularly shoot at +0.33, sometimes up to +1.0, to push the image color detail up as far as I can without blowing highlights. You may want to read Michael Reichmann's Expose to the Right. If you get this right, it can get you significant added shadow detail and reduce posterization.
This is useful, but it can be a dangerous technique. If you aren't checking your histogram after your shots, or don't have the luxury of re-shooting, it is very easy to completely destroy a shot this way (when I was first learning this, I destroyed a third of my shots on a particular outing). Even if you do check your histogram, many cameras (such as my own 20D) give only a luminance histogram, which won't tell you if you blew just one color channel, which can cause posterization. Blown highlights cannot really be repaired, though it may be possible to compensate for single blown color channels. |
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05/21/2005 02:03:07 PM · #12 |
Originally posted by ericlimon:
just keep it in manual mode all the time, and you won't have to rely on exposure comp. |
There is no practical difference between setting an aperture in Manual, then adjusting shutter speed until the light meter shows +1/3, and setting Av mode with +1/3 EV, except that the latter is generally faster. Manual mode is good for keeping a series of shots perfectly consistent, but is usually not the best choice for single images.
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05/21/2005 02:10:18 PM · #13 |
I think it has more to do with what you are shooting and when. I find myself underexposing more shots than overexposing if only because I like the results better. BUT, the exposure is according to my camera - I personally think that my camera tends to overexpose shots. It gets worse when I add a polarizer. Having used film for years without having a histogram, I have no idea what I am looking at anyway. Now that I am finally getting used to my camera and its quirks I am getting back to what I did with film - expose by feel. I'm still trying to work with the fact that you can't choose your film in a digital camera. :) Color and saturation are still throwing me off...
d
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05/21/2005 02:18:17 PM · #14 |
...there I go misinterpreting the thread topic again. ;)
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05/21/2005 02:46:42 PM · #15 |
Originally posted by kpriest:
...there I go misinterpreting the thread topic again. ;) |
And you know you have to meet the topic head-on to score with the voters ... |
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05/21/2005 04:39:14 PM · #16 |
Originally posted by tripl3distill3d:
i understand that now (thanks guys) but i still have another question: since i have a digital slr that i use in manual mode, couldn't i just leave the ev control alone and just open up or close down my lense or did i still miss something? |
In full manual the EV control is not needed, that is true.
I tend to not keep my camera in full manual; preferring to control only the shutter or aperture. Which one I control depends on if DOF or motion blur are my primary concern -- usually DOF. It is generally easier to set the EV than to constantly have to take a reading and then calculate the compensation.
For example, one of the main reasons I bought my camera was to take pictures of my kids. Take them outside to play and set the EV to the desired compensation for their face and start shooting while they play. They are running around in and out of the shade, changing their orientation to the sun, clouds blowing overhead and funning past the white house or the dark fence -- it is generally a nightmare of nearly unpredictable rapid changes of light. With the exposure compensation already set for the tone I want the face to be at and the aperture set I need only meter their face and shoot -- confident all images will be exposed for the same tonality.
There may be an easier way, but I haven't found it. :D
David
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05/21/2005 04:54:38 PM · #17 |
Originally posted by tripl3distill3d:
i understand that now (thanks guys) but i still have another question: since i have a digital slr that i use in manual mode, couldn't i just leave the ev control alone and just open up or close down my lense or did i still miss something? |
Either way, you are doing the same thing. If you have the camera in manual, you are adjusting the EV by setting the shutter and the aperture. Otherwise, the camera is adjusting the EV. I find that manual mode allows me to experiment. I really don't believe that the camera knows what it's doing- I'm a bit smarter, so I take the wheel completely. One reason why I prefer manual transmissions as well. :)
-Nick
Message edited by author 2005-05-21 16:55:17. |
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