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05-29-2009, 10:49 AM   #1
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The three imaging parameters... let me get this right...

Hi Everyone, I've been using my k200d for the past 5 months and teaching myself (with the help of this forum and wanted to get something clear.

The three things that affect the exposure are the following (with their respective effects on the image), please correct me if i'm wrong in any way:

ISO: determines the how sensitive the sensor is to light. the more sensitive, the more noise you get...

Shutter speed: doesn't need explaining, but to let more light in, the more you're exposed to shake, unless you use a tripod

Aperture: how wide the irises are set in your lens, the faster (or more open), the shallower the DOF.

Now, what I'm trying to understand is how metering makes it's way into the equation. For example, the new k7 has 77 metering segments, thus it calculates what level of ISO, shutter speed and aperture should be used based on 77 different parts of the image (if you were to use matrix metering)?

Thanks, I just want to be clear on this.. before i venture off any have no clue what i'm talking about.

05-29-2009, 10:56 AM   #2
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The metering determines the best exposure, there is only 1 exposure for the image, you have the same iso, shutter, aperture for the whole image.

The meter measures the light in 77 parts of the image, then averages them to come up with 1 exposure value, then adjusts the 3 settings to give an exposure of that value.

Think of the exposure as a given number in EV, lets pick 11. So the camera will adjust the iso shutter and aperture to get an exposure of 11, if you think that is wrong you can use the EV compensation to say you want one at 12 or one at 10 etc.

the meter segments are different sensor points, ie you get 77 readings instead of the old 16, when you have more readings/samples you have more information on which to base your conclusion.

so the meter will come up with 1 exposure, a given combination of iso/shutter/aperture yields one exposure, if you, say change just the iso, then the exposure is changed, if you change the iso but also compensate by changing the shutter as well then the exposure remains the same.
05-29-2009, 11:02 AM   #3
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I can add that the meter is color blind, it just measures light. So it has to assume something, It will calculate an exposure value that is correct for 50% grey (some experts differ in what value is correct for each camera, but its a middle grey in any case). 50% grey is average and usually pretty close to correct. but you will get false results if a scene is too far to either extreme. IE> point the camera at a white wall so that the whole frame is white and it should give an exposure that will under expose the wall to make it look grey. point it at a black wall and it will over expose to make it look, yep, grey.
05-29-2009, 11:32 AM   #4
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QuoteOriginally posted by and Quote
So it has to assume something, It will calculate an exposure value that is correct for 50% grey (some experts differ in what value is correct for each camera, but its a middle grey in any case).
Sorry, just a bit confused on what you mean by "...that is correct for 50% grey"

Other than that, thanks for your explanations

05-29-2009, 11:57 AM   #5
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QuoteOriginally posted by esman7 Quote
Sorry, just a bit confused on what you mean by "...that is correct for 50% grey"

Other than that, thanks for your explanations
The meter in a DSLR is a reflective meter, meaning the light hitting the metering sensors is the light that is reflected off the subject.

for example a white wall with a spot light directed at it, the camera is also pointing at the wall, the camera is not directly measuring the light coming from the spotlight, instead it is measuring the light that is reflected off the wall.

That means the reflectiveness of the subject becomes a variable in the calculation.

Now as you probably can imagine, a white wall reflects more light than a black wall. and a grey wall is in between those.

So the meter would measure more light if its pointed at a white wall than a black wall.

But the camera doesnt know if the wall is black, grey or white (or anywhere inbetween). It has to assume something so it assumes the wall is grey which is the middle point.

Now lets consider you have painted half the wall black and half white, aiming the camera such that first you only have white in the frame and then only black, will give 2 different exposures since they reflect different amounts of light back,
However, the amount of light hitting them is the same (assuming we are in a room that is lit by a centered ceiling light) so if the light hitting them is the same that means the correct exposure for the two pictures would be the same, but the values your camera gave you were two different ones, which tells you its wrong.

a different type of metering is incident metering, where you use a hand held light meter and hold it in front of the subject, ie in our case the wall, the meter measures the light hitting the wall, not reflecting off it, so in case of the white and black parts of the wall it would give the same exposure which is the correct result.

I digress a bit here, this is not something to worry about normally, its just something to keep in mind so you can understand why in some situations you can get wrong results and when to use the ev compensation.


This short example from sekonic is a good demonstration of what I have just said
Sekonic Classroom: Metering Techniques

Again, Im not telling you to get a light meter, just saying its useful to know how the camera thinks
05-29-2009, 02:21 PM   #6
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QuoteOriginally posted by esman7 Quote
Now, what I'm trying to understand is how metering makes it's way into the equation. For example, the new k7 has 77 metering segments, thus it calculates what level of ISO, shutter speed and aperture should be used based on 77 different parts of the image (if you were to use matrix metering)?
Matrix metering includes a bit of mystery. It measures light in however many segments - 16 in your camera - then uses proprietary logic to figure out the exposure. The logic is not explained thoroughly, and different manufacturers use different logic. Scene modes may influence matrix metering as well. The camera may look at the data and decide your photo is a sunset or a portrait and adjust exposure accordingly. If I am trying to capture a scene that the camera is unlikely to have in its database, I know I might have to watch matrix metering more carefully.

Your camera has a metering mode that can help you learn something about metering. That's spot metering. The only part of the scene that is metered is between the brackets at the center of the viewfinder. You can aim at the bright part of your subject, get a reading, then get a reading off the dark part. You'll have a better idea of where the other metering modes got their information.

One example of how to use spot metering: I have a black and white dog who is a good subject for practice. If he can find sun, he'll lay in the sun. Matrix or center-weighted metering often gets the reading from a scene like this wrong, so the white fur is really overexposed (blown out). I didn't know why until I used a spot meter for this scene. The white fur can be 6 Ev brighter than the black fur, and my camera can't capture much more than that range. None of the fur is at middle gray where the camera could see its favorite color, and the fur is in a random pattern that won't make sense to a database of exposures. For a decent photo I have to expose the white fur just below overexposure. Then the black fur is bright enough to still have detail. Just knowing that was very helpful in understanding the other meter modes.

Using the numbers from a spot meter is different than using readings in another meter mode. The meter still will give you a setting for middle gray (18%, not 50%, but it doesn't really matter). It only applies to that spot, not the whole scene like the other modes. You have to know what you're pointed at, the range of exposures you can capture, and how you want the subject to look. In my example above, I point the spot meter at the white fur, then make the shutter speed about 2-2.5 stops faster so the white fur will be just short of too bright.

I don't use that mode very often now, but you will learn more with the spot meter than by looking at the photo you just took, adjusting Ev and shooting again.
05-29-2009, 03:51 PM   #7
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Thanks for all that explanation, it really helps; however, I would like to clarify two things:

1. EV's

Is the Exposure Value an actual, calculated number? For example, does the camera give the scene and EV of 1-20 (just for e.g.) and then you adjust by -2 to +2?

I understand now how the EV +/- works for over/underexposing your image; however, why is it a value of 2? Why not percentages to stay proportionate (if necessary?

2. AE-L

Secondly, is the whole idea of using the AE-L to have your camera calculate the exposure of an image based on another scene that resembles the lighting of one you're trying to catch?

i.e.: If there's a house in front of me but the sun is shining from behind it, I could take a picture of house using matrix metering and the house will show up as too dark because there's too much ambient light coming from the surrounding sun. Now, is the whole point of AE-L that I point the camera at an area that's the same "darkness" as the house, hold the shutter button halfway down, then hold the AE-L, take it back to the original scene and take a snap? Then this way, the house will come out right, but the surrounding sky will look overexposed... correct?

Please correct me if I'm wrong, but another way around this is to use spot metering in the first place, but only if the house is in the center of the image. If the subject was off-center, then it would make more sense to use the AE-L button and then move the camera and line up for the shot.

I'm only explaining this to see if my rationale is correct on both EV's an AE-L. Thanks again to the both of you.
05-29-2009, 06:30 PM   #8
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QuoteOriginally posted by esman7 Quote
Is the Exposure Value an actual, calculated number?
Yes. Google "exposure value" and check out, for example, the Wikipedia entry. This system of measurement has been around a while, and is nice because it fairly simply allows one to relate it to the the exposure parameters.

QuoteQuote:
is the whole idea of using the AE-L to have your camera calculate the exposure of an image based on another scene that resembles the lighting of one you're trying to catch?
Yes. Hopefully one that is simpler / more predictable to meter.

QuoteQuote:
Now, is the whole point of AE-L that I point the camera at an area that's the same "darkness" as the house, hold the shutter button halfway down, then hold the AE-L, take it back to the original scene and take a snap? Then this way, the house will come out right, but the surrounding sky will look overexposed... correct?
Yes, except no holding of the half-press should be necessary. In any auto mode, simply pointing at the scene should calculate an exposure; just hit AE-L to lock it in.

QuoteQuote:
Please correct me if I'm wrong, but another way around this is to use spot metering in the first place, but only if the house is in the center of the image. If the subject was off-center, then it would make more sense to use the AE-L button and then move the camera and line up for the shot.
Correct again. Or use spot metering *and* AE-L - point directly at house to center the spot there, then hit AE-L. But note that metering off the house only makes sense if it is medium darkness. If it's a white house, metering off it will result in underexposure as the camera tries to make the house medium gray.

05-29-2009, 11:36 PM   #9
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When using spot meter, remember that the meter is for medium gray and the further what you pointed the spot meter on is away from medium gray, the further off the exposure will be.
05-30-2009, 03:13 AM   #10
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If trying to wrap your head around this stuff doesn't make Sunny 16 appealing, nothing will.
05-30-2009, 02:14 PM   #11
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QuoteOriginally posted by esman7 Quote
Thanks for all that explanation, it really helps; however, I would like to clarify two things:

1. EV's

Is the Exposure Value an actual, calculated number? For example, does the camera give the scene and EV of 1-20 (just for e.g.) and then you adjust by -2 to +2?

I understand now how the EV +/- works for over/underexposing your image; however, why is it a value of 2? Why not percentages to stay proportionate (if necessary?
The Wikipedia entry that Marc suggested is here if you want to skip Google:

Exposure value - Wikipedia, the free encyclopedia

The short answer: Although all the parameters measure wildly different things, for our purposes they have been converted to a scale which relates to the amount of light and how to double or halve it. The scales allow us to gloss over the underlying units and math, except to explain why the aperture scale is f1, f1.4, f2, f2.8, etc.* Exposure value is simplifies that even further and more units disappear, so it looks like just a number. It is still a scale measuring the amount of light, with each whole number increase indicating twice as much light. Each whole number increase is also one stop increase.

Older cameras with Ev compensation have the dial marked so it makes more sense. It will say 4x, 2x, 0, x, x - a better idea of what is happening to the amount of light.

Understanding that Ev isn't just a number helps you understand what changes when you change Ev. Like you said at the start, only three parameters. So let's say you are taking your house photo at ISO200. You want control over depth of field so you put your camera on Av and choose f16 to get everything in focus. You don't want to bother spot-metering or using AE-L because you already figure the meter reading will underexpose by about 1.5 Ev. The meter says 1/125sec. So you just set +1.5 Ev on the camera. What parameters will be used when the photo is taken? What parameters will be used if you switch to Tv mode?

*Because of something about the square root of two. Forget I mentioned it.

Last edited by Just1MoreDave; 05-30-2009 at 02:16 PM. Reason: forgot asterisk
05-30-2009, 06:29 PM   #12
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So a +1.0 EV means twice the amount of light is coming in? and +2.0 means four times?
05-30-2009, 06:30 PM   #13
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QuoteOriginally posted by Mike Cash Quote
If trying to wrap your head around this stuff doesn't make Sunny 16 appealing, nothing will.
Oh and I like this rule, thanks
05-30-2009, 06:51 PM   #14
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QuoteOriginally posted by esman7 Quote
So a +1.0 EV means twice the amount of light is coming in? and +2.0 means four times?
That is correct. Each EV step represents a doubling (or halving) of the amount of light.
06-01-2009, 08:44 AM   #15
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This might help clarify a bit as well. The inputs for ISO, shutter speed, and aperture are all set up so you can make equivalent & offsetting changes in EV steps (or stops).

ISO 1EV steps: 100-200-400-800-1600-3200.
Shutter speed 1EV steps: ...1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500...
Aperture 1EV steps: ...f/2, 2.8, 4, 5.6, 8, 11, 16, 22...

Intermediate steps may be available (usually either 1/2 or 1/3 EV) but the idea is that you can easily make equal and offsetting changes to other parameters to influence depth-of-field, freeze motion, etc.

Example: Meter tells you 1/125 @ f/5.6 with ISO 200. You know your subject demands extra depth-of-field so more subjects are in focus. Without even re-metering, you can "stop down" aperture to f/11 (two stops), increase ISO to 400 (1 stop) and decrease shutter speed to 1/60 (1 stop) and get the same level of exposure.
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