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01-30-2010, 08:28 AM   #1
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spot vs centre vs matrix exposure

Hi is there a guide any one can link to that gives a decent explanation as to when to use each exposure mode or maybe when not to? even better if it explains what to beware of before using a given mode?
The reason I ask is that the Pentax manuals do not really explain very well when to use each mode and, if I am honest, having taken 3 images one in each mode of the same spot in my back garden of a area with heavy shadow and bright sun and every thing in between I can't really see the difference?
I have been following the thread https://www.pentaxforums.com/forums/pentax-beginners-corner-q/88297-need-some...d-shadows.html
with interest because I also with K10 often get a surprise as to the unexpected poor exposure, unlike the OP in that thread I do understand manual mode etc etc
and exposure compensation in fact I mostly shoot with 3 or 5 steps to get the best in the above sort of situation but the modes interest me as I feel I am not making the best use of a feature that obviously was not put there with no thought and even has its own quick change setting lever.
Alistair

01-30-2010, 08:32 AM   #2
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this might help:
Understanding Camera Metering and Exposure
01-30-2010, 11:53 AM   #3
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Here is one on spot metering.

What is spot Metering - Spot Metering Examples
01-30-2010, 01:34 PM   #4
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QuoteOriginally posted by ma318 Quote
Thank you both for the links, the spot meterbook and zone book look like they will make interesting reading, time to save more photo pennies up.
many thanks,
Alistair

01-30-2010, 04:34 PM   #5
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Highly recommended book suggestion:

Amazon.com: Michael Freeman's Perfect Exposure: The Professional's Guide to Capturing Perfect Digital Photographs (9780240811710): Michael Freeman: Books

This assumes you already have a basic understanding of aperture, shutter speed, and ISO (and their relationship), and are ready to think about questions like what you're asking.
01-30-2010, 05:36 PM   #6
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QuoteOriginally posted by adwb Quote
Thank you both for the links, the spot meterbook and zone book look like they will make interesting reading, time to save more photo pennies up.
I've read that book, my library had it. If the web page makes sense to you, the book is not completely necessary, but the book does explain spot metering in complete detail. It's actually pretty simple: the name is accurate.

I have my camera always set on center-weighted metering, sort of by elimination. I don't use spot metering because I have installed a Katz-Eye split prism focusing screen. The screen might be inaccurate with spot metering, and I decided not to figure out whether it was. (I bought a Pentax Spotmeter V which is a very useful teaching tool about spot metering.)

I don't use matrix metering for two reasons. One is, it only works with KA or newer lenses, that is, lenses with an A position on the aperture ring or no aperture ring at all. If I switch to an older lens, the meter automatically switches to center-weighted mode. So for consistency, I decided to just stay in center-weighted. Matrix metering works by metering many parts of the scene, comparing that information to a database of typical scenes, then deciding on a meter reading. I figure I can do that better than the camera.

Center-weighted mode is also what I'm used to on film. It doesn't always give you the right reading but it is predictable. As long as you are paying attention to the scene, you can make up for its limitations.
01-30-2010, 05:45 PM   #7
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QuoteOriginally posted by mattdm Quote
Highly recommended book suggestion:

Amazon.com: Michael Freeman's Perfect Exposure: The Professional's Guide to Capturing Perfect Digital Photographs (9780240811710): Michael Freeman: Books

This assumes you already have a basic understanding of aperture, shutter speed, and ISO (and their relationship), and are ready to think about questions like what you're asking.
thanks again Matt, perhaps a even more interesting read since it has sections that cover what both of the previous books cover.
Alistair
01-30-2010, 05:53 PM   #8
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QuoteOriginally posted by Just1MoreDave Quote
Center-weighted mode is also what I'm used to on film. It doesn't always give you the right reading but it is predictable. As long as you are paying attention to the scene, you can make up for its limitations.
I agree. But I must say I use spot as well, sometimes, since I can hone in on the specific area of the frame I want to expose perfectly. That said, in those sorts of circumstances when lighting is difficult I more recently tend to just use manual mode and guess my way through things. No film to waste and instant feedback on whether I guessed correctly or not. After a few shots I have a feel for the light.

I did all the shots in this thread that way. Manual focus too.

Automation is over-rated. Though useful one time in ten.

01-30-2010, 06:32 PM   #9
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When shooting in Manual mode with manual lens, I find I am doing this a lot of time.

- select spot metering
- select the aperture I want
- set the focus ring to 2 feet (or the minimum if min focus distance is greater than 2 feet).
- hold my palm out in the ambient light (or whatever light that is lighting my subject)
- look thru the view finder and ensure the center circle is on my palm and press "green" button
- decrease the shutter speed by 1 stop (you want 1 stop more light from the metered setting)
- if the shutter speed is too slow, I increase ISO or choose large aperture manually and repeat.


That would give me a starting exposure that should be close to the "correct" one. I take one test shot and check the histogram and adjust if required. Once you get used to it, it is very quick. Its only requirement is that your palm and your subject have to be lighted by the same lighting.

This is the "palm" trick. Because the palm, in most cases, is about 1 stop above 18% gray, you have to increase exposure by 1 stop after hitting the "green" button.

Last edited by ma318; 01-30-2010 at 07:01 PM.
01-30-2010, 07:52 PM   #10
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Here is the situation that made me want to learn more about metering. I get a lens in the mail and take a test shot of a dog lying in the sun. There's plenty of light to not worry about shutter speed, and not much motion. This is an example of the camera's metering, in this case center-weighted but matrix isn't much different:



The white fur is overexposed, setting off the highlight clipping warning.

When I got a spotmeter, I was able to figure out why this happens. The white fur in the sun is about six stops brighter than the black fur. If you were using the camera's spotmeter, you might see the white fur as reading f2.8, 1/1000 and the black reading f2.8, 1/15 if I count my stops correctly. Since the white fur's pattern doesn't match the model used for center-weighted or matrix metering, the meter can't see this extreme difference, so it typically overexposes by a stop or more. With the extreme range of light to dark, any overexposure blows out the whites.

To correctly meter this scene, you can use your spotmeter to see the range of values between the fur colors. You can't just point the meter at the white fur and use those values, though. The spotmeter would show you an exposure that would make the white fur have the same exposure as 18% gray. That would make the black fur too dark. You need to overexpose the meter reading (adding exposure compensation or altering manual exposure) by 1.5 to 2 stops. That makes the black fur bright enough to show detail.

Ultimately, to get the best exposure without taking a dozen shots of everything, you need to analyze the scene in the viewfinder, notice the bright and dark values, and choose what to keep and what doesn't matter.
01-31-2010, 03:23 AM   #11
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QuoteQuote:
Ultimately, to get the best exposure without taking a dozen shots of everything, you need to analyse the scene in the viewfinder, notice the bright and dark values, and choose what to keep and what doesn't matter.
So if I look for high contrast subjects , like your dog example, take several shots with the bracketing comp set to 1/3 and say 5 steps and look at the results the difference in the best image setting from the centre point image will start to give me a feel for how much exposure compensation I need to apply. with practise this will lead to a better understanding of exposure compensation. It also occurs to me if I take the "best" exposure and convert to b/w then I will also start to be able to identify the "mid" grey point in a colour image, A concept I have to admit I find hard to wrap my mind/eyes around.

From the previous posts I also now better understand the benefits of using the 3 different exposure modes and when to use them rather than just leave it on matrix and point and shoot and try to compensate after.

I have to admit I had also got hold of the wrong end of the stick as I seemed to have incorrectly thought that if I used centre focus setting and matrix mode and had the option "link AF point and AE" enabled this set the exposure at the focus point in matrix mode and would correctly expose my subject. I now see that in the dog example that would not work but in a general landscape type situation it probably would be fine.
Alistair

Last edited by adwb; 01-31-2010 at 04:14 AM.
01-31-2010, 09:45 AM   #12
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Things make a lot more sense to me when I realize that it is the intensity (brightness) of the light just before it hits the subject that ultimately determines the exposure setting. That's why incident light meter is usually preferred for very accurate exposure measurement. This measurement is not dependent on the surface of the subject.

But the camera's meter does not measure incident light. It measures the light reflected by the subject instead. The intensity (or brightness) of the reflected light will change depending on how the subject reflects the light. A white object will reflect more light into the camera's meter and a near black subject will reflect very little light into the camera and anywhere in between depending on the type of surface. Regardless of the actual brightness of the metered subject, the camera's spot meter will always try to give you an exposure setting that would make the metered subject appears 18% gray. Thus you need to make some adjustment to the spot meter reading depending on the tone (lightness or darkness) of the metered subject. If the metered subject is in fact lighter than 18% gray, then you need add more light (+ compensation) to the metered setting to make it appear lighter. If the metered object is in fact darker than 18% gray, then you need to subtract light (- compensation) from the metered setting to make it appear darker. How much to add and subtract is an art that can be learned by trial and error testing (i.e. experience).

For matrix and center weighted metering, the relative location of the bright and dark spots in the frame also has an influence on the camera's metered exposure setting. This makes it difficult to use the above rules to figure out exposure compensation.
When I use matrix or center weighted metering, I prefer to use the histogram (and blinking highlights) to determine how much exposure compensation I need to make - if any.

Last edited by ma318; 01-31-2010 at 04:21 PM.
01-31-2010, 11:02 AM   #13
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QuoteOriginally posted by ma318 Quote
Things make a lot more sense to me when I realize that it is the intensity (brightness) of the light just before it hits the subject that ultimately determines the exposure setting. That's why incident light meter is usually preferred for very accurate exposure measurement. This measurement is not dependent on the surface of the subject..
yes point taken.
QuoteOriginally posted by ma318 Quote
When I use matrix or centre weighted metering, I prefer to use the histogram (and blinking highlights) to determine how much exposure compensation I need to make - if any.
yes again I agree , I have been playing all day with increasing my exposure at various step amounts ending up with increasing from the initial the meter reading by 5 steps at .03 per step. On average using the kit 18-55 lens the best exposure seems to be 2 steps up [=0.6 or half a stop or just over is that right?] any way visually what pleases me also gives a fairly consistent histogram either w. r.g.b. or white only and with highlight enabled shows me if I am blowing highlights.
It has been a very very interesting learning curve as I now can, at a glance ,look at the preview screen and see what my exposure looks like compared to what I now know I want to see [histogram wise].
All in all a Sunday off in miserable weather well spent.
I forgot to mention that had I found this yesterday I would not have asked the original question, but I'm glad I did 'cos of all the friendly replies.
http://backroom.renderosity.com/~photo/Tutorials/thomas_haynes_%20spot_metering%20.pdf
Alistair

Last edited by adwb; 01-31-2010 at 11:32 AM.
02-01-2010, 07:28 AM   #14
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QuoteOriginally posted by Just1MoreDave Quote
Here is the situation that made me want to learn more about metering. I get a lens in the mail and take a test shot of a dog lying in the sun
A few quick comments.

1. Many would prefer to meter off the eyes and let the fur take care of itself.

2. A seperate spot meter is going to be much more accurate than the one built into the camera, if only because the angle of incoming light it measures is a lot less. I prefer to think of the built-in spot meter as a "region meter".

3. You would only be able to do so well in this case, unles you add in some fill flash or have a reflector handy to tip light ino the dog's face.
02-01-2010, 09:21 AM   #15
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QuoteOriginally posted by adwb Quote
So if I look for high contrast subjects , like your dog example, take several shots with the bracketing comp set to 1/3 and say 5 steps and look at the results the difference in the best image setting from the centre point image will start to give me a feel for how much exposure compensation I need to apply. with practise this will lead to a better understanding of exposure compensation.
Sort of, but not really. Shots like this are not about being able to understand up front how much compensation will be needed - that's actually going to be next to impossible, because depending on exactly how you frame the shot, the meter is likely to give wildly different exposures. Shots like this are more about understanding the relationship betwene light and shaodw and figuring out how to meter such that you are not subject to those sort of random fluctuations. You may indeed need to also apply compensation, but the real trick is doing the metering in the first palce so as to give consistent results. Eg, spot metering only off the black fur, or only the white - or "substitute metering" (metering off something else that is in the same light as your subject and is has similar value ranges).
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