Very Basic Metering Question

[canoli]

don't know why this is taking me so long to understand but....


Let's assume you're metering an average scene with a trustworthy camera - 40D or better - and let's ignore the quirks of specific bodies for the purposes of this question please.


So an average scene, nothing predominately dark or light. When I meter this shot (regardless of mode) and the EV graph's indicator is in the middle, at zero, that tells me...what exactly?


is it saying, "Shoot at this exposure to capture the scene as close to reality as possible."


or is it supposed to tell me something else entirely?


Is the EV graph really only useful to JPG shooters?


Thanks for any thoughts you'd like to share.

[nrdavis]

There is no single, simple answer to that question but I'll try to give my best two cents.


If you're using Canon's Evaluative Metering, the camera is telling you (to it's best ability) that the indicated exposure is the one you should shoot.


The meter has made an educated guess about the incident light level based on the reflected light it sees off of surfaces that it assumes to be neutral (18%) gray. Of course in any typical scene the various surfaces the camera sees are not all 18% gray. If the image is indeed "neutral" (in that is has no predominately white or black areas) this assumption is good, and will yield a good exposure. If the scene does contain large areas of very light or very dark surfaces, the exposure may be skewed.


Evaluative metering adds one further level of complication to this matter, by dividing the viewfinder up into many small pieces and weighting their contribution to the final exposure decision. For instance, the metering zones on and around your selected focus point carry greater weight.


In all cases, the camera is trying to calculate the correct exposure based on the incident light level, by measuring the reflected light level. Another way to arrive at this exposure would be to use an incident (hand-held) light meter, which would often be more accurate because it does not take into account the reflection of light off surfaces of various reflectivity.


Sometimes, however, because of various circumstances, the incident exposure reading may not be the one you want. If you want to pull additional detail out in shadows (overexpose), or retain color in a very bright sky (underexpose). In these circumstances it is best to know what the meter is trying to do, and then decide how to compensate in the direction you want to go. Luckily, digital photography has allowed those decisions to be made based on visible results (image previews, histogram), where with film a photographer made their best judgement and held their breath.


Knowing how to compensate exposure can be very important, however, in fast-changing light or fast-action sports, where the luxury of adjusting exposure and re-shooting an image is not available.


hope this helps...

[alexniedra]

When I meter this shot (regardless of mode) and the EV graph's indicator is in the middle, at zero, that tells me...what exactly?

This tells you that the camera's exposure system is going to give you what it sees as a proper exposure level based on the chosen metering mode (evaluative, centre-weighted, spot, etc. ). For spot metering, the "proper" exposure level is based on a small area dead centre in the frame.

is it saying, "Shoot at this exposure to capture the scene as close to reality as possible."

Not always. Imagine you are at a wedding, shooting a man wearing a black tuxedo in a garden. If the camera where to meter based on the exposure level on the tux itself, the camera can get fooled into blowing the highlights of the image. Same goes for the opposite - Imagine shooting a white wedding dress. If you were to spot meter on the dress, the camera will think "Wow! Super bright! Better get a faster shutter speed..." and the image could potentially come out underexposed.





There are a few things you can do if the camera's metering does not coincide with your vision or the real light level of the scene:

• Change the exposure value to compensate for over/underexposure - Most likely the fastest and easiest way to control the camera's exposure level
• Switch to Manual mode - I frequently use trial and error when if I have a minute to set up my shot. I check what exposure level the camera will give me in Aperture Priority, and then I dial in this exposure in Manual mode. From there I can change all of the parameters as I like and adjust exposure until I find it's what I'm looking for. I frequently do this when I'm faced with a tricky exposure situation, say, a shot containing a lot of water reflection, a black suit, or a white dress.
  
• Use Flash
• Shoot HDR

I find that I use the frist two options the most. Many times, you may not be able to shoot with flash, like in a church. Or perhaps you don't have the time or a tripod to bracket exposures.

Is the EV graph really only useful to JPG shooters?

I'm pretty sure. You do mean the exposure value "line" (-2 -1 0 +1 +2), right? Not the histogram?


I always rely on the EV, even when shooting RAW.

[canoli]

Thanks a lot you guys. I've read up on it of course, and have been shooting DSLRs for a year or so, so I know the mechanics of metering, the modes, how the selected AF point figures into it (as much as Canon tells us anyway).


But the "line" - the -2 -1 0 +1 +2 in the VF. Is it really helpful at all for RAW shots? except as a rough starting point?

I'm pretty sure. You do mean the exposure value "line" (-2 -1 0 +1 +2), right? Not the histogram?

Yes, the line in the VF or on the LCD.

I always rely on the EV, even when shooting RAW.

You do? You don't rely on the histogram? But maybe you don't ETTR? I always shoot RAW as close to HL clipping as possible, so I barely look at the EV line - I'm concerned with the histogram, making sure I've got plenty of latitude to knock down the exp in post.


That's what brought me to question the whole EV line in the VF. The way I understand it, it's only good if you don't plan on bringing your shots into a PP environment - if you're shooting JPGs.


Does that make any sense?

[hpitka]

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Hello my first post here. Interesting topic. Do you use manual or any automatic mode?

[peety3]

Unfortunately, "(regardless of mode)" doesn't work here - in automatic modes, it shows you what exposure compensation you've dialed in. In manual mode(s), it's showing you that the scene is being exposed correctly (if you're at base ISO...lots of DB lessons are about to come in).

[canoli]

sorry, yeah - let's dismiss auto mode - any of the "creative" modes.


but I'm okay with what I know. I shoot RAW anyway, so the EV graph doesn't do much for me. I may be wrong - and I'd love to know what you guys think - but I go strictly by the histogram, ETTR (shutter speed/aperture adjustments - bumping up ISO only if I can't handhold the shot) and then do what I want in post.


Once I start shooting JPGs I may need to understand what the meter is showing, but till then I won't worry too much about it I guess.


Thanks All!

[hpitka]

I meant do you use any automatic mode like P, TV, AV? Because When I'm using AV I just take test shot and look at the histogram, but in manual mode I have to use this &ldquo;EV graph&rdquo; (build in light meter) before first shot. Right?

[peety3]

sorry, yeah - let's dismiss auto mode - any of the "creative" modes.


but I'm okay with what I know. I shoot RAW anyway, so the EV graph doesn't do much for me. I may be wrong - and I'd love to know what you guys think - but I go strictly by the histogram, ETTR (shutter speed/aperture adjustments - bumping up ISO only if I can't handhold the shot) and then do what I want in post.

Obviously there can be a big difference between the meter and the histogram. However, there should be a relationship between the meter and the histogram (such that if you boost the meter by a stop, the histogram will shift right by a stop), and it can be a useful gauge when shooting in manual.


The other comment I felt compelled to make is that your ETTR/ITTR strategy is good, but do leave yourself flexibility to ITTR if you want a fast shutter speed. You probably do this anyway, I just know that I'm having to remind myself that wide-open apertures (and the corresponding fast shutter speeds) aren't always the way to go (I shot some kids on zip lines Saturday, and learned that 1/60th was the answer).

[canoli]

but do leave yourself flexibility to ITTR if you want a fast shutter speed.

You are so right, and I appreciate the reminder. For the first 10 months shooting DSLR photos I resisted bumping up the ISO - almost never shot above 200. Naturally I had to throw away tons of shots because of blur. Painful, but I kept doing it, thinking all I needed was a better shooting technique. I was so preoccupied with the dreaded noise I just couldn't bring myself to "ruin" a shot using a higher ISO.


Thankfully I learned (here) it wasn't the high ISO, it was the lack of light causing the excessive noise. Once I got past that I found ISO 800 and even 1600 weren't bad at all, they just need a proper exposure.