Light meter indicating a correct exposure

batmura · Oct 22, 2012

I am a total beginner with a Nikon d3100 and I have a question. Sorry if this sounds dumb but I really need to understand this. I have been reading Bryan Peterson's Understanding Exposure lately and I really enjoy his explanations about the photographic triangle, but I am still not clear on one thing. Whenever he explains the logic behind one of his shots, he says something along the lines of the camera's light meter indicating a "correct" exposure or the light meter indicating 1/250 sec as the correct exposure -- I would like to know where this crucial piece of information is displayed. Owning a good entry level camera I think the viewfinder should tell me? I have been trying the manual mode a lot these days and fiddling with the settings a lot. I am getting a lot of under and overexposed shots which I believe is the reason why I still cannot figure out where and when the camera indicates the correct exposure.

I will appreciate any detailed AND simple explanations on this one because I cannot proceed with the book without understanding this key info otherwise.

Thanks!

tirediron · Oct 22, 2012

This will be covered in your camera's manual (You did read it, right?). The meter display is the series of bars at the bottom of your viewfinder. The '0' in the middle is "correct" exposure, according to the camera and the number of bars illuminated either to the left or right indicates a varying degree of under/over exposure.

480sparky · Oct 22, 2012

A proper exposure is any combination of shutter speed, aperture and ISO that will both prevent the highlights from being blown out while retaining details in the shadows.

What Bryan means by 'correct' exposure is choosing from the combinations of ss/Ap/ISO to get the results you want. For instance, if you're shooting fast-action sports and want to 'freeze' the action, you don't want to use 1/30 sec. at f/22 at ISO 400. You would more likely want to open your aperture to f/5.6 so your shutter speed can be changed to 1/500. If DOF will be an issue at f/5.6, then raise your ISO to 1600 and that will allow you to close the aperture to f/11 for more DOF. Each will give you a proper exposure for the image, but only one will give you the results you desire.

Now, given the exact same lighting, you certainly wouldn't want to use 1/500 sec to create a dreamy waterfall look, would you? No. You'd want to keep the ISO as low as possible (100), close the aperture to minimum (f/22) so you could use as long a shutter speed as possible.... in this case, 1/4 sec. If that's not long enough, you get out an ND filter so you can slow the shutter down even further. Again, each will give you a proper exposure for the image, but only one will give you the results you desire.

TCampbell · Oct 22, 2012

When you look through the camera's viewfinder, you'll notice there's an LCD display with lots of information just below the image. In the center of that information is a small scale with a "0" in the middle and a "-" and "+" side to the scale.

You adjust the exposure settings on your camera (shutter speed or aperture or even ISO) so that the meter shows the exposure at the "0" mark in the center. This means that based on the light meter in the camera, you will get the correct exposure. You can deliberately over or underexpose a shot if you want... and sometimes you'll need to do this.

The meter in the camera is a "reflected" light meter. That means it can only meter the light which is reflecting off of your subject or scene and into the camera's metering system. Sometimes that wont be accurate. The true and correct exposure is based on the amount of light falling on your subject... not the amount of light reflecting off of your subject. But there's no way to meter light "falling" on a subject without walking up to the subject and using a meter that records the amount of light landing on that location. A hand-held "incident" light meter does this. A camera makes an assumption about the average reflectivity of objects. Obviously not all things reflect the same amount of light and bright white colors reflect more than dark black colors. Hence, by YOU being aware of this, you can choose to deliberately increase or decrease the exposure when you realize that the meter is probably not reporting the correct amount of light at the scene.

Most of the time you can trust the meter in the camera (there's a reason this solution has worked so well for so many years). But just occasionally it will not -- be aware that it's not fool-proof.

SCraig · Oct 22, 2012

batmura said:
I am a total beginner with a Nikon d3100 and I have a question. Sorry if this sounds dumb but I really need to understand this. I have been reading Bryan Peterson's Understanding Exposure lately and I really enjoy his explanations about the photographic triangle, but I am still not clear on one thing. Whenever he explains the logic behind one of his shots, he says something along the lines of the camera's light meter indicating a "correct" exposure or the light meter indicating 1/250 sec as the correct exposure -- I would like to know where this crucial piece of information is displayed. Owning a good entry level camera I think the viewfinder should tell me? I have been trying the manual mode a lot these days and fiddling with the settings a lot. I am getting a lot of under and overexposed shots which I believe is the reason why I still cannot figure out where and when the camera indicates the correct exposure.

I will appreciate any detailed AND simple explanations on this one because I cannot proceed with the book without understanding this key info otherwise.

Thanks!

The viewfinder DOES display it, as does the information display on the back of the camera. For the information display see the diagram on page 6 of the D3100 user's manual (items 2, 3, and 17) and for the viewfinder see page 10 of the D3100 manual (items 4 and 5)

Big Mike · Oct 22, 2012

I have been trying the manual mode a lot these days and fiddling with the settings a lot. I am getting a lot of under and overexposed shots which I believe is the reason why I still cannot figure out where and when the camera indicates the correct exposure.

If you are using manual mode and changing your settings to get you to 'zero'...then yes, I would expect that you would get a fair amount of under and overexposed shots. The same thing happens when in the auto modes, because all they do is get you to zero. I haven't read Peterson's book, but when he says that the camera is indicating correct exposure...he's probably talking about getting to zero.

Now, it's important to note that cameras are calibrated so that zero will likely be correct for the 'average' photo. If your scene (or what you meter on) is brighter or darker than what we call 'middle grey', then zero will not be a correct exposure.

In other words, camera are designed to get exposures wrong....but they do it in predictable ways, so we can compensate for it...once we know how.

They key is to realize that the camera will always try to make the scene come out middle grey (tone)...if you follow what it tells you (getting to zero). So you need to realize that and know when to go above zero and when to go below it.

To put it another way, when you shoot/meter on something bright, the camera will try to underexpose it, so you have to be above zero. If you shoot/meter something dark, the camera will try to make it brighter and you have to be below zero to compensate for it. So try to remember that...bright means you need to be +zero and dark means you need to be -zero.

Knowing how far to go will come with experience.

Alternatively, check out what a grey card can do. How to use a Grey Card ~ Mike Hodson Photography

jwbryson1 · Oct 22, 2012

MLeeK · Oct 22, 2012

Well, for the most part correct is assumed to be 0 on your meter, however you have to understand how all of the elements of metering come together to create 0 on your camera's meter AND have it be correct exposure.
I'm pretty sure I've pasted this tutorial on here a dozen or so times, but I can't find it for the life of me... This is also using the names Canon gives for it's metering modes. Someone will have to give you the correct terms for Nikon because I can't remember off hand what they are. They are similar and I know you can guess pretty accurately which one is which, but if you have questions ask.

In camera meters don't measure the light in your scene. It's not like a scale or a tape measure that measures the ACTUAL LIGHT in any quantitative measurement. It measures what the reflected light from your subject looks like. A light meter actually measures the LIGHT of your scene and tells you what proper exposure settings should be. It's a ruler for light.

Now you are saying... um... ok.... what is the difference? Let's use snow as an example: A foot of light fluffy snow could weigh 1 ounce. A foot of heavy wet snow would weigh more like 1 pound. They both APPEAR the same-it's a foot of snow!!! But in reality one is heavier. It's the same with light. Things can look the same, but they aren't.

Your camera's meter takes that reflection off of whatever you are metering-could be snow, could be a black backdrop. No matter what it is, your camera tries to make that reading equal middle of the road or middle gray on the light to dark scale. (This is changing in newer cameras, but the basics always will hold pretty true) The camera doesn't know if you are metering a black scene or a white scene. It only knows that average exposure should be middle gray.

Take a look at a black and white photo.

If I had metered off her dress it would have made her dress middle gray... This color:

Her dress would be really underexposed at that color. If I metered the background the camera would have tried to make that middle gray. Her dress would have been blown out. On this particular image I probably would have metered skin if I wanted something to be middle gray.

Now that you know that much, you also have to consider what the metering modes are using your scene and trying to average to middle gray.
Evaluative metering is measuring the whole scene. Sky, water, child playing in the sand... EVERYTHING and it's trying to make it average middle gray. Just like a grade in school it adds all of those pixels up and tries to equal middle gray when it divides them out. On the beach with the sky and water? Probably not going to be extremely accurate. For example this photo of Haley on the beach.

Convert it to grayscale and look at the bright and dark of it:

If I average the pixels out this is the gray they come out.

Compare that to middle gray and it's a lot brighter tone-and there was a lot of dark stone in that one.
If you're on a white sandy beach on a bright day your gorgeous child playing in the sand is probably going to be drastically underexposed. You have lots of bright sky pixels, lots of bright sand pixels and a few dark ocean and child pixels... The bright ones win and hold the average up. In high contrast situations like my beach example evaluative doesn't really work well.
Consider an average setting like maybe your living room (assuming your walls are fairly light) with your kids playing? It might work really well!

Center Weighted average metering takes all of the pixels into consideration, but like a mid term and final test is worth more points than any other test, so are the pixels in the center of your scene. It averages all of them together and counts the center ones multiple times to make them more important than the outer pixels. It would sure help with that beach scene to make the pixels that were of your child more important. Closer at least!!!

Then there is Spot Metering. Your meter only evaluates the pixels you put your SPOT on. This works well if you can train yourself to look at the scene in terms of light and dark and decide what you want to be the middle tone. White, non-african/asian/indian etc skin is not usually middle gray It's much lighter. Think of when you convert to black and white. Is skin ever that color ^ up there? Nope. I find that your average grass is a good middle gray outside. My light oak hardwood floors indoors are a good middle of the road. For the image of Haley I would have metered Haley's skin on the back of her arm and then set my exposure to about +1 or JUST before the skin would blow out.

So, now that you understand metering a little better and have a better friendship with your meter it'll be easier to make it do what you want it to do.