Metering - what is it and how do I use it? :)

EdinburghGary

EdinburghGary

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Sorry, I know Google is only a searchbox away, but I love the way some of you guys lay things out in simple terms anyone can understand!!!

Anyway I took lots of shots today, and the cloudy white sky has been well and truly lost in a bight white light (over exposed?), even though the ground levels look slightly under exposed.

Anyway, I keep hearing metering mentioned, and some people say meter the sky, others say meter the ground.

I am so very confused.
 
Back to basics.

The sensor in the camera has limits on the brightest and darkest parts of the scene it can record - this is called the dynamic range. Anything too bright will be white, anything too dark will be black. On most DSLR's this range is around 8 stops.

If you meter for the sky you might get a reading of 1/8000s @ f/8, ISO 100.

If you then meter for the ground you might get a reading of 1/125s @ f/8, ISO 100.

Now the difference between the two readings in this example is just the shutter speed, it's gone from 1/8000s to 1/125s which in terms of stops is:

1/8000, 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125

Which means the ground is 6 stops darker than the sky.

If you expose for the ground then the sky will be 6 stops over exposed. If you expose for the sky the ground will be 6 stops under exposed. That's the average reading, the brightest part of the sky to the darkest part of the ground will be well beyond the dynamic range of the sensor so you have to sacrifice one or the other.

What you meter for depends on the shot and what you'd prefer to have the most detail. Do you want to keep more detail in the sky or the ground or somewhere between the two?

A common solution to this particular problem is to use a ND grad filter which cuts back the light a certain number of stops on the top and gradually fades off towards the bottom. This lets you balance the exposure between sky and ground so the two are much closer to the same setting.

In other situations (tho not a landscape shot) you could use flash to balance things out, say a shot of a person with a sunset behind them. You expose for the sky and let the flash light up the person.

Sometimes it's just a case of choosing to expose for the highlights or the shadows. Back when I learned B+W printing I was always told expose for the whites and let the blacks take care of themselves and it's kind of stuck with me. There's no real right or wrong answer tho, whatever choice you make someone will suggest you should have done the opposite :nuts: :lol:
 
Thanks for that - now I know what a ND Grad filter is used for :)
 
I have a cokin which I think reduces light by 8 stops. Presume I should have multiple ND Grads for different levels of this problem? I probably should have bought 4 x 2 stops, then I could put them in one at a time?
 
Andy could you elaborate on your line expose for the whites and let the blacks take care of themselves please , do you mean when printing or taking the picture ?
 
EdinburghGary said:
Blinkies?
Click to expand...

In the menus for your camera you should find a setting which causes any over-exposed parts of preview images to flash continuously. It tends to err on the side of caution - but that's no bad thing - it's a useful function.
 
donutagain said:
Andy could you elaborate on your line expose for the whites and let the blacks take care of themselves please , do you mean when printing or taking the picture ?
Click to expand...

Both really. It's probably more important with B+W to get good solid blacks which is a by product of getting the whites right. If you expose for shadow detail things can get a bit murky elsewhere.

Of course a lot of this can be fixed digitally now but I've been doing it for 20 odd years now so it's automatic for me to get the highlights right.

Having said that I will often let highlights blow out because I don't consider the highlights to be the brighest part of the overall scene. I expose for the highlights I want to be correct if that makes sense.
 
Sorry catching up with the rest of the thread. The blinkies are great for checking what's blown...

I should add that by printing I meant the wet process too :thumbs:
 
well from a black and white film point of view i would say the opposite is true ,expose for the shadows , if you expose for the white you would get a thin neg in the shadow areas ,but at least if you expose for the shadows you get a more dense negative and can at least burn in some of the detail in the whites . a thin neg would have no detail in it so not a lot to be got out of it . or am i missing something ?
 
Yeah indeed, its worth looking at HDR after you've taken in pxl8's excellent explanation for metering... I have to say though, I always though it was a little better to err towards overexposing a little bit if you have to go one way simply to keep noise levels in check - you can always darken a picture a bit on your PC (obviously getting it right first time is better detail-wise) but if you take 3 bracketed shots then at least you can cover all bases.

see http://www.hdrsoft.com/resources/tutorial_basic/index.html
 
donutagain said:
well from a black and white film point of view i would say the opposite is true ,expose for the shadows , if you expose for the white you would get a thin neg in the shadow areas ,but at least if you expose for the shadows you get a more dense negative and can at least burn in some of the detail in the whites . a thin neg would have no detail in it so not a lot to be got out of it . or am i missing something ?
Click to expand...

Exposing for the whites does lead to a thin neg, that's the reason why the saying is "let the blacks take of themselves". It's a personal choice but IMO B+W looks better if you hold the detail in the white and have good solid blacks. For me there's nothing worse than a B+W print with wishy washy blacks - all the depth and dimension goes. With a dense neg getting rid of that effect can create a lot of dodging work to protect the whites - the longer you expose to get the blacks the more the highlight areas will suffer. Of course it's a lot easier with digital to fix this and you don't need to worry about repeating the process for each print ;)

This shot:

web_4910.jpg


demonstrates how I expose for the highlights. The fur has kept the detail in the highlights but the darker areas on the subject still have detail - it's only the true shadow areas that have fallen into a good solid black. If you copy/paste the shot into PS and invert it you'll see it's a "thin neg" but the detail is still there in the right places (for me anyway).
 
I usualyset my camera's exposure compensation to -0.3, always better to underexpose then over expose IMO, you can recover lost deail in areas that are a tad too dark, but recovering them from blown out areas is much harder.
 
i believe with digital that to be the case , as chris says above , once you ve blown the highlights they ve gone , but i was thinking in film terms . by the way andy nice pusscat pic , but i would say that it was just a good exposure as oppposed to metered for the bright areas .
 
Lovely shot pxl8... I think I'll probably be trying to emulate that at some point today - I just need to steal one of the neighbours cats ;)

Chris, I agree if it's blown out in the highlights too much then its lost - I just mean err towards the upper end of the histogram :)
 
and could i also add , like you i prefer a b+w pic with good solid blacks , i like a lot of contrast , unlike some you see with hardly any blacks or whites just greys in the middle , , thats why i like those bright sunny ( winter ) days , with plenty of swadows .
 
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