Metering/Exposure

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Dan1502

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I have a Canon 50D and have just been trying out a new lens and polarizing filter in the back garden and whilst doing so was trying to control which part of the image the camera uses to meter.

I understand the importance of picking the right focus point but am not sure of how to lock the exposure/and or metering.

I tried pointing the chosen focus point at the area I wished to meter then pressed the * button and held it then recomposed the shot and took the picture.

I have the camera set to centre weighted average I think. Should I have it set to spot metering? when is the metering point linked to the chosen af point? etc etc

I have a bit of an understanding but want to be sure of what I'm doing.
 
What I find useful, maybe you wont...
I use back button to autofocus (40d) and the half press on the shutter for AE lock. Means I can meter on anything I want, half press, recompose to what I want to shoot, dab the back button to focus (after choosing the AF point first, not enough fingers) and then click.
I do a lot of handheld of landscape type shots so I find it useful to (usually spot meter) on the sky or buiilding or grass etc. so that I know my subject matter is correctly exposed.
 
I thought that if you used spot metering, it meters from the centre point of the viewfinder?

I tend to use spot metering most of the time.

It depends how you shoot. I usually shoot in Av. With spot metering, I usually pick a point in the scene i'm looking to shoot which I want to be exposed properly, and hit the * button. Then recompose & focus etc.

That way, say you're photographing a sunset and want the sky exposed properly, you meter off the sky and lock it, then shoot. Likewise, if I want the foreground exposed properly I'll meter off that, lock it, then recompose. Unless you want everything perfectly exposed, but then you're getting in to ND filters.

I find i'm shooting in manual more now though, which helps to keep things consistend once you've metered.

Sorry if this is drivel!
 
Mat, I think that was what I was thinking. If that is the case then I could set the AF point appropriately for the composition then point the centre at the area I want to spot meter then press the * button (and keep it depressed?) then recompose and shoot.

Edryn, is this a similar alternative. I guess you use spot metering too, then half press with the centre on the metering area (though I'll need to check if the metering point is always the centre or matches the AF point) then recompose and focus using the back focus button then shoot.

I did watch a Canon AF video seminar which explained a lot, including customising the back buttons but it's quite hard to get your head round. I could do with wathcing it again with this thread and the camera manual in front of me (and the camera) - or alternatively perhaps someone can confirm the above.
 
Metering and focusing are basically independent functions, but some cameras, in some modes, do relate the two - working on the assumption that you have focused on an important area of the subject which needs to be properly exposed, so there is some bias towards that.

What I think you are describing is spot metering, which reads the centre of the frame only - usually the central circle. It needs to be used with care and understanding though as it will simply read that area and interpret it as mid-grey, which may not be appropriate. Most people use spot metering to select a highlight area, then modify the setting (say by increasing exposure by three stops) and set that on manual. It's a way of pegging important highlights to a certain exposure value, which suits some subjects and/or your preferred way of working.

Bear in mind that there is only ever one optimum exposure setting for a given situation and it makes no difference how you arrive at it - Av, P, manual, spot, evaluative or whatever. It's just a preferred way of working. Personally I almost always use evaluative/matrix which will get you close even if not quite optimum in some situations, then tweak according to the LCD image and histogram, with blinkies enabled (highlight over exposure warning in the menu). See handbook.
 
HoppyUK said:
Metering and focusing are basically independent functions, but some cameras, in some modes, do relate the two - working on the assumption that you have focused on an important area of the subject which needs to be properly exposed, so there is some bias towards that.

What I think you are describing is spot metering, which reads the centre of the frame only - usually the central circle. It needs to be used with care and understanding though as it will simply read that area and interpret it as mid-grey, which may not be approrpraite. Most people use spot metering to select a highlight area, then modify the setting (say by reducing exposure by three stops) and set that on manual. It's a way of pegging important highlights to a certain exposure value, which suits some subjects and/or your preferred way of working.

Bear in mind that there is only ever one optimum exposure setting for a given situation and it makes no difference how you arrive at it - Av, P, manual, spot, evaluative or whatever. It's just a preferred way of working. Personally I almost always use evaluative/matrix which will get you close even if not quite optimum in some situations, then tweak according to the LCD image and histogram, with blinkies enabled (highlight over exposure warning in the menu). See handbook.
Click to expand...

This all makes sense. It's just a case of me getting to understand what the camera does in each mode so that I know how to get what I want.

Just to make sure I'm correct, when we're talking exposure and not aperture, a stop one stop down is twice the exposure time and two stops is four times etc (just like one stop down in aperture doubles the amount of light)?
 
Dan1502 said:
This all makes sense. It's just a case of me getting to understand what the camera does in each mode so that I know how to get what I want.

Just to make sure I'm correct, when we're talking exposure and not aperture, a stop one stop down is twice the exposure time and two stops is four times etc (just like one stop down in aperture doubles the amount of light)?
Click to expand...

Yes, you've got it.

A 'stop' refers to any halving or doubling of exposure. It's an old term originally relating to apertures, but is now universally applied to all aspects of exposure.

F/2.8 is double f/4, which is double f/5.6 etc etc. The confusing bit is the relationship which is the square root of two, ie 1.414.

The rest is easy - 1/125sec is double 1/250sec. And ISO200 is half as sensitive as ISO400 etc etc.

From which it becomes clear that 1/125sec at f/4, ISO200, gives the same exposure as 1/250sec at f/5.6, ISO800.
 
HoppyUK said:
Yes, you've got it.

A 'stop' refers to any halving or doubling of exposure. It's an old term originally relating to apertures, but is now universally applied to all aspects of exposure.

F/2.8 is double f/4, which is double f/5.6 etc etc. The confusing bit is the relationship which is the square root of two, ie 1.414.

The rest is easy - 1/125sec is double 1/250sec. And ISO200 is half as sensitive as ISO400 etc etc.

From which it becomes clear that 1/125sec at f/4, ISO200, gives the same exposure as 1/250sec at f/5.6, ISO800.
Click to expand...

I struggled with the last one for a second but I get it!
 
I'm having serious issues understanding the * button. Hopefully someone can help clear it up for me :(

This is with my 50D out of the box settings.

I understand the AF on button I think, I press it and it focuses for me much like the half press of the shutter button.

I try to spot meter off the side of say the sun. Press the * button but I get no indication it has locked exposure and when I recompose the light meter hasn't locked and the camera refocuses. I'm presuming you press the * once and don't have to hold it, that doesn't work for me either.

I'm sure I have either miss understood this function or have a set up to do, any help clearing this up?

The whole recompose subject even watching the canon AF videos still has me confused.
 
dannyjo22 said:
I'm having serious issues understanding the * button. Hopefully someone can help clear it up for me :(

This is with my 50D out of the box settings.

I understand the AF on button I think, I press it and it focuses for me much like the half press of the shutter button.

I try to spot meter off the side of say the sun. Press the * button but I get no indication it has locked exposure and when I recompose the light meter hasn't locked and the camera refocuses. I'm presuming you press the * once and don't have to hold it, that doesn't work for me either.

I'm sure I have either miss understood this function or have a set up to do, any help clearing this up?

The whole recompose subject even watching the canon AF videos still has me confused.
Click to expand...

Are you doing this on manual? Doesn't do anything on manual.

I think this might be a case of RTFM :D You've just got to learn what does what, and when it doesn't. But it can get confusing and I often think that some of the multiple functions we've got create more problems than they solve.

This is a very good tutorial about customising the functions of the AF-ON and lock* buttons http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=2286 It's more about AF but might help you sort things out.
 
Thanks Hoppy I will have a read of the link. I was trying it on manual. Tbh I have read a fair bit on it but always get more confused the more I've read.

Why does it not work on manual, is it because you meter from where you like and recompose and just ignore the meter reading.
 
dannyjo22 said:
Thanks Hoppy I will have a read of the link. I was trying it on manual. Tbh I have read a fair bit on it but always get more confused the more I've read.

Why does it not work on manual, is it because you meter from where you like and recompose and just ignore the meter reading.
Click to expand...

The exposure lock doesn't work on manual because because it's already locked ;) Think of manual as 'set & lock' and it makes more sense.

Personally, I think using manual settings is not the best way to learn. It seems to cause a lot of problems and it's easy to go very wrong, especially if you start fiddling with spot metering which many people seem to think is some kind of magic bullet for wonderful exposure control.

It's not. It's mainly an old film technique, one of several methods for getting correct exposure when you needed all the help you could get because you had no idea if it was going to be right until the film was processed.

With digital, I just let the camera do the work - or as much of it as I can, by pre-selecting the most important parameter for the picture eg Av or Tv etc. Then let the camera do the fiddling for me and chimp the result. The LCD, with blinkies enabled (over exposure warning) and the histogram are an absolute indicator of what exposure you have actually got, as opposed to what you hope you might get by messing about with spot/incident/matrix/whatever metering etc.

My advice to newcomers is let the camera do it for you, as it will never be too far out. But you must then make sure you check the settings it's come up with, compare them to the result you've got, and understand what's happening.

That way I think you will get along okay with almost zero knowledge, get encouragement from some half decent results, then learn to tweak and enhance them as you gain knowledge and confidence. The alternative is to set everything on manual, make a lot of mistakes and hope you learn from them before you give up! ;)
 
Thanks again for the advice Hoppy, I'm very happy with exposures I've been getting in manual. Those two buttons Af on and * had just been giving me much confusion. That link you provided cleared it up a lot better than the user guides I had been reading.

Thank you.
 
HoppyUK said:
The exposure lock doesn't work on manual because because it's already locked ;) Think of manual as 'set & lock' and it makes more sense.

Personally, I think using manual settings is not the best way to learn. It seems to cause a lot of problems and it's easy to go very wrong, especially if you start fiddling with spot metering which many people seem to think is some kind of magic bullet for wonderful exposure control.

It's not. It's mainly an old film technique, one of several methods for getting correct exposure when you needed all the help you could get because you had no idea if it was going to be right until the film was processed.

With digital, I just let the camera do the work - or as much of it as I can, by pre-selecting the most important parameter for the picture eg Av or Tv etc. Then let the camera do the fiddling for me and chimp the result. The LCD, with blinkies enabled (over exposure warning) and the histogram are an absolute indicator of what exposure you have actually got, as opposed to what you hope you might get by messing about with spot/incident/matrix/whatever metering etc.

My advice to newcomers is let the camera do it for you, as it will never be too far out. But you must then make sure you check the settings it's come up with, compare them to the result you've got, and understand what's happening.

That way I think you will get along okay with almost zero knowledge, get encouragement from some half decent results, then learn to tweak and enhance them as you gain knowledge and confidence. The alternative is to set everything on manual, make a lot of mistakes and hope you learn from them before you give up! ;)
Click to expand...

Having just read this forum,several times the penny just dropped about the metering,cheers for that hoppyuk:thumbs::clap:
 
HoppyUK said:
Yes, you've got it.



From which it becomes clear that 1/125sec at f/4, ISO200, gives the same exposure as 1/250sec at f/5.6, ISO800.
Click to expand...

Does it!

Or should it be iso 1600 ?

Stew
 
why not ? You are going 3 stops with F number 3 stops with shutter speed but only 2 with iso

Stew
 
stewsmith said:
why not ? You are going 3 stops with F number 3 stops with shutter speed but only 2 with iso

Stew
Click to expand...

No bud. One stop of shutter speed and one stop of f/number, balanced by two stops of ISO.

Where do you get 3 + 3 stops from? :thinking:
 
125 - 160 - 200 - 250
F4 - F4.5 - F5 - 5.6
200 - 400 - 800
 
stewsmith said:
125 - 160 - 200 - 250
Click to expand...

Are third stops. Total one stop.

F4 - F4.5 - F5 - 5.6
Click to expand...

Are third stops. Total one stop.

200 - 400 - 800
Click to expand...

Are whole stops. Total two stops.

A 'stop' is any halving or doubling of exposure, eg 1/125 sec to 1/250sec, ISO200 to ISO400.

F/numbers are related by the square root of two, ie 1.4. Therefore f/4 x 1.4 = f/5.6.
 
Sorry to have hijacked the thread, I get it now. Thanks for the explanation. I must admit I had just woke up after a 17 hour shift.

Stew
 
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