understanding exposure...questions

[fwellers]

LOL.
I'm just a beginner too, so I use these threads to help shore up my understanding too. )

 

[puyjapin]

what exactly do you mean by recompose?
Also why doesnt the camera see a white card and this oh, thats a bit bright and compensate on its own?? that would be a lot easier
im joining a local photo club next week so hopefully someone will show me physically as i can never seem to get to grips with it! thanks

 

[Eldrich]

Let me just try to explain what is meant by metering, the camera 'seeing a white card' and recomposing. When you press the shutter button halfway, the camera will measure the amount of light falling on its sensor. It can do this several ways, depending on which metering mode you are in, for this i'm assuming the camera is in spot metering, which means that it will measure the amount of light coming from exactly one spot (the center) of the camera's view. Now, since the camera has no way of independently knowing what you are looking at, it assumes that whatever you are metering off of is 18% grey, a particular shade of white, that is the only way it can calibrate for the amount of light coming in. Then if you are in manual mode, it will show you how much light is getting to your sensor, for your selected aperture and shutter speed. Furthermore it will tell you how much light is getting to your shutter by comparing it to a 'normal' exposure for your chosen ISO. So if the meter reads 0, then that particular grey spot is shining exactly enough light for a perfect exposure. IF the meter reads -2, then you are getting 2 stops less light than a perfect exposure for that area, and you need a slower shutter speed, or wider aperture etc.

However, the problem is that scenes do not always have the same amount of illumination. So if you metered on a relatively darker patch in the middle of a lot of really bright areas that weren't in the area you metered, then the bright areas will become over exposed, because you set the camera to have 'proper exposure' for the darker areas.

Now, what is meant by recomposing, is that you often choose to meter off of a particular subject. Bryan peterson likes to meter off the sky a lot for example, because he has found that it seems to be reliably close to the 18% grey that the camera already assumes it is. But since you had to chose a particular place to meter, the picture may not be the composition you desired, you may have wanted to take a picture of a person on the ground, but needed to meter off the sky. So after metering, you reposition the camera (recompose the shot) and take the picture using the settings found while metering off of a more reliable (ie, the sky) area of the scene.

Hope that helps

 

[puyjapin]

You have given me something to think about and im very grateful for your indepth explanation. So the metering does not come into play until the btton is pressed halfway? When i point my d40 at different objects the metering goes up and down depending on the brightness without half pressing the shutter....but now i assume tat has nowt to do with metering?
so is it in fact possible to have a dark subject on a bright background all exposed properly in one shot?
thanks

 

[Eldrich]

Its possible that the camera is continuously metering, I'm not too sure about this, I'm not familiar with the d40's settings. If it is, you just need to be careful knowing what you are point at when you look at the reading, of course you always need to be careful about this.

If you want a dark subject and a brightish background, what you'd probably have to do is meter off of the background, then recompose the shot to fit the subject in the way you wanted it in there and take the shot.

Well, your last question requires a definition of proper. In regards to the metering, the camera will set 'proper' exposure by assuming its looking at an 18% grey card. Strictly speaking, its not possible to get that type of proper exsposure for both a darker subject and a lighter background. If the background is twice as bright as the subject, it will be so, no matter what you meter off of. If you meter off of the background, the subject will be twice as dark but the background will be 'properly' exposed. If you meter off the subject, it will be 'properly' exposed, but the background will be twice as bright.

Now, that being said, bryan peterson talks about a 'creatively correct' exposure, where you need to choose what type of shot you want, and meter off of the correct thing accordingly. If you want the subject sillohouetted, then you would meter off the background in this case, but if you wanted to see the subject clearly, you might need to meter off of it directly.

There is a lot of play between how you do this metering exactly, and that is where one of the arts of photography lie. You need to understand what the camera is doing exactly, what you want, and how to get the camera to do what you want. The sections of his book where it talks about 7 different 'proper' exposures, but only one of them being 'creatively correct' for what you want, are talking about this.

by the way, i really hope i am using the same language (proper, and creatively correct) the way he is. I think i am, and I hope its clear from my description, but just as a disclaimer, he may be using slightly different termonology.

 

[rufus5150]

Then push the shutter button half way to lock it in.

At least with my camera, that will only focus. I have to use the exposure lock button or set a custom function to tie exposure lock to the half-pressed shutter button.

 

[JMD]

At least with my camera, that will only focus. I have to use the exposure lock button or set a custom function to tie exposure lock to the half-pressed shutter button.

This is the way it is with my d50 as well. I'm going to guess that the d40 works this way as well.

 

[puyjapin]

what has locking the exposure got to do with metering off of an other area of the area before re composing?

 

[Samanax]

what has locking the exposure got to do with metering off of an other area of the area before re composing?

Depending on the lighting conditions and what you're shooting, you would want to get an exposure reading for what you want exposed correctly, lock that exposure and recompose for the shot. If you didn't lock the exposure before you recomposed, the exposure could be different and your main subject may be over or under exposed.

 

[mrodgers]

Depending on the lighting conditions and what you're shooting, you would want to get an exposure reading for what you want exposed correctly, lock that exposure and recompose for the shot. If you didn't lock the exposure before you recomposed, the exposure could be different and your main subject may be over or under exposed.

.... assuming in any mode other than manual, just to finish the thought there.

Exposure lock has EVERYTHING to do with metering off another area if you are not shooting in full manual mode.

 

[puyjapin]

so focus on subject, lock exposure then meter off something else? makes no sense to me!

 

[Eldrich]

Meter off something to get the amount of light correct, then focus on subject. If you're in some sort of automatic mode, you'd need to lock the exposure after you meter. Too bad you can't talk to someone in person, I think it could be cleared up very quickly.

 

[Samanax]

so focus on subject, lock exposure then meter off something else?

When you lock exposure you're locking the exposure settings of what the meter is getting a reading off of.

If you get a meter reading off the main subject and then lock the exposure you're telling the camera to use those exposure settings (shutter speed, aperture) so you can recompose the shot (and not worry that the meter is setting different exposure settings after you recomposed).

See this article.

 

[EOS_JD]

Although I think you know what you are doing, your explanation is really bad!

for this i'm assuming the camera is in spot metering, which means that it will measure the amount of light coming from exactly one spot (the center) of the camera's view.

This isn't true. With spot metering, the camera will only measure a very small area of the scene - normally around 1% - 9% of the frame. Not a "single spot"

In-camera metering is standardized based on the luminance of light which would be reflected from an object appearing as middle gray. If the camera is aimed directly at any object lighter or darker than middle gray, the camera's light meter will incorrectly calculate under or over-exposure, respectively.

Then if you are in manual mode, it will show you how much light is getting to your sensor, for your selected aperture and shutter speed. Furthermore it will tell you how much light is getting to your shutter by comparing it to a 'normal' exposure for your chosen ISO.

Manual, Semi Auto and P all show your exposure meter
An in-camera light meter can work surprisingly well if object reflectance is sufficiently diverse throughout the photo. In other words, if there is an even spread varying from dark to light objects, then the average reflectance will remain roughly middle gray. Unfortunately, some scenes may have a significant imbalance in subject reflectivity, such as a photo of a white dove in the snow, or of a black dog sitting on a pile of charcoal.

Selected aperture, shutter speed and ISO together make your exposure and contribute to correcting your camera's inability to make accurate decisions.

So if the meter reads 0, then that particular grey spot is shining exactly enough light for a perfect exposure.

That's not the case at all. What if there are no grey spots?

When the camera meter is centred your scene (or part of a scene of using centre weighted or spot metering) is reflecting light back that would be measured (averaged) as a middle grey.

IF the meter reads -2, then you are getting 2 stops less light than a perfect exposure for that area, and you need a slower shutter speed, or wider aperture etc.

Not necessarily. A perfect exposure may require a -2 exposure compensation. A perfect exposure can be many things depending on what you want from the scene.

If you are shooting a standard subject then yes it is likely that your shutter speed, aperture OR ISO is wrong and you can amend any of the three to correct the under exposure.

However, the problem is that scenes do not always have the same amount of illumination. So if you metered on a relatively darker patch in the middle of a lot of really bright areas that weren't in the area you metered, then the bright areas will become over exposed, because you set the camera to have 'proper exposure' for the darker areas.

I see what you are saying although it would be difficult for a newbie to understand.

Now, what is meant by recomposing, is that you often choose to meter off of a particular subject. Bryan peterson likes to meter off the sky a lot for example, because he has found that it seems to be reliably close to the 18% grey that the camera already assumes it is. But since you had to chose a particular place to meter, the picture may not be the composition you desired, you may have wanted to take a picture of a person on the ground, but needed to meter off the sky. So after metering, you reposition the camera (recompose the shot) and take the picture using the settings found while metering off of a more reliable (ie, the sky) area of the scene.

I see again what you are trying to say but it reads as gobbledygook....

Metering off the sky is NOT always ideal because generally it is much brighter than the ground. A sky is certainly not generally a middle grey. f it was then we would all meter for the sky and be happy

Generally if you set your camera meter to expose the sky correctly, your subject on the ground will be badly under exposed (without flash to expose your subject). Bryan only uses metering from the sky in very specific circumstances for a particular image.

Your example of recompose is verhy difficult to follow - last sentence would have been enough.

Not meaning to be cheecky but you need to be sure what you are writing is correct.

There are many websites with great info and probably better to redirect them. I know you have the understandingthere - just the explanation doesn't read very well and there's a little inconsistency there.

Regards
JD

 

[EOS_JD]

You have given me something to think about and im very grateful for your indepth explanation. So the metering does not come into play until the btton is pressed halfway? When i point my d40 at different objects the metering goes up and down depending on the brightness without half pressing the shutter....but now i assume tat has nowt to do with metering?
so is it in fact possible to have a dark subject on a bright background all exposed properly in one shot?
thanks

All cameras measure the light continuously and all camera meters will move around as the light changes. But when you half press the shutter you are generally locking the exposure for the scene you are shooting.

Your camera is metering all the time.