Question on Exposure


kurby

New Member
Dec 19, 2004
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#1
Hi, I am very newbie in DSLR. Got myself a DSLR and have been using it.

I am just wondering - there is a metering in the DSLR to give us the exposure, whether under or over expose....then we will set either aperture, shutter speed, or ISO to get the correct exposure.

My questions are:

1. Do we set exposure different from what the metering tell us? I suppose the answer is yes, but under what circumstances?
2. For indoor shoots, if aperture is already set to wide open, and still under expose, the next logical step is to increase ISO or to increase lighting (through flash)? And how much ISO to increase then depends on the metering, i.e. will increase until the metering shows proper exposure? Right?
3. Is the setting of Aperture, speed and ISO more for for the special effects in photos, such as bokeh?

Pardon me for the silly questions above....
 

Rashkae

Senior Member
Nov 28, 2005
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#2
kurby said:
Hi, I am very newbie in DSLR. Got myself a DSLR and have been using it.

I am just wondering - there is a metering in the DSLR to give us the exposure, whether under or over expose....then we will set either aperture, shutter speed, or ISO to get the correct exposure.

My questions are:

1. Do we set exposure different from what the metering tell us? I suppose the answer is yes, but under what circumstances?
2. For indoor shoots, if aperture is already set to wide open, and still under expose, the next logical step is to increase ISO or to increase lighting (through flash)? And how much ISO to increase then depends on the metering, i.e. will increase until the metering shows proper exposure? Right?
3. Is the setting of Aperture, speed and ISO more for for the special effects in photos, such as bokeh?

Pardon me for the silly questions above....
1. When the Camera is reading an incorrect exposure of course. For example, taking a picture of a white dress may trick the camera and make it underexpose the shot. In this case you would compensate with higher exposure.

2. Yup. But note that flash has a limited range, increasing ISO is better.

3. Nope. Bokeh is not a special effect. Bokeh does not mean blur.


I have 2 tips:
1. Your questions seem to indicate you are shooting full manual. Why? You can try learning from other modes too.

2. Read the photography notes for newbies stickies. Your questions are very basic and are covered there.
 

kurby

New Member
Dec 19, 2004
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#3
1. When the Camera is reading an incorrect exposure of course. For example, taking a picture of a white dress may trick the camera and make it underexpose the shot. In this case you would compensate with higher exposure.

2. Yup. But note that flash has a limited range, increasing ISO is better.

3. Nope. Bokeh is not a special effect. Bokeh does not mean blur.


I have 2 tips:
1. Your questions seem to indicate you are shooting full manual. Why? You can try learning from other modes too.

2. Read the photography notes for newbies stickies. Your questions are very basic and are covered there.
Thanks Rashkae....

Nope I was shooting in "A" and "P"....so I was at an indoor playground and aperture was wide open all the way....and yet still underexposed....so I turn on the flash and fire off....and notice still underexposed...that's where I notice the metering :)P still learning the camera...haha probably need to read the manual again...), so I turned to Auto and fired off and see what settings the camera used...ISO was 2800...that's where I started to bump up the ISO bit by bit and observe the metering...only when ISO was bumped up to more than 2000 then the metering showed proper exposure....so that made me wonder why I dun put to auto if at the end of the day I am changing the settings to what the camera metering wants me to do....

I read the stickies some time back...so will revisit it again....

So the whole idea of changing aperture, etc is to correct situation where we think the camera didn't read the correct exposure?
 

Rashkae

Senior Member
Nov 28, 2005
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#4
kurby said:
Thanks Rashkae....

Nope I was shooting in "A" and "P"....so I was at an indoor playground and aperture was wide open all the way....and yet still underexposed....so I turn on the flash and fire off....and notice still underexposed...that's where I notice the metering :)P still learning the camera...haha probably need to read the manual again...), so I turned to Auto and fired off and see what settings the camera used...ISO was 2800...that's where I started to bump up the ISO bit by bit and observe the metering...only when ISO was bumped up to more than 2000 then the metering showed proper exposure....so that made me wonder why I dun put to auto if at the end of the day I am changing the settings to what the camera metering wants me to do....

I read the stickies some time back...so will revisit it again....

So the whole idea of changing aperture, etc is to correct situation where we think the camera didn't read the correct exposure?
If you are in aperture mode, the camera will automatically adjust shutter speed and iso to achieve what the meter says is correct. You should read your manual, specifically the section on exposure compensation.

Aperture, shutter speed and ISO are all part of the exposure triangle and have different purposes. Read your manual and the newbies guide stickies to understand more.
 

kurby

New Member
Dec 19, 2004
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#5
Oh..correction....I was in P mode when I was shooting at the indoor playground...I should have gotten the same results as Auto if I was in A mode..am I right?

I understand the aperture, shutter speed and ISO....now am wondering what's the purpose of exposure compensation? Why do we adjust the exposure compensation instead of adjusting say the aperture, shutter speed or ISO? Unless we have reach the limit of a particular variable, e.g. aperture already fully open....then why did we use exposure compensation but not increasing ISO?
 

SkyStrike

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Staff member
Nov 29, 2010
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#6
Oh..correction....I was in P mode when I was shooting at the indoor playground...I should have gotten the same results as Auto if I was in A mode..am I right?

I understand the aperture, shutter speed and ISO....now am wondering what's the purpose of exposure compensation? Why do we adjust the exposure compensation instead of adjusting say the aperture, shutter speed or ISO? Unless we have reach the limit of a particular variable, e.g. aperture already fully open....then why did we use exposure compensation but not increasing ISO?
- P (Program) and A (I assume auto?) mode may give you same results... But the diff besides the result is that for P mode, you can tweak the parameters while auto cannot...

- For Exposure Compensation (EV Compensation), it's actually to tweak the parameters as to what the camera have decided for you based on the metering mode selected. I think it's only available when you are in Aperture, Shutter Priority, (not sure about program). The camera based on the metering, will give you a "recommended" setting, and if after taking the shot, you felt that it's either under/over exposed, you can use this EV+- to get the "correct" result you want. But you cannot exceed the maximum parameters your camera can perform..

Of course you can adjust the aperture, shutter speed, ISO, since EV essential is tweaking the same thing...
 

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kurby

New Member
Dec 19, 2004
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#7
Thanks Skystrike....A mode is Aperture...since in A mode the aperture was wide open and in auto mode the aperture was also at it's largest, then I should technically get the same results in both A mode and Auto mode...

So when do we use EV when we could be adjusting aperture, shutter and ISO?
 

SkyStrike

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#8
Thanks Skystrike....A mode is Aperture...since in A mode the aperture was wide open and in auto mode the aperture was also at it's largest, then I should technically get the same results in both A mode and Auto mode...
to what I know, yes..

So when do we use EV when we could be adjusting aperture, shutter and ISO?
I personally find changing EV faster than adjusting aperture/shutter speed/ISO. But some people may find turning the dials easier...
 

Rashkae

Senior Member
Nov 28, 2005
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#9
kurby said:
So when do we use EV when we could be adjusting aperture, shutter and ISO?
If, let's say, you are in P mode and adjust shutter speed, the camera will try to change another setting to get the same exposure as before.

When you use EV, you are telling the camera "look, I know you think this is right, but I actually want it brighter / darker"
 

thoongeng

Senior Member
Jan 26, 2010
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#10
Thanks Rashkae....

Nope I was shooting in "A" and "P"....so I was at an indoor playground and aperture was wide open all the way....and yet still underexposed....so I turn on the flash and fire off....and notice still underexposed...that's where I notice the metering :)P still learning the camera...haha probably need to read the manual again...), so I turned to Auto and fired off and see what settings the camera used...ISO was 2800...that's where I started to bump up the ISO bit by bit and observe the metering...only when ISO was bumped up to more than 2000 then the metering showed proper exposure....so that made me wonder why I dun put to auto if at the end of the day I am changing the settings to what the camera metering wants me to do....

I read the stickies some time back...so will revisit it again....

So the whole idea of changing aperture, etc is to correct situation where we think the camera didn't read the correct exposure?
The 'exposure' may still be underexposed if the camera has reached it's maximum 'auto' settings, like the aperture is already is at it's widest, the shutter speed is the slowest auto mode would allow it (some compacts max out at 1s) and ISO is highest auto mode would allow (my camera max auto ISO is 1600)

Thanks Skystrike....A mode is Aperture...since in A mode the aperture was wide open and in auto mode the aperture was also at it's largest, then I should technically get the same results in both A mode and Auto mode...

So when do we use EV when we could be adjusting aperture, shutter and ISO?
Yes if the aperture/shutter speed/ISO is the same in Auto mode and Aperture Priority mode, then the final exposure is the same (though sometimes the camera sets the white balance / colour settings differently in different modes so the final picture looks a bit different)

'EV' is a combination of the exposure triangle... eg EV 0 is the result of aperture of F1, shutter speed of 1s and ISO 100

I think you are referring to 'EV compensation' AKA 'exposure compensation'... it is used to set the 'correct' EV as perceived by you as sometimes the metering system is fooled in difficult situations, an example which was brought out by Rashkae. So for example metering the white dress the settings were F4, 1/60s, ISO400, but the result turned out too dark by 1 EV or commonly referred to as '1 stop' (as the meter usually will try to average the scene to grey), then you can either:
- dial in exposure compenation of +1 EV (which will work in P, A, S mode but some cameras don't allow compensation in auto mode that's why you need the former modes)
- or switch to M mode and dial in eg F4, 1/60s, ISO800

Have to experiment around and think about it more then you will get faster at these calculations when the camera's metering system disappoints ;)
 

kurby

New Member
Dec 19, 2004
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#11
If, let's say, you are in P mode and adjust shutter speed, the camera will try to change another setting to get the same exposure as before.

When you use EV, you are telling the camera "look, I know you think this is right, but I actually want it brighter / darker"
This makes great sense....thanks...
 

kurby

New Member
Dec 19, 2004
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#12
Thanks thoongeng...I still need to figure out the 1-stop, 2-stop thing :)

I am loving every moment of this new discovery :)
 

thoongeng

Senior Member
Jan 26, 2010
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#13
Thanks thoongeng...I still need to figure out the 1-stop, 2-stop thing :)

I am loving every moment of this new discovery :)
good to hear that! :)

1-stop is a difference of 2x of light. Shutter speed and ISO easier to understand
- 1/60s to 1/30s - double the time shutter remains open, to double the light, so 1 stop difference
- ISO400 to 800 - again double

Aperture slightly more complicated: F1.0 to F1.4 is double the light and 1 stop difference... cos it's related to diameter of the aperture opening, in school we were taught area of circle is 'pi' times 'radius squared', so '1.4 squared' is around 2x ;)
I find it easier to remember doubling 'F-number' is 2 stops difference eg F2 to F4 is 2 stops (doubling the double = 4 times more)

:confused: confused already? hehe...

Check out the newbies guide in this forum, I also like the analogy in Canon's guide (but applies to any brand of camera)
Enjoy! Digital SLR camera
 

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oceanpriest

Senior Member
Apr 24, 2010
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#14
There's nothing such as correct exposure, it's you who decide how your picture will look like, not the camera. If underexposed image with no detail in shadow is what you really want, then so be it.

Most camera's meter is using 18% reflectance (middle grey), so if you meter a white snow, and set exposure according to it, the snow will be grey. If you meter heaps of coal, and set exposure according to it, the coal will be grey. It's your job to tell the camera that you want to shoot white snow or heaps of coal.

The important thing it to know how your camera works. How does it meter? Average? Spot? What's the EV range your camera can measure? Then you can tell your camera what you want.
 

kurby

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Dec 19, 2004
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#15
I understand the concept of aperture, shutter speed and ISO...what I am still trying to figure out is which one to adjust when you need more light....it can be either of the three...it can also be the EV compensation...

Today I was searching for the exposure indicator when using the A mode...haha...now then I realize the exposure meter will not show in A mode, Auto, P or S mode cause all these mode the camera will "Auto" get the exposure right....unless the flash is turn on...
 

Rashkae

Senior Member
Nov 28, 2005
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#16
kurby said:
I understand the concept of aperture, shutter speed and ISO...what I am still trying to figure out is which one to adjust when you need more light....it can be either of the three...it can also be the EV compensation...
EV compensation will still adjust one of the 3 variables.

If you really understand the concept of the 3, you will know which you need to adjust, as each has a different impact on the image.
 

Mar 1, 2012
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#17
For a start, use P mode and adjust exposure comp if the metered exposure is not to your liking.

Then move to A or S mode and adjust exposure comp. The difference between here n in P mode is that one of the variable (shutter speed or aperture) is fixed by u, and exposure comp changes the other 2.

When u are comfortable, then step up to M mode and adjust everything yourself. Usually I will keep iso fixed, set aperture to wide open if I want subject isolation or stopped down for landscapes, and change shutter speed to get the exposure I deem right. It's intuitive to do it with my mirrorless camera, not too sure about traditional DSLRs...
 

daredevil123

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Oct 25, 2005
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#18
I understand the concept of aperture, shutter speed and ISO...what I am still trying to figure out is which one to adjust when you need more light....it can be either of the three...it can also be the EV compensation...

Today I was searching for the exposure indicator when using the A mode...haha...now then I realize the exposure meter will not show in A mode, Auto, P or S mode cause all these mode the camera will "Auto" get the exposure right....unless the flash is turn on...
When in P, A and S mode, ISO does not automatically change by default. It will only automatically change IF your auto-ISO is turned on.

So traditionally, when you are using A mode, the camera meter will determine the shutter speed for you. When you dial in EV, that EV setting will tell the camera meter to increase or decrease the exposure by the amount you entered. This is before the days of auto-ISO.

With auto-ISO, the ISO will be raised by the camera if the shutter speed is too low. As the ISO raises, the shutter speed can be faster.

Without auto-ISO, you will have to change the ISO manually. THe camera will not even touch the ISO at all.
 

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kurby

New Member
Dec 19, 2004
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#19
Thanks for all the information....

So, if I am in A mode....changing the size of the aperture will not change the exposure (unless hit limit of either speed or ISO) since the camera will set the exposure back to the same (as metered by the camera) by changing shutter speed or ISO (if on ISO auto). Changing aperture size in this case is only to get the necessary effects that is managed by the aperture. To change exposure will then have to do a EV compensation cause that will tell the camera that I want an exposure higher/lower than what was metered...correct?
 

edutilos-

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Dec 28, 2010
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#20
I understand the concept of aperture, shutter speed and ISO...what I am still trying to figure out is which one to adjust when you need more light....it can be either of the three...it can also be the EV compensation...

Today I was searching for the exposure indicator when using the A mode...haha...now then I realize the exposure meter will not show in A mode, Auto, P or S mode cause all these mode the camera will "Auto" get the exposure right....unless the flash is turn on...
It may be easier for you to understand with Manual mode. Not saying that it's a better mode, it just allows you to tweak each individual component and understand the concept quicker when indulging in the practical aspect.

Anyways, a while ago, someone (can't remember who) used this analogy that I think works pretty well if you read it slowly. You can think of exposure (which really relates to the amount of light collected) like water entering a pail, and every pixel is an individual pail (of light). The camera and its lens are the hose conveying the water/light.

1. ISO relates to the speed at which the water/light flows through the hose to fill the pail/pixel.
2. Aperture relates to the DIAMETER of the hose.
3. Shutter speed, intuitively, relates to the amount of time that the hose is kept running for.

Incidentally, each of these 3 affect other things that photographers will be concerned with. Correspondingly in the same order:
1. ISO affects the amount of noise in the picture. The higher the ISO, the higher the amount of noise present.
2. Aperture affects the DEPTH OF FIELD in the picture. The larger the aperture, the shallower the DOF (and the smaller the F-stop number)
3. Shutter speed affects the amount of motion captured in the picture.

Now back to exposure - remember the analogy of the pail/pixel? If your 3 parameters, be it ISO/aperture/shutter speed (or speed of water/diameter of hose/time for hose to run) result in the pail being overfilled, the pixel will be overexposed. The result is there is too much light, so the pixel turns white. If the pail is underfilled, then the pixel will be underexposed and turn black.

There is nothing wrong with overfilling some pails and underfilling some pails, as long as most of the pails are filled nicely. As others have mentioned, you can deliberately overfill and underfill the pails. Example of deliberate overexposure in the picture: Juxtaposed | Flickr - Photo Sharing! ; example of deliberate underexposure in the picture: Alone | Flickr - Photo Sharing!

After understanding this, then understand what the different modes do. M allows you to control all the 3 parameters individually, A and S allow you to control 2 out of the 3 (aperture-priority means you control ISO + aperture, shutter speed priority means you control ISO + shutter speed; the camera determines the last one automatically based on its metering, but you can override this via EV compensation).

I hope this helps.
 

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