Exposure compensation question

[Goldcoin79]

Hi all, why I was thinking about exposure compensation a question came into my head which im hoping someone has a answer for. The question is that if you set the exposure compensation to say -2 (I assume this means reduced by 2 stops) how does it adjust adjust to -2 stops if it doesn't change shutter speed, apeture or iso?

James

 

[MLeeK]

It does change something. If you are in Aperture Priority it changes shutter; if you are in shutter priority it changes aperture. If you are using auto ISO it can change that too.

 

[Judobreaker]

Doesn't do anything while in full manual as far as I know. He might've been trying it in that mode.

 

[Overread]

An exposure in a camera is only affected by 3 things - the aperture, shutter speed and ISO.

Now if you're in a situation with constant and unchanging light you can easily test things (if you try outside or in any variable lighting you can get different results because the meter reads things differently). To test I would set your camera up on a tripod at a fixed subject in good stable lighting - that way you can see how the settings affect things without the light or the subject affecting your meter reading.


As Mleek says, if you're in aperture or shutter priority modes the camera will adjust the setting(s - if auto ISO is used) so that it makes the exposure fit the amount of exposure compensation you select based on the meter reading.
If you have a NIKON camera you can also adjust the exposure compensation in manual mode as well - however note that since you have control over all three settings yourself in this mode the camera won't actually do anything. What will change is that now the 0 point on the exposure scale for the meter reading is now reading the + or - exposure compensation value you selected (if you've a Nikon camera with Auto ISO you can use this so that you can have a fixed shutter and aperture setting in full manual and then let the camera auto set the ISO based off them and the meter reading).

 

[SCraig]

Doesn't do anything while in full manual as far as I know. He might've been trying it in that mode.

Actually it does do something in manual, it changes the meter readout. If you have +2 EV set then the meter will read 2EV high so that when you adjust aperture / shutter speed / ISO so the meter reads zero you will actually be shooting 2 EV overexposed. True that in manual it doesn't directly affect the exposure unless the meter reading is used to adjust the exposure.

 

[unpopular]

^^ this is not the case on my a350, though I wish it were.

ETA - just got y a700 a few minutes ago, it does this the right way. WOOT!

 

[Goldcoin79]

So if you are in apeture priority and change the expose compensation what I am being told here is that when you adjust the exposure compensation the shutter speed is changed, so why not adjust the shutter speed yourself if it is doing the same thing?

 

[MLeeK]

That's how I shoot. I can't shoot for the life of me in a priority mode. I totally SUCK at it and I shoot EVERYTHING, even sports, in full manual. It's just a matter of which settings you are most comfortable adjusting when it comes down to it. There are those who are the opposite of me and HATE full manual, but can rock out Aperture or Shutter priority like the king himself.

 

[panblue]

I thought I found a way recently of tricking a Pentax KM to go higher than the permitted available ISO..can't remember how to do it now (..??) it was either using EV comp or auto bracketing.

 

[unpopular]

So if you are in apeture priority and change the expose compensation what I am being told here is that when you adjust the exposure compensation the shutter speed is changed, so why not adjust the shutter speed yourself if it is doing the same thing?

That isn't really how AE works. If you change the aperture, the shutter follows. If you change the shutter, the aperture follows.

Under normal circumstances, in Av mode the shutter is increased one exposure value for every stop you open the aperture. With -1ev EC, then the shutter is increased by two exposure values. For example, if you're reading something that is one stop darker than middle grey, and your initial exposure is 1/60 f/8, adjusting the exposure by one stop would get you 1/120 f/5.6, but the subject will still be over exposed. Using exposure compensation at -1, the aperture is still adjusted to f/5.6, but the shutter would instead be adjusted to 1/250 - one stop under what it was without EC.

 

[KmH]

Doesn't do anything while in full manual as far as I know.

On Nikon cameras, setting an exposure compensation value biases the exposure meter.
Adjust the EC while looking in the camera viewfinder at the meter.

 

[TCampbell]

Exposure compensation changes the METERING that the camera will use.

In other words, suppose you're using "Program" mode and you set the compensation to -2 but the camera is allowed to set aperture, shutter, and ISO. If the shot metered for, say, ISO 1600, 1/125th, and f/4, then "-2" says to shoot at TWO full stops below whatever the meter reads. So it might actually shoot at ISO 400, 1/125th, and f/4 (assuming it was allowed to change the ISO.)

You could turn off auto-ISO so that the camera cannot alter the ISO you set and can only play with f-stop and shutter speed.

If you want to force the camera, you could turn off auto-ISO, and use, say, shutter priority. This would force the camera to drop the two stops by playing with aperture (so it'd have to shoot at f/8 instead of f/4) or you could use aperture priority and that would force the camera to drop two stops by playing with shutter speed (so it would shoot at 1/500th.)

Your camera probably does not have exposure compensation in "manual" mode (mine does not) -- as this wouldn't make any sense because the camera can't actually adjust the exposure in manual mode... only you can. If you want to compensate to -2 when shooting in manual, then you just use the needle in the viewfinder so that it lines up with the "-2" position on the graph instead of at the "0" or center position on the light meter graph.

Use exposure compensation when you recognize that your camera is really metering off a subject that would reflect a neutral gray value. For example... if you're shooting a dark scene or subjects dressed mostly in blacks then going to the minus side will help correct the exposure. I shoot most concert events in the minus range... usually between -1 and -2. You'd do the opposite in strong "whites". You might shoot an outdoor winter snow scene in the +1 or +2 range.

If you _really_ want to know the correct exposure, use an "incident" light meter (a hand-held meter) and not the reflected meter built into the camera. An incident light meter is basically always right.

 

[Goldcoin79]

Thanks for all your comments I now understand, another question I can tick off my list that the photo forum has answered for me.

James

 

[2WheelPhoto]

Camera Exposure: Aperture, ISO & Shutter Speed

 

[TCampbell]

Exposure Compensation performs a single function...it biases the meter. The camera will then perform as it normally would with such a meter reading.

One of the more useful concepts you can learn is Exposure Value.

Good point.

Exposure Value is based on very simple addition and a table of the "full stops" of exposure.

"Aperture value" + "Time value" = "Exposure value", or: Av + Tv = Ev

(which is why the model dial on Canon cameras doesn't say P A S M but instead says P Av Tv M.)

The table (based on ISO 100) is as follows:

Av​

0​

1​

2​

3​

4​

5​

6​

7​

8​

9​

10​

Tv​

1.0​

1.4​

2.0​

2.8​

4.0​

5.6​

8.0​

11​

16​

22​

32​

0​

1s​

1​

2​

3​

4​

5​

6​

7​

8​

9​

10​

1​

1/2​

1​

2​

3​

4​

5​

6​

7​

8​

9​

10​

11​

2​

1/4​

2​

3​

4​

5​

6​

7​

8​

9​

10​

11​

12​

3​

1/8​

3​

4​

5​

6​

7​

8​

9​

10​

11​

12​

13​

4​

1/15​

4​

5​

6​

7​

8​

9​

10​

11​

12​

13​

14​

5​

1/30​

5​

6​

7​

8​

9​

10​

11​

12​

13​

14​

15​

6​

1/60​

6​

7​

8​

9​

10​

11​

12​

13​

14​

15​

16​

7​

1/125​

7​

8​

9​

10​

11​

12​

13​

14​

15​

16​

17​

8​

1/250​

8​

9​

10​

11​

12​

13​

14​

15​

16​

17​

18​

9​

1/500​

9​

10​

11​

12​

13​

14​

15​

16​

17​

18​

19​

10​

1/1000​

10​

11​

12​

13​

14​

15​

16​

17​

18​

19​

20​

So for example, a Sunny 16 exposure would be Ev 15. You'll notice that if you go to the Av=8 column (where the f-stop is f/16) and then look down until you find the "15" and then look to the left margin, you'll see the shutter speed is 1/125th and the Tv=7. If the Av=8 and the Tv=7 then 8 + 7 = 15 (Ev=15). Notice that this table has 6 different cells which all have "15" in them. All of those are equivalent exposures (assuming you leave the ISO untouched.)

When I used to shoot with the Hasselblad 500CM, both the shutter speed and the aperture were rings on the lens (not on the camera body). The two rings could be interlocked. On one area of the rings it had the Av & Tv settings, but if you followed around to the other side of the rings you'd see the Ev values. You could meter the "Ev" values of a scene, interlock the rings at that Ev value, and then every setting on the camera would still produce a correct exposure.

It's rare to find a camera that uses Ev values anymore, but it's still good to know about them because the concept still applies today. Most good hand-held light meters will still report Ev readings if you want them.

(Edit: I just noticed that the upper-left box on that table should have had a zero in it. The table I copied it from (a spreadsheet I created) does have a zero in the box, but for some reason when I save the post, the zero is erased and the box is empty.)