what is 2/3 stop ?

[fwellers]

Ok time for a very beginner question.
I'm reading Bryan Petersen's Understanding Exposure. He talks about adjusting exposure by 2/3 or -2/3 stops. For instance he likes to meter on green grass sometimes. So he says to meter on the grass, and expose for -2/3 stop, then recompose and shoot.

I get the concept, no problem, but what is 2/3 exposure ? What I'm thinking is that on my D90 I would have the meter centered ( at zero ) while metering off the green, and then adjust until I get 2 or 3 bars to the left ?
Are each of those bars one stop ? Or are each of the 1/3 stop ? Or am I missing something else going on ?

It seems so basic. I am pretty sure he is not talking about using the exposure comp button, because that one isn't effective in manual mode ( I think ).

Thanks,
Floyd

 

[tirediron]

A stop is a doubling or halving of the amount of light reaching the sensor. One stop of exposure can be either one stop of shutter speed, or aperture or combination thereof. Assume your camera tells you to expose a scene at 1/125 @f8. One full stop of exposure (increasing) could be either 1/250 @ f8 or 1/125@ f11, or a combination thereof (say 1/200 @ f9). 2/3 of a stop is exactly that. Increasing or decreasing the amount of light reaching the sensor by 2/3.

 

[Jaszek]

How many small bars do you see between the larger bars? If its two than its 1/3 for each bar if its only one than its 1/2 stop. You can change that in the cameras custom functions

 

[Eldrich]

basically 1 stop means half, or twice as much light, depending on if its -1 or +1. Some cameras have the ability to change in half stops, meaning, the next notch will provide 50% more light than the last (or will be 75% of the original stop etc.), and some have the ability to do 1/3 or 2/3 stops, providing 33 or 66% more or less light respectively. Some cameras even have the ability to choose between 1/3 or 1/2 stop increments. Your light meter should have some notches, then some should be labeled -2, -1, 0, 1, 2. Those are labeled for the full stops.

If you understand what a stop is, then 1/3 stop is just what it sounds like, its 1/3 of the effect of a full stop.

does that make sense?

 

[fwellers]

How many small bars do you see between the larger bars? If its two than its 1/3 for each bar if its only one than its 1/2 stop. You can change that in the cameras custom functions

Ok thanks Jaszek. Just wanted to make sure about that.
Appreciate it.

 

[Jaszek]

no problem.

 

[fwellers]

basically 1 stop means half, or twice as much light, depending on if its -1 or +1. Some cameras have the ability to change in half stops, meaning, the next notch will provide 50% more light than the last (or will be 75% of the original stop etc.), and some have the ability to do 1/3 or 2/3 stops, providing 33 or 66% more or less light respectively. Some cameras even have the ability to choose between 1/3 or 1/2 stop increments. Your light meter should have some notches, then some should be labeled -2, -1, 0, 1, 2. Those are labeled for the full stops.

If you understand what a stop is, then 1/3 stop is just what it sounds like, its 1/3 of the effect of a full stop.

does that make sense?

Yea thanks. I know what a stop is, I just didn't know how to measure it on the meter. Actually I didn't even know for sure if the lines on the meter were representative of stops ( or partial stops ). It seems that yes they are !!

So far I've been using mostly ap mode and if I felt like it, just using the ev button to compensate exposure. But now I am into only shooting manual and have been simply metering by zeroing the lines.
I'll have to look again, I don't remember there being numbers on my meter view that I see in the viewfinder. Just lines. But I'll check.

Thanks.

 

[Dao]

Go take a look at EV table from the following link

Exposure value - Wikipedia, the free encyclopedia

For example.

You are using manual mode and want to set the aperture at F/2.8 with ISO100

After you use a spot meter mode meter the grass. The camera come back and suggest, lets say, using shutter speed of 1/500 to obtain a correct exposure. That is EV12 (The exposure level should be in the middle)

Now, if you adjust it by -1 FULL stop (down) EV11, then according to the EV table, you can do F/2.8 with 1/250 or f/2.0 with 1/500.

And that is one stop down. So how about 2/3 stop down. In that case, that is some where in between.

If you want to keep the same aperture, then the shutter speed will be around 1/320 to 1/330.

Hope this help. And please correct me if this is right.

 

[Eldrich]

But now I am into only shooting manual and have been simply metering by zeroing the lines.
I'll have to look again, I don't remember there being numbers on my meter view that I see in the viewfinder. Just lines. But I'll check.

Yeah, it sounded like you were pretty close to getting the whole picture, just not sure about the specific definition of the partial stops...

Like you said, in manual, the control to stop up or down won't necessarily be the same thing as the 'exposure compensation' knob in the semiauto modes, but the principle is the same.

and I thought most camera's would have numbers, but I could be wrong, perhaps it just has a thicker line for the full stops or something. at any rate, you should be able to figure it out by looking at the camera settings or manual if its not obvious from the screen itself.

 

[fwellers]

As Jaszek said, I have 6 lines. 2 lines then a line with a dot over it, then 3 more lines. So each line is probably 1/3 stop.

As an aside, while I was checking that out in my laundry room, I decided to make a test exposure for what the meter showed, which was 1.3 seconds shutter speed. I did what I learned today which is elbows in, and let out the breath to the very end just at the relaxing point then press. I couldn't believe it. I've never had such a sharp pic at such a crazy slow shutter speed. This was at 18mm focal length but still. ( and I zoomed all the way in ).

patting myself now. hahahaha.

Thanks all.