Understanding the Triangle Approach to Exposure

[Hannah]

Hi guys,

I've been reading the updated "Understanding Exposure" by Bryan Peterson and he talks about setting the aperture to gather the "correct" shutter speed as indicated by the camera - then adjusting from that point.

My question is how exactly do you get that initial correct shutter speed from the camera once do determine what aperture you want to use? Do you do this in AV mode to get a reading and then switch over to manual mode to adjust it creatively based on that measure?

He doesn't really explain this so I'm sure it's obvious and I'm missing something major.

Thanks in advance,

Hannah

[hmhm]

I've got the original version of that book on my shelf, but the new one seems to have just come out last month, so I'm not sure how many people here will have it.

I'm not entirely clear on your question from your post.

In general, if you choose the aperture first, say, to set your desired depth of field, and you dial that in Av mode, the camera will then give you what it thinks is the correct corresponding shutter speed.

Now, when you talk about "adjusting from there", you might mean that you're adjusting the shutter speed to make corrections for what the internal metering suggested, e.g. to raise or lower exposure either for an intended effect or because you think the camera's metering was "fooled" and you need to correct it (possibly in response to a look at the histogram from a prior shot). In this case, you'd most likely leave yourself in Av mode and use exposure compensation up or down to apply an "adjustment' to what the camera's metering chose.

Alternately, you could switch to manual mode and apply those offsets manually, either by remembering what the camera suggested in Av mode and dialing in a shutter speed above or below that value, or by using the display in the viewfinder that shows you what the internal metering thinks is correct, and then dialing up or down from the middle of the display. I think most people would stay in Av mode and use exposure compensation, though.

If, on the other hand, when you say "adjust from there", you're talking about trying different corresponding aperture/shutter-speed pairs, all based on the camera's metering suggestions, then you can do that from Av mode (or Tv or P) by just selecting different apertures, and the camera will change the shutter speed to match your selected aperture.

Or maybe I don't understand the question... :)
-harry

[scottbergerphoto]

The Triangle consists of the Aperture, Shutter Speed and ISO. Peterson has you select a medium toned(18% Grey) part of the subject to meter off of. Unfortunately that isn't always possible as there may not be one.
Your camera meter is set up to make whatever it sees, black, white and all shades in between, 18% Grey. That is called a "Normal Exposure". Test it out for yourself. Put your camera on P and fill the viewfinder with a black, then white then grey objects. They will all look grey. You need to get to a "Correct Exposure" or what the image should actually look like.
In a simplified Zone System, you use your camera on partial or spot metering to fill the partial/spot metering circle with a major tone(your reference tone) in the subject. A tone that is the most prominent or most important in the image. Take a reading in Manual Mode. Be sure to fill the partial/spot metering circle with that tone. Then adjust your Normal Reading based on the difference between the reference tone you selected and 18% Grey

Black(4.5%)- Dark Grey(9%)- 18%Grey- Light Grey(36%)- White(72%)

From the above, select the tone your reference tone is closest to. Adjust your "Normal Exposure" based on where the reference tone you selected is in relation to 18% Grey. For Black, reduce the exposure by 2 stops, Dark Grey, reduce by 1 stop, etc. In a simplified Zone System, white is two stops from 18% Grey. To get a Correct Exposure for white, you need to open up your aperture or slow your shutter speed by two stops. Each tone listed represents one stop.
The principle is that if one of the major tones in a subject is correctly exposed, the rest will all fall into line.

Regards,
Scott

[PacAce]

Hi guys,

I've been reading the updated "Understanding Exposure" by Bryan Peterson and he talks about setting the aperture to gather the "correct" shutter speed as indicated by the camera - then adjusting from that point.

My question is how exactly do you get that initial correct shutter speed from the camera once do determine what aperture you want to use? Do you do this in AV mode to get a reading and then switch over to manual mode to adjust it creatively based on that measure?

He doesn't really explain this so I'm sure it's obvious and I'm missing something major.

Thanks in advance,

Hannah

What you do is set the camera on Manual. You then set the aperture (or shutter speed) to whatever you want. In manual mode, the "gauge" you see in the viewfinder serves as an exposure meter. To get the correct exposure as determined by the camera, you adjust the shutter speed (or aperture) so that the "needle" in the gauge is dead center (on zero). You can now get creative with your exposure by adjusting either the aperture or the shutter speed to your liking. When the needle moves to the left, you are decreasing exposure. When it moves to the right, you are increasing the exposure. For each notch you move off center, you will be changing the exposure by 1/2 or 1/3 stop, as set by your custom function).

If you don't want to use the M mode, then you can switch to Av or Tv and then use the exposure compensation (with the big wheel at the back) to increase or decrease exposure.

You can also use the P mode and adjust the shutter and aperture but it's a little more tedious since you'll likely end up using both wheels to make the adjustments.