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Old May 23, 2006, 9:46 PM   #1
Join Date: May 2005
Posts: 84

I just want to make sure I am getting this right. I just bought a Sekonic L-358 and it's giving different Fstops than my Rebel XT. To test it, I meter a scene that was almost 100% white. When I used the XT, it said I should take the picture for 1/5 seconds at F5.6. (very low light, sorry). This produced a histogram with a very sharp spike right in the middle of the histogram. The XT is trying to average the scene to 18%, right?

Now with the sekonic, I took a incidence reading, and it suggested that I was underexposing and it wanted me to make the shutter speed 1/2 a second at F5.6. When I look at the histogram on this image, the very sharp spike is almost all the way to the right. That means it didn't average the scene to 18%, it averaged the light falling on the scene to 18%, and therefore exposed the wall correctly.

Am I right in all of this, or do I need to do more studying?

I meant to post this in Gen Q&A, sorry for double post.
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Old May 24, 2006, 3:30 AM   #2
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Join Date: Jan 2004
Posts: 897

As you say the XT measures the reflected light and 'assumes' that the metered scene is 18% grey. This works well for most scenes particulary with the algorythms used for matrix metering. However there are some scenes which a predominately light or dark that will fool the meter.

style="BACKGROUND-COLOR: #000000"An incident metre will always give the correct exposure setting as it measures the light falling on the subject. The 18% grey doesn't really figure with incident meters because it is not measuring the reflected light. You may have to measure the light at various parts of the scene and carry out the averaging yourself, if there is shade the amount of light in different parts of the scene will be different.
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