On board camera metering

[ryan_rochette]

<p>I have a Canon 7D with metering set to 16 seconds with a maxim 30 minute meter time. I can snap a photo as soon as I turn the camera on. How does the camera meter for 16 seconds let alone 30 minutes? How do you determine after metering a scene that the sky is to bright so you may need a two stop or three stop ND filter to correctly expose the scene?</p>

[randy_settle1]

<p>That's in Live View mode, turn off live view and look thru viewfinder, live view eats battery life so you can set exposure meter to turn off to save battery life.<br>

I'll let someone else do the ND question as I don't shoot much outdoors.</p>

[john_deerfield]

<blockquote>

<p>How do you determine after metering a scene that the sky is to bright so you may need a two stop or three stop ND filter to correctly expose the scene?</p>

 

</blockquote>

<p>I am not sure I understand the question. You meter the sky but you don't know if you need a ND filter? The only thing a ND filter will do is lower the EV allowing you to use a slower shutter and/or wider aperture. So if you meter the sky and you want to use a slower shutter and/or wider aperture and you are already at your lowest ISO, you need a ND filter. Usually, if you are shooting a landscape/cityscape where the sky is brighter than the rest of the frame, you want a <em>graduated </em>neutral density filter. This lowers the EV on the top half of the frame but not the bottom. How much you want to lower it all depends on the light. You can meter the sky and then meter a spot on the "ground" and find out what the EV difference is, and that would tell you how much of a GND to use. As example, if there is a 3-stop difference you want a 3-stop GND.</p>

[jaydesi]

<p>The camera begins metering the scene when you half depress the shutter button. It will continue to meter until the shutoff time, or until you turn off the camera. A longer time would be useful when you're using a remote trigger that doesn't activate the focus or metering, so you can get those things set and then walk away to take your shot, without having to worry that the sun breaking through the clouds a second later will cause an overexposure.</p>

[waltflanagan]

<blockquote>

<p>I can snap a photo as soon as I turn the camera on. How does the camera meter for 16 seconds let alone 30 minutes?</p>

</blockquote>

<p>I'm not sure what you're asking. The meter reading is basically instantaneous. It's not averaging over 16s or 30min. Assuming a constant scene like a studio with one light on the meter reading will be exactly the same after 1s as it will be after 30min or after 1 year.<br>

The camera provides electricity from the battery to run the meter. When the meter shuts off you use less electricity so the battery will last longer. If the meter only stayed on for 2 seconds before shutting off it would be annoying because you would have to constantly half press the shutter release to get it to turn back on.</p>

<blockquote>

<p>How do you determine after metering a scene that the sky is to bright so you may need a two stop or three stop ND filter to correctly expose the scene?</p>

</blockquote>

<p>If you know the dynamic range of your camera or film you can spot meter the various areas of the scene and use Ansel Adam's Zone System<br>

http://en.wikipedia.org/wiki/Zone_System<br>

I suspect you don't want to do that... You have a digital camera. Take a picture, if the highlights are blinking in the sky then you're overexposed so use a ND grad filter. Whether you use a 2 or 3 stop or something else all depends on how much brighter the sky is and what you want the final picture to look like.</p>

[richard_john_edwards]

<p>Film Characteristic curves determined how much variance of light the film could capture, the camera exposes for mid grey. Digitally the sensor should have a limited range as well. what you need to do is spot meter the scene and determine the scene luminance range and go from there. you need to ensure that the exposure for the shadows is not blocked up, in doing this you need to understand where a tonal value lies from mid grey. ie is the shadow detail about 2 - 3 stops below mid grey, check you meter with spot metering, then evaluate where the highlights are, once you get an idea of the zone system you can then make a determination on weather the scene luminance range exceeds the capacity of the camera to record it, if the problem is in the clouds and sky, the you should know by how much and use the appropriate ND filter.</p>