Ambient Metering Question

[steve_elms]

<p>Please excuse my ignorance here but I'm just starting to get a grip on using flash effectively. I have a question that I'm hoping you guys can answer for me.<br /> <br /> I've been doing a lot of reading and what I've gathered so far is this:<br /> <br /> For indoor shots with flash I should be metering the ambient light first, then adding the flash. My understanding is that this will give me a properly exposed background and main subject. If I want to brighten the background then I slow down my shutter speed and vica verca. Is this correct?<br /> <br /> So am I to assume that I do the same for outdoor shots when I'm using the flash for fill? I guess I'm just looking for guidance on how to meter properly for outdoor fill flash.<br /> <br /> Thanks in advance!</p>

[mark_sirota1]

<p>It's the same thing indoors or out, with two caveats related to the issue that it's generally a lot brighter outdoors:</p>

<p>(1) Your flash needs to be more powerful to compete with the ambient. You might run into the power limit of your flash, or have issues with recycle time or overheating.<br>

(2) You might run into sync speed issues outdoors, where you can't make the exposure time as short as you'd like it to be. Or depending on your camera and flash, you might be able to use high-speed sync to address this, but then you might run into flash power issues.</p>

[steve_elms]

<p>Thanks for the reply. I should have added that I'm using an SB900 with a Nikon D700. My understanding is that once I go over my sync speed my flash loses signifigant power?</p>

[Ed_Ingold]

<p>You can "pull" the shutter speed to increase the contribution of ambient light, indoors or outdoors. Indoors, you have a problem with the color of ambient light vs the daylight color of the flash. This makes pictures too yellow, or pasty looking where the flash predominates. You can overcome this by putting a colored gel on the flash to better match room lighting. Alternately, you set a higher shutter speed so that the flash dominates the light reflected by the subject.</p>

<p>"Pulling" the shutter works for one simple reason. The flash duration is so brief that the shutter speed doesn't matter (within limits), so that only the aperture controls flash exposure. Ambient light is controlled by both the aperture and shutter speed. Once you set the aperture according to the flash, you can adjust the shutter speed, for that aperture, to control the ambient exposure.</p>

<p>The D700, and nearly all DSLRs, have focal plane shutters which work by moving a slit across the film or sensor. The speed of the slit is constant, so the exposure depends on the effective* width of the slit - the less the slit, the shorter the exposure. The maximum flash speed is that where the width of the slit is equal to the width of the film or sensor. At any higher speed, only part of the film/sensor will be exposed by the flash.</p>

<p>Ordinarily, flash is used outdoors for fill, at least a stop below the ambient light. The SB900 should have ample power if used correctly. Since the flash is affected only by the aperture, it helps to use a higher ISO setting to reduce the flash requirements, provided that the shutter speed does not exceed 1/250. I find that ISO 400 is a good compromise between speed and image quality for outdoor use with flash (and/or without a tripod).</p>

<p>Nikon recommends that you limit the use of the flash to 10 full-power pops in 15 minutes or so, to avoid overheating. You can do 40 quarter-power pops in the same time frame, which is typical for fill-flash applications. Neither are hard limits, but sensible guidelines.</p>

<p>* The width of the slit is adjusted by the delay between release of the leading blade and that of the trailing blade. It is not physically bigger than the film gate, even at slower speeds.</p>

[jim_stewart3]

<p>Realistically you want to keep your shutter speed under 250 to avoid going into FP mode as you loose so much power when you do. You are better off using ND filters to get the shutter speed where you need it. </p>