Flash power in full frame camera

[analox]

<p>I wonder if a full frame camera (e.g., Canon 5D) will require more flash power to light a subject than crop-frame (e.g., 4/3 camera)?<br /> <br /> Let assume I use 2 cameras to shoot a subject (e.g., person)<br /> <br /> - Camera 1: full frame Canon 5D, at f=24mm, f4, ISO100 (shutter speed is not an important factor here)<br /> <br /> - Camera 2: 4/3 camera, at f=24mm, f4, ISO100<br /> <br /> Well, since I think<br /> <br /> - the aperture radius of the lens is the same for both settings (~ 24mm/4)<br /> <br /> - the sensor size of FF is 4 times larger than 4/3 sensor<br /> <br /> Then if the flash power is 1/16 to well expose the subject in 4/3 camera, we need to set flash power to 1/4 in case of using full frame camera.<br /> <br /> Hmm... is the math correct here? Please advice :D<br /> <br /> Thank you all in advance :)</p>

[NK Guy]

<p>Sure. Lots of modern cameras and automatic flash units will adjust their zoom head settings to produce less coverage area when used with subframe DSLRs, in order to save battery life and shorten cycle times.</p>

[jimstrutz]

<p>If you use the same lens, subject distance, aperture, and flash head's zoom setting; the flash requirements are the same. But a lot of that light is spilled off to the sides of the image with a smaller sensor camera. However, as NK Guy notes, smaller sensor cameras can use a narrower zoom setting to focus the light tighter, and therefore need less flash power.</p>

[mike dixon]

<p>The amount of light you need to have falling on your subjects is the same in both cases. If you have your lighting [flash(es) and modifiers] set up so the subject is properly exposed at 1/4 power, then the exposure remains the same whether you're using a full frame or 4/3 camera.</p>

<p>With the full frame camera, you will be photographing a much wider area of the scene. If the lighting is somehow more focused (only covering a smaller part of the scene) for the 4/3 camera, then you could use less power. But for this to be true, you would not simply be changing the power setting; you would also change the positioning or modifiers for your light source.</p>

[andrewg_ny]

<p>I don't think more flash power is inherently required--if you're shooting with an equivalent field of view (which in your example you're not), like 16mm on APS-C or 12mm on four-thirds, you would get a similar picture and require similar flash power. I wouldn't think of this in terms of sensor size, I'd think of it in terms of field/angle of view.</p>

<p>I'd also consider that 24mm 135-equiv is sort of pushing the limits of most accessory flashguns in terms of angle-of view; many require an additional diffuser/wide panel to gain full coverage at this wide an angle.</p>

<p>Another consideration it is that your full-frame camera likely has better high-ISO capabilities than its smaller-sensored brethren and possible availability of faster glass (this would depend somewhat on focal length in use) so by taking advantage of these may be able to make do with <em>less </em> flash power.</p>

[mark_sirota1]

<p>The important difference is depth of field. On a larger sensor you generally stop down a bit more to get equivalent depth of field, and stopping down requires more light.</p>

[Alan Marcus]

<p >Your question presupposes that the exposing light intensity at the film or chip plane will differ due to image size. There is optical truth to this judgment however countermeasures have been employed that render this fact somewhat moot. </p>

<p > </p>

<p >We and the camera’s chip logic uses the f/number system. This method takes the chaos away. The f/number is a ratio that takes into account lens focal length and lens working aperture. The f/number system assures that any lens set to the same f/number as another will deliver the same exposure. I won’t do you any harm to study the f/number system in depth. </p>

<p > </p>

<p >First let’s explore the image size difference full frame vs. 4/3” CCD sensor. By the way, the first application of the CCD was to replace the Vidicon vacuum tube of the miniature television camera. The 4/3” describes the tube’s diameter. We continue to use this obsolete categorization. The image dimensions are taken from a light sensitive rectangle situated atop the tube. Kodak and Olympus jointly marketed a 4/3” CCD.</p>

<p > </p>

<p >The full frame CCD has an image size that measures 24mm by 36mm with a diagonal of 43.3mm. The 4/3” CCD measures 13mm by 17.3mm with a diagonal of 21.6mm. The 4/3” CCD is ½ size i.e. 43.3 ÷ 21.6 = 2.0 meaning a 50mm lens uses with this chip has a 2x crop factor or magnification factor thus on this camera it performs as to angle of view akin to a 100mm on a full frame i.e. like a lens 2x longer. </p>

<p > </p>

<p >How does this information effect flash exposure? The intensity of the flash will be highly dependent on the efficiency and shape of the built-in reflector. If highly polished and shaped to yield a tight beam, the flash acts somewhat like a spot light. This design carries further but likely the center of the picture will be correctly exposed while the edges will certainly be underexposed. Thus your built-in flash has a reflector shaped to achieve better uniformity. Further the shape must accomplish uniformity over the zoom range. No small task if interchangeable lenses can be fitted to the camera body. As a result the built-in flash won’t carry as far as specialty off-camera flashes. </p>

<p > </p>

<p >OK the bottom line: The key factors that determine exposure - intensity of the flash (power measured in effective-candlepower seconds. Flash duration (averages about 1/1000 second. Reflector shape and polish. Room size taking into account reflectivity of walls and ceiling (or lack of same). Camera ISO setting.</p>

<p > </p>

<p >Don’t forget subject distance. This is the most mysterious and foremost factor. Flash intensity is subject to the law of inverse square. Each doubling of flash-to-subject cause a four fold drop in light intensity at the subject plane (two f/stops). Thank goodness for cameras with built-in sensors and chip logic. In the old days we used a guide number. Your flash likely has a guide number of about 100. We estimated subject distance and divided distance into guide #. Say your principle subject was 13 feet away, thus 100 ÷ 13 = 7.7 we rounded to 8 and set our aperture to f/8. This method remains viable but camera chip logic has rendered it old-fashioned. </p>

[jpo3136b]

<p>No. Flash to subject distance will drive exposure. <br /> Light loses intensity with distance, not size of intercepting surface area (cf. The Inverse Square Law of Light).</p>

<p>Changes in fields of view may affect averaging methods of exposure; but basic strobe exposure is driven by the intensity of light striking the subject.</p>

[jpo3136b]

<p>This is why we can run a test strip during print enlarging and not have to adjust the times or the f/stops. If we put a small recording surface in the cone of light, and then a large recording surface in the cone of light, without changing anything else, then the areas that match will show the same exposure.</p>

[dbcooper]

<blockquote>

<p>"I wonder if a full frame camera (e.g., Canon 5D) will require more flash power to light a subject than crop-frame (e.g., 4/3 camera)?"</p>

 

</blockquote>

<p>+1 for 'no'. Exposure is exposure. If you think about it, <em>f</em> /4 is<em> f</em> /4, and <em>f</em> /8 is <em>f</em> /8, and so forth, regardless of lens focal length, lens or camera brand, or sensor size. We don't give any camera any more or any less natural light exposure because of the sensor size, so why would we do it for flash?</p>

<p>As an example: properly used, any decent hand-held flash meter will give a correct exposure setting applicable to any decent camera without regard to flash brand, flash size, camera brand, digital or film, sensor size, aspect ratio, focal length, etc.</p>

<p> </p>

[analox]

<p>Thank you all for your explanations!<br /> <br /> Guess I'm a bit confused when posting this question :D. In summary,<br /> <br /> a. Yup, sensor size does not play a role in light exposure, so the it is not required to adjust light power in my question (assuming same equivalent flash zooms are used)<br /> <br /> b. In other situation when FF camera needs to preserve angle of view + depth of field (maybe in group shot in wedding), the flash power may need to be adjust according to the change in aperture. For example:<br /> <br /> - Camera 1: full frame Canon 5D, at f=24mm, f8, ISO100 (shutter speed is not an important factor here)<br /> <br /> - Camera 2: 4/3 camera, at f=12mm, f4, ISO100<br /> <br /> In this case, if the flash power is 1/16 to well expose the group in 4/3 camera, we need to set flash power to 1/4 in case of using full frame camera.<br /> <br /> It it correct now?</p>