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Photography Question 
Jen Topp
 

Portraits: How to Get White Background


I have four lights for my home studio. I would like to shoot children's pictures on a white backdrop (with the "white out" high-key look). I was also going to purchase plexiglass so the subject looks grounded but wasn't sure how big of a piece I needed ... is 3 x 8 OK on a 10 x 12 backdrop? I am not sure what f/stop to set all 4 my lights on in order to get the affect of a "white out". Can anyone help me?


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2/7/2008 11:53:03 AM

 
W.    You need 3 stops difference in light between the subject and the background.


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2/7/2008 12:07:52 PM

 
Alan N. Marcus   Hi Jen,
As you know, “High Key” is achieved when the majority of tones are reproduced light. When children are the subject the light should be highly diffused, this adds an even softer touch.
Two schools of thought on “High Key” Some maintain the image must not contain any blacks. Some claim a black is needed to key off the light tones to give an illusion of extra brilliance. In any event the lighting should maintain a feeling of depth accomplished with the 3:1 ratio or perhaps slightly less.
It is advisable to position the main light a little closer to the camera than normal as this position more fully illumines frontal areas of the subject. The main is kept high to simulate midday sun. The fill is positioned at lens height as near as possible to the lens with out getting in the picture. The lighting ratio main to fill is set to 3:1 or slightly less. Best accomplished using a light meter. We want the fill light to arrive at the subject 1 f/stop weaker (50% less light) than the main. No meter? Set both main and fill at equal distance from the subject and set the power of the fill to ½. No power adjustment? Accomplish brightness difference by setting the two lamps at different distances. Assuming main and fill are identical fixtures, set the main closer. To achieve the correct ratio, measure fill-to-subject distance and multiply by 0.7. The answer is the distance main-to-subject this math places the main closer so that its energy arrives at the subject plane 100% greater than the fill i.e. 1 f/stop. This achieves the target ratio of 3:1
As Mr. Smith told you, the background is illumed quite bright. You want to light the background uniformly with 3 f/stops more energy (300% more) on the background as compared to the main. Not an easy task. Best to use a light meter. You can use more than one light to brighten up the background. If you are using the distance method, this assumes all lamps are equal in power and design, multiply main-to-subject distance by 0.35. Place back ground lamp at this revised distance; this achieve the desired 300% brighter background.

Exposure is based only on the fill i.e. turn off all lamps except fill and measure subject’s skin. Set camera to this setting. It is always wise to bracket your exposures meaning you should shoot a series using aperture adjustments.

For “High Key” overexposure is your enemy as you need to record the delicate tonal difference. These will be lost if you overexpose.

Best of luck,

Alan Marcus, Anaheim, CA (dispenser of technical gobbledygook)

Editor's Note: Also check out BetterPhoto's excellent Understanding the Tools of Lighting course!


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2/7/2008 11:25:00 PM

 
Ronald H. Musser
BetterPhoto Member Since: 10/28/2006
  HI Jen,
two of your lights should be set as to evenly iluminate the background in a cross lighting pattern. set at f22. the main light should be set at f11 and be above the camera on the same axis as the nose of the subject, the fill should be at a 45 degree axis to the camera and set at f8.


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2/12/2008 7:47:16 AM

 
Alan N. Marcus   Photography, as we use it, is a two dimensional media. In portraiture we are taking a picture of a three dimensional subject. We need shadows in the final image as shadows give an illusion of depth. The time honored method is light simulating late afternoon sun. This lighting is too harsh for our photographic media thus the shadows will reproduce too dark. A fill lamp is required to soften the depth of the shadows. We fill only for the cameras, thus the fill is set close to the camera, at lens height. Otherwise, we are filling from the prospective of an observer standing off axis from the camera; this is a common but not too severe mistake.

A 3:1 lighting ratio is universally accepted as best as it takes full advantage of the dynamic range of films and chips. Additionally the ratio is set, keeping in mind, that the final presentation may be on paper as a (hard copy). Such a presentation is generally best if the lighting is 3:1.

While the 3:1 ratio is best for most presentations. The next ratio is 5:1 a bit more dynamic being more contrasty.. Next is 9:1 a very contrasty presentation sometimes called “theatrical presentation”.

How is 3:1 achieved? Say the main illumines the frontal areas of the subject with 100 unit of light (for the unit you can substitute watts or watt seconds or candle power etc.). Thus the main shines directly on the forehead and cheeks and other frontal parts of the face and delivers say 100 units of light. Shadows formed by the nose and valleys of the face (dimples etc.) are areas blocked from the light of the main. These shadows reproduce as too dark because they exceed the current dynamic range of film and digital chip unless somehow independently illumined.

The fill is intended to perform this feat. The fill is adjusted to arrive subordinate to the main. Generally the fill is set to ½ strength as compared to the main. Thus at this setting it delivers 50 units of light. In this position it illuminates both the frontal areas plus the shadow areas as seen from the cameras prospective. The frontal areas thus receive 100 units from the main plus 50 units from the fill for a total of 150 units. The shadow areas however only receive light from the fill thus they receive only 50 units. The light on the subject is thus: 150 units’ frontal areas and 50 units in shadow areas. This is expresses as a ratio 150:50. A ratio is actually a fraction that can be reduced to a simpler form. 150:50 reduces to 3:1.

If the fill is reduced again in half it will only deliver 25 units. The main is unchanged and delivers 100 units. Now the subject receives 100 units main + 25 units fill for a total of 125 units’ frontal areas but only 25 units in shadow areas. The ration is 125:25 which reduces to 5:1.

If the fill is reduced again in half it will only deliver 12 ½ units. Now the lighting is 100 units from the main plus 12 ½ units from the fill for a total of 112 ½ units on frontal areas and 25 units in then shadow areas. The ration is 112 ½ : 12 ½ which reduces to 9:1.

Alan Marcus, Anaheim, CA (known to deliver marginal technical advice otherwise known as gobbledygook)
ammarcus@earthlin.net)


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2/12/2008 9:23:08 AM

 
Alan N. Marcus   Correction 9:1 explained:
total of 112 ½ units on frontal areas and 12 ½ units in then shadow areas. The ratio is 112 ½ : 12 ½ which reduces to 9:1.

Sorry about that --
Alan Macus


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2/12/2008 11:15:12 AM

 
Amanda Murray   Alan, I think it is absolutely wonderful that you made the time to answer this question so thoroughly. I certainly learned a lot from reading your response! How encouraging for all betterphoto members.


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2/13/2008 10:01:12 AM

 
Vik Orenstein   hi, all! not to muddy the water, but I find that IF the subject is properly exposed and IF the backdrop is lit evenly and IF the white balance is correct, then a 1:2 ratio (the backdrop one stop brighter than the subject) is just perfect. at one stop overexposed, the drop is completely blown out. at higher ratios, wrap around light can be created and the subject can start looking "mushy" around the edges and the crisp separation between subject and backdrop is lost. if that's the look you are striving for, then that's great. but I like to teach my students first how to get a crisp, well-separated subject since this seems to be the most difficult style to achieve, and then encourage them to experiment from there. jen, hth! ;-)


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2/14/2008 12:14:41 PM

 
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