F-stops and studio lighting

Discussion in 'Lighting Equipment' started by michelle_n|1, Feb 7, 2013.

  1. Hello Everyone!
    I am a photography student and a new member to this website.
    I have kind of a silly question.
    I know that when taking photographs the the lower the f-stop the more light is let in. For example f/2 a lot of light
    and then f/8 not as much light. I get confused in the studio though.

    For example if I want to create a white background and the subject is f/8 and then the background is f/11 I know
    that is has to be 2 stops brighter than the subject. Naturally I would think 2 stops brighter would be f/ 5.6
    because you are letting more light in. However, letting more light on the background means an f/stop of f/16?
    Aren't you then letting less light in?

    Please help me to clear up the confusion!

    Thanks!
     
  2. I think you may have it a little backwards. An F/stop is telling you how large the iris is going to open when the shutter releases. Therefore the larger (smaller the number) the f/stop the more light that hits the sensor during exposure. A light meter is going to tell you what a normal exposure will be with x amount of light. If it is super bright in the room, it will tell you f/11 or so to cut down the amount of light hitting the sensor and vice versa if it is dark. If you want to do a high key and you want your background to be 2 stops brighter then your meter reading should say two stops higher to compensate.
    Hope this helps.
     
  3. I am very sorry Michelle! Please do not feel offended. Since you are a photography student why don't you ask your teacher and his/her answer will also benefit your classmates.
     
  4. [T]he subject is f/8 and then the background is f/11 ....
    This statement strikes me as an incomplete description and a potential source of confusion--what exactly do you mean by it? How are you getting the values f/8 and f/11? How are you controlling the lighting? One of the fundamental challenges for any newbie is to understand what people mean when they use a particular field's (photography or whatever) lingo, and then be able to communicate your ideas back, clearly and precisely, using that lingo.
    I suspect that what you mean is that you are metering the subject and being told (by a hand-held meter, or with contnuous lighting, by the camera's meter) that, using studio strobes (or continuous lighting and some fixed shutter speed), for your camera's sensitivity setting or the film's "speed" (often incorrectly referred to as "ISO"*), a given aperture (f/8 or f/11 or whatever) will give you a theoretically 'correct' exposure.
    But all meters assume (relative to sort of telling you what aperture to use) that all subjects are middle-gray (instead of dark or light or whatever), and in the case of incident meters, that all subjects reflect the same proportion of the light that hits them (instead of being more or less reflective or 'shiny'). So suppose you're in a typical studio, and your hand-held incident (continuous or flash) meter, when placed in front of your subject and pointed back at the camera, reads f/8. Doing the same thing at the background gives you a reading of f/11. If the subject and the background are the same darkness or lightness, in the picture the background will appear one stop brighter--visibly but not hugely brighter. Assuming a medium-brightness subject, but instead a white background, to make it appear pure white in the picture, you probably need to get a reading two or three stops brighter at the background than at the subject; so if the subject is f/8, you need to increase your background lighting until the meter reads f/16 or even f/22. All else being equal, the higher the f-stop number the meter reads, the more light you have, because what the meter is telling you is that you have to stop down more to get a 'correct' exposure. Of course, you set the camera to f/8, so the subject is properly exposed, and then you get a white background.
    This involves various assumptions and over-simplifications. Indeed, you'd probably benefit from spending some time with your instructor in the school's studio. Good luck.
    *ISO means International Standards Organization. The ISO adopted, as international standards, means of measuring the sensitivity to light of film (more-or-less by adopting the pre-existing American Standards Association (ASA) and German (DIN) standards), and more recently of digital sensors. So "ISO 100" refers to the sensor's or film's sensitivity to light, and means that a certain fixed amount of light will result in a specific response under specific conditions.
     
  5. If you have a white background - like Super White background paper - and you are using an incident type meter (the kind
    where you measure the amount of light coming at the meter or subject - you only want the meter reading for the
    background to be about 1/2 stop brighter than your main light on your subject.

    This is because it is white and going to be reflecting a lot more light back at the camera than your subject will.
     
  6. Besides what has been posted already go to sekonic.com and have a look at their instruction videos and articles.
    They manufacture light meters so they know what they are talking about.
     
  7. Hi, Michelle.
    First a recommendation that you find any materials you can (ebay, website, etc) from the late Dean Collins. Dean was the absolute master of complete scientific understanding and control on lighting and ratios in the studio and on location. I had been involved in commercial photography for a few years and was pretty good at the lighting, or so I thought, until I went to one of Dean's seminars. The first half hour was a complete revelation of how much I only understood intuitively, compared to actually methodically understanding and planning each and every facet of using light in an image.
    I cannot stress highly enough how much this material will help you totally understand how to achieve what you want in the studio!!!!
    Now to your actual question about background.
    For one moment, forget about the camera's f stops. We will get back to that shortly in terms of final exposure.
    First, I hope you have a flash meter with both incident and reflective capabilities. Incident alone is not the best way to measure the difference between subject position brightness and background brightness.
    To get to full and pure white on the background is a matter of the ratio of brightness created by the lighting on each point. That is, you need to have that full (but not over) two stop different between background and subject. It is simplest if you begin with a white seamless for the background, but you can also do it with a pure black background (or any other tone) IF your have enough power to throw enough light on the black paper to create the two stop difference in reflectivity. (You can also create a pure black background tone with a pure white seamless if you just eliminate all light on the background.) It is all purely a matter of relative brightness in your exposure.
    Technically, measure the subject brightness with the incident dome on the meter. That gives you your base from which to work on creating your needed background brightness. Now we are going to briefly bring the camera f stop back into the mix, but only in terms of being sure that you have enough brightness on the subject to allow for the depth of field you wish to use. If too much light, power it down. If not enough, increase the power until you have the f stop brightness you wish to use.
    Now you have established that f stop of choice, forget about the camera again. Now it is your task to light the background to achieve the two stop greater exposure than the subject.
    Meter the background using only a reflective reading, period. If you don't have a pure white paper, using the incident dome at this point will only give you relative brightness balances of the light, NOT relative reflectivity which must be your goal. Using the reflecetive method gives you exactly what the background is throwing off in terms of brightness which is the only way to be sure of a perfect white background.
    Example: Subject plane reads at f5.6 with an incident reading.........balance the brightness on the background so that it reads f11 with a reflective reading.
    It all sounds much more complicated than it is. Once you've done it the first time, it all falls into place very easily.
    Deans's materials will quickly teach you how to do the same kind of foreground to background exposure controls to achieve mid tones, and by the use of gelling either the lens or the lights with color filters, create any level of hue density on the background from the deepest blue that you just barely see, to the lightest pink, etc. This is listed under his teachings on "Chromazones". He also shares a massive amount of information of balances within the subject, creative tricks to achieve all kinds of effects such as "haloing" a subject, many different lightning items that one can build from simple things like PVC pipe, etc.
    Good luck.
    Tim
     
  8. Hi Michelle,
    First let's list the f/stops in order:
    f/1 - f/1.4 - f/2 - f/2.8 - f/4 - f/5.6 - f/8 - f/11 - f/16 - f/22 - f/32
    f/1 is a giant lens opening allowing tons of light to play on film or chip. f/32 is a tiny aperture greatly restricting the amount of light that can enter the camera. The f in f/number is an abbreviation for Focal Ratio. In optics, we must take into account both the lens focal length and the diameter of the aperture. These two factors tell us how much light energy can transverse the lens. The f/number takes these two factors into account. The f/numbers are based on a 2X change in light level. In other words, f2.8 allows twice as much light to play on film or chip as f/4 whereas f/5.6 allows only half this amount.
    The aperture restrictions inside the lens are designed to mimic the human eye pupil. Your pupil changes its diameter automatically as the light level around you changes. We are taking about the colored portion of the human eye that surrounds the black pupil. This adjustable aperture is named iris for the Greek goddess of the rainbow.
    When we do portraiture, first we desire a natural, lifelike representation of skin tones. Thus, when we light and adjust exposure, it will be a value needed to properly expose the face. Once we know this value, we set out camera. Let us suppose the principal subject's exposure setting is f/8. Now generally we light a portrait using two lights. A main or key set high to simulate afternoon sunlight. The second light is called a fill. Its job is to illuminate and soften shadows cast by the main. Generally, the fill is placed close to an imaginary line drawn subject-to- lens, and set a lens height. This fills shadows from the camera's viewpoint. The fill is set subordinate to the main. Most often, it is set to half power as compared to the main. To accomplish this we can measure main to subject distance and multiply by 1.4. Thus if we are using two equally powered lamps, the main at 4 feet from the subject, the fill at 4 x 1.4 = 5.6 feet from the subject. If we meter the light falling on the face with the main off, it will read f/5.6. This tells us the light from the fill is half as powerful as the main. Thus, the basic portrait lighting set-up consists of two lights. Our exposure is based on a reading taken off the face.
    Now for the background: We can cause the background to become lighter or darker by adjusting a third light devoted only to lighting the background. We can either adjust the background light's brightness by a setting on the lamp or by adjusting lamp-to- background distance. Moving the background lamp further away darkens the way the background reproduces.
    The distance multiplier is again 1.4. Let us say we meter the background and it reads f/8. To make it go darker we need to reduce the intensity of the background light. Say it is 8 feet from the background, moving it back 8 x 1.4 = 11 feet, reduces its brightness at the background by half. Now a meter reading reveals f/5.6. This is true because the meter is telling us to open up the camera lens to compensate for the dimmer background light. To darken the background more, multiply the 11 feet by 1.4; the revised lamp to background distance is 11 x 1.4 = 16 feet. Now if you meter the background it will read f/4. Maybe you recognised that the distances follow the f/number set. Each f/number is 1.4 times its neighbor going left. The value 1.4 is the basis of the math dealing with doubling of halving light energy using distance as the adjustment. Also 1.4 is the increment used in the f/number set. Ask you teacher why this value is so important in photography.
    More gobbledygook from Alan Marcus
     
  9. Whoops --- I forgot to clarify how to lighten (drive background towards white).We
    simply brighten the light on the background. To accomplish we either up the
    power by a knob on the lamp fixture or move the background lamp closer to the
    backdrop. When adjusting lamp brilliance, we like to move the fixture in 2X increments
    (1 f/stop). To accomplish by distance, we measure the expanse,
    lamp-to-background and divide by 1.4 (you can also multiply by 0.7, same answer).
    If you want a 2 /f stop change, divide the measured distance by 2. This method
    follows the math of the inverse square, the law that governs light propagation
    (based on the square root of 2 which is 1.4 rounded.
     
  10. +1 on Tim's suggestion for Dean Collins material. They released a DVD set with Dean Collins videos in 2006 that is still available.
    The images he makes are from the 80's but the information is top notch and still as useful today as they probably where in when he made them.
    http://www.software-cinema.com/training/photography/dean-collins/4/the-best-of-dean-collins-on-lighting
     
  11. Hi Michelle
    A lot of words on the subject, are you confused yet? There is another solution to trying to sort out the meaning of all those words. The method I ended up using to work out f stops was the old tried and true "Suck it and see method". In other words I took a series of photos with the range of apertures my lens had at set lighting levels. Printed the result on 4x6 paper and pinned them to the wall for a quick reference. You will be amazed at how soon you come to terms with settings and stop looking at your prompts. Happy shooting.
     
  12. Lots of good explanations here but I'd like to add one observation to the suggestion of moving the light source closer/farther away from a subject: in moving the light source, you change the quality of that light; a light that is closer to the subject has a softer quality. As that light moves farther away, the quality of light gets harder.
     
  13. You are correct when you say 5.6 is brighter than f-8 if it where just one light source and it was reading f8 and then you took a picture at 5.6, it would be overexposing the picture though. When you work with flash the power is based in wat seconds which we read on a meter in f-stops. The more power the higher the f-stops. If you have two lights and one is lighting a subject at f-8 and the other is lighting the background at f-8 you have a perfect balanced exposure. (keep in mind the color of background will determine if you are to add or subtract light to make it look visually balanced.) When you start to decrease the power and the f-stops start to go down 5.6 and 4 the background gets darker. When you go the other way and increase power 11 and 16 ect. the background gets brighter. This is all based on your first lights exposure on the subject at f8. You need two lights to do this type of control in the studio.
    by the way don't get confused with the 2 stops overexposure on background to make it white. This is not entirely true. The more important thing is to have the right color or shade of color background to begin with and expose that correctly based on your subject. Only when the shade is off and you need to visually make it brighter do you overexpose. The other time is when you want to completely blow out the background and all detail then you can start to go towards the 2 stop overexposer technique if that is what you are going for. but never use this approach as your first and only technique.
     

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