F/stops and lighting?

Discussion in 'Beyond the Basics' started by Sarah23, Jun 17, 2008.

  1. Sarah23

    Sarah23 TPF Noob!

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    OK...so I have been reading and re-reading Strobists Lighting 101 section, trying to get it down..and I am stuck. Maybe I just missed since my brain has turned to mush from all the info I am taking in.

    I am just going to paste the paragraph that is confusing me, so I dont screw it up. Here he is talking about hard light, and soft light:

    Say you had an environmental portrait in an office. You might bounce one small strobe off of the ceiling, softly bringing the room up to, say, f/4. Then you put your other strobe on a stand, point it directly at your subject's face, and dial it down until it gives you an exposure of about f/5.6. You may wish to limit the area the hard light will hit by moving the flash up close and zooming the head to an 85mm coverage angle. Or use a quickie snoot made out of a piece of cardboard.

    So, you'd be shooting at f/5.6, with the shadows lit to f/4. The effect will be crisp light on the face of your subject, with nice shadow detail everywhere. Brownie points for thinking to cool the bounced strobe down a little (with a cooling gel) and warming up the harder accent light. (You'd then have contrast in color, direction and hard/soft quality of the two lights.)


    I am SO confused about the f/4 and f/5.6....what the crap is he talking about??? What does all that mean when it comes to lighting??? I'm assuming its NOT the cameras aperture in this instance??? I think my head is going to explode! :meh:

    Here is the link:

    http://strobist.blogspot.com/2006/03/lighting-101-hard-light.html
     
  2. shutter1000

    shutter1000 TPF Noob!

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    Your exposure will be F5.6, your background will be F4. Meaning-There will be a one stop difference between the light on the face and the background. (the background will be one stop darker than the subject)
     
  3. Sarah23

    Sarah23 TPF Noob!

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    Sooo...if you were to meter the background, it would tell you to set your f/stop at 4...and if you were to meter the subject (or what you want exposed correctly) it would tell you to set it to 5.6? Is that referring to your aperture still????

    Im so confused.
     
  4. christopher walrath

    christopher walrath No longer a newbie, moving up!

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    It's hard to say exactly what this refers to. It's still out of context it seems. If you have white caucasian skin and a black background there would be more than merely a single stop's difference in exposure between the two, ESPECIALLY with flash. It sounds like the author is trying to refer to exposure zones but they are getting their terminology screwed around backwards somehow. Maybe it's just us.
     
  5. Sarah23

    Sarah23 TPF Noob!

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    ok, i added the link to my original post, so if anyone can read it and explain what hes talking about, it would help. I guess I dont get how the f/stops relate to exposure?
     
  6. NateWagner

    NateWagner TPF Noob!

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    As I understand it (and I'm new at strobe lighting) lighting a room to a certain f stop such as f/4 or f/5.6 will be where you have the shutter speed on your camera set to a certain specific speed (such as 1/60 or your high speed sync) you will then set the aperture of the camera to say f/4 and then you will adjust the flash (or perhaps the ISO) until it properly exposes the area you're trying to expose at that shutter speed and aperture. (at f/4 obviously it would need less light, or flash power, than at 5.6 for the same shutter speed).

    Thus, this sort of thing will be obviously done on full manual mode on your camera.

    Towards the bottom of the comments on that page I think they give a pretty good example on how to do it (if you have two strobes of course) I don't think this is something you could do with only one strobe, though I may be mistaken.
     
  7. Helen B

    Helen B TPF Noob!

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    Yes, it is referring to aperture.

    Both Nate and Shutter1000 gave good answers, so I'll just expand on them a little.

    With flash/strobe lighting the exposure can be measured solely by aperture, because the exposure time is set by the brief duration of the flash

    One of the commonly-used methods of describing the contributions from different lights is to describe the f-number they produce on their own. It doesn't matter what they are illuminating - it is a measure of the light's effective output at that location. Typically it is done with an incident meter, but even if you don't have a meter, it can be used as a description of the strength of the light.

    The lens is set to the 'key' light - ie the brightness of the light on the main subject. In this case f/5.6. If you went up to the subject and took an incident reading, you would read f/5.6, irrespective of the colour of the subject's skin or clothes.

    The background is lit at half the brightness level of the subject, ie f/4. If you wanted the background properly exposed, you would set your lens to f/4. If you had only the background light on, and you took an incident meter reading in the background, the meter would read f/4. In this case you do not want the background exposed properly, you want it to be slightly darker than the main subject so that it does not distract, but it won't be so dark that it will be gloomy - you will be able to see detail.

    One would usually choose the brightness of the key light first, then set the background relative to that. For example, you could decide that you wanted to take the shot at say f/5.6 to give the appropriate depth of field. The key light on the subject would be set so that it is bright enough to give an exposure of f/5.6. Then, with the key light off you would set the background light to the brightness you wanted it to be, and one stop darker than the key is a good rule of thumb. A light that is one stop darker than a light that gives f/5.6 would be a light that gave f/4.

    Stating the f-number for a light is simply a short-hand way of describing how bright it is.

    The writer is using another common technique: lighting the background with slightly cooler (bluer) light than the main subject. This also has the effect of bringing the subject 'forward' in the viewer's consciousness.

    I hope that this makes sense. Once you have the hang of it, it is a useful method of working and communicating.

    Best,
    Helen
     
  8. Garbz

    Garbz No longer a newbie, moving up!

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    It is also more consistent than simply stating the power output which is often a first reaction. One flash's 1/1 output may be the same brightness as another flash's 1/2. Hence taking the flash power out of the equation and using the fstop described above.
     
  9. andrew99

    andrew99 TPF Noob!

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    Thanks Helen, you answered a couple of my own questions there! :)
     
  10. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    Good explanation.
     
  11. Sarah23

    Sarah23 TPF Noob!

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    Thank you Helen! I think I am starting to get it now.

    So basically, instead of stating the actual flash strength/settings (like 1/1), you use the f/stop that you are trying to achieve in the picture.

    Lets say I was taking a shot, and wanted an aperture of say f/3.5...I would then set the flash so that I can use an aperture of 3.5 and get the correct exposure on the subject....is that right??

    I am so new to lighting and strobes and all of this...I am really trying to learn, but am still in that overwhelmed phase.
     
  12. Village Idiot

    Village Idiot No longer a newbie, moving up!

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    Yes. And you use Guide Numbers to approximate the required power of each flash to get that aperture at that distance. GN = Distance x F stop or F stop = Guide number / Distance. Very useful way to also setup your strobes so you're close to a proper exposure instead of just firing and adjusting and guessing to get it to where it needs to be.
     

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