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In-studio light adjustment
Old 12-15-2008, 09:00 AM   #1 (permalink)
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Hi,
beeing an amateur I always ask a simple question but therefore I learn lot of things here on GG...

just for my better knowledge, what does it mean or where I can find the explanation of changing the light source power by 1/2 or 1 stop down or up.... This question is connected specially with lightmetter SECONIC 758 that I use in my studio work....

For example: when someone says 1/2 stop down from f8...that is what??? 7.1 or 6.3 or 5.6....

I need to understand that lesson....

Regards Zak
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Re: In-studio light adjustment
Old 12-15-2008, 02:41 PM   #2 (permalink)
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Quote:
Originally Posted by Zakd View Post
Hi,
beeing an amateur I always ask a simple question but therefore I learn lot of things here on GG...

just for my better knowledge, what does it mean or where I can find the explanation of changing the light source power by 1/2 or 1 stop down or up.... This question is connected specially with lightmetter SECONIC 758 that I use in my studio work....

For example: when someone says 1/2 stop down from f8...that is what??? 7.1 or 6.3 or 5.6....

I need to understand that lesson....

Regards Zak
In theory 6.7. 7.1 and 6.3 are 1/3 stops.
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Re: In-studio light adjustment
Old 12-15-2008, 03:50 PM   #3 (permalink)
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Typical one-third-stop f-number scale

F/# 1.0 1.1 1.2 1.4 1.6 1.8 2 2.2 2.5 2.8 3.2 3.5 4 4.5 5.0 5.6 6.3 7.1 8 9 10 11 13 14 16 18 20 22


Typical one-half-stop f-number scale

f/# 1.0 1.2 1.4 1.7 2 2.4 2.8 3.3 4 4.8 5.6 6.7 8 9.5 11 13 16 19 22

I hope this helps.. Just compare the two scales

Mike
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Re: In-studio light adjustment
Old 12-15-2008, 03:53 PM   #4 (permalink)
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There's a mathematical formula to figure out what an f stop is. The list is the easy way, it is generally found on your lens. It is:

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/45, f/64, f/90, f/128

One stop down from É8 is É11. It is down because it lets less light in. One stop up from É8 is É5.6. One half stop down is like É9.5.

These numbers represent the size of the aperture, i.e. the size of the opening which lets light in. It is good to know that each f-stop doubles the amount of light from the one before. So, if you go out into the sun with ISO 125 film/digital, your basic exposure is going to be É16, at 1/125 of a second exposure. If you need a faster shutter speed, no problem. Use 1/250 of a second which lets half of the light in as 1/125 of a second, but now you will need to use É8 which is twice as big.

Now most cameras, light meters, and the like today use incremental steps. Instead of an f stop they use 1/2 or 1/3 f stop increments. My camera can be set at either 1/2 or 1/3 stops. Some of my equipment uses, 1/2 increments, and some 1/3 increments. It is very helpful to have all equipment speaking the same language, that is using the same increments.

Now you have the list you can figure out what a half stop, or a third stop is.

Your lights should have the ability to power down to 1/2 stop or 1/3 stop. It may be on the back of the light. It may be in the manual. If perhaps you don't have that ability then you can still manage by moving the light. Googling "law of light inverse square" will help you figure out what the distance is going to be. As the light gets further away from the model it drops of very quickly. And you could figure out by distance where your half stop, or third stop distances would be.

When you go to a photo studio there are often tape marks on the floor where lights go. It will save you time once you figure out where the lights should be. Later on, you will just know.

On where to go to find out more, read any manuals that came with your camera, light meter, and flash units first. Then googling "f-stop" is not too bad of a start. One web page is:

http://www.uscoles.com/fstop.htm

Also, there is a surprising amount of stuff on Youtube.com.

After that, the library, or books stores have much info on the basics of photography.

I hope I haven't confused the issue too much.
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Re: In-studio light adjustment
Old 12-15-2008, 07:53 PM   #5 (permalink)
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You may already have your answer but I thought that I'd throw in a bit of trivia.

If you don't have a F-stop chart in front of you, you can compute F-stops by two simple numbers: 1 and 1.4. All of the other F-stops are binary multiples of those two numbers; for example 2*F1.0 = F2, 2*1.4 = F2.8, and the next sequence is 4*, then 8*, then 16*, then 32*, etc.

Since the area of a circle (your aperture) is 2*pi*radius^2, then if you double the radius (the aperture is twice as wide as it previously was) then because of the radius squared rule you let in 4 times as much light (2 squared). That is equivalent to 2 F-stops. To get 1 F-stop, you have to take the squareroot of 2, which is 1.4. So if you increase your aperture diameter by 1.4 times its present size, you have let twice as much light in and that is 1 F-stop. That's why the two important numbers to remember is 1.0 and 1.4. All of the rest are based off of those (actually F1.0 is the only REALLY important one).

Light works the same way. If you move your light source back 1.4 times further back (for example from 6 feet to 8.4 feet away) then the light intensity is half, which is 1 F-stop. If you move the light source back 2 times further away (from 6 feet to 12 feet), then the light intensity is 1/4th, or 2 F-stops. In the same way, moving the light source back 1.2 times further (from 6 feet to 7.2 feet) is 1/2 F-stop reduction in intensity.

Good luck on your future shots !!!

Thanks,
PTL,
Lee
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