F/stops and lighting?

[Helen B]

Oooooo....ok...I think I am starting to get it.

So depending on what mm setting I have the flash on (lets just say 50mm) I then use that corresponding guide number (which was 34 I believe) in the F=GN/D equation...correct? So if I WANTED to use an aperture of 5.6...I would divide 34 by 10 (if its 10feet) and get 3.4 (which would be the aperture)...so to get the aperture at 5.6, I would move the light closer?

Is that anywhere close to right?

Careful with the units. The GN is 34 metres, not 34 feet. I've given the calculation in my post above (post #28). You need to move the light further away, or reduce the power of the flash. You could also use a diffuser, or set the zoom to a wider setting than 50 mm and move the flash away just a little instead of a lot. Some of these adjustments will affect the nature of the light as well as the power.

Best,
Helen

 

[Sarah23]

Ooops. Ok...I think thats part of whats tripping me up.

So do I need to convert the distance to meters?

 

[Big Mike]

So depending on what mm setting I have the flash on (lets just say 50mm) I then use that corresponding guide number (which was 34 I believe) in the F=GN/D equation...correct? So if I WANTED to use an aperture of 5.6...I would divide 34 by 10 (if its 10feet) and get 3.4 (which would be the aperture)...so to get the aperture at 5.6, I would move the light closer?

As Helen said, that's in Meters, not feet...so be careful.

Also, the GN is with the flash on full power. You can turn the power of the flash down. In fact, I'd recommend trying to find a set up that allows you to shoot at a much lower power setting like 1/8 or 1/16 etc. That way, you have faster recycle times and less battery usage.

 

[Big Mike]

One Meter = 3.28083 feet.

 

[Sarah23]

OK...yeah mine can go down to 1/64 I believe

OK...so in that situation above, I could just just lower the power, and use that guide number to do that little equation and figure out if I need to still move the light?

 

[Sarah23]

HOW DO YOU REMEMBER ALL OF THESE NUMBERS?!??!?!

This is all if its on ISO 100...if I have to go to 200 or 400, this all changes, right?

 

[Village Idiot]

HOW DO YOU REMEMBER ALL OF THESE NUMBERS?!??!?!

This is all if its on ISO 100...if I have to go to 200 or 400, this all changes, right?

Yeah. But you shouldn't need to change your iso unless you're shooting in darker circumstances and are also trying to aim for getting the ambient exposure as well.

 

[Big Mike]

This is all if its on ISO 100...if I have to go to 200 or 400, this all changes, right?

Yup.

Like I said earlier....just practice and use the old guess and test method. That will get you in the ball park, then you can fine tune it from there.

Once you have things set up, you can go about changing the settings and see the affect it has. That might be a better learning method than trying to figure it out in your head.

 

[Sarah23]

OK. Thanks!! I think I am getting it. I will keep practicing! I think the biggest thing that I wasnt getting was I thought the guide number was some number that the flash spit out depending on the circumstances...but its a set number. (1/1 = 34)