Can someone put this in simple terms for me please.
- Thread starter lovejoy
- Start date
pistnbroke
Banned
divide the guide number (28) by the f number of the lens say F2.8 and you get 10 so it will expose ok up to 10 metres .. at 100 asa you can raise this distance if you use a higher ISO setting......
The 35mmFX is a way of telling you it will cover the view area of a 35mm lens on an FX camera ..about 25mm on a DX like your 5100 but often you will find its ok down to 18mm ..just try it in your living room to check
The 35mmFX is a way of telling you it will cover the view area of a 35mm lens on an FX camera ..about 25mm on a DX like your 5100 but often you will find its ok down to 18mm ..just try it in your living room to check
Take the GN (28) and divide it by the aperture you're shooting at to give you the distance the flash will fire;
So on a f/1.8 50mm lens for example; 28/1.8 = 15.6m (at an ISO of 100).
If you were to increase the ISO to say 200 then you need to multiply the GN by 1.4... so in this example it becomes; (28x1.4)/1.8 = 21.8m
Here's another example, you know how far away your subject is (2.93m) so you need to know what aperture to use: GN/2.93m
So in this case; 28/2.93 = f/9.5 (not that you can get an f/9.5 but you then use the next closest aperture.
Also, I'm assuming from your earlier figures you have the SB-700, in which case have a good read through the manual - there's some informative pages in there that explain all this
So on a f/1.8 50mm lens for example; 28/1.8 = 15.6m (at an ISO of 100).
If you were to increase the ISO to say 200 then you need to multiply the GN by 1.4... so in this example it becomes; (28x1.4)/1.8 = 21.8m
Here's another example, you know how far away your subject is (2.93m) so you need to know what aperture to use: GN/2.93m
So in this case; 28/2.93 = f/9.5 (not that you can get an f/9.5 but you then use the next closest aperture.
Also, I'm assuming from your earlier figures you have the SB-700, in which case have a good read through the manual - there's some informative pages in there that explain all this
pistnbroke
Banned
remember lovejoy you dont need to do any calculations as your flash gun is iTTL so it does it all for you
Yes I know, but I need to understand the limitations.
You have seen the wallies at rock concerts who are 1/2 a mile from the stage and trying to photograph the band with a kodak instamatic
Not just limitations - but also if you want to go manual, you need to understand all the numbers
Not just limitations - but also if you want to go manual, you need to understand all the numbers
One thing at a time Sam, I'm sticking with the iTTL for the time being
Instruction book says this of my flash unit:
[SIZE=+1]Flash Output [/SIZE]Guide Number rated at 28 meters, 92 feet, at ISO 100 at normal 35mm FX setting.
Could you give me a sample or two of what this means in the simplest layman terms
I think I read that you had a SB-700. Good flash. It has many Guide Numbers, this is only one of many, depending on the manual power level selected and flash head zoom and ISO for that matter.
SB-700 manual page H-23 and H-24 shows much of that. Page H-6 explains it.
If you need any help, see Four Flash Photography Basics we must know - Guide Numbers
GN = distance x f/stop, which gives the correct exposure (computing in the inverse square law). This is the overall idea of it. Direct flash only.
Normally, if for the power level and zoom and ISO you are using, if that GN is say 80, and if your subject were at ten feet, then the division GN80 / 10 feet = f/8, which is the aperture that gives you the correct exposure. It only works for direct flash, and Manual flash mode of course.
Note SB-700 manual page C-11 to C-15 describes a flash mode called GN. This mode already knows zoom and aperture and ISO, so you just dial in the distance, and it sets the power level automatically. This is still considered to be Manual flash mode,, you are setting the flash power for the distance. Again, direct flash only, but it is good stuff for that, because it is independent of the subject.
To compare, TTL mode is automatic, it first fires a preflash that the camera meter measures, and sets the flash power level for the shot. But reflective meters see the light reflected from the subject. A lighter or white subject reflects a lot, and the meter cuts it back to come out gray. A darker or black subject reflects very little, an the meter boosts it up to come out gray. So we often have pay attention, and use Flash Compensation as needed. TTL does however handle bounce flash well.
But GN just relates the flash power and distance and f/stop to be the right light at the subject, no matter what the subject is. Black things will come out black, and white things will come out white. Which is all generally pretty fine, if you know the distance, and if using direct flash. It is common to guess the distance, and then tweak a bit if necessary. Indoors though, you will probably like TTL Bounce flash better.
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