How much light?

Peter69

Peter69

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Now this is probably a very dumb question (mine normally are).

Is there an equation that will allow me to calculate the amount of light needed for a particular shoot?

I.e. I'm in a completely dark room. I'm shooting @ 100ISO at F22.
As flash are measured in GNs I guess I need distance and GNs to figure it out.

You will probably need to start from the very beginning
 
•
There should be, on older flash, a scale indicating the aperture to use given
the distance. Newer flashes may display the same info on a screen. TTL
compatible units make this simpler.

The remaining point is to read the distances in the dark! ;-)
 
Doesn't have to be complicated, what are you shooting and how far away are the lights from it?
 
I'm just posting because I want to see what others have to say...
If you're trying to determine what power lights you will need, I personally don't know of another way without GN's.
 
example:
using 90mm Macro @iso 100, f28 (for dof) on a DX.

now what flash do I need at 500mm closest to be out of shot. No ambient light
 
It can be almost any off camera flash (so there is no shaddow cast from the lens and/or better modeling).
If the flash has dialable power settings that is even better.
Shot with a 100mm macro lens inside our house at night with ambient light of one 40W flouro (not a lot). The ambient doesn't really matter except to help focus and see the subject.

#1 @ f22 and 1/200 (max synch speed) and ISO 200. Canon 580EX flash gun with a diffuser and external to the camera. I can't remember what flash power I was was using (or if I just let TTL do its job (poosibly with some flash exposure compensation dialed in))
In our Family room this morning (2) by Richard Taylor, on Flickr

#2 Same settings however minimum focus distance (1:1)
In our Family room this morning (1) by Richard Taylor, on Flickr.
 
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sk66 said:
Guide number is 46ft... divide 46 by the desired f-stop (28) to determine the max distance at ISO 100... about 1.5ft.. a hair over your 500mm requirement.
Click to expand...
This is the kind of info I was after
 
GN, Ratios, Power and all that

Guide Numbers (GN)

In the days before many had meters they needed a simple way to work out what the exposure from a flashgun was. Most flashguns had something akin to a small slide rule fitted that you could move to calculate the exposure.

The output of a Flashgun is given as a guide number normally in metres but in the past it was in feet and this is usually at ISO 100.

So typically you would see something like, Guide Number = 56 (metres)

If we know the distance then we can work out the exposure from

Aperture = GN/distance

So at 10 metres we would have 56/10 = 5.6 or better f5.6

Ratios

A bit old school but many photographers quote the power of individual lights as ratios so that a 3:1 ratio would mean the key light is 3 times more powerful than the fill light. As many photographers now just work with meters that measure the exposure it can be useful to know what the f stop difference is

1:1 Both lights same power
2:1 Key light 1 stop brighter than the fill light
3:1 Key light 1.5 stops brighter than the fill light
4:1 Key light 2 stops brighter than the fill light
8:1 Key light 3 stops brighter than the fill light

EV (Exposure Values)

Sometimes we let the camera decide what is the right amount of flash i.e. exposure via TTL (Through the Lens metering). The chosen exposure by the camera is often called 0EV so if we want the flash brighter we dial in +EVs and conversely darker we dial in –Evs. A classic example, main light is left to the camera decide and fill is 2 stops darker so that light (the fill) is adjusted to -2EV and maybe a hair light at -1EV i.e. 1 stop less. (light from behind is often said to be twice as bright so a simple starting point is to have the hair light 1 stop less)

Percentages

On some flash meters it is possible to get the percentage of flash to ambient light

0% All Ambient 6% is a 16:1 ratio i.e. Ambient is 4 stops brighter than flash
10% is a 9:1 ratio i.e. Ambient is 3 1/3 stops brighter than flash
20% is a 4:1 ratio i.e. Ambient is 2 stops brighter than flash
30% is approx 2:1 ratio i.e. Ambient is 1 stop brighter than flash
40% is a 3:2 ratio i.e. Ambient is 2/3 stops brighter than flash
50% is a 1:1 ratio i.e. both lights the same
60% is a 2:3 ratio i.e. Flash is 2/3 stops brighter than ambient
70% is approx a 1:2 ratio i.e. Flash is 1 stop brighter than ambient
80% is a 4:1 ratio i.e. Flash is 2 stops brighter than ambient
90% is a 1:9 ratio i.e. Flash is 3 1/3 stops brighter than ambient
94% is a 1:16 ratio i.e. Flash is 4 stops brighter than ambient
100% All Flash

20-30% is often accepted as a good starting point for adding flash to existing light which is great if your meter gives the percentage but if not there is a trick how you can do this.
First take an ambient light reading and then introduce flash and re-meter.
Note the increase in exposure.
The following is approximate;
20% is approx ⅓ stop increase
30% is approx ½ stop increase
50% is approx 1 stop increase
70% is approx 2 stop increase
90% is approx 3 stop increase

As an example we meter at ISO 100 and we have 1/125 at f5.6 and want to add flash to be 30% i.e. we need to get an increase by 1/2 stop We add enough flash so that the meter reads 1/125 at f6.7 OR We add enough flash so that the meter reads 1/180 at f5.6

Watt/s

Unfortunately studio lights are usually quoted in Watt/s which is quite a different measure of power. It is very difficult to convert from GN to Watt/s and vice versa and whereas you can measure the GN of a flash using a meter many believe that some manufacturers get an engineer to measure the Watt/s and other companies get a salesman to do it. As soon as you change the reflector or modifier on a studio flash you are changing its output spread and hence the light you can measure. Contrary to what many say a studio light of 200-250 W/s is ideal for a small studio whereas 500- 600W/s is needed to overpower sunlight.

Sunny 16

For the really old amongst us when you purchased a roll of fill on the inside of the cardboard box there were some diagrams that gave some basic exposure guides. Sunny 16 gets its name from the fact that on a very sunny day you set your shutter to the ISO of the film and your aperture to f16

Expose to the Right (ETR or ETTR)

Due to the way that digital cameras record information there is far more detail in the highlights than in the shadows. The theory is that instead of exposing so that it looks right on the LCD screen that you base your exposure on the histogram being as close to the right as possible. Given the right sort of image the shadows will get a lot lighter. When the image is then processed the tones are shifted back to where they should be. This needs a much more detailed explanation to really understand but in part it has its roots in the zone system Zone System Used by Ansel Adams to get the optimum exposure recorded on black and white film so that as much information as possible could be printed. There are numerous books written on the subject and not being that relevant to digital you can do your own research. This is really a study of how the technical can help to produce fantastic images Chimping Looking at the LCD screen and going ooh-ooh look at the image. Unfortunately the image you see on the LCD screen is the in camera JPEG conversion and has little to do with any RAW file being shot

Mike
 
I played last night with my old Amaran Halo HN100.
It is supposed to be
Guide Number: 49' at ISO 100
Correct exposure was achieved at 1/60 f/5.6 iso100 500mm (1.64') distance.
working on the equation f/5.6=GN59/1.64 doesn't work.
Given the distance is right and aperture is right the true guide number would be GN9.24 (fully charged batteries, full power). Which is confirmed as another shot was f/11@≈275mm.

Thanks for the link and good explanation @mike weeks
 
Peter69 said:
I played last night with my old Amaran Halo HN100.
It is supposed to be
Guide Number: 49' at ISO 100
Correct exposure was achieved at 1/60 f/5.6 iso100 500mm (1.64') distance.
working on the equation f/5.6=GN59/1.64 doesn't work.
Given the distance is right and aperture is right the true guide number would be GN9.24 (fully charged batteries, full power). Which is confirmed as another shot was f/11@≈275mm.

Thanks for the link and good explanation @mike weeks
Click to expand...
Remember that the GN is also zoom dependant. The GN is generally quoted at max zoom and reduces dramatically at 'normal' zoom head settings.
 
Peter69 said:
I played last night with my old Amaran Halo HN100.
It is supposed to be
Guide Number: 49' at ISO 100
Correct exposure was achieved at 1/60 f/5.6 iso100 500mm (1.64') distance.
working on the equation f/5.6=GN59/1.64 doesn't work.
Given the distance is right and aperture is right the true guide number would be GN9.24 (fully charged batteries, full power). Which is confirmed as another shot was f/11@≈275mm.
Click to expand...
That is an LED ringlight, not a flash... I don't believe that GN at all. And the manufacturer doesn't seem to provide one. They say 1020 lux at 500mm which isn't much of anything for a flash.
A flash doesn't care about SS... The GN assumes the flash is providing the only light.
 
@sk66 in the booklet I got with it states 49' iso 100 & 1020 lux. The 'flash' has 2 functions, on & flash (which is on low and then flashes with shutter). The flash mode appears to supply much light then the constant. On the same settings I used in my previous post the subject was only just visible. I guess in that mode though you can use longer shutter speeds
 
Kodiak Qc said:
•
There should be, on older flash, a scale indicating the aperture to use given
the distance. Newer flashes may display the same info on a screen. TTL
compatible units make this simpler.

The remaining point is to read the distances in the dark! ;-)
Click to expand...
Be careful using older flash guns, they can fry modern digital cameras, some have a trigger voltage over 250v.
 
Peter69 said:
@sk66 in the booklet I got with it states 49' iso 100 & 1020 lux.
Click to expand...
Yes, but that doesn't mean it's accurate.... "Lux" is not a measurement for a flash, and 1000lux is ~ equal to shade on a sunny day. 1000lux is 1000 lumens in a square meter, and they measured it from only 50cm away.

There's just no way that equates to a standard flash GN of 49ft as is normally specified.
 
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