Help understanding Flash and light modifiers.

Jabfinger · May 28, 2011

Hi all,

I have been trying to learn to take photograph's using flash manually. I have three flashes, two sb-700's and one Promaster 7500EDF and a
couple of stands.

I asked my brother to come over the other day so I could shoot him with/on his bike. Although I had read a great deal about shooting with flash (No really... Volumes of posts on the net) when we got to the location and setup I had no clue how to set the flash power to get a correct exposure. I spent more time running back and fourth to adjust the flashes than I did taking pictures. I must have walked three or four miles adjusting flashes and never got it right. Good workout though. Here is a shot from that day:

So after that I forced myself to buckle down and learn how to set up flashes. So what I am doing now is:

Aperture = GN/Distance

Measure the distance from the flash to the subject and use the aperture that u are shooting at and plug those into the formula. Use a little Algebra to get the GN then look up flash power and zoom head position in the flashes GN table. There are many combinations of power and zoom that you can choose from in the GN table for the number that you calculate. Amazingly enough this gives near perfect exposure on the subject.

So for a little practice I set up my flashes in the driveway and took a picture of my son's basketball:

I set the flashes up as described with Main on the camera right and fill on the left. I set the main to the calculated GN and set the fill to the calculated adjusted down 2 on the power. Of course these two flashes had different GN's due to the difference in distance. I also lit the fence in the background with the third flash and as you can see by the blown out portions I did not make the required calculations for that one. As far as the exposure goes on the basketball I am happy and satisfied, and for the next shoot I will calculate the background flash as well.

My question is pertaining to setting up the flashes with light modifiers. What happens to my calculations when I flash through an umbrella or a light panel? It would seem my calculations will fall short and I will be underexposing. Say the umbrella has a one f-stop material on it how does that figure in to the calculation and where do i measure the distance to the subject from, the flash or the front of the umbrella? For the light panel what happens to the calculation as I move the flash back away from the panel?:scratch

I know this is a lot and I would sure appreciate a little insight into this as I am pretty sure the articles on the net are not going to get this deep into it. I do not want to guess and have to do a 5k every time i do a shoot with modifiers.

Thanks

Art Scott · May 29, 2011

time for a flash meter, like a sekonic L358 or a Wein 500.......it takes the guess work out of the equation....really no more guessing or tape measures.
A light Meter will just make every thing so simple for you.......I quit measuring and calculating over 30 yrs ago........

pathfinder · May 29, 2011

Art is giving great advise.

The Sekonic 358, with the addition of the built in flash trigger, lets you set up your flashes triggered by a Pocket Wizard, pop a flash via the 358 one time. Take the reading from the 358 and put it in your camera in Manual Mode, and you are ready to rock and roll.

As long as your flash to subject distance does not change, neither does your exposure settings. This is what I do to.

Having said that, with just a bit of practice with guide numbers, you will quickly learn what setting for specific flash to subject distance to use, and find just a few quick shots, and a quick examination of your histogram will tell you what you need to do.

Jabfinger · May 29, 2011

Thanks Art Scott,

When I was younger and in Air Force Tech School(They were teaching me to be a Radar Tech) they said to me " Don't be afraid to ask questions Airman cause there are no stupid questions". A few days later when I was trying to figure out what was going on I started asking questions and they told me "We aren't training to be engineers here we are training to be technicians".

I guess what I'm trying to say is that sometimes I have a need to go deep in order to understand certain things. I think this is one of those times. I know that if I purchased a light meter tomorrow and everything worked out simply I would still want to know why. This is why I will probably be stuck in my driveway shooting basketballs instead of out makin a few bucks shooting real people for some time to come.:D

Anyway any other enlightenment would surely be appreciated...

Thanks

Jabfinger · May 29, 2011

Thanks pathfinder,

Sounds like you have a great system going for you. I was reading some of the sticky's and you as well as Art Scott have a great deal of knowledge concerning this.

My system goes like this.

1. With aperture kill the ambient.
2. Read aperture
3. Get distance to flash.
4. plug aperture and distance numbers into formula and figure GN.
5. look up GN on flash GN table to find most efficient zoom and power based on coverage angle.
6. Set zoom and flash power.

This works for me. Near perfect exposure. I can now play with shutter speed to adjust ambient without affecting the exposure on the subject.

I think...

And while you are out tomorrow shooting real people I'll be here shooting my kids Basketball or maybe I'll switch it up and shoot the soccer ball.:D

Ohiohiker · May 29, 2011

If you don't have the money to invest in a flash meter, just practice, practice, and practice some more. I have never used a meter but through using my lights in different setups and using the same lights for over a year now, I can setup the lights with the modifier I want and set my flash power and aperture and be close(within a stop) of where I want to be. Basketballs, Soccer Balls, Teddy Bears and any type is still life is good practice. Set up an object and then move your lights around it, move them closer, move them up, move them down, put modifiers on them. Do this over and over again and you can learn to "see the light". BTW if you keep using your kids over and over again, eventually they dread it every time you want to take their picture.

Hope this helps.

jiro · May 29, 2011

... With aperture kill the ambient.

This works for me. Near perfect exposure. I can now play with shutter speed to adjust ambient without affecting the exposure on the subject. I think...

I think you got it right on the second statement rather than the first. Shutter speed controls the ambient lighting, Aperture control the flash exposure. SAAF - Shutter = Ambient, Aperture = Flash. Hope this helps.

Jabfinger · May 29, 2011

Thanks again everyone I really appreciate you guys taking the time to set me on the right path.

@jiro
Thanks for pointing out step one of my system. Step one assumes you are set to max sync speed which in my case (without being in FP sync) is 1/250 and also lowest ISO (in my case 100). 1/250 wont kill ambient, no more room on ISO, so its on to aperture to kill the ambient.

Doing it this way is good(for me anyway) because I now have one value to plug into the formula Apeture = GN/Distance. Of course the other value is distance from flash to subject which is a simple measurement. I am using speedlights and I don't think they are ever going to be very far away from the subject outside. Now I have two parts of the equation Aperture and Distance and the equation becomes GN = Aperture * Distance. This to me is as simple as it gets. It takes all the guesswork out of bare flash setup. The exposure is dead nutz. Using the GN table for your flash in this way is also going to allow you to use your flash in the most battery efficient way possible because you can go down the flash output side of the GN table and pick the lowest possible flash output for your flash using your calculated GN. Can the light meter do this? I don't know but I have spent hours upon hours trying to get my head around this manual flash setup and this method make sense to me right now.

If I wanted to make my life simple I could use Nikons built in CLS and get pretty good if not great results and that would be free to me as I have that capability right now. But in doing so I would learn nothing and be at the mercy of that system. I'm not sayin light meters and CLS is not the way to go. Just not for me right now.

ziggy53 · May 29, 2011

It does sound like you have a fair grasp of the use of flash to illuminate your scene. It also sounds like you may need to adjust the ambient lighting to meet your goals for the project.

You can use time-of-day to help moderate the ambient light and you can use siting/location to choose a better ambient light situation.

For strong and direct daylight, try to find a site with a shaded overhang or some other form of open shade. In this way you can use the daylight as a cooperative light source. Adding both reflectors and your flashes can give you pretty good lighting control in this case.

The "sweet light" at the beginning and end of the day is another good strategy and it can give you both an exposure advantage as well as potential background advantages in a carefully chosen scene.

If all 3 flashes allow FP/HSS mode then you might try that mode of operation and faster shutter speeds. While it does cut the available flash output and effective GN, it can offer situational solutions.

Finally, if you really want to overpower direct sunlight it takes a lot of flash-power to achieve that goal. Either purchase additional speedlights or purchase/rent some very powerful studio strobes. I use a pair of 1800 effective Watt Second studio monolights for that purpose (key light to overpower the sun), but I generally also try to find open shade so I don't have to use full power.

ziggy53 · May 29, 2011

Jabfinger wrote: »

...

My question is pertaining to setting up the flashes with light modifiers. What happens to my calculations when I flash through an umbrella or a light panel? It would seem my calculations will fall short and I will be underexposing. Say the umbrella has a one f-stop material on it how does that figure in to the calculation and where do i measure the distance to the subject from, the flash or the front of the umbrella? For the light panel what happens to the calculation as I move the flash back away from the panel?

...

I don't see this particular question addressed in previous responses.

If you know the amount of loss a modifier induces you also need to know how the modifier affects light spread. Ideally you should either use a light meter, which you suggested you would rather not do, or use your camera as a light meter in a controlled situation first.

Get something in a neutral gray, a photographic gray card is best but anything neutral gray will work. Find a test area with little additional ambient light. Photograph the gray target with a single bare flash at known output and use manual mode on the camera. Adjust the manual exposure until you achieve a spike in the center of the camera's histogram. Use this as your starting point.

Test at different distances as well but return to your starting distance at the end of your tests.

Now add your light modifiers, one at a time. Revert to the original starting distance. Photograph the target and adjust exposure until you can again get the spike in the center of the histogram. Note the difference in exposure between the bare flash and the flash-plus-modifier. That's your loss offset from the bare flash.

Now test using different distances with the same modifier. You should be able to develop a chart of relative loss for each modifier and each distance.

You will probably find that the bare flash varies with distance somewhat differently from the modified and diffused flash. The reason is because the bare flash light is fairly well collimated and follows a fairly tight beam. The diffused light is much more scattered at the diffuser and the spread is much wider, resulting in a faster reduction in light output per unit distance.

Your chart will give you the "relative" changes and then you can apply those relative numbers into your starting guide numbers to give you a somewhat better idea of the starting guide number needed for a particular modifier and distance to subject.

When all is said and done, you will understand the need for a light meter in this situation.

Jabfinger · May 29, 2011

Thank you ziggy53,

The test that you describe will be a good way for me to determine the f-stop loss of the rip stop fabric I purchased for the light panel I am constructing.

When all is said and done, you will understand the need for a light meter in this situation.

Thats Funny

pathfinder · May 30, 2011

One thing worth knowing, aperture ( f stops ). Come f2.8, f4, f5.6, f8, f11 etc and alter the light by one stop. If your flash, in manual, is moved from 2. 8 feet to 4 feet, the light decreases by 1 stop also. Move the flash from 4 feet to 5.6 feet, and the light drops a second stop. This knowledge will help you set up and move your light stands quickly.

Jabfinger · May 30, 2011

Great stuff pathfinder!

Even though my original question has not been answered yet the thread has a lot of really valuable information and I thank everyone for taking time to post.

The other day when I finally got a grasp of how to set up my flashes manually was a really big deal to me and I am sure there is a calculation for setting up the flashes with a modifier somewhere. I will continue my search for that but in the meantime I think setting up my flashes with modifiers is going to start by setting them up without modifiers. I think that by knowing the f-stop for the material used I will be able to either adjust the flash power up by that amount or using the method, explained by pathfinder, move that much closer and be really close exposure wise.

Great stuff everyone thanks abunch:D

pathfinder · May 30, 2011

The amount of exposure change caused by a specific modifier should be quite consistent with that specific modifier, so once you know your exposure with the bare flash, you should quickly learn the amount to adjust for the modifier. As a rough guess, would guess ~ 1-2 stops of light. If 2 stops are needed, it would mean that you move your bare flash from 11 feet to 5.6 feet with the modifier.

Different modifiers will vary in their effect, but the effect should always be consistent. With some practice, you will find you dial things in pretty quickly. I would suggest starting with your flash at specific distance from the subject, and learn to move the flash to and from the subject at first, to get an intuitive feel for how accurate the distances I mentioned above are. If you can adjust your exposure by flash movement, adjusting it by varying it's output will make more sense.

Be aware, when you move your flash toward and away from the subject, you will also change the quality/ softness of the light as well.

divamum · May 31, 2011

I didn't see anybody mention that guru of all things small flash (and a Nikon man to boot), www.strobist.com. He has addressed a few of these questions in posts over the years, so it's welll worth browsing his site if you're not already familiar with it!

Jabfinger · May 31, 2011

Yes,

Thank you divamum www.strobist.com is definitely a great resource for flash work. I have spent many hours reading that site. I am sure it was that site that inspired me to recently blow my budget on flashes,triggers and stands rather than get the 85 f1.4 lens I wanted. I'll have to sneak that one in when the wife aint payin attention:D

Jabfinger · May 31, 2011

Yesterday I finished the light panel that my wife and I have been building and decided to put some of this new flash knowledge to practice. The basketball was uncooperative and I couldn't find the soccer ball so I brought out the bike.

I like how it turned out. I set this up with three flashes, the new light panel, and a shoot through umbrella. I set up the three flashes, one camera right, one camera left and the other behind the bike for the background. The right and left flashes were measured for distance from the air cleaner. The camera was set initially at 1/250 shutter speed, 100 ISO, and I killed the ambient light with the aperture. Using the aperture setting from the camera and distance measured to the right and left flashes I calculated the GN.

Aperture = GN/Distance

or

GN = Aperture*Distance

Using this GN number I referenced the flash manual to find the correct flash settings and set them into the right and left flashes. I did not measure the background flash or use it in the first test shot. I took a test shot and the exposure was good. I then placed the light panel between the flash and the bike and put the umbrella into the holder. Estimating the materials of the panel and umbrella at ~f1.5, I left the power of the umbrella flash where it was and doubled the power of the flash behind the panel. I turned on the background flash and set him to full power then took another test shot and the background flash was way to bright so I lowered the power on him.

This was the initial setup of the three flashes and it was close enough for me. It did not take long to set this up. Compared to the other day when I shot my brother and his bike this was nothing. I was in control of the light and was happy.

I did take other pictures of my bike last night and I did make minor adjustments to flash settings and I also moved the light panel around a bit but I think that was par for the course and I got some decent pictures of my bike in the process.

Thank's again for all the great feedback:D

Qarik · May 31, 2011

last thing..it is awesome to learn GN's and the manual settings for your flash. Depending on your goals though..you may just want to shoot TTL and adjust the EV. TTL will get you in the ballpark and then 1 or 2 adjustments would be all you need. This works with or with out modifiers.

Help understanding Flash and light modifiers.

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