"Zone VI," Weekly Photography Assignment for 8 - 14 March 2010

[keithsnell]

Hi Michele,

I think the use of a "guide number" for your strobe would help with your objective to be able to consistently control your flash photos.  Usually, a photographer uses the guide number, and their aperture and ISO to calculate the effective distance of their flash for a specific setting.  Since you don't have a guide number, I can help you determine one so that you can use it in the future to help determine the appropriate strobe power and aperture settings.

So, normally a guide number works like this.  If the guide number for my flash is 38 (in meters) at an ISO of 100, then I can calculate it's range for any specific aperture by dividing the guide number by the aperture value set on my camera.  So at an aperture of f8 (and ISO of 100), the range of the flash for a normal exposure will be 38/8 = 4.75 meters.  If my aperture is f16, then the range is reduced to 38/16 = 2.375 meters.  

So, if you don't have a guide number, but you know your ISO and aperture, and can determine the distance for a "normal" exposure with the strobe at a predetermined power setting, then you can calculate a guide number for that power setting.  You will need a prop like my "photo bear" or a person that is willing to sit through some testing in order to let you determine the proper power setting for a specific ISO, aperture and distance.  I would recommend a white stuffed animal, or a person dressed in white.  You will need something white at the appropriate distance in order to determine if you are "blowing out" the whites.  Once you arrive at the setting just below the threshold of blowing out the whites, then you have successfully determined the power setting for your "normal" exposure at that ISO, aperture and distance.  You should check skin tones as well in order to make sure they look "natural" at that power setting.

I would recommend starting at ISO 100 on your camera, and choosing a "typical" aperture for individual or group shots.  For standard school portrait type shots I typically use an aperture of f8, and for group shots I will use an aperture of f11.  Set up your flash at your "normal" working distance from the subject.  If you don't have a "normal" distance, then select a comfortable working distance for the conditions that you will normally be working in (size of room, etc).  Typically you want to be far enough back that you aren't in someone's "personal space."  And far enough away so that you can get "group" shots without using too wide angle of a lens.  I find that I am often shooting from a distance of about 12 to 14 feet or so (3.66 - 4.27 meters), but recommend that you adjust that distance based on your individual circumstances.  For weddings, I will set my tripod in a position that can be used at wide angle for the group shots and zoomed in for the individual portraits.  It's a pain to be constantly moving your tripod, and it takes too much time, so finding a distance that will work for both is optimum.

The point for this exercise is to pick a "known" distance that you can use to calculate the guide number of your strobe.  You might find it convenient to use a distance of 4 or 5 meters, especially if one of those numbers is listed on the distance scale on your lens.  If you chose a distance that is listed on the scale of your lens, then it makes it easier to consistently setup you flash at that distance for future events.  (Just estimate the distance, measure using your autofocus, check the scale on your lens and then adjust as necessary.)  

So now let's calculate the guide number for a specific power setting.  I'll use the following numbers for an example:  ISO = 100, Aperture = f8, distance = 5 meters, power setting on the strobe = 1/4 power.  Let's say we get lucky, and these settings provide the perfect exposure on your test subject.  That means that the guide number for your strobe at 1/4 power is 8 x 5 = 40.  Next time you go to set up your strobe, you can calculate the appropriate settings, because now you know the guide number.  

Let's say the distance to the altar is now 10 meters from where you are allowed to photograph the ceremony, and you want to determine the appropriate settings for your camera and strobe.  You know the guide number (at 1/4 power) is 40, so dividing the guide number by the distance will give you the aperture you need to use to get a "normal" flash exposure.  40/10 = 4, so that is the aperture you should use.  If you decide that you need to use an aperture of f8 instead (2 stops smaller) then you can compensate by increasing the power by 2 stops (going from 1/4 to full power on the strobe, since a "stop" of exposure is equal to doubling the amount of light), or by increasing your ISO to 400 (doubling the ISO twice = 2 stops).  Does this make sense?  Once you know the guide number, then you can always use your ISO and aperture to determine the "effective distance" for a normal flash exposure.  The math might seem intimidating at first, but it becomes intuitive with practice.

One more key point to make.  If you double the distance to your subject, you need to quadruple the power of your flash to compensate.  Let's go back to our examples again.  At our initial settings of ISO 100, aperture f8, and distance of 5 meters we were able to set the strobe at 1/4 power.  If the subject is 10 meters away (double the distance) then we needed to increase our power by four times (from 1/4 power to full power) in order to use the same aperture and ISO.  

Please try this out and see if it makes sense to you.  Please let me know if you have any questions and I will try to clarify this explanation.

Have fun!  

Keith

P.S.
I wouldn't worry about using a cable.  It won't provide any additional capability for the camera to meter the flash, and as you said it will just get in the way.

[Michele]

Thank you so much.  This information is golden.  I am going to try it out as soon as I can.

Here is the church.  See the windows that line both sides?   How do you handle such uneven lighting?  Ceremony is at 9:00 - 10:00 in the morning, so on the right side has these beams of light coming through at an angle that almost reaches the middle aisle.  The center is always in the dark. 

By the way, I am not sure I am allowed to use a strobe in the church.  I know I can use my dinky flash (which never works out for me).  I wonder if the priest would allow the strobe.  I am going to ask.

[keithsnell]

Hi Michele,

In a situation like this, where you have mixed light from sunlight and flash, you will need to treat the flash as "fill flash."  You should set your ambient exposure (without the flash) to ensure you don't over-expose any sunlit areas, and then use the flash for "fill."

If you cannot use the strobe in the church, you can try your "dinky flash"  using the same concepts.  (Determine the guide number, and use that to determine your future settings, etc.)  Given the lower power of your flash, you will have to modify your settings to try to maximize the range of the flash.  You will want to use the fastest shutter speed possible given the sync speed of your camera (1/250th of a second) and then determine what aperture is required to control the ambient exposure.  If your subjects are directly in one of the shafts of light from the windows, it will be a real challenge, since this will require an aperture in the f11 to f16 range (for an ISO of 100) which will limit the range of your flash.  Depending on how powerful your flash is, you might be able to get effective "fill" light out to 3.3 meters or so.  If you are using the flash for "fill light" instead of as the primary light source, then you can normally extend the range by 1.4 times the range for a "normal" flash exposure where flash is the primary light source.  (The flash just needs to fill in the shadows, and can do so effectively without being "full power.")  For example, if your normal range for "full power" flash is 4 meters, then you can expect to get good "fill" flash at 4 x 1.4 = 5.6 meters.  Make sense?

If you are allowed to use even your "dinky" flash, I suspect that doing so will improve your images, and help to even out the contrasty light in the church.  Just be careful about changing distances from the flash to your subject once you have determined a good flash exposure.  Keep the distance from flash to subject about the same, and use zoom if you need to change the framing, etc.

Does this help?

Keith

[Michele]

Yes, it helps.  And I just bought a Speedlite 430 EX II EOS today.  Hopefully it is as good as the reviews say.  You really are gold, you know.  This helps sooo much.  I don't know how many times I can say thank you, but you know, thank you.  X 1000000 etc.

[Michele]

By the way, my mother wants to say thank you as well.  She is so happy that I finally have people I can ask questions to about photography.  She specifically said on the phone to be sure and thank them. 

[keithsnell]

Yes, it helps.  And I just bought a Speedlite 430 EX II EOS today.  Hopefully it is as good as the reviews say.  You really are gold, you know.  This helps sooo much.  I don't know how many times I can say thank you, but you know, thank you.  X 1000000 etc.

Thanks.   

I suspect the 430 EX will make a tremendous difference in your flash photography.  The nice thing is that the camera and flash will now calculate the appropriate flash power automatically.  This will be great for those flash photos where you are on the move, and constantly changing distances to your subject.  For "static" subjects where you can control the distance, I still prefer using manual flash since I can eliminate "surprises."

Have fun with the new flash.

Keith

[Michele]

I expect I can change it to manual  too,  I hope.

[Michele]

Hi Keith, I just got the flash today.  Yup, it changes to manual.  Yup, I am excited.  I will try to use it today at my daughter's ballet class.

I better read the manual after lunch.  I have to say, I wish you lived in Switzerland.   

Michèle

[keithsnell]

Hi Michele,

I'm sure you will enjoy the flash.  I'll try to help if you have any questions.

Keith

[Michele]

Do you think that the flash would be enough in the church?  Can one set up a strobe and the flash if the strobe in not Canon?  (I have a trigger for the strobe that attaches to the camera, would it trigger off the flash too, or does it have to be a Canon?  I am just asking instead of taking everything out to test it.

[keithsnell]

Hi Michele,

From what I have read, the 430 EX II has a guide number of 43 meters when the camera is set to ISO 100 and the flash is zoomed to 105mm (it will zoom automatically when you zoom your lens).  If there is no direct sunlight in your scene, the flash will be plenty strong enough, since you will be able to adjust your ISO and aperture to increase the flash range if needed.

The most challenging situation will be if there is direct sunlight in the scene.  If there is direct sunlight, then your ISO and aperture will have to be set to keep the areas in sunlight from being overexposed, and you will have to use a lot of flash power to "fill" the shadows.  

A good guide to estimate the exposure settings required to control the direct sunlight is the "sunny sixteen" rule.  This "rule" says that in bright sunlight, if you set your aperture at f16 (thus the "sixteen" in the "sunny sixteen" title), your required shutter speed will be the inverse of your ISO.  So if your ISO is set at 100, the shutter speed required to provide a properly exposed scene in bright sunlight will be 1/100 sec.  Since the sync speed of your 50d is 1/250th of a second, you will be able to adjust your shutter speed faster, and open up your aperture by a corresponding amount in order to keep a balanced ambient exposure while extending your flash range.  By setting your shutter speed to 1/250th of a second, you can reduce the ambient exposure by 1 1/3 stop.  This gives you the ability to "open up" your aperture by a corresponding 1 1/3 stop, from f16 to f10.  Since you will be adding a bit of light from the flash to the ambient exposure, it is probably a good idea to underexpose ambient by about 1/3 stop, so I would recommend only opening your aperture up to f11.  

So now we've calculated what ISO and aperture you will need to control the ambient (sunlit) portion of the scene, and now we can determine if your flash will have enough range at that aperture/ISO setting.  Using the guide number of 43 (meters) we can determine the range by dividing the guide number by the aperture.  43/11 =  3.9 meters.  So the range at which your flash will be able to provide the proper amount of light on the scene as a primary light source is 3.9 meters.   If you will be using the flash for "fill" (assuming your are using the flash to fill in shadows from the direct sunlight) then you only need the flash to be about 1 stop less than "full" exposure; therefore you can extend the range by a multiplier of 1.4, knowing that the flash will have an intensity of one stop less at 1.4 times the distance.  So for fill flash, your flash will have an effective range of 3.9 x 1.4 = 5.46 meters.  So, although I don't know the layout of the church, I can say that you should be able to get good exposures as long as you can be within 5.46 meters of your subject.  

As long as you can get within 5.46 meters of your subjects, and the on-camera flash will provide sufficient power, I wouldn't complicate matters by trying to use the strobe too.  You will have to be careful that you give the flash enough time to fully recycle between exposures, since you will need almost all the power the flash can provide.  (Watch the "ready" light in the viewfinder or on the flash to determine when the flash is fully recycled and ready to fire again.)  If you can get closer than 5.46 meters, then the flash won't need to use full power, and the recycle times will be shorter.

Does this help?

Keith

[Michele]

Hi Keith,

Here is where I am at...  For all those interested in some icky pictures with my new flash, you are welcome to visit www.michelebollhalder.com and look at my daughter's dance class.  (although my son (in the corner playing Nintendo) and my daughter are beautiful)).     I obviously have a lot to learn.  It is very uneven and I have some strong, strong shadows.  Also, I thought when I meter, the flash would be included, but it really isn't.  Many of those bright shots are on -2.  I fiddled with the camera and with flash, but it doesn't seem to meter.  At least to me.  By the way, the last two circles with the girls are without the flash. 

It is very strong even when I brought it down by half.  Or maybe it's my position.  Of course, whatever is closest to me will be lit much brighter than farther back.  But it seems so odd as a photo, like the light is wrong.   I know it is just the first series, but I need to get a handle on this.

I would post the photos here, but I don't know where.   Any suggestions on how I can really get the metering to work?

Thank you again.

[Michele]

It's under Testing Flash.  Do you think a diffuser for the flash might cut down on the harsh shadows?

M.

[keithsnell]

Hi Michele,

Do you mind uploading a few of your "testing flash" photos into a personal gallery on this site?  To set up your personal album, click on the "Albums" link in the top center of the Gallery page,  Select the "My Gallery" option in the drop down menu, click on the "New" button in the bottom of the dialog, Highlight the words "new album" that appear in the bottom of the window and type in the name for your new album, then click on the apply modifications button.

Once you have started a new personal album, you can upload images to that album with the "upload" button you use to upload images into the weekly assignments albums.  Just select the name of your personal album when you get to that step in the upload process. 

I know this is a nuisance, but it will allow me to look at the exif data in your images so that I can try to help you determine what might be wrong with the flash settings.

Thanks!

Keith

[keithsnell]

Do you think a diffuser for the flash might cut down on the harsh shadows?

M.

I was surprised that the flash didn't come with a "dome" type diffuser in the package, or at least with a small reflector panel built into the flash.  Hmmm, I guess I've been spoiled by the flashes that I have bought in the past.

You might want to consider something like this:  http://www.bhphotovideo.com/c/product/602483-REG/LumiQuest_LQ_122_Quik_Bounce_Light_Modifier.html  This reflector appears to be very versatile, and I would probably use it with the "barn doors" open most of the time in order to bounce a portion of the flash off the ceiling and provide semi-directional lighting that has softer shadows.  (I haven't used one like this, since my flash has a built in reflector panel, but I'm guessing that it would work well.)

When I'm doing "serious" flash work, I use a pair of flashes bounced into a white umbrella (or two), but that is a whole different ball game, and much more hassle.  I'd try the Lumiquest diffuser that I linked to and see if that works for you.

Keith