Understanding manual flash unit

[Satyaa]

Hi. I am not a regular user of flashes or studio lights. Over the recent years I learned the basics and am able to use these lights under controlled conditions in make-shift basement setup. I had only one scenario for such setup and it worked well.

All my flash units are tall, big and not really portable for travel/emergency needs.

Now I am interested in a small portable/pocketable flash like these:

Godox IM20 Mini Flash (Black) IM20 B&H Photo Video (built-in battery).

Godox iM30 Mini Flash IM30 B&H Photo Video (two AAA batteries).

I have searched online and this forum, but it is not clear to me how to determine the settings on a manual flash. Specifically, given that the camera is trying to meter for available light (it's not aware of the flash unit) how do I match the flash setting for a proper exposure? I can see that the flash is overpowering at full power, and mostly even at 1/2 power. I am unable to estimate how much to reduce it, short of trying each step. I want to be able to make this fast when the subject is not going to patiently wait for me.

Is it just trial and error by experimenting few shots or is there a rule of thumb, at least as a good start? For example, if I use desired ISO, aperture and shutter speed, let's say the camera shows that my shot would be under exposed by 1.66 EV. Is there an easy way to translate that into what power level the flash unit should be set to?

Thanks.

--
See my profile (About me) for gear and my posting policy. My profile picture is of the first film camera I used in the early 80s, photo credit the internet.

 

[Jigal]

We use a Godox MS-300 studio light which is pointed directly up towards the ceiling of our living room. It's used for photos of pups which are playing in the living room, so the subjects have a limited space to freely move around.

A Godox TT-658s II is used as both controller for the MS-300 and as a fill flash (for the little light in eyes).

The lighting settings were indeed trial and error for me. I managed to find settings which lead to usable photos in the entire area where the pups are allowed to walk around.

If you use multiple light sources it's harder to guess exposures.

 

[Ellis Vener]

We use a Godox MS-300 studio light which is pointed directly up towards the ceiling of our living room. It's used for photos of pups which are playing in the living room, so the subjects have a limited space to freely move around.

A Godox TT-658s II is used as both controller for the MS-300 and as a fill flash (for the little light in eyes).

The lighting settings were indeed trial and error for me. I managed to find settings which lead to usable photos in the entire area where the pups are allowed to walk around.

If you use multiple light sources it's harder to guess exposures.

 

[tugwilson]

I'm guessing that you are using you lager flashes in ttl mode. In which case you might consider the iT20/iT22

 

[Gato Amarillo]

You are basically on the right track. It will take a lot of trial and error at first, but with some experience you will be able to get it with only a couple of test shots.

Manual flash can work well if the flash is off the camera, not so great for flash on the camera. So if you will ever be using the flash on the camera go for a TTL unit, as already suggested.

Remember that flash exposure is consistent so long as the flash is the same distance from the subject. If the flash to subject distance changes you'll have to change the power setting. With the flash off the camera you can keep the distance and power setting constant, but with the flash on the camera you likely want to vary the camera to subject distance, which means changing the power setting.

Here's how I do it for off-camera manual flash, one possible way. I keep the flash on a light stand at the same distance from the subject. If I move the subject I move the flash to keep the distance constant. I almost always set up with the light 6 feet from the subject, so I pretty much know the power setting. With the flash off and the camera on manual I find the desired overall exposure. Often I'll let the background go a bit dark. Then I turn on the flash and test to dial in the power level. Since I can control the power from the transmitter on the camera it's pretty quick and simple.

Overall, it's best to buy a TTL flash that can also do manual. That gives you the most flexibility.

Gato

 

[Satyaa]

You are basically on the right track. It will take a lot of trial and error at first, but with some experience you will be able to get it with only a couple of test shots.

OK, thanks.

Manual flash can work well if the flash is off the camera, not so great for flash on the camera. So if you will ever be using the flash on the camera go for a TTL unit, as already suggested.

OK, thank you and @tugwilson

Remember that flash exposure is consistent so long as the flash is the same distance from the subject. If the flash to subject distance changes you'll have to change the power setting. With the flash off the camera you can keep the distance and power setting constant, but with the flash on the camera you likely want to vary the camera to subject distance, which means changing the power setting.

Yes, understood.

Here's how I do it for off-camera manual flash, one possible way. I keep the flash on a light stand at the same distance from the subject. If I move the subject I move the flash to keep the distance constant. I almost always set up with the light 6 feet from the subject, so I pretty much know the power setting. With the flash off and the camera on manual I find the desired overall exposure. Often I'll let the background go a bit dark. Then I turn on the flash and test to dial in the power level. Since I can control the power from the transmitter on the camera it's pretty quick and simple.

Overall, it's best to buy a TTL flash that can also do manual. That gives you the most flexibility.

Gato

Thanks again. That's helpful.

 

[nobru1958]

Hi Satyaa,

Your flash's guide number can give you an indication.
For example, the Godox IM 30 has a guide number of 15 at ISO 100.
I'll use 16 for simplicity in my examples.
This means that at full power at ISO 100, it can adequately illuminate a subject 2m away at f/8 (2x8=16). If the subject is 1m away at f/16, etc.
If the power is halved, then there will be half as much light to cover, i.e., at 2m f/5.6, and at 1m f/11.
This only applies if the flash is illuminating the subject directly without reflection or shaping, and if the zoom head (if there is one) has been left in the position for which the guide number was calculated.

Here's a rule of thumb

Regards.

Bruno

 

[Satyaa]

Hi Satyaa,

Your flash's guide number can give you an indication.
For example, the Godox IM 30 has a guide number of 15 at ISO 100.
I'll use 16 for simplicity in my examples.
This means that at full power at ISO 100, it can adequately illuminate a subject 2m away at f/8 (2x8=16). If the subject is 1m away at f/16, etc.
If the power is halved, then there will be half as much light to cover, i.e., at 2m f/5.6, and at 1m f/11.
This only applies if the flash is illuminating the subject directly without reflection or shaping, and if the zoom head (if there is one) has been left in the position for which the guide number was calculated.

Here's a rule of thumb

Regards.
Bruno

Excellent. I will try how this works with an existing flash in manual mode before buying anything new.

Thank you.

 

[rumple]

Is it just trial and error by experimenting few shots or is there a rule of thumb, at least as a good start?

1/4 power as a rule of thumb.

Beyond that, if you want to learn manual flash: https://strobist.blogspot.com/2006/03/lighting-101.html - there are no shortcuts.

 

[Jigal]

If you use multiple light sources it's harder to guess exposures.

If only someone made a light meter that also measured flash!

That works in an extremely controlled environment (aka normal studio shoots).

If you have subjects running around an area in a room then you can't go up to them measure the flash, adjust the camera and take a shot.

You could prepare yourself with replacements (stuffed toys in my case) and take measurements in a lot of places, but in the end it remains a bit of guessing.

With some strobes/flashes you can use TTL but if the scene changes a lot and if you use multiple lights then this could also influence the balance and the desired effect.

 

[fotowbert]

I was curious if your question could be answered using some math relationships. Yes it can but the process seems like way to much effort considering the speedlights you linked to have so few power level choices available, either 5 or 7. Instead why not just take test shots starting at the mid power level. That will tell you which direction to go leaving you with either 2 or 3 choices left to try. By paying attention to what works best you are essentially developing your own "rule-of-thumb" to use in similar situations.

Continue reading if you are interested in a math based solution.

I separated the question into 2 parts, A) how much light is needed to offset the ambient underexposure and B) how to set a manual speedlight to produce that amount of light.

Part A is based on the work first reported here since it also involved flash raising the ambient light level by ΔΝ stops.

The amount of flash needed to offset an ambient underexposure is given by









Where:
ΔF = flash needed (in stops below shooting aperture)
ΔΝ = camera reported ambient level (stops below shooting aperture)

The ΔF value then needs to be converted into a specific F value relative to your desired shooting aperture. Aperture F values have the general form







where SC is the stop counter

Use the calculated ΔF amount to offset the SC for your shooting aperture and solve for the flash F value. (Interger values of SC produce the well known F value sequence 1, 1.4, 2, 2.8, 4, etc.)

Part B uses a Guide Number approach to compare the amount of light needed from part A to your speedlight capability and returns the required power fraction setting.







where:
PF = required speedlight power fraction
F = F value from part A
d = flash to subject distance
GN(SL) = speedlight full power GN

When PF calculates to 1 or less, that PF is the power fraction providing the light needed. A PF greater than 1 indicates the speedlight has insufficient power and needs to be moved closer to the subject.

Now applying the above process to your example, the amount of flash illumination needed to bring the exposure up 1 and 2/3 stops to your desired shooting aperture calculates out to be a .55 ΔF stop underexposure.

Assuming in your example you wanted to shoot at F8, you offset F8 down by .55 stops. The SC for F8 is 6 so calculate F for SC = 6 - .55 = 5.45 which gives F = 6.6, the F value the flash needs to produce.

Assuming you want to shoot at 5 feet with the iM30 (GN 49.2). Calculate the power fraction, PF, as square of (( 6.6 x 5 ) / 49.2) = .45 You would use the iM30 at half power.

Notes:

The equations will most likely return F values other than the typical 1/3 stop increments used by todays electronic cameras. An additional complication is the 1 stop increment for the power settings of the iM20 and iM30 speedlights. Use the closest available value.

To avoid doing tedious math by hand during a shoot there are probably several ways to automate this entire math process on your phone. Or you could go old school and carry reference cards and graphs having worked out typical situations beforehand. Here are ΔF values for 1/3 step increments of ambient underexposure. Also listed is the amount of flash in the total exposure for your reference.



ΔΝ ······· ΔF ···· %Flash
⅓ ······· 2.28 ···· 21%
⅔ ······· 1.43 ···· 37%
1 ········ 1.00 ···· 50%
1⅓ ····· 0.73 ···· 60%
1⅔ ····· 0.55 ···· 68%
2 ········ 0.42 ···· 75%

If nothing else this exercise has increased my appreciation for the convenience provided by my flash meter and TTL capable flash gear. Not having to struggle with flash exposure allows more attention to be directed towards artistic issues such as light quality and placement and in addition more time to connect with your subject.

• John

--
"[If you don't sweat the details] the magic doesn't work." Brooks, F. P., The Mythical Man-Month, Addison-Wesley, 1975, page 8.

 

[Satyaa]

Hi. I won't understand this as easily as you have explained, but I will go over it few times.

Thank you!

 

[kciuk aka Ken60]

Lots of things to unpack. Several little rules that when you get to understand and practice will help you .

Rule : 1

Light dissipates by the inverse square..,.,.,,.., so double the distance to the light and you will receive 1/4 of the illumination, or 2 stops. In the old days before flash meters and light meters they would use tests to gauge exposure and then tie string to the light , placing knots along the length of the string to offer idea of f stop to use. Remember everything in manual.

Rule 2:

Aperture controls flash as the duration of the flash is too short and happens while your shutter is open. Your shutter speed on a camera is limited to only exposure times when the entire shutter is open.

Shutter speed controls the ambient part of the exposure , so short shutter speeds create a flash dominant image and long shutter speeds blend the two .

Flash units for this sort of manual play need to be able to be set to reduce their output to control their brightness , but also the flash recycle times are faster at lower settings. At full power the recycle time for a portrait will cause a lot of delays and possibly you firing before it is ready.... your flash on full power might be referred to as " phasers on stun " and your model will be blinded. So look for ability to do 1/2, 1/4, 1/8 1/16 etc

Using small "on camera " style flashes " is limiting the sort of light shaping accessories you can fit. But for the moment try aiming the flash at a piece of white card and bouncing it off the card onto your subject..... for a harder look cover the card in silver foil or gold foil for a warm look. Loo roll folded and taped over the flash will soften the light as foil sweet paper will add colours.

Enough to go on there, lets see some results.

 

[Gato Amarillo]

[No message]

 

[Chimping Aberratation]

Forget the guide numbers and math equations. Instead, read some Strobist articles or books, watch a few basic YouTube videos, or—better yet—take a flash photography course or hire an experienced photographer to teach you the fundamentals. Doing things hands-on, with someone guiding you, is one of the fastest ways to learn the basics. Once you have those down, everything else becomes easier.

You can also "cheat" by using TTL mode. Modern flashes handle it surprisingly well, and modern cameras make flash photography much easier. Just check the results on the back of your camera—if it doesn’t look good, adjust your camera or flash settings and try again.

There are also many light meters that measure flash power. However, with modern digital cameras (and their displays for instant review), they’re no longer essential.