What strobe power for my simple one light setup ?

Akzidenz · Oct 23, 2021

Hi

I need to choose from the Godox flash lineup. My choices are 100, 200, 300 WS output.

Project

I would like to be able to shoot portraits for a specific project but I really want the minimum power to do the job.
The setup is just a single light in a 125cm/49 inches white umbrella with diffuser at 2 meters from the subject (Elinchrom Deep).

Constraints

- I use 50 to 400 asa films
- I use 6x7 medium format so being able to use an aperture of f8 would be very convenient
- I'd like to use 1/250 or 1/400 at least

My question
What minimum power should I get for a tight portrait (head and shoulders)? And if i need to frame wider to get the whole body (and moving back the light a bit) would it be very very different ?

EDIT (add specifics to the project)

I talked about my shutter speed because I use a Mamiya RZ67 and its giant mirror with a 250mm lens. I know it's less relevant for flash work so please ignore it.

Here are the detailed specs that I found for the lights and also more details for my setup :

STROBE 200WS
Guide Number At Full Power: 170.6' / 52 m ISO 100

STOBE 400WS
Guide Number At Full Power: 236.22' / 72 m at ISO100 with Reflector

100WS
minus 1 stop comparing to the the 200WS.

300WS
I suppose the 300WS unit is falling between the 200 and the 400.

My worst case scenario :
- Flash mounted on 125cm/49 inches white reflecting umbrella with a diffuser (-2 stops to be confirmed)
- Light at 1.8m/ 6 ft from the subject max
- Medium Format 6x7 film camera with 250mm focal plane shutter at f8
- 50 ASA PanF film

I know I should take the 400WS or the 600WS version to be safe but really the form factor is important and it's just a project, even if it's an important one. Bigger head needs bigger boom stand and I really want to work lightly. I already have a Mamiya RZ and a 250mm lens to handle ! This is why I'm so picky with my choice.

Thank you all

removed account4 · Oct 23, 2021

Hi Akzidenz

Do you have a digital camera that can use your flash without having the electrical system ruin your camera?
I'd do test exposures with a digital camera as your 21st century polaroid and when you are confident you have the right numbers fine tune it with your film camera.
If you can purchase a flash meter, I would. They don't cost very much and you will have a better grasp of what you need to do THEN you can use the 21st century polaroid . )
Good luck !
John

Akzidenz · Oct 23, 2021

Hi Jnantz

I don't have any digital camera but I have an overpriced sekonic meter that can help me with exposure and I'm confident to be able to shoot with strobes. But my questions were mainly about the power output of the flash to buy.

I really want to buy the good one and the least powerful for my needs. I'm not a big fan of "if it can do more, it can do less" (that is in fact not very accurate for flashs ) and prefer "the right tool for the job".

Thanks

removed account4 · Oct 23, 2021

If you haven't been there, you might go to the strobist (https://strobist.blogspot.com )...

good luck !
John

Akzidenz · Oct 23, 2021

I will look at this website, but it seems to be pretty loaded on information and find something so specific looks tedious :/

Thank you very much

Pieter12 · Oct 23, 2021

A big modifier and diffuser is going to need some extra power. And it depends on your space and how far the light is going to be from your subject. I would recommend going with the 300WS option. I use 1500WS units, usually not at full power but when you need it there's no substitute. You can always throw an ND gel over the strobe head if you have too much light.

wiltw · Oct 23, 2021

Akzidenz said:
Hi

Project

I would like to be able to shoot portraits for a specific project but I really want the minimum power to do the job.
The setup is just a single light in a 105cm white umbrella with diffuser (Elinchrom Deep 105 white).

Constraints

- I use 50 to 400 asa films
- I use 6x7 medium format so being able to use an aperture of f8 would be very convenient
- I'd like to use 1/250 or 1/400 at least

My question
My choices are 100, 200, 300 watts output.

What minimum power should I get for a tight portrait (head and shoulders)? And if i need to frame wider to get the whole body (and moving back the light a bit) would it be very very different ?

Thank you all

The major information left out of your statement is

WHAT DISTANCE from light to subject?
what size area are your illuminating?

The coverage area determines to some extent how far back your light has to be from the subject(s) for adequate coverage of the illumination.
The DISTANCE is the biggest fact in the question. Let us assume a 500 watt-second light which at ISO 100 gets f/13 at 10' (Guide Number 130)

at 10' shooting distance f/13
at 20' shooting distance f/6.5

...so 500 w-s studio flash is inadequatt at f/6.5 at 20' with ISO 100 film, but is fine at ISO 400 with f/13 at 20'
But bouncing into umbrella loses -1EV at best, so no better than f/9.5 (and maybe f/8!) at 20'. That helps quantify a studio head.

But if buying a speedlight, history and many individuals with many tests using flashmeters show the typical stated Guide Number is OVERstated by typically +1EV. An ISO 100 guide number of 200 on a zoom speedlight head is only guide number 130 at 50mm FL coverage, and that is +1EV overstated! Really ISO 100 guide number is 94, not 130. (f/8 at 12', not f/11 at 12'). Taken to ISO 400, that same power uses GN 188 for computation for direct flash; but again, at least -1EV due to umbrella bounce.

If your camera has a leaf shutter in the lens...so shutter speed means NOTHING in altering flash exposure with RB67, but you are shutter speed limited to X-sync speed as max for Pentax67

MattKing · Oct 23, 2021

Akzidenz said:
Constraints

- I use 50 to 400 asa films
- I use 6x7 medium format so being able to use an aperture of f8 would be very convenient
- I'd like to use 1/250 or 1/400 at least

Are you referring here to the shutter speed, or the flash duration?
Are you using a camera with leaf shutter, or focal plane shutter?
Are you trying to balance ambient light with flash?

Pieter12 · Oct 23, 2021

MattKing said:
Are you referring here to the shutter speed, or the flash duration?
Are you using a camera with leaf shutter, or focal plane shutter?
Are you trying to balance ambient light with flash?

I was wondering about the shutter speed, too. Unless there is ambient light to consider, the shutter speed is irrelevant. Most cameras reliably synch at 1/60th and I have never found the need for a different speed unless I am working outdoors or looking for a special effect.

Akzidenz · Oct 23, 2021

Hi all and thank you for the answers

I talked about my shutter speed because I use a Mamiya RZ67 and its giant mirror with a 250mm lens. I know it's less relevant for flash work so please ignore it.

Here are the detailed specs that I found for the lights and also more details for my setup :

GN 200WS
At Full Power: 170.6' / 52 m ISO 100

GN 400WS
At Full Power: 236.22' / 72 m at ISO100 with Reflector

100WS
minus 1 stop comparing to the the 200WS.

300WS
I suppose the 300WS unit is falling between the 200 and the 400.

My worst case scenario :
- Flash mounted on 125cm/49 inches white reflecting umbrella with a diffuser (-2 stops to be confirmed)
- Light at 1.8m/ 6 ft from the subject max
- Medium Format 6x7 film camera with 250mm focal plane shutter at f8
- 50 ASA PanF film

I know I should take the 400WS or the 600WS version to be safe but really the form factor is important and it's just a project, even if it's an important one. Bigger head needs bigger boom stand and I really want to work lightly. I already have a Mamiya RZ and a 250mm lens to handle ! This is why I'm so picky with my choice.

Thank you again.

Akzidenz

MattKing · Oct 23, 2021

The real issue is how much usable light output you will have available once the umbrella is included in the setup.
A flash used straight with a fitted reflector and pointed toward the subject will provide a lot more light at the subject than one that is pointed toward and bounced off a reflecting umbrella.
Which in turn will provide a different amount of light than the flash used that way with an additional layer of diffusion.
The answer to your question depends first on those efforts you are making to modify (diffuse and otherwise change the shape and size) the light, and then on the light actually supplied.

Chan Tran · Oct 23, 2021

Akzidenz said:
Hi all and thank you for the answers

I talked about my shutter speed because I use a Mamiya RZ67 and its giant mirror with a 250mm lens. I know it's less relevant for flash work so please ignore it.

Here are the detailed specs that I found for the lights and also more details for my setup :

GN 200WS
At Full Power: 170.6' / 52 m ISO 100

GN 400WS
At Full Power: 236.22' / 72 m at ISO100 with Reflector

100WS
minus 1 stop comparing to the the 200WS.

300WS
I suppose the 300WS unit is falling between the 200 and the 400.

My worst case scenario :
- Flash mounted on 125cm/49 inches white reflecting umbrella with a diffuser (-2 stops to be confirmed)
- Light at 1.8m/ 6 ft from the subject max
- Medium Format 6x7 film camera with 250mm focal plane shutter at f8
- 50 ASA PanF film

I know I should take the 400WS or the 600WS version to be safe but really the form factor is important and it's just a project, even if it's an important one. Bigger head needs bigger boom stand and I really want to work lightly. I already have a Mamiya RZ and a 250mm lens to handle ! This is why I'm so picky with my choice.

Thank you again.

Akzidenz

Make a test. Use a flash meter.

Pieter12 · Oct 23, 2021

Akzidenz said:
Hi all and thank you for the answers

I talked about my shutter speed because I use a Mamiya RZ67 and its giant mirror with a 250mm lens. I know it's less relevant for flash work so please ignore it.

Here are the detailed specs that I found for the lights and also more details for my setup :

GN 200WS
At Full Power: 170.6' / 52 m ISO 100

GN 400WS
At Full Power: 236.22' / 72 m at ISO100 with Reflector

100WS
minus 1 stop comparing to the the 200WS.

300WS
I suppose the 300WS unit is falling between the 200 and the 400.

My worst case scenario :
- Flash mounted on 125cm/49 inches white reflecting umbrella with a diffuser (-2 stops to be confirmed)
- Light at 1.8m/ 6 ft from the subject max
- Medium Format 6x7 film camera with 250mm focal plane shutter at f8
- 50 ASA PanF film

I know I should take the 400WS or the 600WS version to be safe but really the form factor is important and it's just a project, even if it's an important one. Bigger head needs bigger boom stand and I really want to work lightly. I already have a Mamiya RZ and a 250mm lens to handle ! This is why I'm so picky with my choice.

Thank you again.

Akzidenz

Depending on the speed lights used, this might be a solution to consider:

Or a battery-powered power pack with a separate head (like a Hensel Porty or a Elincrom ELB 1200) that would be lighter and easier to mount on a boom than a monolight.

removed account4 · Oct 23, 2021

Akzidenz said:
I will look at this website, but it seems to be pretty loaded on information and find something so specific looks tedious :/

Thank you very much

Your welcome ...
Well I guess it is tedious to learn what you want to learn then.
The strobist site t has exactly what you are looking for and shows through VIDEO how to do it without spending 1thousand euros. They even sell a lighting KIT that has everything you need.
Good luck!
John

David A. Goldfarb · Oct 23, 2021

The W-s rating doesn't really tell you what the output is, except within one system, all things being equal. It tells you the power going into the head. The output depends on that and the efficiency of the head and the modifiers, such as reflectors and umbrellas you are using.
A big umbrella with a diffuser is probably going to eat 2 stops. A tight head and shoulders portrait on 6x7 will require around 1/3 stop extra exposure due to magnification or "bellows factor." So if this is your only light, get the biggest one you can manage and afford, then a film of a speed that works with what you've got. Alternately, if you have a strong preference for a particular film, start with that and figure out how much power you need to shoot with the speed of your preferred film.

So what's the whole situation? Are you in a studio or outdoors? If in a studio, is the background dark or light? If you want it to be light, you'll also need to illuminate the background, so you'll need another light. If it's dark, what color is the model's hair? If the hair is also dark, you might want a kicker to get some separation from the background. Maybe think about getting a second-hand 800 W-s or larger pack system with two or three heads, and that will give you a lot of options for many different portrait setups with medium format film.

Akzidenz · Oct 23, 2021

David A. Goldfarb said:
The W-s rating doesn't really tell you what the output is, except within one system, all things being equal. It tells you the power going into the head. The output depends on that and the efficiency of the head and the modifiers, such as reflectors and umbrellas you are using.
A big umbrella with a diffuser is probably going to eat 2 stops. A tight head and shoulders portrait on 6x7 will require around 1/3 stop extra exposure due to magnification or "bellows factor." So if this is your only light, get the biggest one you can manage and afford, then a film of a speed that works with what you've got. Alternately, if you have a strong preference for a particular film, start with that and figure out how much power you need to shoot with the speed of your preferred film.

So what's the whole situation? Are you in a studio or outdoors? If in a studio, is the background dark or light? If you want it to be light, you'll also need to illuminate the background, so you'll need another light. If it's dark, what color is the model's hair? If the hair is also dark, you might want a kicker to get some separation from the background. Maybe think about getting a second-hand 800 W-s or larger pack system with two or three heads, and that will give you a lot of options for many different portrait setups with medium format film.

@David
I think you miss my second post where I'm more specific. I'll edit my first post if you want to take a look. Thanks!

@jnantz
Actually I wish I could take the time to learn but this is why I'm here, to take shortcuts I just need advice to pick a strobe to get the job done.

@Pieter12
Thanks but I prefer a dedicated flash unit !

@Chan Tran
Hmmm I need advices to buy a strobe with a set of constraints. I'm not sure about what you want me to test ?

@MattKing
Thanks yes indeed a modifier produce light loss. I was just thinking that the basic settings I have in mind (one light setup, umbrella with diffusion layer at 2m./6ft max from the subject) could be simple enough to pick the best strobe for my set of constraints.

Thank you all !

removed account4 · Oct 23, 2021

Akzidenz said:
@jnantz
Actually I wish I could take the time to learn but this is why I'm here, to take shortcuts I just need advice to pick a strobe to get the job done.

good luck with that --

Mr Bill · Oct 23, 2021

Akzidenz said:
Here are the detailed specs that I found for the lights and also more details for my setup :
...
GN 400WS
At Full Power: 236.22' / 72 m at ISO100 with Reflector

Akzidenz said:
My worst case scenario :
- Flash mounted on 125cm/49 inches white reflecting umbrella with a diffuser (-2 stops to be confirmed)
- Light at 1.8m/ 6 ft from the subject max
- Medium Format 6x7 film camera with 250mm focal plane shutter at f8
- 50 ASA PanF film

Hi, given that you have some guide numbers, along with scenarios, you can calculate the situation. The only question is whether the guide numbers are good. (If this is professional studio gear I'd guess that they are.) Plus, you are apparently making the assumption that your umbrella assembly will lose 2 stops of light.

Let's start with the 400Ws spec, w/Reflector, and ISO100 film. Since I'm from the US I like to work in feet, so you gave a guide number of 236 feet.

Now, in order to calculate a new guide number, for each time you cut power in half, or cut film speed in half, you divide the guide number by 1.41 (this is the square root of two).

So here's some numbers, just to confirm that they match yours...
400 Ws @ ISO 100, GN = 236
200 Ws @ ISO 100, GN = 167

Then, carrying them further...
100 Ws @ ISO 100, GN = 119, and then finally for ISO 50...
100 Ws @ ISO 50, GN = 84

Now, you are figuring that the umbrella/diffuser combination will cut your light by 2 f-stops; this will cut the guide number in half (same thing as dividing by 1.41 two times), so...
100 Ws @ ISO 50, then losing 2 f-stops, GN = 42 (feet).

Now, for anyone who doesn't know how to use a guide number, you can divide it by distance to get the f-number (or divide by f-number to get distance). You wanted to be able to shoot at f/8, so GN 42/8 = 5.3 feet. This is pretty close to your "worst case" situation of light to subject distance of 6 feet (max).

So it looks to me like you would be able to get by with the 100 Ws flash unit. But... in the event that the guide numbers are overly optimistic, or you lose more than 2 f-stops out of the umbrella, then a more powerful flash gives you a little "cushion."

Ps, you mentioned using a focal plane shutter. Your camera should have an official "sync speed," which is the fastest shutter speed allowable with an electronic flash. (The shutter must be fully open, so the entire film surface is illuminated by the flash.) So this may give you problems if you are photographing in bright ambient light - the ambient exposure may tend to overpower your flash, meaning that you may get motion blur with the relatively long shutter speed.

Akzidenz · Oct 23, 2021

Mr Bill said:
Ps, you mentioned using a focal plane shutter. Your camera should have an official "sync speed," which is the fastest shutter speed allowable with an electronic flash. (The shutter must be fully open, so the entire film surface is illuminated by the flash.) So this may give you problems if you are photographing in bright ambient light - the ambient exposure may tend to overpower your flash, meaning that you may get motion blur with the relatively long shutter speed.

Hi @Mr Bill
I'm sorry it was a mistake, english is not my main language. It's a leaf shutter with the Mamiya Z lenses.
Your answer is perfect. 100ws can be used but a small "cushion" would be appreciated. I will take the 200ws flash unit.
Thank you very much !

Mr Bill · Oct 23, 2021

Akzidenz said:
Hi @Mr Bill
I'm sorry it was a mistake, english is not my main language. It's a leaf shutter with the Mamiya Z lenses.
Your answer is perfect. 100ws can be used but a small "cushion" would be appreciated. I will take the 200ws flash unit.
Thank you very much !

You're welcome, hopefully that does the trick. You may want to double check my math, just in case I botched something up.

Chan Tran · Oct 23, 2021

I asked the OP to test and he can't because we no longer have brick and mortar store. Back in the day we could simply go to the store and ask them to set up and see which one fit our need.

cowanw · Oct 23, 2021

Mr Bill
Would 100 to 200 be a 2x cushion or a 1.4x cushion?

Pieter12 · Oct 23, 2021

Not sure what you mean by dedicated flash unit. That usually refers to a speedlight that electronically communicates to a specific brand or model of camera, having automatic functions that sometimes can be controlled by the camera body. I don't think that is what you have in mind for if there are such units for the RZ, the choice is probably severely limited.

Mr Bill · Oct 23, 2021

cowanw said:
Mr Bill
Would 100 to 200 be a 2x cushion or a 1.4x cushion?

Hi, well it's a 2X cushion with regard to exposure. But the guide number only increases by 1.4X.

AgX · Oct 23, 2021

MattKing said:
The real issue is how much usable light output you will have available once the umbrella is included in the setup.

Already with a/their standard reflector studio flash lights make different use of the capacitator energy (Wsec). So that value is only a rough guide.

What strobe power for my simple one light setup ?

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