Help with flash exposures

[lilac22]

I'm struggling with determining correct exposures when working with manual flash off camera, especially when my flash represents the primary source of light in the scene.

Could someone please work me through it please?

A common problem is if I meter for ambient only my camera will need to be setup way beyond where I am happy, in terms of ISO and speed. So rather than setting a perfect ambient exposure, I get as close as I can without pushing my ISO and shutter speed too much and then I end up chimping with my flash firing to get enough light.

The downsides are in processing I realize my exposure on my subject can be up or down some ways; the LCD lies and it takes time and looks unprofessional.

I have a Portaflash meter for flash, but with no correct ambient exposure to work from don't understand how I can use it to resolve the above issues.

I would really appreciate some help, thank you.

 

[FotosbyMike]

The best way to approach off camera flash is meter for ambient first by adjusting shutter and ISO which affects ambient light. Now adjusting for the flash you need to change the aperture or the flash power output. F-stops like 1.8-2.8 etc need very low power from the flash some times as low as 1/128 or lower which flashes don't go any lower, F-stop of F11-F22 would need 1/2-1/1 power from the flash. ISO can also affect the flash settings too.

So say you are shooting in bright day light ISO 100, shutter speed no higher then 1/200, and f-2.8 the flash power can't go low enough to not over expose your subject you would need to bump the f-stop up to f/8 or so to get a good exposure.

Now say you are shooting indoors with some good ambient ISO 400, shutter 1/60 and f-4.0 you should set your flash output to 1/32-1/4ish and get a good exposer.

The best way for us to help is for you to post a few images with all settings; ISO, Aperture, Shutter speed, flash output power, and was the flash in a light modifier or was it bounced...etc.

 

[tirediron]

Mike's suggestion above is on the right track, but expressing it in terms of fractional flash output is, IMO, a iittle confusing since output will vary in relation/distance to the subject. Use your camera's internal meter to set the correct exposure for the ambient light (ensuring that you use a shutter speed no higher than your camera's flash sync speed, normally 1/180 - 1/250) and then bring the flash in and using your flash meter, adjust the output so that it matches.

Vis... Let's say your ambient light exposure is ISO 200, 1/200, and f4.5. With your camera in manual mode, apply those settings. Now place your flash where you want it, and using the Guide Number method (You don't have to do this, but until you've had some experience, it saves a bit of time) set an approximate exposure on your flash, just to get your close, and then do a series of test exposures with your meter until your flash shows 1/200, and f4.5 at ISO 200. You will then have perfectly balanced flash and ambient.

 

[Braineack]

A common problem is if I meter for ambient only my camera will need to be setup way beyond where I am happy, in terms of ISO and speed. So rather than setting a perfect ambient exposure, I get as close as I can without pushing my ISO and shutter speed too much and then I end up chimping with my flash firing to get enough light.

When/where are you shooting that you're struggling with "acceptable-to-you" ISO and SS outdoors?

You're capturing two exposures when you use flash on a subject: the background/ambient and the subject.

you want to setup your camera so youre exposing for the ambient, then most people stop the camera down one or two stops--it's a matter of preference. Now you'll use that incredibly rudimentary, over-complicated, Portaflash meter and match your flash exposure on the subject to match the ambient.

Since you've now set the power on the flash to match the camera settings, the exposure on the subject will be correct and the background/amibent will also be captured correctly.

The best way to approach off camera flash is meter for ambient first by adjusting shutter and ISO which affects ambient light. Now adjusting for the flash you need to change the aperture or the flash power output.

Changing your f-stop changes your ambient exposure--this is somewhat poor advice.

So say you are shooting in bright day light ISO 100, shutter speed no higher then 1/200, and f-2.8 the flash power can't go low enough to not over expose your subject you would need to bump the f-stop up to f/8 or so to get a good exposure.

the problem isn't the flash power not being low enough, the problem is the aperture being too wide with a limit of 1/200 and you're going to be overexposing the ambient--so much that the image would just be pure white. In this situation you'd need to be at ~1/4000sec to bring the ambient exposure down when using such a wide aperture.

 

[FotosbyMike]

Changing your f-stop changes your ambient exposure--this is somewhat poor advice.

I would have to disagree that this is poor advice, this is why light meters give you a f-stop reading. F-stops can change your ambient exposure but it so little I wouldn't say it is that important.

Lilac22 - Check out a few pros who shot a lot of off camera flash:
Karl Taylor - Karl Taylor - YouTube
Gavin Hoey - Gavin Hoey - YouTube
Mark Wallace - snapfactory - YouTube

 

[tirediron]

Changing your f-stop changes your ambient exposure--this is somewhat poor advice.

I would have to disagree that this is poor advice, this is why light meters give you a f-stop reading. F-stops can change your ambient exposure but it so little I wouldn't say it is that important.

Lilac22 - Check out a few pros who shot a lot of off camera flash:
Karl Taylor - Karl Taylor - YouTube
Gavin Hoey - Gavin Hoey - YouTube
Mark Wallace - snapfactory - YouTube

I think you're a little confused Mike. A change of aperture by one stop (eg f4 ->5.6) means that there is now an increased depth of field and half the amount of light coming through the lens. Given how critical both elements can be in studio composition, I would submit that relying on changing aperture to accomodate balanced exposure is going to lead to much disapointment.

 

[FotosbyMike]

Sorry Tirediron, I am not confused I know how aperture affects exposure and DOF. I was trying to keep things some what simple for someone new to off camera flash. But just like most post/forums everyone wants to through the whole book at them instead a page or 2 and have them test and play.

 

[tirediron]

Sorry Tirediron, I am not confused I know how aperture affects exposure and DOF. I was trying to keep things some what simple for someone new to off camera flash. But just like most post/forums everyone wants to through the whole book at them instead a page or 2 and have them test and play.

In my experience, short cuts, while they may cut to the chase initially, aren't really always that beneficial in the long run, and there's really no experience for learning how to do things properly, even if it takes a little longer. That said however, and putting aside our disagreement on the potential importance of aperture in relation to flash exposure, I'm curious as to why you advocate that rather than simply adjusting light output. After all, a setting is a setting...

 

[FotosbyMike]

From what I have seen in the past most new shooters have a 50mm 1.8 lens and they are trying to shot at the widest f-stop possible trying to get a shallow DOF. And they can lower the flash output to 1/128 and it will not help.

 

[Braineack]

Changing your f-stop changes your ambient exposure--this is somewhat poor advice.

I would have to disagree that this is poor advice, this is why light meters give you a f-stop reading.

yeah, and you increase/decrease the flash exposure until the f-stop on the meter matches what the camera is set to.

F-stops can change your ambient exposure but it so little I wouldn't say it is that important.

you're joking right? so you're saying that if I went from f/2.8 to f/11 and cut out 4 stops of light from the ambient that it insignificant?

You need shutter speed, aperture, and ISO in order to even determine your ambient exposure. Any change to any of them, flash or not, will change the exposure.

If I take my ambient at 1/200 and 100 ISO, like you suggest, then I need to alter the aperture to even come up with the exposure value to bring the light meter where I want it.

If I then start changing it to adjust the flash output, then I've impacted the orignal ambient exposure settings.

 

[Braineack]

From what I have seen in the past most new shooters have a 50mm 1.8 lens and they are trying to shot at the widest f-stop possible trying to get a shallow DOF. And they can lower the flash output to 1/128 and it will not help.

It's mostly because they are limited to a 1/200sec shutter sync speed, where they are unable to reduce the ambient low enough in the first place.

again: 1/200sec, iso 100, and f/1.8 on a bright sunny day = overexposed in the first place. You need a black hole flash in this instance to suck away light.

This is why people use ND filters with flash or shoot hypersync, so they can reduce the ambient while using larger apertures and still be able to use flash to exposure the subject properly.

and the key is "new shooters", so why do we care about the blind leading the blind here?

 

[MartinCrabtree]

So you argue and confuse the OP more. Nice job.

 

[Designer]

Could someone please work me through it please?

Hi, lilac22, and welcome!

As you see, there are conflicting posts here. Please do not allow that to confuse you.

 

[Braineack]

So you argue and confuse the OP more. Nice job.

I'd rather be confused now than getting advice, without challenge, that will lead to more confusion in practice.

 

[KmH]

Using the Guide Number (GN) of the flash unit :
GN = distance × f-number

and using some basic algebra:

Distance = f-number / GN
f-number = Distance / GN