Shutter speed and ISO in manual flash mode

[qleak]

Okay after a couple of hours of pure goofing around, I think i finally understand something about flash photography. Here's my realization:

• Shutter speed is all about controlling the ambient exposure. The slowest duration flash for an SB700 is 1/1000 but it's T.1 ~ 1/305, since the sync speed is 1/320 max before going to high speed sync mode, you really won't be freezing the action any better than the flash.
  
• Below both are f8 1/16 manual flash. This is 4 stops lower than 1/1 power which has a guide number of 92ft so it's equivalent to dividing the GN by a 4 stops lower f-number f32 92/32 ~ 2.9ft. I set the flash on a stand 2 ft away. Picture 1 is 2" exposure Picture 2 is a 0.1" exposure. The exposure on the plant is of course changed a bit by all the extra ambient light in #1.

• The aperture changes the guide number GN (the effective range of the flash), and the ambient exposure.
  
• ISO also affects the guide number, but every stop change in iso is sqrt(2) change of GN.
• Distance from flash to subject should be less than or equal to the guide number. GN mode on a nikon flash is really handy for getting a baseline on this. I wanted to use my old gossen luna-pro f but the flash mode does not work correctly!
• ND filters can be used to widen open up your aperture while maintaining the same ambient exposure.
  
• Both of below are shot at ISO 100, 1/60th shutter. Picture #1 is at f8 and picture 2 is at f2.8 with a 3 stop ND filter. I think i stacked too many filters and introduced some vignetting too.

 

[Braineack]

correct.

ISO and Shutter to control ambient

Aperture and Flash Power to control flash exposure.


I would have rather just brought my flash down 3 stops worth of power from 1/16 to 1/64 instead of using the ND filter.

 

[qleak]

correct.

ISO and Shutter to control ambient

Aperture and Flash Power to control flash exposure.


I would have rather just brought my flash down 3 stops worth of power from 1/16 to 1/64 instead of using the ND filter.

That's a really good point about reducing the flash power 3 stops. With most speedlight flashes that will probably be the best option because it will reduce the flash duration also.

Thanks for your input

 

[Braineack]

curious as to why the flash duration would necessarily matter?

at 1/64 power, my flash duration is 1/25000sec--You're shooting at 1/60sec; a heck of a lot slower than the flash cycles.

They only time, I think, this starts to matter is when you break the sync speed, and shoot at say 1/1 power (1/1000sec duration) but at a shutter of 1/4000sec.

That means the flash output will be significantly lessened because the shutter is opening and closing faster than the entire flash duration, but the trade off is being able to shoot at fast shutter speeds if you so desire.

 

[qleak]

curious as to why the flash duration would necessarily matter?

at 1/64 power, my flash duration is 1/25000sec--You're shooting at 1/60sec; a heck of a lot slower than the flash cycles.

They only time, I think, this starts to matter is when you break the sync speed, and shoot at say 1/1 power (1/1000sec duration) but at a shutter of 1/4000sec.

That means the flash output will be significantly lessened because the shutter is opening and closing faster than the entire flash duration, but the trade off is being able to shoot at fast shutter speeds if you so desire.

You're right once you're past 1/1 I suppose that's not really much of an issue. Especially shooting still life like my example, not very much action happening there.

1/25000 is that your T0.5 flash duration time? I've been looking up a measured T0.1 flash duration for my model flash here.

Thanks again for your input, I'm still trying to wrap my mind around the whole flash photography concept.

 

[TCampbell]

Very nice!

You can combine the guide number along with the f-stop, power setting on the flash, and ISO to just work out a sum "effective" f-stop. Just don't mix in the shutter speed to the math since the shutter is collecting light continuously while it's open but the flash is only providing a momentary burst of light regardless of how long the shutter is open.

e.g. if using f/8 with ISO 400 (2 stops up from 100) and 1/4 flash power (2 stops down). Then it's effectively 8 - 2 + 2 or just 8 again.

It may seem silly to increase the exposure sensitivity by 2 stops (increasing the ISO) but then drop the flash output by 2 stops, but in reality it does change everything because changing the ISO increases the sensitivity both to the flash AND to the ambient background. But decreasing the flash power decreases the flash power but retains the same rate of flash "fall off" (whereby each time the distance from flash increases by the square root of 2 the amount of light at that more distant location is halved.)

E.g. if you're getting just the right amount of light on a subject 2 feet away, then at 2.8 feet (2 x 1.4 (1.4 being our liberally rounded value for the square root of 2) which works out to 2.8 feet. Anything 2.8 feet away gets half as much light compared to something 2 feet away. At 4 feet away it halves again (or 1/4 of the light).

By cutting flash output power by 2 stops but increasing the ISO by 2 stops you neutralize the effect of the flash at your intended subject distance (the plant will seem neither brighter nor darker) but the background will seem brighter even with the same shutter time. You could ALSO just leave the flash and ISO alone and increase the shutter exposure by 2 stops.

As for the use of ND filters... that's good advice.

Sometimes when I'm shooting outdoors in bright-isn light, I'll use my flash as "fill", but this then restricts me to my flash sync speed and THAT forces me to use a smaller aperture. If I want the background blurred, then I'll attach an ND filter to allow me to open the aperture back up so I can have a reduced depth of field.

 

[qleak]

TCampbell: Thanks! I appreciate your perspective on this

 

[qleak]

Something I haven't had a chance to try yet. Bounce flash. Do you just use the total distance traveled from the flash to the surface to the subject for guide number calculations?

 

[TCampbell]

Something I haven't had a chance to try yet. Bounce flash. Do you just use the total distance traveled from the flash to the surface to the subject for guide number calculations?

No. Unfortunately unless it's a surface you've already dealt with and know it's reflectivity, you'll have to test it.

The uncertainty in bouncing is the reflectivity of every surface may be different. A glossy white painted ceiling is going to be more reflective than a flat paint.

 

[photoguy99]

Use the total distance and then open up a stop to account for light lost in the transaction. Generally get you in the ballpark!

 

[WayneF]

Bounce is probably two, and maybe three stops more. TTL mode is great for figuring that out.

 

[Bebulamar]

Something I haven't had a chance to try yet. Bounce flash. Do you just use the total distance traveled from the flash to the surface to the subject for guide number calculations?

No. Unfortunately unless it's a surface you've already dealt with and know it's reflectivity, you'll have to test it.

The uncertainty in bouncing is the reflectivity of every surface may be different. A glossy white painted ceiling is going to be more reflective than a flat paint.

Even when you know the reflectance of the surfaces involved it is still very complicated. The GN applied only to a point light source that is a light source which is significantly smaller than the distance between the light source and the subject. When the light hit the bounce surface it turns the bounce surface into a larger light source which the GN method no longer valide.