[adwb]

I know I am missing something simple here but please set me right with this.

manual flash and camera on manual. settings for example are.ISO 200 shutter 1/125 aperture f4.5

This gives a nice light balance between the ambient light and my subject who is in heavy shade. and shows the subject and the pretty back ground.

Now something happens in the back ground and I now need to make the back ground much darker.

I know altering shutter and aperture affect both ambient and flash, altering the aperture affects the flash only and altering the shutter speed affects the ambient only.

But since I am using 1/125 I can only go to 1/180 to stay within the sync speed and that adjustment is not enough to darken the back ground sufficiently.
So how do I do this?

If I had started with a lower shutter speed I could get there but I am hand holding and using a 100mm lens so I am wary of shake if I slow down much more? or do I need not to worry due to the flash duration?
If that is correct is there a minimum sync speed?
I assume there is some trick with ISO but when I try it seems to affect both rather than just the ambient light.

[enoeske]

You can crank the shutterspeed up to 1/180th but beyond that, you have to darken the ambient with the aperture. This means you need to crank up the power of the flash to counteract that smaller aperture. You can't do much except get a more powerful strobe beyond that.

[steve1307]

To make the background (lit by ambient light) darker you need to decrease the ambient light coming into the lens.

Increase the shutter speed.
Leave apeture, ISO and flash power the same and the flash exposure will not be affected.

The limit is 1/180sec shutter speed of course (unless you have a hotshoe flash with Hi Speed Sync capability)



If you decrease the shutter speed as you mentioned above this will make the ambient lit background brighter not darker.



If at 1/180 the background still is not dark enough you will have to decrease (stop down) the aperture or lower ISO and then adjust your flash power to light the forground subject the same as you want it.

[adwb]

If you decrease the shutter speed as you mentioned above this will make the ambient lit background brighter not darker.

What I meant was if I started with a slower shutter speed that would give more latitude to increase to darken the background hence my question

is there a minimum sync speed?

[enoeske]

What I meant was if I started with a slower shutter speed that would give more latitude to increase to darken the background hence my question

No, because in order to start out with a slower shutterspeed, you would have to have a smaller aperture. So you'll just end up in the same place.

There is no minimum sync speed. Anything less than 1/180 and the whole sensor is exposed. You do have the option of having the flash go off when the sensor is first exposed, or right before the exposure ends. This is front and rear curtain flash.

[SOldBear]

You can also use lower ISO setting (e.g. 100) and/or smaller aperture (e.g. f/11).

Of course you need more power from the flash; and the smaller aperture will render deeper DOF (that can be good or bad).

[maxfield_photo]

How 'bout a ND filter on the lens and boost the flash output?

[alohadave]

How 'bout a ND filter on the lens and boost the flash output?

That would work as well. This method is used when you want to keep your aperture open for shallower DoF.

[pop4]

I know altering shutter and aperture affect both ambient and flash, altering the aperture affects the flash only and altering the shutter speed affects the ambient only.

Incorrect. In general, altering shutter speed has no real affect on the flash exposure, only the ambient exposure. E.g. whether you're shooting at 1/180th or 1/20th or 1 second, the flash still only fires for that split second in the flash exposure, so makes no real difference to the total exposure.

It's aperture and ISO that affect both ambient and flash exposures. Here's a really good resource that I find explains things quite well: http://neilvn.com/tangents/flash-photography-techniques/

So back to your first question, you're shooting at ISO 200 shutter 1/125 aperture f4.5 and want do darken the ambient but keep the same exposure for the subject.

Here's a few things you could do:
- decrease ISO only, but you'll have to compensate by increasing flash power or move the flash closer to the subject.
- chose a bigger f-stop, e.g. f8, f11. But once again, you'll have to compensate by increasing flash power or move the flash closer to the subject
- a combination of both of the above, and compensating with more light from the flash. And you change the look of the photo due to different DOF.
- plus, there's other ideas, like the ND filter as mentioned above