Slow shutter speed to balance flash with ambient light - motion blur

[dubcat]

Hi - I have been getting better results since I started using higher ISO and slower shutter speed to expose the background better when using a flash. I would prefer to use the lowest ISO possible in order to minimise noise. This means slowing my shutter speed down then. I read that the flash freezes action and so you can use a slow shutter speed. However, at some point surely the shutter speed is so slow that the ambient light hitting the subject causes motion blur. Is that right? Clearly I would raise the ISO before getting to this point - but how can i predict what the slowest shutter speed I can safely use is without suffering from motion blur?

 

[Garry Edwards]

I'm guessing that you're talking about using a hotshoe flash outdoors?

Yes, of course there's a point at which the level of ambient light is high enough to create movement blur even though there's a flash. Generally though that's not a bad thing because if the flash isn't at full power the flash duration will be pretty short and so will create an underlying sharp image even if there is movement blur too - which can make many shots look more natural.

but how can i predict what the slowest shutter speed I can safely use is without suffering from motion blur?

Just experience and some quick mental arithmetic
Personally I'd be more worried about camera shake than subject movement blur, but you can predict the level of blur quite easily. It will depend of course on the speed of the subject, the direction of travel, the magnification of the lens and the magnification of the image.

 

[dubcat]

No - i was talking about flash indoors.. like at parties and stuff where you don't want the shot to look like it was taken in a cage. I just slow the shutter speed down to expose the background.. i ramp up the iso too..

 

[Garry Edwards]

No - i was talking about flash indoors.. like at parties and stuff where you don't want the shot to look like it was taken in a cage. I just slow the shutter speed down to expose the background.. i ramp up the iso too..

Well, it's actually easier to answer questions if the question explains the problem, but not to worry.

It's exactly the same answer, but with the very low level of ambient light you'll need a much longer shutter speed, which will usually cause problems with camera shake - unless of course you use a very high ISO setting, which will cause its own problems. Using a hotshoe flash like this indoors is a very well established and useful technique but it does need a steady hand - or a handy door or wall to lean against.

 

[dubcat]

OK... well I guess IS should help and i have that on my camera.. i will just experiment.

 

[CanonDad]

IS is good but wont freeze action.
IS just compensates for hand shake at slow shutter speed on subjects that arent moving.

 

[dubcat]

IS is good but wont freeze action.
IS just compensates for hand shake at slow shutter speed on subjects that arent moving.

Yup I get that. I was just replying to garys comment about camera shake. I remain confused about how to judge when photographing my moving kids when I am letting in enough ambient light to result in motion blur. I guess it is something I will have to experiment with.

 

[HoppyUK]

Hi - I have been getting better results since I started using higher ISO and slower shutter speed to expose the background better when using a flash. I would prefer to use the lowest ISO possible in order to minimise noise. This means slowing my shutter speed down then. I read that the flash freezes action and so you can use a slow shutter speed. However, at some point surely the shutter speed is so slow that the ambient light hitting the subject causes motion blur. Is that right? Clearly I would raise the ISO before getting to this point - but how can i predict what the slowest shutter speed I can safely use is without suffering from motion blur?

I think you've got a pretty good understanding of the issues there.

In situations like that (low ambient indoors, longer speeds) camera shake is usually not nearly such a problem as subject movement, especially with IS which will run you down really quite long with a standard/wide lens.

Up to a point, a bit of subject blurring is good, and the flash will overlay a sharp stamp on it which often looks quite good, but that quicky desends into a blurry mess. Then, if you've run out of aperture and ISO options to keep the shutter speed up, you have to choose between either more ambient blur, or letting the background go dark with the flash taking prominence.

You just have to learn about movement blur. There is science to it, but it doesn't really apply in practise. For example, a car driving across the frame will blur very easiliy, but coming towards you much less, and at greater distance hardly at all. That theory is fine, but with people, hands and legs blur easily but that doesn't usually matter too much, but blurred faces are never acceptable.

If the room isn't too large, sometimes you can effectively supplement or even overwhelm the background ambient light by bouncing the flash as widely as possible, plus a bounce card or something for the direct flash element. Stofens are good for that, spreading the light fairly indescriminately but pretty much everywhere which is what you want. Most diffusers will do similar. You need a lowish ceiling and a powerful gun.

 

[helen_s]

It's just a trick of balancing ambient light with flash. I usually set the exposure before turning the flash on, so that I am roughly 2 full stops underexposed. Then turn the flash on in ETTL. That seems to do the trick usually. Bouncing the flash off a nearby wall softens the light too. If you find your exposure is not spot on, adjust the compensation of the flash, rather than adjusting your settings again.

H x

 

[dubcat]

Helen - that's a great tip!!! Thanks I will try the 2 stop thing out. Makes perfect sense too.