October 7, 2008

flash exposure is controlled by aperture … but only for manual flash

Quite often the short-hand descriptions of the fundamentals of flash photography become misleading ‘facts’.  I frequently see the following statements repeated on the various photography forums and in questions that are emailed to me:

  • flash is controlled by aperture
  • shutter speed controls ambient light.
    (This is sometimes twisted around to a completely misleading version:
    ambient light is only controlled by the shutter speed.)

Even though these descriptions are half-right, they are also half-wrong and will obscure a proper understanding of how shutter speed, aperture and ISO interconnect for ambient light and flash photography.

But first, for the love of all that is good on this planet, if your camera has the ability to set 1/2 stop indents for shutter speed and aperture, please take it off that setting and change it so that your camera has  1/3rd stop indents for everything.  This is essential.

This becomes important in situations where you already have correct exposure, but you want to change any of your settings in tandem.  Then you can change either your shutter speed or aperture or ISO (or any combination of them), simply by counting clicks.  Eg: If you want a full stop more shutter speed (ie 3 clicks), then you can count 2 clicks wider on your aperture, and 1 click more on your ISO, to still give you correct exposure for your new shutter speed.

The topic of TTL flash vs manual flash has been covered here before. However, this time I want to concentrate specifically on those two statements mentioned earlier:

  • flash is controlled by aperture

This depends on whether you are shooting manual flash or TTL flash. There are distinct differences.

With TTL flash, if you are within the range of what your flash can put out, then your choice of aperture and ISO in effect becomes transparent.  (Please re-read that statement until the implications become clear.)

With manual flash however, aperture does have a direct influence on flash exposure …
but so does ISO.  We should not leave ISO out of the equation with manual flash.

Shutter speed has no effect on flash exposure (below max sync speed),
BUT … with ambient light, any chosen shutter speed (at a chosen ISO), will dictate a specific aperture. Therefore your choice of shutter speed clearly does have an indirect influence on your flash exposure.

For example, the reason why we choose max flash sync speed when working in bright ambient light, is that at max sync speed we have the widest possible aperture (while remaining outside of High-Speed Flash settings), and therefore have the most range from our flash.  Which is also the reason why it makes most sense to shoot in manual exposure mode on our cameras when doing this, even when using TTL flash.

So in the case of mixing flash with available light, the shutter speed does dictate the aperture, and therefore affects our flash exposure, or how hard our flash has to work.

Now, about the other comment:

  • shutter speed controls ambient light.

This is true.   But it neglects to mention the two other players here - aperture and ISO.  This somehow slips by quite often in descriptions of balancing flash with available light, and it becomes another misleading short-hand description.  And as explained earlier, choice of shutter speed actually often does have an important bearing on flash photography.

The theme of this post has been extended by a subsequent article, explaining it with some images:
when aperture does not control flash exposure.  This might help to make it even more clear.

 

help support this website

 

{ 10 comments. } Add a Comment

1 Lucian October 7, 2008 at 10:09 pm

Hey Neil, another insightful read!

It’s not a false statement for me to say that your site has been my greatest source of insight for flash photography and even certain aspects of photography (eg. exposure, metering, shooting in manual mode).

Your teaching has done wonders to my pictures and in the end, that’s probably what matters most!

Reply

2 Iztok Grilc October 8, 2008 at 4:08 am

Neil – good article.
In the middle of summer, I also take some time and wrote simple article on my blog about flash. If someone want to read about simple flash using, welcome to
http://kacnje.blogspot.com/2008/06/canon-flash-system-simple-way.html

Reply

3 Alessandro October 8, 2008 at 7:59 am

Ok,

let me see if I get this right.

Assuming:

Manual Flash
Constant flash-subject distance
Constant flash power
Constant ambient lighting on subject
Changes made keep subject within flash’s usable range for given settings

given a reference shot taken at mid iso, mid Aperture, mid Shutter speed …

Changing Shutter Speed alone will affect ambient exposure but not flash exposure. In essence Shutter Speed controls ambient light

changing ISO will affect BOTH flash and ambient lighting equally, in essence ISO *controls* both flash and ambient

Changing Aperture alone will affect BOTH flash and ambient equally, in essence Aperture *controls* both flash and ambient

Given the peculiarity of Shutter speed only affecting Ambient exposure, you can use that in order to negate the Ambient component of a change in either Aperture or ISO.

So given the reference shot I mentioned above…

Change the shutter speed alone faster or slower and you get a shot exposed no differently in terms of flash but darker or lighter in terms of ambient.

Change the Aperture or ISO and make a corresponding *negating* change in shutter speed (open lens increase shutter, stop down lens decrease shutter, +iso faster shutter, -iso slower shutter) and you get a shot exposed no differently in terms of ambient but darker or lighter in terms of flash.

I think I’m correct

and if so I think this little thought exercise was INVALUABLE to me as I was *stuck* on just the shutter for ambient and aperture + negating shutter for flash… I had NOT realized that ISO can also be used with a compensating shutter move to affect flash (and as a result, keeping a given DOF!).

Thank you!

And please do correct me if I’m wrong (though I will undoubtedly play a bit… but I think I’m right )

Reply

4 Pat October 8, 2008 at 9:27 am

Thanks SO much for all the time you put into this site. It’s an incredible resource!

Reply

5 Paul Hodgson October 8, 2008 at 4:55 pm

Neil, as far as I’m concerned, use as many words as you like…you type and they will come!

Reply

6 Pla October 9, 2008 at 10:47 am

I don’t understand why shutter speed does not affect the flash exposure.

Why…

If I shot @ 1/250s @ f/4 with flash settled to correct exposure my subject

and….

if I shot @ 1/125 f/5.6 (so I increse 1 stop shutter speed and aperture for more dof)

I change the flash exposure ?

I reduce the aperture by one stop, so this mean half light.. in order to compensate it I increase the shutter speed by 1 stop. So, the flash light sould seem to be equal.. or not ?

I’m missing something…. sorry Neil…

Reply

7 Alessandro October 10, 2008 at 8:30 am

Pla,

I think I can answer this one :-)

ambient light is a contiunous source… the sun is on for the whole time of that 1/250 and it’s on for the whole 1/125, and the intensity of the sun doesn’t change (relatively speaking).

The result is that if you shoot at 1/125 the sensor is exposed to a constant amount of light for twice as long as if you shoot at 1/250… hence you get one stop brighter exposure.

Flash is an instantaneous (for lack of the technical term) source. For the purpose of this post we’ll assume that the flash burst lasts 1/4000th of a second. If you set your camera to 1/125 you get a flash burst that lasts 1/4000th of a second and then for the remainder of the exposure there is no flash. If you set the camera to 1/250 you get a flash burst that lasts 1/4000 of a second and then for the rest of the time there is no flash. Therefore your exposure will get the same exact amount of light from flash regardless of what shutter speed you select.

Hope this Helps

By the way… what you posted is true assuming MANUAL flash. If you set your flash to TTL it will most likely give you the same flash exposure in both cases by varying the intensity of the flash.

P.S. and that last bit is interesting in and of itself. I was doing some reading up on this last night and found out that most (if not all) flash (at least portable strobes anyway) accomplish their varying intensity (power) feature not by actually varying the intensity of the light… but by varying the DURATION of the flash. So for instance say that at 1/4 power your flash fires for 1/8000th of a second. Then at full power it might fire for 1/2000 of a second or thereabouts…. but the INTENSITY is for all intents and purposes, constant. I thought that was pretty interesting.

Reply

8 Neil November 15, 2008 at 2:09 am

Pla ..

It is like Allesandro explained. But to add a bit to it …

Shutter speed has no effect on flash exposure (while remaining below max sync speed.) This has to do with the way a shutter works with two curtains opening. The duration of how long the curtains are open, determines the shutter speed duration.

However, flash can be considered to be nearly instantaneous and hence needs the entire frame to be open / clear so that entire frame will receive proper exposure from the flash.

This means that as long as you keep to ANY shutter speeds such that the entire frame is open for that instantaneous burst of flash, then you will have your frame evenly exposed. These range of shutter speeds are ANY shutter speed below max sync speed. Above max sync speed, one of the shutter curtains will partially obscure your frame, and you won’t get even flash exposure.

(This really needs a good diagram to facilitate the explanation, and is something I properly cover during the workshops, and will also cover properly in a future posting on this blog.)

Neil vN

Reply

9 Neil November 15, 2008 at 2:12 am

Pla ..

Re the example settings you mention. It is as I said in the original posting here:

In the case of mixing flash with available light, the shutter speed does dictate the aperture, and therefore indirectly affects our flash exposure, or how hard our flash has to work.

Therefore in your example, the flash exposure is affected by the change in aperture .. which happened as a result of your change in shutter speed. But shutter speed in itself doesn’t affect flash exposure (for manual flash), or how hard your flash works (for TTL flash).

Neil vN

Reply

10 ShoShots November 21, 2009 at 1:58 pm

You have to get out there and try it for your self, on in experimenting can you really understand this concept..

Great article Neil..

Reply

Leave a Comment

Previous post:

Next post: