the effect of having a higher flash sync speed …
… why 1/500th @ 200 ISO is better than 1/250th @ 100 ISO
A question I often see asked on the internet forums, is whether having 1/250th flash sync at
100 ISO (on a camera like the Canon 10D), is the same as having 1/500th flash sync but being limited to 200 ISO, (such as with the Nikon D70).
In short, the answer is .. no, it’s not the same.
The camera with the higher flash sync has a distinct advantage.
Not only in being able to stop motion better, and giving you better control over depth of field,
but more importantly, the higher flash sync gives your flashgun greater range.
The reason for this is that a change in ISO will affect both ambient light exposure and flash exposure. As does your choice of f-stop. But flash exposure is not affected by the choice of shutter speed (as long as you remain within flash sync range). This higher shutter speed will enable us to get a wider aperture – and hence greater flash range.
At first glance, it seems logical that 1/250th @ 100 ISO might effectively be the same as
1/500th at 200 ISO, but it isn’t that simple. Here’s an example to help us step through this, and let’s base it on an actual scenario.
The actual EXIF data for this image, taken with the D2H says, 1/250th @ f6.3 @ 200 ISO,
but let’s keep it to round figures for simplicity’s sake.
In our example the couple is getting married inside a venue, and the sliding doors are open and you can see New York’s skyline in the background. And we want to balance them perfectly with the background skyline. We need to use flash, or else the background will wash out completely if we expose only for ambient light on the couple. Or, alternately, without flash, the couple will appear silhouetted – a cool idea for a photo or two, but won’t be acceptable for an entire sequence.
Let’s say we have light outside that gives you 1/250th @ f5.6 (100 ISO), for the background.
(ie, 1/250th @ f8 for 200 ISO)
And let’s say our flash’s guide now is such that at the distance we are standing away from them,
ie flash to subject distance, is such that we can squeeze f4 maximum out of it at 100 ISO.
So now we have a flash that can only give us (at this distance),
f4 (100 ISO) = f5.6 (200 ISO) = f8 (400 ISO)
So the photographer with 1/250th flash sync, can get
f5.6 ambient, but only f4 for flash, (100 ISO), or ..
f8 ambient, but only f5.6 for flash, (200 ISO).
It is important to notice here that the photographer with a max flash sync of 1/250th in this example, will always be one stop under for his flash exposure compared to the ambient light.
Or seen in another way, his background here will be one stop over-exposed compared to his flash exposures. It’s also very important to note that raising ISO up or down does nothing in terms of balancing flash with ambient .. simply because raising the ISO or lowering the ISO, affects ambient exposure and flash exposure equally.
Now, on the other hand, the proud owner of the D70 with 1/500th max flash sync, just bumps his shutter speed to 1/500th of a sec. His ambient light becomes .. 1/500th @ f4 (100 ISO).
Aaah, but you say he is limited to 200 ISO ?? Not an issue ..
this becomes .. 1/500th @ f5.6 for ambient exposure in this example, for 200 ISO.
And our flash in this example, can give us … f5.6 at 200 ISO.
Tadaaaaa .. in this instance we can perfectly balance flash with ambient ..
because we could raise our flash sync speed. ISO had nothing to do with it.
So someone who has only 1/125th top flash sync (like on the Fuji S2), has a very real (2 stop) disadvantage compared to the 1/500th flash sync speed like that found on the Nikon D70.
By the way, in the example photo above, I had to trigger other strobes for additional light
and to give some degree of modeling that straight-on flash wouldn’t have.