when aperture doesn’t control the flash exposure
The two truisms regarding flash photography that get thrown around most often, are:
- ambient exposure is controlled by shutter speed,
- flash exposure is controlled by aperture.
I’ve railed against this before in a post on what I called the urban legends of flash. My take on it is in that in over-simplifying, and bandying around those two statements in a perfunctory manner, we actually lose understanding of how flash exposure and ambient exposure inter-relate.
My problem with the first statement is that it disregards that aperture and ISO both control ambient exposure as well.
Where this ‘short-cut’ came into being, is that with manual flash, our exposure is controlled by 4 things: aperture, ISO, distance (from light source), and power of our light source, and this leaves shutter speed as our only independent control for ambient light.
It is essential to understand that with TTL flash, things change a little. Actually, things change a lot, since exposure control for manual flash and TTL flash are entirely different.
This specifically is where I feel that the second truism there: “aperture controls flash exposure” is entirely misleading.
With TTL flash, your camera and flash work together to control /calculate flash exposure .. and then adjust the power the flash emits, to give you (what it deems to be) correct exposure … regardless of your choice of aperture, or your choice of ISO, or the distance of your flash to the subject. Of course, these things have to be within reason. You have to work within the capabilities of your flash.
Let’s see how this works. Here are two images that I used in my book on flash photography techniques .. and I’d like to use them with a slightly different take.
With our model, Jackie, positioned in front of a window, we have two things to expose for:
- we have the ambient exposure to consider for the outside background,
- and we have flash exposure to consider for our subject, Jackie.
Ambient exposure for the background – shutter speed, aperture and ISO. Simple enough.
Flash exposure settings to light our subject – well, this will depend on whether we are using manual flash or TTL flash.
As mentioned, if I had used manual flash here (for example with a softbox), then I would have to meter my manual flash for the specific aperture and ISO and distance and power setting of my flash. And then juggle those four things to give me correct exposure as metered with a flashmeter. (Or perhaps checked via histogram.)
But in this case, I used TTL flash that I bounced over my left shoulder into the room.
And I purposely set out to use TTL flash here in a way to illustrate a point:
With TTL flash, aperture effectively becomes transparent.
In other words, with TTL flash, our choice of aperture, (within reason), has no effect on flash exposure. (This is also true for our choice of ISO.)
Let me explain ..
Let’s look at my choice of exposure settings for those two images:
LH: 1/125th @ f2.8 @ 800 ISO
RH: 1/125th @ f5.6 @ 800 ISO
Traditionally, if we wanted our background darker while using flash to light our subject, we would change our shutter speed. Normally I too would’ve changed my shutter speed from 1/125th to 1/250th before I would’ve made any changes to aperture or ISO. That would gain me a stop already on my background exposure, without my flash having to work harder. It absolutely makes sense to change shutter speed first to change ambient exposure, before changing any other exposure control.
But in this example, I willfully went the other way. I kept my shutter speed constant .. and changed my aperture.
Now, if this had been manual flash, I would’ve had to change my ISO or bring up the power on my flash or bring the flash closer to my subject to compensate for the change in aperture.
But with TTL flash, the flash and camera calculated the exposure it thought I needed, and automatically compensated for the change in aperture. I did not have to change anything. My camera and flash did. I did have my flash exposure compensation set to +1.0 EV for every image in the extended sequence.
As I changed from f2.8 to f5.6 .. I could continue shooting and get the same (TTL) flash exposure. My camera and flash followed my change in settings seamlessly to give me exactly the same flash exposure at f5.6 as it had at f2.8
Sure, the flash had to work much harder. In fact, it had to dump 4x the amount of light at f5.6 as it had at f2.8 … but my exposure remains the same. And this is the reason why even with TTL flash, we’d rather change the shutter speed first to affect our ambient exposure .. even if we don’t “have to” like we would “have to”‘ with manual flash.
And as you can clearly see, in changing our aperture, our background exposure changed.
So back to those two truisms about flash, but this time in regard to using TTL flash as I had in this sequence.
And my sincere belief with this is that if we can completely understand how flash and ambient exposures inter-relate, we will … well, we will have exactly that .. complete understanding of how flash and ambient exposures inter-relate. :^)
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