Understanding camera flash

I would have no hesitation in saying that TTL flash photography is probably one of the most misunderstood and misused aspects of modern digital photography. The core of the problem is that you are dealing with two separate exposures in one photograph – one exposure for the background and one for the foreground. So from the outset, you will make your life easier if you can remember that ….
FLASH LIGHTS THE FOREGROUND, NOT THE BACKGROUND.
Whether the background is under-exposed, over-exposed or properly exposed depends on how appropriate your shutter speed, aperture and ISO selections are relative to the ambient background lighting.

Just to complicate things further, here’s another thing that might surprise you. Shutter speed, aperture and ISO affect your background exposure. However, in theory with TTL flash mode, none of these affect your subject’s exposure. This is because, within reason, the idea of TTL flash is that it will blast enough flash at your subject to light your subject properly irrespective of what combination of shutter speed, aperture and ISO you have chosen. Of course if your settings are such that the image would be over-exposed without flash, it stands to reason that the flash is not going to darken it down for you!
The photo below was taken at 6:14PM on 4th Feb 2012 in Ireland. In other words, even though I could see clearly, light was starting to fall. I deliberately put my camera in P-mode (programme-shift) and set my flash unit to TTL with no compensation applied. ISO was set to ISO-100.

P-mode with TTL-Flash (ISO-100, F/4.0, 1/60th sec)

Analysis:  When you put your camera into P-mode with the flash in TTL mode in falling light, the camera will attempt to get you a ‘hand-holdable’ shutter speed – in other words about 1/30th or 1/60th sec. To achieve this, it will open up the aperture as wide as it can (or as wide as it needs to ) and then if that is not going to adequately expose the subject, it will blast just enough flash at the subject to properly expose your subject. In this mode of operation, the camera does not care about your background. It is simply concerned with lighting your subject. This can be seen in the photo above. I could clearly see my subject and the background but yet the photo shows a heavily under-exposed background.

So, what do I do if the background is important to me? Say for example, I’m photographing a wedding couple beaming with pride in front of the castle they have paid a small fortune to hire for the occasion. They won’t be happy if the castle does not appear in the background.

Here are the steps with photos to show it works:
Step 1: Switch your camera to manual M-mode and although not necessary, you might like to start with the settings that failed miserably in P-mode just to have a reference starting comparison point. Keep your flash in TTL mode. If you take a photo now, you will get the exact same result as you did in P-mode because you are essentially forcefully replicating the settings the camera chose for you in P-mode.

Step 2: You now have four ways of increasing the background lighting while maintaining the proper subject exposure. These are:
(i)  increase your ISO
(ii)  slow down your shutter speed
(iii) open up the lens aperture
(iv) a combination of the above

Let’s look at these in turn and discuss some advantages and disadvantages of each.
(1) Increase your ISO:  In manual M-mode, if you leave your shutter speed and aperture setting unchanged but increase your ISO, the background will start to brighten up as you would expect when you increase ISO. Interestingly, your subject does not brighten up. This is because your camera knows the ISO has been increased and this information is communicated automatically to the flash unit. As a result, the flash unit knows that to maintain proper exposure on the foreground, it must slightly reduce its flash power output (in reality it reduces the duration of the flash but this is effectively the same as reducing power). So the subject lighting remains unchanged when the ISO is increased but the background gets brighter. This is because the flash has no effect on the background so the fact that it has reduced in power is irrelevant to the background. The background is brighter simply because the ISO has increased. While this same ISO increase in theory also brightens your foreground subject, this is compensated for by a reduction in flash power resulting in zero net change.

The series of photos below shows what happens as I progressively increase my ISO value without changing anything else. As can be seen, we go from having no visible background to a normal visible background with negligible change in subject lighting.

M-mode with TTL-Flash (ISO-200, F/4.0, 1/60th sec)

Now let’s try increasing the ISO from 200 to 400 without changing anything else.

M-mode with TTL-Flash (ISO-400, F/4.0, 1/60th sec)

We are now clearly starting to get a brighter background. Let’s increase the ISO again by one stop – i.e. from ISO 400 to ISO 800.

M-mode with TTL-Flash (ISO-400, F/4.0, 1/60th sec)

The photo above is probably closest to the actual light as perceived by my eye.

Now let’s try one final increase from ISO 800 to ISO 1600.

M-mode with TTL-Flash (ISO-1600, F/4.0, 1/60th sec)

I’ve now reached the stage where I am arguably over-exposing the subject because my ISO, shutter speed and aperture combination are over exposed for the ambient light on the subject.

Let’s summarize on ISO:
By progressively increasing the ISO, the background became progressively brighter. The foreground subject lighting remained more or less unchanged because in each successive photo, the flash intensity or power was reduced to exactly cancel any brightening as a result of higher ISO.
The disadvantage of doing this is that the photo will become ‘grainier’ or exhibit more ‘ISO noise’. How much noise it exhibits depends on the camera model and whether it is a full-frame camera or an APS-C or other camera. It also depends how recent the model is as there are regular improvements in high ISO performance with each new sensor technology released.

Now, let’s look at changing the shutter speed.
(1) Slowing down your shutter speed:
Let’s start with our reference photo again to set the starting reference point.

M-mode with TTL-Flash (ISO-100, F/4.0, 1/60th sec)

 Now let’s slow down the shutter speed from 1/60th sec to 1/30th sec. This allows more time for light to hit the sensor and will result in a brightening of the background. In the case of the foreground, the flash simply fires with slightly less power resulting in no noticeable foreground exposure change.

M-mode with TTL-Flash (ISO-100, F/4.0, 1/30th sec)

Let’s slow down the shutter even more and watch the background brighten further below.

M-mode with TTL-Flash (ISO-100, F/4.0, 1/15th sec)

And now from 1/15th sec to 1/8th sec.

M-mode with TTL-Flash (ISO-100, F/4.0, 1/8th sec)

And finally from 1/8th sec to 1/4 sec.

M-mode with TTL-Flash (ISO-100, F/4.0, 1/4th sec)

Now let’s summarize on shutter speed:
By progressively using a slower shutter speed, the background became progressively brighter. The foreground subject lighting remained more or less unchanged because in each successive photo, the flash intensity or power was reduced to exactly cancel any brightening as a result of effective increases in ambient lighting.
The disadvantage of doing this is that unless you are using a tripod (as I was), you are going to get a progressively more blurred photo because I’ve yet to meet anyone who can hand hold a camera at 1/4 second and consistently get sharp images that can stand up to enlargement. Even with a tripod, you have a problem if your subject moves. That said, most adult subjects can hold a pose for about a second if requested to do so.

Overall Summary (so far)
As you can see from the photos, increasing ISO and slowing down your shutter speed both increase background lighting without (up to a point) affecting foreground exposure. Obviously you can temper the disadvantages of either method (ISO noise on the one-hand and potential blur on the other hand) by changing both a little bit instead of changing one on its own in a more extreme way.

You might also reasonably ask, what’s the point of flash if I’m simply going to adjust my exposure control settings to the point where I am exposed properly for ambient as is the case for the last one or two photos in both the ISO and shutter speed variation cases. Well, let’s reconsider one of the original scenarios I considered where a couple have hired a castle as their wedding venue. What I want to achieve here is at least a sense of the background. If the background is black or near black, no-one will see it. If I go too far with my ISO or shutter speed and either get a very ‘noisy’ photo or a  noticeably blurred photo, then this is not good either. If I go for a in-between trade-off where the couple are nicely exposed and the castle is somewhat underexposed but clearly identifiable as a castle, then everyone will probably be reasonably happy.

Photography is all about trade-offs and unless you know and understand your camera and flash settings, then you are not in control of the trade-offs.

Okay, there’s a lot more to flash and a lot more to come in this article.

Update: Actually there isn’t more to come in this article because I’ve decided to put the rest in separate articles. As of today (6th Feb 2012), I’ve put the first of these related posts covering the topic of manual flash and guide numbers in the following article.