Table Topping (stuff about DOF)

Are you stuck/safe at home pondering such questions as “Does the penalty for mariticide diminish the longer you are forced to share each others company…?” If so then why not get your camera out and have some fun instead?
To assist your endevours, here are a few hint’s on how to maximize DOF with close ups. Mostly based on high speed flash work but still applies to regular macro shooting.

photo of paint splash

Topic One: More DOF for your buck.
One of the biggest problems we face when trying any form of table work (macro in particular) is a profound shortage of depth of field. This needn’t be the case.
The shot on the right was taken at an effective shutter speed of approx 1/20,000th of a second, with apperture set at f-20.
So what do I mean by effective shutter speed? Well, this is one of those shots taken alone in the dark with a speed-light. As such the shutter on the camera is set to about 1 second. As we know, in darkness the shutter speed is largely irrelevant as with no light there can be no exposure.
The speed-light therefore creates the exposure and it is the length of time the flash is illuminating the scene that creates the effective shutter speed. In this case a manual flash (Yongnuo 560 MkII) was set to 1/128th of it’s maximum output which gives an illumination duration of just 1/20,000th of a second (claimed).
“So that shot was taken at an aperture of f-20, which is quite a small hole for the necessary light to get through, with an exposure time of just 1/20,000th of a second. Surely sir you are taking the…”
No I’m not, here’s why. I’t’s all down to the fact that the speed-light was positioned a mere 50mm(±) away from the object (two small drops of paint awaiting a smack from below). And that helps because…If you reduce the distance between light source and subject by 50% you don’t double the amount of light hitting the source you increase the illumination by a factor of four, not two1.
That means that whatever illumination a subject receives at say, 800mm (flash to subject distance), that can be multiplied by 256 times by the time the flash is moved to 50mm (light to subject distance).
In a nutshell, if you need more depth of field, close down your aperture. This means you’ll need more light. To get that, move your light source up close and personal with your subject.

Topic Two: Choosing your f-stop
OK so we now know how to get more depth of field for a given amount of light but how do we know how much DOF will be produced for a given aperture?
Thanks to modern technology this couldn’t be easier. Type in DOF Master into Google and you’ll see a link to the on-line app I use. There are also many downloadable apps for your phone if you prefer to carry your tools around.
If we look at the image on the right we can see the settings that govern how much depth of field can be achieved for a predetermined set of circumstances. The variables are:

  • Format, this tells the software the size of your image sensor. Bigger sensor, more DOF
  • Lens, the longer the lens the less DOF
  • f-stop, the bigger the number, the smaller the hole, the greater the DOF
  • Focus distance, the greater the distance (subject to focal plane) the more the DOF

The settings shown were those used when taking the image discussed previously. As can be seen the available DOF was about 3cm.2

Topic three: It’s not all about high speed flash.
For those with an attention span exceeding 1/20,000th of a second, this “in your face” positioning of lights is less of an issue. For when you’re not alone in the dark, but still want greater DOF, try:

  • Use multiple strategically positioned speed-lights to either eliminate unwanted shadows or to just get more light on the scene3
  • Set your speed-light to a higher power setting
  • User a longer/slower shutter speed to give the light more time to get through the small hole

1 For a more detailed explanation of how speed-lights work and more importantly how distance to subject effects the effective illumination, see this article.
To see an example of the relationship between light source to subject distance and effective illumination see the image to the right.
Remember, a speed-light always fires at maximum intensity/brightness. When we turn down the power we reduce the length of time we expose our subject to that intensity. Speed-lights are not like dimmer switches. They are always “full-on”, it’s just the duration we change.

2 There are implications for altering the settings as shown. For example, use a 50mm lens to increase the available DOF. That works but you then have to move closer to the subject to fill the frame as before. This reduces DOF so you’re back where you started. Or, move further away to increase DOF, but then use a longer lens to fill frame. Again, back where you started

3 The reason multiple speed-lights aren’t generally used in high speed flash photography is due to the fact that identical lights with identical settings don’t necessarily fire at the same time. Imagine the go button is pressed on whatever you’re using as a flash trigger. Theoretically both guns fire together and all is well. However, if your guns aren’t perfectly in sync and one fires a fraction of a second (eg 1/20,000th sec) after the other, your supper fast exposure is now a long winded 1/10,000th of a second. Believe it or not this can be enough to allow motion blur to creep in or even worse, ghosting. Given that the event being photographed can last milliseconds or even fractions of a millisecond, I personally prefer to use one flash to eliminate the potential variable of synchronization.