Macro photography and DOF

MACRO PHOTOGRAPHY and Depth of Field

The photograph below of a knob tailed gecko shows shallow depth of field in which only the eye is sharp and the sand below the eye. I used an aperture of f/2.8. Some photographers like this look where a single feature, or a narrow plane, is sharp and the rest of the image is blurred. I do this once in a while, but when photographing macro subjects I think it’s much more impressive and visually compelling to see all of their intriguing and beautiful detail with tack sharp clarity. Besides, anyone can blur a picso your ture. That takes no talent or skill. Capturing extensive depth of field with small subjects is a different story.

Depth of field strategies

1. Small lens aperture. We all know that a small lens aperture means increased depth of field. But there are ramifications of using, say, f/22 or f/32. First, so much light is reduced that one of three things has to happen to compensate for this light loss.

exposures are correct: a) a much slower shutter has to be used to allow more light onto the sensor, b) a higher ISO has to be chosen for the same reason, or c) a flash has to be used.

If you choose a slower shutter speed, that necessitates the use of a tripod and, at the same time, the subject can’t move. Not even a little. Macro photography often involves significant magnification, such as the eye of a water dragon, right, and that means the slightest movement of a small subject will translate as a blur. Movement is magnified, and the greater the magnification (such as when you added extension tubes to a macro lens) the more blur you’ll see in the pictures.

If you are shooting outdoors, for example, even the slightest of breezes can disturb a flower, seed pod, blade of grass, insect, or water drop just enough to be unsharp in the photographs. In the case of poison dart frogs, like the Peacock frog above, their necks pulsate as they breathe and this, too, will cause blur.

If you don’t slow the shutter speed down but instead raise the ISO, you’ll see increased noise. With modern mirrorless cameras and

intelligent post-processing, the noise can be significantly reduced or eliminated. But consider this: If the aperture is closed down from f/5.6 to f/32, that’s a 5-stop change. If you compensate for the loss of 5 f/stops of light starting from, say, 400 ISO, the newly adjusted ISO would be 12,800! That’s a lot of noise to deal with in post-processing.

The last option is to use flash. This provides enough light to enable you to use a low ISO for maximum picture quality as well as a small lens aperture. There are many types of flash to use. I prefer a ring flash (not a ring LED light which doesn’t have enough light output) because it’s small, mobile, and it simulates diffused daylight. Shadows are kept to a minimum especially when the flash is used very close to the small subjects. The closer the flash is positioned to the subject -- like 4 to 6 inches -- the softer the light will be. Even a ring flash will approach a point source of light if it’s placed several feet away.

Lens diffraction

For my semi-annual Frog and Reptile workshop, which is essentially a macro photography experience, I recommend the participants shoot with the smallest lens aperture available. This is usually f/32. This provides maximum depth of field, notwithstanding focus stacking. Even f/16 doesn’t provide enough depth of field when moving in very close to a small subject as you can see in the image above. The green arrows shows soft chameleon scales. They should be sharp.

The problem, though is that small apertures cause diffraction. Lens diffraction is an opti-

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cal interference occurring when light passes through a tiny opening. In other words, a photograph grows progressively less sharp at small aperture values.

So, is f/32 less sharp than f/8? Yes, it is. F/16 is also less sharp than f/8. However, shallow depth of field means much of the picture is unsharp as you can see on page 4. What is the solution, then, for maximizing sharpness?

Focus stacking , which is a composite of multiple exposures, usually at f/8, in which the sharpest slice of each frame is composited together to make a competely sharp image, can only be used if the subject isn’t moving. There can be no wind and, if the subject is an animal, it has to be absolutely still over the course of at least 15 seconds. Frogs, reptiles, insects, and other types of small animals are most likely not going to remain perfectly motionless for long periods of time.

Therefore, the only option is to use a small aperture. I used f/32 for the photo of the frog on page 5 as well as for the cinnamon frog below. In both images there is focus fall-off, as you would expect, but the subjects are completely sharp. Are they as sharp as a focus stacked image at f/8? No, but given the limitations of optics and the fact that the animals usually aren’t motionless for long periods of time, this is the best that can be done.

In post-processing, if I want to add additional sharpness, I’ll use Topaz Sharpen AI or, if I want to do it quickly, in Photoshop I’ll choose Filter > sharpen > unsharp mask.

Parallelism

When doing macro work, especially extreme magnification of 1x or larger, another factor in determining depth of field is the angle of the camera back (i.e. the plane of the digital sensor -- we used to call this the ‘film plane’) to the subject plane.

Look at the illustrated sketch below. The camera was rotated counterclockwise a few degrees to make the digital sensor as parallel as possible with the side of the chameleon. The reptile’s side isn’t perfectly flat, of course, but the general plane of the animal’s lateral surface is angled. To maximize depth of field, the camera should be angled the same way.

In the photo of the chameleon on the next page, notice how the eye and the center of the side is sharp while the crest and the hind foot are blurred. This occurred because the back of the camera was not parallel with the chameleon’s lateral surface.

This is a simple aid in increasing depth of field, but in my experience most photographers forget to take advantage of it. Even if they are aware of this concept, angling the camera properly to make it as parallel as possible to the plane of the subject isn’t easy. Many times in my photo workshop I’ll have to point out that the camera isn’t as parallel as a participant thinks.

Don’t try to judge parallel angles by looking through the viewfinder. Instead, hold the camera as you think it should be positioned and then look at it from one or two feet away. It will be much easier to determine the ideal angle from this vantage point.

Study the pictures of the emerald tree boa on the next page. I took the photograph at the top of the page from the nose looking back toward the eye. The plane of the digital sensor was significantly oblique to the plane of the snake’s face and, even though I took this at f/16, the depth of field is quite shallow. Having Important elements in the foreground out of focus ruins the picture, in my opinion.

The photograph at the bottom shows what happens when the plane of the digital sensor is parallel with the side of the snake. I used the same f/stop, and now the entire surface is sharp.

There are times, of course, when you want an oblique angle on a subject for compositional purposes. That’s fine, but be aware of the consequences -- depth of field will be compromised in the extreme. The greater the magnification and the more oblique the camera angle, the shallower the depth of field will be.

Even though I’m talking about two-dimensional planes of focus -- the digital sensor and an imaginary focus plane on the subject -- the camera can be rotated in 3-D space. You have to check the parallelism from left to right as well as from top to bottom. In other words, swing the camera from left to right and from top to bottom of the subject. At some point, you’ll see the back of the camera at the parallel angle. This in addition to a small lens aperture, will give you the maximum depth of field possible in a single exposure. Macro photography is very precise because it’s all about depth of field. §

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