More Depth of Field – Advanced Information

Fast Glass: Lenses that offer faster apertures are valuable tools for depth of field control. Apertures of f/1.4 and f/1.8 are often associated with fast single-focal length lenses: 24mm, 50mm or 85mm. Zoom lenses with f/2.8 apertures that remain constant at any zoom length are incredibly popular too. Common are: 17-50mm, 28-75mm, 70-200mm and more.

ND Filters: In order to get the most out of fast-glass lenses outdoors photographers need to use Neutral Density filters. To use the widest apertures outdoors the amount of light entering the lens must be reduced. Digital cameras can only reduce ISO sensitivity so far (usually ISO100), the photographer then must use an ND filter in order to use f/2.8 or even f/3.5. Most photographers will have both ND4 and ND8 filters to fit their fast-glass lenses.

Nit-Picking Tech Note: ‘Depth of field’ and ‘depth of focus’ are not the same thing. While depth of field describes a range of focused space in an image, ‘depth of focus’ describes the range of distance inside the camera between the lens focus node and the film / sensor plane that provides acceptable focus. Depth of focus is also called focus tolerance.

One Big Argument for Full-Frame Sensors: Perhaps the most compelling argument for owning a camera with a full 35mm sized image sensor is found in depth of field. An APS-sized sensors doesn’t really magnify the length of a mounted camera lens, it crops the image instead. If we look at the image below we see that a 50mm lens provides the exact same image circle to either an APS or Full-Frame sensor camera.

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To achieve the same composition on an APS-sensor camera as on a Full-Frame camera the photographer must either back up to increase the distance between camera and subject or keep the same working distance and choose a shorter lens length. Either choice increases the depth of field in the final image. This is true at any lens length or working distance when we compare the two sensor formats. For any specific composition the Full Frame sensor will always provide the option of shallower depth of field.

Bonus Info – Hyperfocal and Depth of Field: Hyperfocal length is the closest distance at which a lens can be focused while still keeping objects at infinity acceptably sharp. (Also called the maximum depth of field of a lens.) This point will vary with the lens aperture; as the aperture gets smaller the hyperfocal point gets nearer to the camera increasing the maximum depth of field. The image below is of a wide angle lens, the callout box shows the focus range indicator.

We see the lens is focused to infinity and we also see that there are aperture numbers arranged on either side of the focus mark. This is where hyperfocal length can come in handy. What the aperture scale is indicating is the depth of field on the focus ring for the combination of aperture and focus setting.

On the focus ring we see that the distance scale in yellow numbers indicate ‘Feet’. Let’s assume that we have the camera set to use f/16. We read the depth of filed scale in this way: the focus indicator is pegged at infinity and on the aperture scale below the window we see that on the right side of the focus mark aperture f/16 lines up close to the yellow “4” on the focus ring. Reading this scale tells us that focused at infinity with an aperture of f/16 everything from four feet to infinity will be in focus.

Now here is the hidden trick used by those in the know. Since the aperture scale below the focus ring is indicating the lens’ hyperfocal length we can use this information to get even more of our scene in focus. Rather than focusing at infinity we move the focus ring on the lens so that infinity lines up with our chosen aperture on the left side of the focus indicator, in this case f/16. In doing so we have changed the depth of field in our final image from “4 feet to infinity” down to “2 feet to infinity”. This is a huge advantage in landscape photography, by using this technique the photographer is able to capture images with enormous depth of field.

There aren’t many lenses left that still sport a depth of field scale, those that do tend to be prime lenses and not zooms. The scale is accurate for either full-frame or APS-sensor digital cameras. As lens length gets shorter this technique will have less effect.

1/3 and 2/3 – The depth of field of an image isn't equally distributed around the subject. Actually the range of acceptable focus is unbalanced with 1/3 of the range in front of the focus point and 2/3 behind it. This is important to know especially when shooting up close and at wide apertures. For example: a close portrait image at a 105mm lens length and a f/2.8 aperture shot from four to six feet away. If the photographer focuses on the bridge of the subject's nose the ear will likely be out of focus. Focusing instead on the cheek or eye farthest from the camera will often be more successful in achieving an overall focused image.

Because of this imbalance in the range of focus, group shots will often look sharper overall if the photographer chooses the second row of the group as a focus target.

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