Camera Techniques

About apertures and depth of field

A question from a reader of my Lightroom 2 book:

“Dear Mr. Coalson,
I just recently purchased your book, Lightroom2. I notice that many of your beautifully printed images are done with small aperture settings.

For years, I have shot many landscapes at f22. This seem to go against the grain of conventional wisdom. Many serious photographers refer to a sweet spot in the lens, about f8 or f11.

I shoot with Minolta professional glass; all of my lenses are 2.8s. now I’m using them digitally with a Sony A700 body.

What is your opinion?

As I develop a work flow, I find myself unsure of how much to stop down my exposures on landscapes. I want to keep as much as possible in any given scene in sharp focus.”

My reply:

“Thank you very much for your email, and for purchasing my Lightroom book.

I will be happy to answer your questions re: apertures.

What we’re dealing with here is depth-of-field (DOF). DOF refers to the range of “acceptable” sharpness (or maybe better said “perceptible” sharpness) within the photo. The lens settings for focal length and aperture determine the DOF, based on the distance at which the lens is focused.

First, wide angle lenses inherently have more depth of field than do longer (telephoto) lenses. For instance, f/8 at 24 mm results in more DOF than f/8 at 100mm. In other words, the higher the magnification, the lower the DOF at a given aperture. With wide angle lenses, you may often be able to set your aperture to f/8, focus at infinity, and everything will render sharp.

In order to maximize DOF, it’s essential to select the correct focusing distance. The DOF needed to render the scene fully-sharp depends on how far you are from the nearest object. Generally speaking, I try to set the focus at 2x the nearest object in the frame. For example, if there’s a tree in the scene that is 2 meters away from me, I set the focus at 4 meters. Do this using manual focus, going by the focusing scale on the lens barrel.

Then, based on the focusing distance and the contents of the framed scene, selecting the ideal aperture is based on the focal length set on the lens. Look at the markings on the lens to see what focal length you’re using. If you’re shooting at 24 mm, you may be able to use a relatively large aperture (lower numbers) to render the entire scene fully sharp. If you’re at 100mm, you may need a smaller aperture (higher numbers).

The DOF necessary will depend on the distances between objects in the scene. If your nearest object is 2m away, and the farthest object is on the horizon (at “infinity”) you will need a higher aperture than if you’re trying to capture a range of only several meters.

In much of my work, the objects in the scene are on the same visual plane; at least as far as the camera is concerned. In this case, as long as I select the correct focusing distance, the aperture doesn’t matter, and I will use the largest aperture possible in order to produce fast shutter speeds with lower ISOs. For example, if I am shooting the side of a mountain that is .5 km away, I can shoot wide open (say, f/2.8) and get everything sharp.

Here’s a tip: if you look through the viewfinder (or on the LCD preview) and everything is sharp, you can shoot wide-open. Remember that unless you press the DOF preview button (which stops-down the aperture to the current setting), what you see through the viewfinder is always wide open aperture.

Using these methods, over time I have found that I can often use apertures much lower than f22 and get fully sharp scenes. But it all depends on the distances and magnification involved.

Keep in mind that apertures at both ends of the range (say, f/2.8 and f/22) produce “softer” results than apertures towards the middle of the range. This addresses the “sweet spot” you referred to – all lenses will be their sharpest at the middle of the aperture range.

I’d suggest working towards always using the widest aperture possible. Especially when shooting landscapes, f/22 is rarely necessary, or ideal. You may find that with wider-angle lenses, the highest you’d ever need to go will be f/16.

Here’s a web site with lots of useful info about this stuff:
http://www.dofmaster.com/

I hope this helps explain the concept of depth of field and sharpness. Feel free to reply with any questions or comments you may have.”

1 Comment

  1. If you are really interested in DOF don’t miss this link: http://www.trenholm.org/hmmerk/download.html

    You can download a free pdf file with an astonishing amount of information.

    Happy photos,

    Patrick.

    Reply

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