Saturday, 19 May 2012

On Exposure Part III: Perfect Sharpness, Depth of Field and Hyperfocal Distance

Aperture, Depth of Field and Shutter Speed
As we have seen in Part I - perfect exposure requires mastery of three principal areas: ISO, Aperture and Shutter Speed along with an understanding of how they interrelate. Beyond that, you must also master metering light properly but that's for Part V! For now, let's move on with the Photographic Triangle and a feature of aperture selection - where to focus to achieve a sharp exposure. 

As we saw in Part II, when dealing with aperture generally, a smaller aperture opening results in greater depth of field. Thus, a greater depth of field in an image is usually obtained from f/16 and f/22 (on most lenses). However, as less light is brought onto the sensor, shutter speeds will be lower. This has an impact on handholding at these apertures as we have seen.

However, the optimum sharpness for most lenses is 2 or 3 stops above the widest aperture. This translates to optimum sharpness between around f/8 and f/11 for most lenses. I have personally noticed very little difference in depth of field sharpness between f/8 and f/11 which therefore means f/8 is ideal as shutter speeds will be 1 stop faster. 

The reason the greatest sharpness is at these apertures is partially as a consequence of diffraction. Diffraction is a softening effect when looking through a narrow opening - squint and you'll get the idea. For now, don't worry too much about diffraction - experiment with apertures having read this article and you'll get a sense of what suits you best.

I should point out all the exposures here have been taken in RAW format with no processing at all beyond a conversion to JPEG to put them on here. No sharpening, no contrast adjustment, no tone curve adjustment and no exposure adjustment.

The exposure below was taken at f/8 (at 24mm). The whole image seems sharp but when viewed at 100%, the rocks are clearly somewhat sharper than the masts of the yachts in the distance.

In order to achieve sharpness in an exposure, you will need to adjust the focus on a particular point until it is sharp (either by eye or by using the AF, autofocus, function on your camera and lens - I almost always use AF if I can as, on Canon L-series lenses, the AF is excellent). However, if you want the entire scene to be sharply in focus, where do you focus?

Where to focus
Many photographers, adopting a simple rule of thumb, would suggest focusing one third into the scene to achieve sharpness throughout an image with a smaller aperture opening. Some, including me, find this method not sufficiently accurate. By all means use it, but if you do use that rule of thumb, don't read any more - it will give you a headache.

There are two other methods available - hyperfocal distance focusing and infinity focusing.

Hyperfocal Distance Focusing (from here referred to as "HFD" for Hyper Focal Distance) essentially means ascertaining by way of a mathematical calculation the optimum place to focus for a given focal length, a given camera sensor (reflected by a value called the Circle of Confusion, which is explained below) and a given aperture.

There are two definitions of HFD:

Definition 1: The hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp.

Definition 2: The hyperfocal distance is the distance beyond which all objects are acceptably sharp, for a lens focused at infinity.

There are many ways to work out the HFD, but the most common is by way of Depth of Field charts or tables, as well as smartphone applications which replicate those tables. In essence, you work out what parts of the scene you need sharp (the foreground, the middle ground and/or the background), assess the distances concerned and you are then given the distance you need to focus at by these DOF calculators once you tell them what camera you are using and what aperture. 

Let's say you are looking at a beach with a castle in the distance. You want the rocks and sand on the beach in front of you (say 2m away) to be sharp as well as the rest of the coast and the castle in the far distance. Where do you focus? using a DOF calculator, you would enter the focal length you're using (let's say 17mm on your 17-40mm lens), the aperture you are using (let's say f/13) and the distance of the principal subject on the foreground you want sharp (let's say 6m away - some driftwood which looks interesting). You then adjust the values (usually the aperture) until the limit of acceptable sharpness is 'infinity'. Then the castle will be "acceptably sharp" as will your foreground and subject.

Sounds simple? Especially if you've got a smartphone - just tap in some distances and away you go? Sadly not, in my experience. It all revolves around the phrase "acceptably sharp".

Let's look at the basic principle which can be said to be:

Foreground interest will be sharp at half the distance of the HFD - so if the HFD is 3m, foreground interest will be sharp from 1.5m. The trick is to get the background sharp to infinity and that is what the HFD seeks to achieve.

So in the exposure below, I could see the stones in the water beneath me were 2m away. I set my focal length to 24mm, and manually set my focus on my Canon 24-105mm L-series to between the 3m and 5m marks (i.e. 4m) and used f/16.

One of the issues is your camera sensor. Almost all DOF calculators take this into account. But another is the size of print you want to make and how sharp you want it to be. All three can be reflected in the Circle of Confusion value but most DOF calculators take into account the sensor only.

The Circle of Confusion
This really is rather complicated and, in fact, you need to know very little about the Circle of Confusion in order to understand where in a given scene you should focus to get sharpness throughout. However, it's useful to know about it if you can stand the complexity.

Depth of Field ("DOF") charts are based on a set of assumptions about your camera's sensor, the size of the print you want to make, how good your eyesight is and at what distance you are viewing the print. In order to keep the tables a simple as possible, DOF tables assume a particular viewing distance (how far you are from the print you're looking at), how good your eyes are, and how much you're going to enlarge the image itself. Once you step outside these assumptions, the tables lose their value. 

Put another way, if you're looking to produce a print larger than DOF tables assume or if your criterion for what is 'acceptably sharp' is more rigorous, the results at infinity, i.e. the far distance in your print, will appear unsharp if you actually focus the camera, when making your exposure, on the hyperfocal distance.

My favourite online calculator which takes into account all of these issues, rather than just your camera sensor, is to be found at Cambridge in Colour. You should click on "show advanced" to get the full table. I'll explain why it's relevant.

To assist in DOF calculation, a value has been developed to represent various factors and it is known as the Circle of Confusion. This value, the "CoC", which changes depending on a camera’s sensor, is a complicated mathematical construct which does not particularly require complete mastery for an understanding of its role. The CoC, as stated by Canon, for the 60D is 0.019mm. This is a value that you will enter into whichever DOF table you're using - some will just ask you for the name of your camera.

However, many professional photographers take the view that where an image is going to be printed to A3 size or beyond, meaning they want sharpness beyond "acceptably sharp" a lower CoC value should be used - perhaps even 0.010mm or lower. Why? 

A lower CoC effectively changes the HFD and, consequently, where you should focus. 

Usually, a more exacting (i.e. lower) CoC means you will need to focus further into the scene. What this means is, if you want the background to be sharper than simply "acceptably sharp", you will need to focus further into the scene than most DOF tables suggest. This might result in a corresponding lack of sharpness in the foreground, depending on the aperture used. 

An example:

At f/8, with a focal length of 17mm, for a Canon EOS 60D, the hyperfocal distance is 1.92m. This means I would focus, to get sharpness throughout the image, at 1.92m (although without a measuring tape, that might seem impossible, so say 2m). The foreground should be sharp from 1m away (half of 2m) and everything else should be "acceptably sharp" all the way to infinity. Yet, in practice, I find that when the background at the furthest part of the scene is viewed at 100% (i.e. zoomed right into the picture to see how sharp it really is), the sharpness is not actually that good. Focusing at 3m (i.e. 1m beyond the DOF table's HFD) improves this. Foreground interest remains sharp as anything in foreground will be sharp from 1.5m and I am unlikely to be including anything 1.5m away from me.

The background sharpness can be improved further still by focusing deeper into the image, say 8m. Of course this means anything closer than 4m will not be sharp but you get that now.

Adopting a more exacting CoC takes into account a larger print size, or more assiduous inspection of the print by the viewer and results, when using the Cambridge calculator in a higher value for the HFD. 

Taking the earlier example, for a print 50cm in width (i.e. A3), viewed from 1m, for a 17mm lens, and someone with 20/20 vision, the HFD at f/8 is 5.5m. I ask myself, is there anything 3m away from me I need to be sharp? If the answer is no, I focus at 6m knowing that I'll get the background sharper than if I focused at 2m or even 3m.

That's why I use the Cambridge in Colour calculator because it allows me to take into account a more rigorous standard than just "acceptably sharp".

You can achieve the same effect on other DOF calculators by setting the CoC at 0.010mm or 0.009mm.

So - Problems with HFD
Why does this work this way? The problem with HFD is that objects in the far background are on the extreme outer edge of the depth of field. These objects only just meet the definition of "acceptably sharp." Detail is not as sharp as it should be because the standard definition of acceptably sharp is actually not that sharp. Sharpness at infinity is particularly important for landscape images where the background is just as important as foreground.

Also, when making a hand-held exposure, aperture and shutter speed limitations mean scarifies and these situations call for quick judgment - HFD is not always the best option. I cannot use f/16 or f/22 for handheld images, in reality. f/8 is far better on a mountain.

Here, I wanted the sky to be highlighted along with the high dynamic range in the foreground so I used a 3-stop ND Grad filter (i.e. a filter that is 3 stops darker at the stop and normal at the bottom and graduated in between to be a smooth transition between the two). To take account of this and still be able to handhold, I used f/8. The foreground was so far away from me (around 10m) and the background so minimal that loss of sharpness would be almost imperceptible that I could easily use f/8 and be happy with the result:

Infinity Focusing
There is light at the end of the tunnel and there is a very good alternative option.

That option, particularly if the depth of field which needs to be sharp extends some way and there is little foreground interest because the foreground actually starts some distance away, is to focus on infinity. Focusing on infinity means MF (manual focus) and turning the focus ring to the infinity mark (not somewhere within the infinity range on your lens which is the line after the infinity mark). For mountaineering, this is likely to be your best, and least complicated, option.

Here, I focused on the woman in the distance, for all the world focusing on infinity, and all the stones came out sharp. This was at f/8:

The exposure below, also taken at f/8 but focused on infinity instead, when compared with the first exposure in this post, has the background much sharper with no appreciable loss of sharpness in the foreground.

Mountains vs Landscapes
Taking all of these features into account, several things can be said about outdoor photography for those undertaking outdoor pursuits such as mountaineering and hillwalking.

To achieve the greatest sharpness and depth of field, a smaller aperture such as f/13, f/16 or even (diffraction notwithstanding) f/22. This will require a range of measures as a consequence of low shutter speeds - tripod, mirror lockup and remote/timed shutter release. This is impractical for most mountaineers and hillwalkers - getting a tripod out whilst climbing or mountaineering is often impossible. Additionally, it is unnecessary as few images sought will require depth of field extending into the very near foreground (below 4m). A landscape image might well require that depth of field, but a mountain image will usually not.

For a mountain image, where the depth of field is likely to range from 6-8m to infinity, f/8 allows easy handholding with relatively fast shutter speeds and sits in the perfect sharpness optical range for most lenses. In most mountaineering images, foreground interest (probably someone climbing or walking ahead of the photographer) is unlikely to begin closer than 6-8m and so focusing well ahead (or in fact on the infinity mark itself) will render the whole of the scene sharp.

Landscape images, conversely, may well require significant foreground interest as well as background interest. Tripods, mirror lockup and remote shutter release all come hand-in-hand with the time and convenience to work out the precise HFD on your smartphone (or perhaps on a printed table on laminated card).

Depth of Field Master
Luminous Landscapes
Cambridge in Colour


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