Focus, Aperture and Depth of Field

We all know the difference between 'in focus' and 'out of focus'. We see it every day in a thousand still and moving images -- and if we wear glasses, we are all too aware of it with our own eyes.

 

We are however normally unaware of refocusing our eyes, doing it automatically when we switch from one subject to another. Our brains therefore perceive everything as sharp (subject to individual eyesight) from near to far. Although a camera can likewise be focused on near or far subjects, the difference between what is in focus and what is out of focus is much clearer.

Fortunately, we can use focus to mimic the way our attention works, with shallow focus to draw attention to something we want to mark as important, or 'deep field' focus when we want everything sharp from front to back.

 

American Civil War Re-Enactor

 

What is behind him? We don't know -- and we don't really want to know. Deliberately throwing the background out of focus by using a wide aperture is often a useful trick for concentrating attention on the principal subject, especially with portraits, because it reflects the way we talk to someone: concentrating on their face, ignoring the rest of the surroundings.

Here, Frances used a 90mm f/3.5 Voigtländer Apo Lanthar on her Voigtlander Bessa-T, shooting on Ilford XP2 Super and printing the image on Ilford Multigrade Warmtone which she then sepia-toned. The advantages of black and white are clear enough for this sort of subject, but exactly the same considerations apply in colour.

 

 

re-enactor

 

Depth of field is a bit more complex. It's the zone in front of and behind the sharply focused subject. The smaller the aperture (see the free Basics module on lenses), the greater the depth of field. The easiest way to understand it is with a simple diagram.

d-o-f chart

Depth of Field

On the left, a large aperture. The cone of light focused by the lens is quite 'fat'. On the right, with a small aperture, the cone is much slimmer.The net result is that as you depart further from the plane of optimum focus -- the thick line -- the size of the out-of-focus blob is smaller at a small aperture than it is at a large aperture. As long as the blob is small enough to look sharp, the picture looks sharp.

 

st. martin interior

 

 

This also explains why depth of field varies with enlargement size and viewing distance: the bigger the enlargement, or the more closely you look at the picture, the easier it is to see the out-of-focus blob as a blob, rather than as a point.

Strictly, this drawing illustrates depth of focus (the maximum permissible displacement of the plane of focus) rather than depth of field (the zone that is in focus on either side of the point of focus) but as the argument is the same in both cases, and this is much easier to draw, it will do.

 

Church of St. Martin, Noizé

 

With a picture like this, we expect everything to be in sharp focus from front to rear, because that's the way we normally look around an ancient church.

Although the picture looks tilted, the camera (a KowaSix with 85/2.8 standard lens) was carefully leveled with a spirit level, and it's actually this 1000-year-old church that leans to one side. The 18th century wooden altarpiece that you can see through the arch is upright... Film was Maco Cube 400; print on Ilford Multigrade Warmtone.

Depth of Field and Focal Length

Strictly, depth of field depends solely on the magnification of the image on the film, so you will get the same depth of field with a 50mm lens at 2 metres as with a 100mm lens at 4 metres.

In practice, few people use their lenses like this (except for macro photography) so the depth of field with a wide angle lens at a given distance is greater than the depth of field of a longer lens at the same distance.

Focusing Mounts

Apart from bellows-mounted and other specialist lenses, most lenses nowadays come in a focusing mount. In theory, a focusing mount can focus as close as the designer likes, and indeed there are focusing mounts that go down to life size (1:1 on the film) and beyond. Most stop long before this, for one or more of four reasons.

 

infinity

 

 

extended

 

Vivitar Series 1 lens, 135mm f/2.3

You can easily see the extra extension that is needed for the 135/2.3 Vivitar Series 1 at its nearest focusing distance of 90cm (3 feet). This is a lot closer than most 135mm lenses, being only 6.7 focal lengths (see below), and it does make for a big, heavy focusing mount.

First, as you get closer than about 10 focal lengths, the effective speed of the lens starts to fall detectably. This is because the size of the aperture (see the free lens module in Basics) stays the same, but the effective focal length increases. Think of a 50mm f/2 lens (25mm aperture) that is wound out 10mm on the focusing mount, so the effective focal length is 60mm. Now, the effective aperture is 60/25 = f/2.4, half a stop slower. This explanation is open to theoretical objections but it's a simple way of explaining the idea.

Second, the performance of the lens may deteriorate unacceptably. All lenses are optimized for a particular distance, and while performance may only deteriorate very slightly across a wide range of distances, there comes a point where performance falls so low that the manufacturer deems it unacceptable. Some lenses are fine from infinity to life-size and bigger; others (especially ultra-fast lenses) may show significant deterioration at 1/10 life size.

Third, the focusing mount itself tends to get disproportionately big, heavy and expensive as it winds the lens further and further out. Again, about 10 focal lengths is a convenient guideline: 50cm (20 inches) for a 50mm lens, 1.35 metres (53 inches) 2 metres (six foot seven inches) for a 200mm lens.

Fourth, with direct vision cameras and TLRs (see the free Cameras module in Basics), the discrepancy between what the viewfinder sees and what the lens sees will become unmanageable when you get too close, so close-focus limits are rarely much less than 70-80cm, even for wide-angles, and may well be 1 metre even for a 50mm lens.

Depth of Field Scales

Years ago, all but the cheapest lenses had scales indicating depth of field (hereafter d-o-f). The idea was that you set your aperture and focusing distance, and the d-o-f scale would tell you how much of the image on either side of the focused point would be acceptably sharp. Of course, a lot depended on what the camera manufacturer (and the photographer) considered 'acceptable', and this is further discussed in the free 'How do I...?' module on zone focusing.

 

vivitat 135

 

 

 

D-o-F scale on Vivitar Series 1 135/2.3

With the aperture set to f/16, as here, and the 'infinity' sign against f/16, you can see that according to the manufacturers, everything from about 20 metres (green scale) or 60 feet (white scale) will be in focus. Because this is a lens of long focal length, the d-o-f is small. With a wide-angle lens such as the 21/4 Voigtländer Color-Skopar, everything from 0.5 metres/about 19 inches to infinity is in focus at f/16, while with the 50/1.2 Canon RF lens, it's everything from about 8 feet (2.5 metres) to infinity. Equally, of course, you could re-focus the Vivitar on the right and have everything from 30 to 60 feet in focus at f/22.

 

Today, fewer and fewer lenses (or cameras) carry d-o-f scales. This is partly because they are difficult to provide on zooms, and partly because of the rise of autofocus. But if you have them, and know what they are, and know how to use them, they are well worth having. For more examples of d-o-f scales, go to the free Zone Focus module in the How Do I...? group.

 

 

Depth of field scale on Graflex TLR

On cameras with fixed lenses and a focusing mount built into the camera body, you can build the d-o-f scale into the camera body too. Because the image is magnified more on the film, but enlarged less on the print, assumptions concerning depth of field with a 6x6cm negative are not necessarily the same as those for 35mm.

 

graflex d-o-f

 

Depth of Field Previews

Something else that was more common in the past than today is depth of field preview buttons or levers on SLR cameras. These stop the lens down to its taking aperture, so you can get a better idea of what will be in focus and what won't. They are more of a guide than an absolute guarantee, and of course the focusing screen is a lot darker when you use them, but like d-o-f scales they are very much worth while.

 

porst a-m switch

 

 

porst a-m switch 2

Depth of Field Preview

In the left hand picture above, the switch on the lens is slid to uncover 'M' ('Manual') and the lens diaphragm is set to whatever value is marked on the diaphragm ring, thereby providing a depth of field preview. On the right it is slid to uncover 'A' ('Automatic') and the lens stays at the full aperture until the moment of exposure, when it automatically stops down to the value marked on the diaphragm ring.

The d-o-f preview for Pentaxes and some other cameras is on the lens, as here; on other cameras (below), it is on the camera body, such as the small button to the right of the lens mount on a Nikon F (below left) or the button between the shutter release and the prism on a Nikkormat (below right). Both stop the lens down for only as long as they are depressed. On neither the F nor the 'Mat (below right) is the function of the button marked... The d-o-f preview on the F is the top button -- the other controls are the mirror lock, right, and the self timer, left)

 

nikon d-o-f

 

nikkormat d-o-f

 

 

On very old cameras, there was no auto-diaphragm to stop the lens down from full aperture (for viewing) to working aperture (for shooting). Instead you had either a fully manual aperture or a 'pre-set' with two rings, where you set the shooting aperture on one ring, and twisted the other to switch between aperture for viewing and working aperture for shooting.

An intermediate form was the semi-auto diaphragm. This stopped down automatically when you pressed the shutter release, just like an auto-diaphragm, but had to be winched open again manually. Otherwise they just stayed at the working aperture.

58/2 Biotar for Exakta with semi-auto diaphragm

The lever in the long slot opens the diaphragm to the maximum aperture; the auto stop-down is actuated when the shutter release (which is on the lens, and connects with the body release on the camera) is pressed.

 

Exakta Biotar

 

Optimum Apertures

All lenses deliver their best performance across a limited range of apertures. At full aperture, they are rarely at their best: a certain amount of performance (in terms of sharpness and contrast) is sacrificed to maximum speed. At minimum aperture, diffraction limits their maximum resolution.

As a general rule, the better the lens, the wider the aperture at which it achieves its maximum performance. Take three f/2 lenses, and the best may already have reached its maximum performance before f/4; the average lens will be at its best by f/5.6 or maybe f/8; and the worst may still be improving at f/11, where the other two are just beginning to show the first signs of deterioration as a result of diffraction.

across the neva

Saint Petersburg across the Neva

Quite honestly, Roger has forgotten which lens he used for this picture, which rather argues that you can get too excited about these things. It was probably Frances's 35/2.8 PC-Nikkor on a tripod-mounted Nikon F loaded with Fuji RFP ISO 50 film. The likeliest aperture was f/8: depth of field clearly wasn't an issue, so it was merely a matter of using it at the optimum aperture for sharpness -- which is hardly visible on a computer screen anyway...

 

Other general rules include the following:

Most good lenses deliver their best performance at 2-3 stops down from their maximum aperture, e.g. f/4 to f/5.6 for an f/2, or f/8 or maybe f/11 for an f/4 -- though the best slow lenses may be at their best at only 1 stop down, e.g. f/4 for an f/2.8 or f/5.6 for an f/4. Even then, the deterioration at a stop further down is unlikely to be perceptible.

For a given optical design and quality, a slower lens will usually deliver better quality at any given aperture than a faster one -- though a really slow lens may use a simpler design, and not be as good as a medium-speed lens.

 

ss peter & paul

 

 

 

Standard and long-focus lenses normally deliver better quality at any given aperture than wide-angles (see the free lens module in Basics for a definition of these terms) though once again a good-quality wide-angle may beat an average-quality standard or long-focus lens and telephotos often lack contrast and resolution as compared with 'plain' long lenses.

Zooms normally deliver the worst quality of all at any given aperture. The best zooms are very good indeed, better than many prime lenses, but the best prime lenses are better than even the best zooms.

 

Domes of Cathedral of St. Peter and St. Paul, St. Petersburg

Very long lenses -- this was taken with an 800mm f/11 Vivitar Series 1 'Solid Cat' mirror lens on a Nikon F -- are sometimes castigated as lacking contrast when in reality, the problem is just atmospheric haze. Nowadays, it is comparatively easy to get around this problem with post-processing in Adobe Photoshop, turning up the saturation and the contrast, though you have to resist the temptation to go too far. Things are further complicated here by the fact that it the picture was shot on a very grainy, low-saturation film, Ferrania 1000D (no longer in production). Roger, who took the picture, felt that this best reflected the rather faded glory of St. Petersburg in those days (about 1990). The camera was of course tripod mounted.

Choosing a working aperture

Sometimes you want a really wide aperture for minimum depth of field, so that a sharply focused subject stands out against an out-of-focus background. At other times you may want a small aperture for maximum depth of field.

 

In either case, you have to weigh the loss of quality against the effect you want. As a general rule (again) go for the effect, not the theoretical maximum qality. It is usually a better idea to produce a picture that is technically flawed, but full of drama and interest, than a technically perfect image that is devoid of passion.

 

 

Water-slide, Igal, Hungary

Your choice of aperture may be forced upon you by the need for a high shutter speed. Roger used 1/500 second for this picture, necessitating an aperture of around f/5.6 on the 90mm f/2 Summicron, given that he was using ISO 100 slide film (Kodak Elite Chrome EBX ISO 100).

 

 

water slide

 

The Bottom Line

As with so much else in photography, it's easy to make depth of field sound more difficult than it is. Focus is a pretty easy concept to grasp, whether you do it by scale; on a ground-glass; with a coupled rangefinder; or automatically. If you shoot the same picture at a large aperture and a small one, making the necessary adjustments to shutter speed if you aren't using shutter priority auto-exposure, you'll soon see how depth of field works too. After that, as with everything else in photography, it's down to taking pictures and more pictures -- though there's also a paid module on focus.

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© 2007 Roger W. Hicks