Depth of Field - page 2

Depth of field in practice

Older lens for 35mm and medium-format cameras used to carry depth of field markings which enable the photographer to both assess the real depth in a situation and also, if preferred, set the focus manually to the hyperfocal distance. Partly because the lenses today are fitted to cameras with a variety of chip sizes and partly because everything these days is dumbed down, the markings are not always provided. It is thus useful to have a working idea of your depth of field and so we show here a trio of scenarios. It is one of life’s puzzles that most photographers will grab two stops of speed, when offered (ie a possibility to move from 400ISO to 100ISO). Those same photographers often fail to realise that they can have a couple of stops extra ‘speed’ in the form of a wider aperture if they understand depth of field and exploit it.

Consider then the three scenarios using a small chip DSLR
(eg Nikon D200, Canon 350):
· A tightly cropped facial shot.
· A head and shoulders portrait of a single person.
· A group shot of about 30 people arranged in three rows.

Write down the aperture you would expect to need to keep both ears and nose sharp in the first two and the whole group in the third scenario. Go on write it down! Imagine you are standing in front of a wedding, what aperture are you going to use? If you ask “what focal length?” go to jail, you have not been listening – go back and read point three above! Pay your £200 to get out of jail, then look at the bottom of the next page for the answers. As a professional photographer, hired by your client to do a job for them, it is your duty to know the answer to such questions. Most photographers underestimate the first, get the second close and over-estimate the requirements of the group shot. If you were badly out then do some tests and stick a small label on the back of your camera reminding you of what aperture you need. If you can capture the group on the altar at f2 imagine how much more scope you have to bounce a flash off a high church ceiling and get away with it. If you have the camera on ‘Program mode’ you are totally out of control of the situation.

The fact that you can get sufficient depth of field at f1.8 does not mean you are recommended to use such an aperture on an outdoor, sunshine, group shot – this would require a shutter speed of 1/32,000! However, you can make the best attempt at blurring down that advertising hoarding across the road from the registry office! Your fill flash will be the determining factor in setting your shutter speed. If it is an oldfashioned unit with a synchronisation requirement of 1/250 you are going to be stuck with quite a small aperture and a lot of background clutter. Conversely the loss of effective flash power from a multi-burst modern unit may not matter if you can grab the group at f2 or f2.8.

DECREASING DEPTH OF FIELD

1. The starting shot. No opportunity to move back, a 30-foot drop behind the bride and a cluttered backdrop.
2. First make a mask to silhouette the bride and bridesmaid and soften it with 3px Gaussian Blur.
3. Make a gradient mask running deep grey though to black then out to white at the top.
4. Combine masks 3 and 2 to create a composite.
5. Use the Pen Tool to create a mask for the handrail and fi ll it with solid black.
6. Add the handrail to the mask made at [4].
7. This fi nal mask is an alpha channel and is used by the lens Blur Filter.
8. This controls the strength of blurring – maximum where the mask is white, no blur at all where the mask is black.
9. The various masks are saved as alpha channels as they are made so that they could, if necessary, be adjusted or remade.

One of the most often heard criticisms of portraiture is the lack of differential focus. Exploiting shallow depth of field adds depth to an image, isolates the subject and brings emphasis to those parts of the picture you choose to focus on. Occasions arise which force you to use a wider angle, smaller maximum aperture lens – typically a group inside a church, with no ability to step back from them. The wide angle has three effects - the smaller focal length produces a very small increase in depth of field; the lens may quite likely have a restricted maximum aperture (hence a large increase in depth of field) and the wider view behind the group might contain a larger selection of distracting elements (ie three fire extinguishers instead of one, or none – you might be able to hide one behind the groom, hiding three more gets exponentially trickier!).

This is where technology comes to the rescue – the Photoshop Lens Blur Filter.

The Lens Blur facility provides a highly realistic way of blurring a background in just the same way as a very wide aperture lens might. It may not be anywhere near as fast as getting the depth of field down ‘in-camera’, but, providing you master the technique, it will do an equal or better job. Indeed, if using a zoom at f2.8 carries too much penalty in edge-to-edge sharpness and general image contrast, then shooting at an optimum f5.6 and decreasing the depth of field in Photoshop may provide a superior result.

The basis of the method is that you make a mask to control the depth of field blurring of the filter in a realistic way. A mask with an underlying gradient in it can be used to blur the foreground a little, the subject (in the middle distance) not at all and the background quite a lot. To do that, the gradient mask needs to be lighter in the foreground, fully dense-black across the subject, then tapering out to white in the distance parts of the image. Applying Gaussian Blur produces a similar but less sophisticated effect – for one thing the blurring makes those parts of the image look grain-less and has the effect of dulling out-of-focus highlights towards a mid-tone. The Lens Blur filter allows control over the grain (noise) you put back, the qualities of out-of-focus highlights and even the shape of the lens aperture. Old lenses with very fine, multi-bladed aperture diaphragms produced more refined highlights than cheaper alternatives with diamond-shaped apertures.

INCREASING DEPTH OF FIELD

Normally you increase depth of field by stopping the lens down. Eventually you run out of steam with this method as the effects of diffraction start to degrade the image – you get more depth of field in an otherwise fuzzy image! Diffraction becomes noticeable after f16 on most lenses. In a macro image, as you move the lens further away from the camera, to increase the magnification, the aperture ‘seen’ by the chip is even smaller (it is further away). Thus at 1:1 an aperture set at f16 is really f32 with the depth of field benefits that this bring along with a bucket load of diffraction. In many instances therefore it is just not possible to get sufficient depth of field, even with limitless amounts of lighting power available.

A solution, developed in the 80s was to place the subject on an accurately movable table and make multiple exposures with a sheet of light from a laser. Apart from the obvious disadvantages for field photography, the system cost hundreds of thousands of pounds. Today’s equivalent is a software fix in which clever imaging analysis is used to reassemble a series of shots, made with the camera moving through the subject focus pane. Almost unbelievable this software is free – how cool is that? It is the brainchild of a gifted amateur photographer and computer programmer called Alan Hadley and is called Combine ZM. The work put in by Hadley over the past six years is quite remarkable but it does show the power that can be harnessed by skilled hands using the computing power we have available today – and he did it just for fun! There are some wonderful examples of work made with Combine ZM if you Google for them but try http://www.dgrin.com/showthread. php?t=61316 for starters. We include here some of our initial attempts.

ABOVE: The results for a subject such as this chronograph are spectacular. The series of shots on the right was taken using a Nikon Focussing Rack to increment the camera forward between each of the frames and then Combine ZM was used to assemble the sharpest portions of the image and correct for the change in magnification and perspective.

THIS PAGE: With over 100mm of depth to be covered this lily presented a real challenge for conventional macro even at small apertures. Five shots were made at 20mm increments before combination using Combine ZM. The large shot below is what was created without any clean-up. Note the repeat pattern of the image outside the white annotation line and the slight ghosting in the petal at point A. These may be cleaned up by cropping and cloning. The side-on shot in this box shows that the software can be fooled when there is inadequate detail and contrast – here the two stems have been merged into one.

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Photo Quote: To photograph truthfully and effectively is to see beneath the surfaces and record the qualities of nature and humanity which live or are latent in all things. - Ansel Adams