Articles & Tutorials

Focus - the alternative approach

One of the main drivers of SLR sales is the shallow DOF effects that are obtainable via the larger format and I well understand the appeal as shallow DOF allows us to better pick out the subject from the background and at the same time draw attention to the subject.

Using a larger format is not the only way to achieve the shallow DOF look, the alternative is to blur the image post capture. Using post capture processing does not give the same look as true shallow DOF and can be quite difficult to do well, but it is in some ways a preferable method of achieving a similar look.

The core problem with shallow camera based DOF is accurate focus can be quite hard to obtain, as the aperture gets wider the focus placement becomes more critical and time and again I have seen images where the actual critical focus point is misplaced either due to poor technique or technical limitations within the camera itself.

If the image is out of focus where it should be, it will be impossible to re-constitute the appropriate sharpness level regardless of what software you might use, although there are some programs that can do a passable job so long as the de-focus is not huge.

On the other hand if we have an image that has more sharpness than required throughout the entire image depth we can always add blur to reduce it, which in the scheme of things could be an advantage.

There are limitations with normal DOF changes that are hard to resolve at the time of shooting. First DOF is linked closely to magnification, which means shallow DOF is very hard to obtain if one wants to use a very wide angle lens. The maximum usable aperture is probably f2.8 on most wide angle primes (f3.5 for many kit zooms) but even wide open most will give quite extensive DOF.

A further limitation is actual lens performance. Most lenses when opened up wide enough to give very shallow DOF do not perform all that well in the middle of the image and even more poorly around the edges of the image area, which means if you actually want critical sharpness and shallow DOF at the same time you may well be out of luck.

Generally the way sharpness falls off as the distance moves beyond the focus point is a product of the lens focal length and how close one is focused, this is pretty much not adjustable without making substantial changes to the composition or position of the camera. Obtaining a very steep fall-off or more gradual fall-off may only be obtainable by changing the actual format of the camera being used, for example choosing medium format instead of APSC formats.

Of course cost comes into the overall equation as in most cases if one wants to obtain a steep fall off and high in-focus sharpness there will be little choice but to buy top draw-ultra fast optics or perhaps use a large format camera with tilt/shift ability, (which of course is going to cost even more).

Recently I have been playing with an application on the iPhone called Tilt/Shift Generator, which basically works by blurring the image either along a set of straight lines or in a circular fashion. The options are limited and it by no means fully simulates what one would get via a much larger format but by gosh the results more often than not are very pleasing and look the part on screen and in print.

The iPhone, like all phone type cameras has pretty much unlimited DOF unless one focuses on an extremely close subject, this makes focusing a breeze and allows one to put off the whole desired DOF requirement until later.

Now there are a myriad of ways one could introduce blur into an image in post capture and here in lies one of the other great benefits of post capture DOF settings. When one uses a DSLR set up for shallow DOF the rendering of the out of focus areas will always be much the same, the only way to change that is by changing the lens you are using. Many photographers talk about Bokeh, which is a fancy term for how the blurry parts of an image look, bokeh is however a product of the lens type, the number of blades in the iris of the lens, focus point and the optical arrangement of the lens. Some lenses produce highly desirable bokeh effects while others look downright ugly.

We have had for years the software tools to alter apparent DOF and lately there have been a number of stand-alone programs devoted to the process, virtually all of these tools are capable of amazing results and probably more importantly they provide a way around the limitations of conventional lens based DOF control.

So what then you might wonder are the advantages and disadvantages of software solutions, first the disadvantages.

Created DOF effects via software will always involve extra time and in some cases this could be considerable, overall however an experienced operator may take as little as a minute to obtain a reasonable result.

The second disadvantage which for many negates the whole process is that it is quite difficult to obtain effects that look exactly like those created by the DOF limited camera/lens combo. This doesn't bother me much as who is to say the camera version is the best and only valid DOF effect that we should use from an artistic point of view.

Lastly some images will be very hard to work successfully because they have elements in the background that are intermingled with the foreground making selection of either quite hard, for example a portrait where the subject has lots of stray hairs that cut into the background. Overall however I have found a surprising number of the images I take are easily "DOF’d" in a short period of time and end up looking great.

The advantages are enormous and should not be dismissed as irrelevant. First you have the option to decide just what is and is not in focus after the event, this can work amazingly well because you have a lovely big screen image to look at. Importantly it means that you can get really creative with DOF, for example you could have near and far elements in focus with the middle ground out of focus.

The falloff of the focus is completely controllable, meaning you can have very steep or very gentle fall off, or even a different fall off on front of the focused point to what you have behind.

Bokeh effects are fully controllable, do you wish to simulate the effect of a 5 blade aperture, circular aperture, or any other possible permutation, no problems.

If cost is a consideration then it is simply no contest, you can obtain the look you want for absolutely minimal outlay, as all you need is a good quality compact with a nice sharp lens and these are incredibly cheap these days.

Don't write me protest emails telling me that the results are not the same as what you get with your ultra fast and expensive DSLR lens, I am not saying it is, but in many ways the final image will be just as satisfying and importantly not limited to the normal rules of DOF and the money one has to spend on the hobby.

Extending the Idea a Little Further

Shooting with a smaller format camera that renders virtually the entire scene in reasonable focus offers you the chance to actually change the point of apparent critical focus after the capture. This can be done via both selective blurring and sharpening.

The hybrid approach, here you shoot with a smaller aperture than you would ideally like to use so that you render a bit more in focus than ideal, then add blur to accentuate the affect that the lens naturally provides. This allows you to use slower lenses than ideal or shoot with an aperture that provides optimal sharpness levels for those points which are supposed to be in focus

Shoot with wide angle lenses that normally give great DOF and then blur to provide the look of a much larger format, for example 8 by 10 inch film, without of course the large formats true resolution.

Make better use of camera phones which typically have so much DOF that it is difficult to get separation of the subject from the background.

Generally with larger format cameras once you zoom in on the image on screen you can see that the apparent sharpness falls off obviously on either side of the actual focused point. In other words it isn't sharp across a wide depth of focus and then suddenly completely out of focus. An effect that can be created synthetically with compact digital formats is to have a large plane of critically sharp focus and then a very rapid fall off into unsharpness either side of this. Larger formats cannot however simulate this look as the areas that you want in focus will always be slightly softened as you move away from the critical focus point, unless you use a very small aperture and even then the effect is not the same.

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