Questioning ‘big’ f numbers

optimum sharpness in its aperture range and it’s often considered to be around f/11, or a ‘sweet spot’. What that means is entirely separate to the focusing of an image, using smaller apertures (bigger f numbers) beyond that sweet point will inevitably produce a softer overall image. Clearly there is good reason for using smaller apertures – generally being to increase depth of field. But are we often under-estimating particularly the far limit of focus at any given f-stop?

It’s time for some calculations, the results of which may surprise you. The focal length of our given image here was 35mm, which in rough terms means that the frame begins about 4-5m in front of the camera position. Let’s look at the reference table (right) which is a depth of field calculator aligned to this exact camera set-up.

Terms of reference

Hyperfocal distance is the closest point at which a lens can be focused while keeping objects at infinity sharp. The hyperfocal near limit refers to the distance between the camera and the first element that is sharp when focusing at the hyperfocal distance. Depth of field (of course) is the distance between the farthest and nearest points which are in focus whilst the depth of field near limit is the distance between the camera and the first element that is sharp and depth of field far limit being the distance between the camera and the furthest element that is sharp. Depth of field in front is the distance between the near limit and the focus plane. Depth of field behind is the distance between the focus plane and the far limit.

Questioning the shot

The first element to think about is that as the aperture gets smaller (bigger f number since it’s expressed as a fraction) the depth of field near limit will get shorter Camera Focal length Aperture Subject distance

Hyperfocal distance Hyperfocal near limit

Depth of field near limit Depth of field far limit Depth of field Depth of field in front Depth of field behind

DOF near limit 2.1m DOF in front 2.9m

Subject distance 5m Canon EOS 5DS 35mm f/11 5m

3.64m 1.82m

2.1m infinity infinity 2.9m infinity

DOF behind = infinity

Depth of field

(or closer to the camera) and the depth of field in front (between subject and camera) will get longer. This is why many will tend towards a smaller aperture. But these distances have to be applied to the picture you are producing and your chosen focal length. In our instance, the depth of field in front is more than we need and arguably the same is true for depth of field behind – the scene does not extend to a far horizon, the trees probably being no more than a couple of hundred metres away or indeed less.

Let’s now have a look at what happens to these figures if we open up a few stops. At f/10 the depth of field far limit remains at infinity and the near limit only 2.25m. At f/9 the depth of field far limit still remains at infinity with the near limit still under 2.5m. It is only when we open up to f/8 that the depth of field far limit drops to 181m, but as

we’ve discussed that could well be absolutely fine. Interesting, then, that technically speaking this shot was potentially set up all wrong – and there’s likely many of us out there who would not naturally in our heads have immediately gone for f/8 when putting this shot together.

Now it’s time to have a look in detail at this composition and find out if opening up to f/8 would have had any benefits in terms of image sharpness. Keep in mind that it’s very hard to show comparisons such as this in print, but the above sections show a definite increase in sharpness between f/13 and f/8 – it’s particularly noticeable in the flaking paint of the upright – and that gain is costing little or nothing in the background trees. This, it should be said, is looking at the files at 200 per cent in Adobe Photoshop but think of it like this: if we can be producing a better image with an improved understanding of our equipment and the principles of photography, then we probably should. With this exercise we’ve learned that the sweet spot of this particular lens is quite distinct at f/8 and so to get the best out of it a depth of field table popped in the collective back pocket can assist in making the decision to use a smaller aperture only when actually required, rather than as a default – test it for yourself. The tale of the photographer who shot everything at f/22 no matter what remains for another day…

Expose to the right-hand side

Under-exposure can have devastating consequences for image quality that transcend the ability to ‘fix’ in post-production

In our photography, the term dynamic range describes the luminance range of a scene being captured, from the black point all the way to the white. Simultaneously it can describe the limits of luminance range that a digital camera can capture. There are, of course, methods of ‘expanding’ dynamic range: traditionally through the use of graduated neutral density filters resulting in increased detail in the shadows (this does not increase the fixed dynamic range available on the camera, but expands the usable dynamic range in practice) and on the digital side by combining multiple exposures of the same scene in order to retain detail in light and dark areas. So dealing with dynamic range is important, but what about a couple of questions: how much have we got and where is it?

If you think of dynamic range as the mosaic blocks of an image, that can help form an approach. In other words, if you have more smaller blocks then your resulting image will be finer with better representation of subtle details than if your image is made up of bigger blocks – with the latter situation corners will be harsher and colour transitions more sudden. That much is quite simple, then, and cameras in their iterations have generally improved dynamic range capabilities. For example, Canon’s 5D Mark II offered 11.9 Evs, whilst the Mark IV provided 13.6 Evs – a 15% improvement.

Now we know what we’re discussing and why it matters, we can turn to the big moment. If we assume there’s 12 stops available to us, you could be forgiven for assuming that this value was spread evenly across all tones from black to white. The thing is, it’s not. Dynamic range is heavily biased towards the highlight end of the histogram – hence there’s many more building blocks available to create the right-hand-side of the histogram than there is the left.

The two zoomed in images, right, are radically different captures. On the far-right the image has been under-exposed significantly and then ‘corrected’ or normalised in post-production. The left-hand image was exposed to the right-hand side and then also normalised in post. Now we know where the majority of dynamic range lives we know that the right-hand image at capture has missed out on most of the camera’s available dynamic range to build the image, whilst the left-hand image has maximised its use of available dynamic range by exposing so that the majority of the image lives where the majority of dynamic range also resides. The result is a far finer construction of the image with smooth tonal graduations.

Are you lazily under-exposing your images, perhaps because they look better on the back of the camera? Time to change your ways and let the camera produce the image quality you paid all that money for!