30th July 2015 – Portishead Marina and Focusing

Apologies for late posting, certain technical problems rather got in the way.

Portishead Marina was our destination this week where the old Power Station has been demolished and developed into a series of housing developments around the old harbour. Portishead’s two other claims to fame are the band of the same name and the fact that the first Trans Atlantic wireless signal was received here and it became a famous name in maritime communication with the developments that followed it. No big beach to speak of, unlike last week at Weston Super Mare,  nor any motorcycle show of any size, at least that I could see. Certainly there were more boats than to be found at Weston and more expensive ones at that. The weather was better too, in terms of the amount of sunshine, but that brings challenges of its own. A good walk around in good company and a discussion on depth of field, exposure locking and back button focusing. I will deal with back button focusing next week along with exposure locking, but this week it’s about what is in focus and why….

 

Depth of Field is often blithely referred to as if you can’t be a photographer without it (whereas you can’t be a photographer only without something to focus light and fix an image in reality). Certainly if you aim to take your photography more seriously and you are using something with variable aperture glass, then depth of field (DoF as we shall refer to it from here on) is something you need to be able to manipulate to enhance your images.  In this way it is a third dimensional compositional aid (which is no mean thing in a two dimensional field). It is also one of the first skills that we master.

 

Your lens only really focuses on one point, think of it as the pin point in pin sharp and what we refer to as DoF is that part of the image we find to be “Acceptably sharp” i.e. it looks in focus. The brain acting on what the eyes transmit to it makes that decision with small variations from individual to individual but we can generally agree on what is and isn’t in “focus”.  This is known as “The Circle of Confusion” (I trust you haven’t joined it yet) or more, accurately the “Acceptable Circle of Confusion”, that part of the image that our brains process as being the same or indistinguishable from the actual point that the lens focused on. When we talk about an image being soft we are talking about the margins of what we perceive as being acceptably sharp which is at or beyond the limits of the circle of confusion, but transitions are gradual and there is no defined point at which sharp becomes unsharp and vice versa. DoF is what we see in an image from the closest to the furthest point we see as being sharp (within the acceptable circle of confusion).

 

Using  a lens with a variable aperture we can vary the circle of confusion. Aperture is the size of the hole that we squeeze the light through in our lenses on its way to the sensor. It acts as a regulator on how much light gets through. What fraction of the available light gets through we measure in F-Stops. F stands for focal and it’s the amount of light that is getting stopped. Hence F-Stop. This also determines how much in front and behind the focus point is acceptably sharp. The F-Stop tells you exactly how big the hole is that is letting the light through. It is calculated by dividing the lens focal length by the apertures diameter. So a 50mm lens at F8 gives you an aperture diameter of 6.25mm. A 135mm lens at F8 yields an aperture of 16.88mm. Of most interest to lens manufacturers, but it does stand for something important that we otherwise take for granted. And also it relates to that question that is forming in the back of your mind.

 

But why isn’t DoF the same on all lenses? Good question and it’s down to the DoF and its relation to the DoF. What? There are two? Oh yes indeed, the Depth of Field is directly linked to the Depth of Focus of a lens.  As photographers we are interested in Depth of Field, but it would not be possible without lens designers understanding Depth of Focus (and that’s not helped by the fact that a lot of people think they are the same thing). Depth of Focus is easiest thought of as the point at which the lens you have on the camera places the focal point. Remember that a lens actually only focuses one precise distance (point), that pin point in pin sharp, and what we accept as being in focus is down to the Circle of Confusion. The smaller the pin point in relation to the circle of confusion the deeper the depth of field. The larger the shallower. That’s why your small sensor compact camera has a deeper depth of field at any given aperture than your APS-C sensor DSLR, which is deeper than your 35mm “Full Frame” sensor and why you will see F128 on a 10 x 8 inch plate camera lens but not on any of the others in this list. Aperture also controls the circle of confusion.

 

On the top of most (all?) DSLT/DSLR camera bodies somewhere is etched a circle with a line through it. This isn’t for decoration it tells you whereabouts in relation to the rest of the camera body the sensor plane, aka film plane is. The sensor to the end of the body, the bit where you attach your lens most importantly, is a fixed distance. This allows you to attach different lenses of different focal lengths to the same camera body and the focusing ring/auto focus mechanism moves that fixed point backwards and forwards. When we see an object in focus it means that the three things that determine what is in focus,  are all aligned, (aperture focal length and the distance to the object) in relation to the fixed point that is the sensor plane and accommodated by the acceptable circle of confusion. When the pin point in pin sharp is in front of or behind the sensor and beyond the circle of confusion your object appears out of focus.

 

So much, so theoretical. How can we turn this into something practical? Well with something that sounds as if you need tablets for, hyper focal distance.  Hyper Focal Distance is much loved by landscapers, but that isn’t the only time you can use it.  Essentially it is based on the mathematics of the sensor size, depth of focus and depth of field and is the closest distance a lens can be focused that yields sharpness at infinity at a given aperture. Now infinity is a theoretical concept which, in practice,  means the horizon, or the furthest point in the frame from the camera. You can use your lens’s distance scale – if it has one, if it doesn’t you can’t – and on longer lenses this is probably the best option. On the shorter ones, especially wide angle lenses, a tape measure, or I use the length of my pace (2 steps is 5 feet but only 10 toes) as the Hyper Focal Distance is shorter with wide angle lenses.  A lazer measure might be a reasonable  investment if  you do a lot of this.

My 8mm I just set at 2 feet and snap away. Everything is in focus  from 50cm (19.5 inches) to infinity at f4  when the lens is focused at 3 feet (actually 38.5 inches, or 0.98 m). Do a lot of it then a laser range finder might well be a good investment.  The 50mm lens, or a zoom set at 50mm, has a hyper focal distance of 39m (19m to infinity) at f4, A 135mm, set at f4 has a hyper focal distance of 278m (139m and beyond).

But how do we know what distance is hyper focal? Well we need to know the sensor size, focal length and aperture. If you have a smart phone, then you can get a hyper focal distance calculator with something like Photo Tools (Android and I-Phone, free), or you can memorise the relevant distance from tables  suitable for your lenses (and crop factor) – it helps that there is a linear relationship between distance and aperture, so whatever the hyperfocal distance at an aperture of f2.8 it will be half that at f5.6 a quarter at f11. As long as you know what the distance is with your lens wide open you can work it out for other apertures.  As lenses tend to be at their crispest all round between f8 and f11 that is probably a good place to commit to memory for the times when you don’t have the tables to hand.

It is surprising just how useful hyper focal distance can be. Try it this Thursday when we shall be meeting outside the main doors of Bath Abbey at 19:00 (7pm).