6th August 2015 – Bath Stone, Back Button Focusing and Exposure Lock

You cannot deny the poetry of Bath stone in a soft sunset, with the canal and the river yielding mirrored Georgian realities on a pleasantly warm and breeze-less evening. At least until you get home and look at your images to finally yield to the notion “What the hell was I thinking?”. Last minute change of venue with the Bristol International Balloon Fiesta closing roads around the intended destination of Leigh Woods, and a fine suggestion from Vice Chair Myk Garton saw us decamp the dozen miles or so to the World Heritage City of Bath. Two topics left over from last week to discuss this: back button focusing and exposure lock, both of which were useful tools on the night.

 

There are two buttons on the back of most DSLR/CSC’s from the hobbyist models upwards that are rarely if ever troubled by the majority  of hobbyist photographers. One is the “Back Button” AF and the other the AEL (which may be represented on your camera by a star or asterisk). Both are incredibly useful and are really worth cultivating as default, in the case of the back button AF, and useful tool in the case of AEL, that can help increase your number of keepers. A little bit of background is useful in understanding both.

 

Back Button Focusing (refer to your manual for the native translation in your Camera’s Brand-Speak) does exactly what it says on the incredibly expensive magnesium alloy tin, or plastic camera body as befits your pockets/needs/delusions of grandeur. It is a button on the back of your camera body that activates the camera’s focusing system in isolation from the shutter release.  When you operate via the shutter release a half pressure triggers the autofocusing system (assuming you are not mounting  a manual lens) and a full depress activates the shutter release. Usually the shutter will not fire until the camera processor detects all the algorithms are in place to produce a point of focus and an acceptable circle of confusion (i.e. something is in focus as we discussed last week).  The button itself is usually marked AF or a version thereof and is normally accessible with the right thumb (I’ve never seen one on the left but then I haven’t conducted a survey in any depth). And it’s on the back of the camera.

 

So far so blindingly obvious. Also, so what? Well, my skeptical friend, for that we shall have to take a brief incursion into the Trinity of Focus you played with once, got annoyed with then left on single shot. That, incidentally, is a perfectly acceptable solution because it’s your picture, your way. You want to do it the hard way, then suffer on for your art. It will not take better pictures, only you can do that, but it will help you be prepared for those better pictures, if only by speeding the whole process up and in more than one way. It is a little like buying a fully spec’d DSLR and leaving it permanently on auto or programme. In one way an expensive choice, but if you know what it does in those modes and you know how to override it in the situations where you think you need to push something to get the effect you want, then, pilgrim, it’s not such an expensive choice. It’s one that is made with a purpose rather than one made unthinkingly.

 

There are in our CSC/DSLR and some bridge cameras usually three flavours of autofocus. They may have slightly different sets of initials and names but they will be essentially the same. They will be in the menu system and may be programmable, at least the menu access may be programmable, to a button on the camera body.  RTM (Refer To Manual). The auto focus programmes in your camera were designed because your subject is doing one of three things. It is either static or it is moving. Or you are. Or you both are.  The important dynamic is that between the subject and the sensor.  What the back button does for us is allow us to switch between modes as required without having to resort to the menu system.

 

So, lets take the modes in alphabetical order, starting with AF-A. -A is where the camera decides, as is the case with all auto and programme modes, and it chooses between the other two modes depending upon what is closest to the idea set by its algorithms, i.e. is the subject static or moving relative to the sensor.  On modern cameras it is pretty good but it only responds to what is, it won’t anticipate your next move, so it will always be playing catch up. -C is continuous. When the subject is moving the focus moves.  -S is for when the subject is still, like a portrait. What the back button does is take a step out of the process of firing the shutter, the focus (at half way down)  is done by the back button and so the shutter button becomes just that. Without disabling the focus step the lens will attempt to refocus that which is in focus (which you have already framed as part of your composition). Doing things twice, slows things down. You are effectively switching between –A and –C at a press of a button. There is always manual focus of course, but this tends to be easier with manual lenses which have a longer throw (the barrel twists further because they are geared lower to make them easier to focus by hand) and no where near as rapid.  In the words of Professor Fate, “Push the button, Max“.

 

We’ve talked about exposure before and its relation to colour saturation, detail contrast and so on so I won’t go over that in this blog.  I want to concentrate on an option rather than an effect, though the two are linked, of course. The exposure lock button found on most DSLR/CSC cameras and some bridge cameras. Sometimes the exposure you need is not the one your camera is showing you.  This can be down to the metering mode you are using and the fact that it is metering an average from the area you have selected to measure from in terms of both the subject and the percentage of the sensor coverage involved.  There will be a wide, centre weighted and a spot option. Wide is usually factory default. Spot takes a very small area in the middle.

 

Essentially, in tricky lighting situations, probably but not exclusively covering a wide variation, you meter from the most important part of the scene to your image, keep the AEL AFL or * button depressed then recompose and take your image. The overall exposure will be governed by that part you selected. You don’t have to use spot to do this, just influence that part of the scene you want to feature to your satisfaction (this is far, far easier on a CSC/DSLT where what you see is what you get or in your live view if it allows composition other than pure review).  It is very effective and can be used with ISO compensation for fine control. On our Bath perambulation it was very useful when we were presented with a glorious sun set and allowed for much of the foreground to be silhouetted whilst retaining the exposure for the sky. Yes this could have been done in manual, but it was done far quicker using Aperture Priority and AEL. Try it, it is surprising useful.

 

Next week a change in programme where Nick Hartley has negotiated access to the Tannery where he works. Details on Facebook and the club Flickr page as numbers are limited. If your name’s not on the list your not coming in. Who’d a thought leather had a door policy?