13th & 20th August 2015. On Poor Light and ISO

Two away days to cover as last weeks scheduled blog got displaced. Hopefully back on track now. An unexpected opportunity to tour a Tannery courtesy of member Nick Hale replaced our scheduled Millennium Square trip the week before last and a trip to Blaise Hamlet last week both threw up some tricky light conditions, namely the lack of it and flat light with low contrast. The Thomas Ware & Sons Tannery was formed 175 years ago. The process and buildings are fantastic material and I look forward to seeing members images after the summer’s road trips. Thanks Nick, that was a fascinating evening and all the members were suitably impressed, I thought. Blaise Hamlet was built to house the workers who serviced the John Nash designed main house of the Blaise Castle Estate when they retired. John Harford bought the estate land in 1795 for £13,000 (a bargain £1.2m in today’s coin, using the Retail Price Index, but the average wage in 1795 was circa £20 per anum, in 2014 £25,000 – Source EH.Net ) The castle itself was a folly built as part of the evolving grand design of the estate.


The sun was hidden by a uniform blanket of rain-threatening cloud for both events, which was a pity, as the plentiful sky lights and doorways held the promise of some photogenic lighting in the tannery and the warm coloured stone of the main house and the intertwining of nature and construction to be found at the Blaise cottages (cue debate on the use of buildings to reinforce social order) offer a lot of subtleties that contrasting light brings to the fore. So, if photography is about light, and it is all about light physically, what do we do in the absence or limitation of it? The obvious answer to this is to provide our own, but this is not always feasible, so this week we are going to look at shooting in low light situations, what can and what cannot be reasonably achieved and the costs of doing so in terms of quality. We shall be looking more closely at ISO, the more mysterious member of the Exposure Triangle.


The options on camera are, basically, open the aperture, select a lower shutter speed, or select a higher ISO. The other useful option is to use additional, artificial lighting, either constant light or flash/strobe. A tripod can help with longer exposures. The two other options that spring to mind are focus on details rather than panoramas or switch to black and white, but these are variations, though very useful ones (yes photography is about details and exclusion but we are talking about large buildings here remember and in general at the moment, not in particular). Then there is the pack-up-go-home option and its local variant, pack-up-go-home-come-back-another-day. But where’s the challenge in that and where the learning opportunities? Are you a photographer or a Sherpa?


ISO stands for the International Standards Organisation, doesn’t just apply to cameras, it does exactly what it says on the tin, publish standards for a huge variety of items, systems and products. One of them covered film “speed” or the way that film reacted to light, more specifically, the sensitivity of the crystals in the emulsion applied to the transparent film base react to light. The most widely used standard was the American Standards Association (ASA now known as ANSI, the American National Standards Institute) and that was eventually adopted by the ISO (DIN or the Deutsches Institut für Normung e.V. was used by Agfa among others and had a different numbering system) ISO was carried over to digital processors by the manufacturers, the familiar making sense when introducing a new way of doing something.


Of the three parts of the exposure triangle ISO is the one that acts directly on the sensor. The other two parts, aperture and shutter speed work by throttling the amount of light before it reaches the sensor. ISO directly plugs into the sensor to alter its sensitivity to the amount of light reaching it with its own particular characteristics. With aperture it is depth of field and with shutter speed it is motion blur. ISO can boost it. It can also cut it to a point. There is a constant where the sensor will provide the best quality image, usually around 100 ISO and a group of ISO numbers where very little difference is made by the sensor manipulating the light received, but gradually, as with faster film emulsions, there comes a point where the image will begin to noticeably deteriorate with the incursion of noise. Noise is a product of the signal moving around the sensor, and is a function of all electrical circuits. How much signal (desired data) there is and how much poor data there is in relation to it. As poor light produces more poor data (noise), and the chip amplifies that data to produce the image, the quality starts to deteriorate as artefacts generated by the process become more apparent. All sensors will have a certain amount of noise present at any ISO, it’s part of the mechanics of the sensor. The amount of noise as a proportion of the overall signal determines the reproductive quality of the image. When the light is good and the ISO is set at or close to the speed of the chip then the signal is strong and the noise is low but gradually this inverts the more boost is applied. When you amplify the signal you amplify the noise in the circuit, when the signal to noise ratio is good this doesn’t matter much, but noise will increase as you boost the signal and it will be an increasing amount of what is going on. Hence, well, noise at high ISO’s.


On the part of the photographer getting to know your cameras useable limitations – and it is a judgement thing rather than a given absolute – we have to judge how much noise we are prepared to put up with in an image. There are ways of limiting its effects in post production, also in camera with some models, but the pay-off is a softening of the image. Also do not forget the idea of an optimal viewing distance, as a rule of thumb 1.5 – 2 times the length of the diagonal of the viewing area (works for tv’s too) AND the minimum pixels per inch – calculated by dividing 3438 by the viewing distance). So that is pretty much it, without getting over my head in technical details. ISO and noise.


Tonight – Millennium Square. Meet under the big shiny ball at 19:00 hours (7 PM).