Are mirrorless cameras noisy and inefficient?

Q Mirrorless cameras depend entirely on live view operation – in other words the sensor has to remain powered up all the time the camera is switched on. With a DSLR, the sensor can remain in standby until it needs to take a picture. This prompts me to ask two questions. First, do mirrorless cameras have noisier sensors because they’re always- on? Second, do mirrorless cameras drain their batteries faster because the sensor is always on and the viewfinder also needs power? Ken Carlton

A It’s true that if the heat generated by an imaging sensor can’t be dissipated efficiently it can cause an increase in image noise. However, manufacturers do a lot of thermal modelling when they design their cameras, with heat sinks and ventilation designed to manage sensor temperatures. In general, there is little evidence that a sensor similar in specification used in both mirrorless and DSLR cameras produces noticeably different levels of noise under comparable conditions. But it’s undoubtedly true that mirrorless cameras require more battery capacity to cope with the needs of the electronic viewfinder and/or rear screen being on, as well as the sensor. However, steady improvements in electronic circuit efficiency and battery performance means that the more recent mirrorless cameras can be used normally before requiring a battery change without inconveniencing the photographer unduly. Sony, for example, has doubled the battery life in its latest full-frame models since the original Alpha 7 was introduced five years ago.