Are my photographs blurred?
Q I have returned to the photography hobby after quite a gap. For family photos and holidays I’ve had a point-andshoot digital compact for some time but I’ve recently bought my first digital SLR, an Olympus OM- D E- M10 Mark II with the standard 14- 42mm zoom lens. My original hobby camera was an Olympus OM-2n film SLR. I had a variety of lenses and I remember the golden rule of stopping down to around f/8 in order to get the best sharpness. I’ve been told that’s all different now and that stopping the lens down can make photos less sharp. This sounds very odd to me. Could you provide some technical explanation? Pat Lowry
A The demands placed on lens optics by digital cameras has resulted in much better performance, even at the widest apertures, compared with lenses designed for film. There is much less need to stop the lens down to avoid wide-aperture softness. This is just as well because the nature of digital sensors means the use of particularly small apertures should be avoided because of a phenomenon called diffraction softening. Image forming detail in light focused onto a digital sensor can be visualised as spots over the microscopic photosites that collect photons to form images. The area that these spots cover at the sensor level is known as ‘Airy discs’. Ideally, Airy discs should not be significantly larger than the area of a photosite. However, diffraction can spread the light out causing the Airy disc to expand. A primary cause of increased diffraction is the lens iris as it gets smaller. As the Airy discs get larger in relation to the photosites the sensor steadily loses its ability to resolve details in the image as the discs overlap several photosites. With Micro Four Thirds cameras like your E- M10 there is a measurable fall- off in image resolution from around f/5, though it may not be noticeable in real terms until f/5.6 or f/8. With larger sensors, the visible onset of diffractions comes at smaller apertures of around f/11 on APS- C and f/16 on full frame.