TechSupport

Enable Noise Reduction in the shooting menu to avoid ‘hot pixels’

Starry starry night?

QI recently took some night/ blue-hour photos of the Opera House in Oslo. This was my first serious attempt at getting such shots. I have an entry-level Nikon D3200, with a Tamron 18-200mm VR lens. I used a tripod, set the ISO to 100, and used f/11 to f/22, with exposure from 10-20secs and with the VR turned off. I was initially very pleased with the results as they are in focus, sharp and nice ‘star bursts’ from the lighting. However, when viewed in Lightroom at 100% the sky seems to be filled with tiny bright white stars even though it was cloudy! I can fiddle about in Lightroom with noise reduction and sharpening, but is that the only way? What is causing this phenomenon? Could it be a dirty sensor, or my settings? Jon Lipinski

ADuring long exposures of several seconds and more the image sensor in the camera heats up and inherent faults in the sensor matrix, otherwise usually invisible, become prominent. The result is in so- called ‘hot pixels’ and this is almost certainly what you are seeing. It’s a perfectly normal phenomenon and camera manufacturers know how to deal with it via a process called ‘dark frame subtraction’. After a long exposure the camera will make a second sensor capture of equal duration but with the shutter closed. The camera will then locate all the hot pixel bright spots on the dark frame and mask them out in the fully exposed frame. If you are seeing long exposure noise at ISO 100, it’s likely you have not enabled Noise Reduction in the shooting menu. All is not lost as you can do the dark frame subtraction manually in Photoshop, for example. Take a picture with the lens cap on for the same exposure time as the affected image and then import the original image and the dark frame as layers and use the Subtract blend option. With luck, the bogus stars will have disappeared.

Focal length weirdness

QI use a Panasonic Lumix GX7 camera and I have a Panasonic 100-300mm zoom. I’m trying to understand how this lens can have a focal length as much as 300mm when the physical length of the lens is well under 300mm even when zoomed to the 300mm setting. My basic understanding is that the focal length of a lens is the distance between the optical centre of the lens and the frame or sensor. That has to be much shorter than 300mm, surely? Zane Jacks

AYou’re quite right. The optical characteristics of a camera lens, which is typically formed of multiple lens elements and element types, means the physical lens barrel dimensions may not be anywhere near its focal length rating. Telephoto lenses can be a lot shorter than their focal lengths indicate and, conversely, wideangle lenses can be a lot longer. As a general rule of thumb the stated focal length can be compared to a pinhole camera where the pinhole is the optical centre and so the focal length is the distance from the pinhole to the film or sensor plane. The field of view will be determined by the focal length and the size of the frame the pinhole projects onto. This is why, regardless of the physical size of a camera lens, a given focal length should deliver the same field of view when that view is covered by the same frame size.