Noise Reduction in Digital Photography

Noise Reduction in Digital Photography is Quite Frequently Discussed Topic. There are some ways which we can use to cover up this issue. We will suggest the readers to read our previously published articles like Noise in Digital Photography, How to take Good Pictures at Night on the Road, Practical Tips on Digital Photography for Night Shots, Image Processor and Image Processing Engine, Image Averaging in Image Processing to gather more idea.

In practical life, we need to take some pictures at high ISO or those who loves night photography needs to use long exposure. In both cases, one will face the same common problem – noise. Some DSL manufacturers gives two default option which can be accessed via menu – Noise reduction at high ISO and Noise reduction in long exposure. Unfortunately, sometimes, they are not even closer to solve the real problem but add more difficult to solve problems in post processing. In this article, we will explain when and why one will use the built in noise reduction feature of a typical DSLR camera.

Noise Reduction in Digital Photography : At High ISO

We are taking that you have read our article –  Image Processor and Image Processing Engine. Hardware and Firmware combination matters a lot for realizing the feature. For noise reduction at high ISO, the software tries to remove the chrominance noise and luminance noise. This is the same type of reduction that can be done using a post-processing software, such as Adobe Photoshop. There are reasons for the reduction on camera than using any post-processing software later.

As always happens in case of digital photography, the camera software is much less powerful than you can install on your computer. The image processor is also weaker than your computer’s hardware. In addition, on camera solution does not allow fine tuning needed for noise reduction. One can not apply a mask unlike Adobe Photoshop. This means that the image quality will be much better if you remove the noise using Adobe Photoshop instead of using the on camera solution for noise reduction at high ISO.

Another, issue is that, for this type of noise reduction, the camera uses only images of JPEG format. If normally one shoots in RAW, they will have a preview on the camera for the applied processing. So,t when one will open the RAW image on Adobe Photoshop, the noises will be visible again.

An useful information is, if one is shooting in RAW the camera save the settings related data including that was used for noise reduction as metadata. Some of these metadata are readable by the programs of post-processing softwares from the manufacturer. These unfortunately are closed source softwares.

So, unless you use their proprietary software, if you shoot in RAW you can not see the effects of automatic noise reduction but you will get an idea – How Much Better the Image Can Be Done in Post Processing. So, take it as a Plus Point of the feature.

Noise Reduction in Digital Photography : For Long Exposure

In case of long exposure, the shot will have a shutter speed which will last for few seconds or more. The minimum number of seconds depends on the camera model and manufacturer. Longer the exposure is, greater the chance of acquiring noise. Visibly they will resemble like that of noise at High ISO. However, there are two other phenomenon which occurs in the case of long-term exposure, giving birth to the terminologies – hot pixels and amp glow.

Hot pixels appear as colored bright dots visible when the sensor heats up. Longer the exposure is, the sensor will go more warms. The amp glow is a glow of light visible at the corners of the frame when the sensor heats up. These are fully related to currently used sensor technologies and were never present in Film SLR cameras.

These two problems have a feature that makes their removal easier. If you look at two photos taken with the same ISO, same shutter speed and similar temperature, hot pixels and amp glow will be present in both at the same positions – this will not happen with noise at high ISO or rather what we commonly understand when we are talking about noise.

This fact is exploited by the on camera long exposure noise reduction in better way. Actually the software make a simple subtraction and remove hot pixels and amp glow from the photo. This procedure does not alter the quality of the photo. But as can be understood, this algorithm does not work for removing the high ISO noise, which is possibly the most annoying factor for us. Unlike the previous method of noise reduction, this can work both on RAW and JPEG format images.