How much light is reaching the sensor at ISO6400?

Last update:- 29th April 2023

Did you know your ISO does not add more light (image data) to the sensor?

I was listening to an astrophotographer talking about his camera settings. Most astrophotographers are comfortable with the technical aspects of digital cameras. He demonstrated how he shifts the histogram to the right (ETTR) to improve his image quality and record more tonal data.

Many photographers use the ISO and Exposure Comp to apply ETTR. Those who follow this blog know the ISO amplifies the image signal from the sensor. If you like to control the light reaching the sensor, fix the ISO and use the aperture and/or shutter speed. Increasing the reflected light on the sensor means a higher SNR and a more Saturated sensor. The histogram shows the reflected light on the sensor with a fixed ISO. Consider this as "managing" the sensor's ISO to SNR ratio.

Study the illustration below and visualize the reflected light on the sensor at the different ISOs. What do we learn from this illustration? Experienced M43 photographers think of reflected light (luminance) on the sensor. The sensor might receive no reflected light at ISO12800. We always need a light source to illuminate the subject and reflected light to expose the sensor. No reflected light means no recordable image information. The next time photographers quote ISO12800 or ISO20000, ask how much luminance (reflected light) was available at the image sensor?

This illustration shows what the actual scene looks like at each ISO amplification.

The key is to study the technical basics of image sensors. For example, all image sensors have a noise floor, and we control the final visibility of this noise floor. It helps to know the different types of image sensors and the importance of pixel area. Pixels capture Photons and NOT the size of the backplate housing them. The size of the sensor only determines the Lens Image Circle that drives the OPTICAL differences between digital cameras. It's important to know that Pixel Area is one of many Quantum Efficiency variables that determine Sensor Sensitivity.

Saturating the sensor with light means a higher SNR and less image noise. A higher SNR also means we record more tonal data. Digital photographers should know these basics and how to manage them with digital cameras. For example, the technical design aspects of the image sensor are always the same. What are the main Optical and Technical differences between image sensors?

Why does the ISO shift the histogram? Because it's a variable in the exposure equation.

It should be clear why we risk having noise at ISO6400. The above "Photons to Electrons" graph shows what happens with less light on the sensor. Too little reflected light means the sensor's SNR is lower. How do we manage noise at higher ISOs? We typically increase the ISO when we need higher shutter speeds. The key is light. It's safe to use higher ISOs with enough available light. Photographing a Formula One event in daylight is a good example. It's safe to change the ISO and shutter speed with enough light. Also, keep the sun behind you and always have a flash in your camera bag.

What happens at higher ISOs and shutter speeds? We reduce the light on the sensor and increase the image signal amplification (ISO) to maintain the camera's exposure level. 

TIP:- The histogram follows the light on the sensor at a fixed ISO. It's incorrect to say the histogram shows jpeg data in Live View. Consider this typical fact statement from undisclosed promoters. When do we have a jpeg file? There's no jpeg file in Live View before you record the image...

Study this basic example of how to apply this knowledge:-

Consider Bird in Flight (BIF) photography. We can safely increase our exposure with 2 stops (ETTR) when photographing birds against a blue sky. If your final adjustment is between ISO800 and ISO1250, which will you take? ISO800 is the better option because it allows more light on the sensor. More light increases the SNR and sensor Saturation. The info in this article enables photographers. Compare that to those advising M43 photographers to "never go above ISO800"...

For more information on ISO and exposure, see this article - link.

See the 7 Points each photographer should know about digital cameras - link.

Have you asked why forum promoters use Photons to Photos graphs when they roam photography forums? Why do they never talk about the basics in this article? Pointless presentations based on calculations do NOT improve image quality. Always ask this simple question. Does the presented information enable photographers or only those manufacturers selling full-frame cameras?

For example, technical aspects like Sensor Type and Effective Pixel Area could be important when selecting the right camera for low-light conditions. Micro Four Thirds cameras use Live MOS sensors, and most APC cameras use Standard CMOS sensors. The Live MOS sensor has up to 75% less control wiring at each pixel. This means M43 cameras typically have a smaller noise floor. Live MOS sensors also have a larger Effective Pixel Area. This explains the Live MOS sensor's relatively good noise and low-light capabilities. Another interesting technical variable is Sensor Readout Speed.

The size of the sensor drives the optical differences between cameras. The technical aspects of the camera are the same for ALL sensor sizes. Why would some say, "Ignore the technical aspects discussed in this article?" Are they simply dumbed-down fanboys or undisclosed promoters?

Best and God's Bless

Siegfried