Do larger sensors collect more light? Yes.
If we talk about the amount of light (number of photons): the larger the capture surface, the more photons incident per unit of time.
Assuming that the illumination is the same in two sensors of different sizes and that the exposure time is the same: the larger sensor will collect more light (more photons) overall.
Each sensor cell will collect only the number of photons corresponding to its surface.
If we have two sensors of different sizes, their photosensitive cells have the same capture surface: each cell will collect the same amount of light in the two sensors.
But overall, the larger sensor will have collected more light.
So the image from the larger sensor will appear brighter, clearer? No.
The apparent brightness of the image (if it appears darker or lighter) is known as exposure ( perceived exposure if we also take into account ISO ) and does not depend on the total amount of light but the amount of light per unit area reaching the sensor.
If you want to see it more intuitively:
The larger sensor receives more light but spreads it over a larger surface area
The smaller sensor receives less light but extends it over a smaller surface area
Exposure vs. the amount of light
The exposure of an image depends on the intensity of light ( illuminance really, but think of it as photons per second ) that the sensor receives by the time it is exposed to that light.
The exposure is not dependent on the sensor’s size (assuming no other factors that limit the amount of light received by the sensor).
In cameras, the actual exposure (total photons per unit area) depends on the aperture of the diaphragm ( f-number ) and the exposure time (shutter speed)
The apparent exposure (perceived exposure) of the final image also plays a role in the film’s sensitivity or sensor (ISO value).
By varying the ISO value, we make the image appear lighter or darker, but it is somewhat artificial ‘ since we start with the same number of photons.
Different cameras, exposure, and sensor size
All digital cameras have the sensor calibrated so that it adjusts (more or less) to the behavior of photographic film.
- Exhibition time
All cameras will generate an image with an ‘identical’ exposure.
I put it in quotes because other factors influence, for example:
- The aperture of the objective does not take into account the light that is lost when passing through the lenses that form it (the lenses are not transparent)
- Nor does it take vignetting into account: the central area receives more light than the areas farther from the center
- Each manufacturer calibrates its sensor (ISO values) with a certain tolerance margin
Sensor size, exposure, and image quality
So if the exposure is not dependent on the sensor’s size, what advantage does a larger sensor have?
To understand it, we have to talk about the signal to noise ratio: SNR ( signal to noise ratio )
The signal is the information, the actual scene that we are photographing.
Noise is something that is part of nature. The light itself includes a noise component: the number of photons reaching each point on the sensor fluctuates statistically (Poisson distribution).
There are other sources of noise that affect the electrical signal (once the sensor converts photons to electrons)
We will never have an image that is faithful to reality; it will always contain some noise.
Signal-to-noise ratio (SNR) and image quality
The signal-to-noise ratio, SNR, is used to evaluate the image quality.
This relationship is usually measured on a logarithmic scale, in dB (it could also be measured in light steps, EV, which is another logarithmic scale).
When the signal’s amplitude is double that of the noise, its SNR would be 6dB.
An image is considered to have an acceptable quality starting at around 20dB signal-to-noise ratio.
An image with an SNR above 30dB is considered excellent, and the noise is hardly noticeable.
Noise is seen in the image as grainy and colored dots that do not correspond to the scene’s actual color.
Photonic noise and thermal noise
We then return to noise.
Fluctuations of light at the photon level are known as photonic noise ( shot noise ). Photonic noise grows proportionally to the square root of the number of photons.
If we receive 100 photons on average in a sensor cell, the fluctuations will have an intermediate level of 10. The adjacent cell can receive, for example, 93 photons, another around 105, etc.
If we receive 1000 photons on average, the fluctuations will have a mean of about 32 (square root of 1000).
Besides, we must add the other thermal noise sources, which do not depend on the amount of light. But let’s say the main contribution will be photonic noise in most cases.
As you can see, the noise level does not scale linearly with the amount of light (signal).
The more light the sensor captures, the more difference between the signal level and the noise level.
The SNR increases with the amount of light. The more photons the sensor captures, the more SNR and the better image quality.
SNR at cell level or sensor level?
This is the eternal question that you will find since the beginning of time in forums and bar counter conversations of brothers-in-law and photographers (or photographers in-law).
Let’s first think about the image as a whole.
Imagine that we have two cameras positioned so that they see the same frame of a scene. One of the cameras has a full-frame sensor.
The other is a mobile camera with a tiny sensor. Let’s further assume that the two sensors have the same cell size.
In the two cameras, we adjust their exposure parameters precisely: aperture (whatever the mobile has), exposure time, and ISO.
The two resulting images will look very similar in terms of medium brightness (exposure)
Which of the two images will have more ‘quality’?
If we think of the image as a whole, the large sensor image has been created from many more photons.
If the SNR grows with the number of photons, it seems clear that the large sensor’s image will have a higher signal-to-noise ratio, therefore better image quality.
Are we going to notice it visually by comparing the images?
Imagine there is a lot of light in the scene, and we expose it correctly.
Let’s suppose that for the small sensor’s image, we get 35dB of SNR, and for the image of the large sensor, an SNR of 55dB (I am inventing the values, it is to give an example).
The two images will be of excellent quality, and we will probably not notice any appreciable differences. Indeed the optical part or any other external effect will have more influence.
Now let’s imagine that there is less light in the scene or that we need to set a short shutter time. We raised the ISO on both cameras to compensate.
Now the amount of light is going to be less. Fewer photons, remember that with the ISO, we rescale or amplify the signal.
Suppose that now, for the small sensor’s image, we stay at 25dB of SNR, and in the large one, we go down to 45dB.
Although the image from the small sensor is still of acceptable quality, the noise will be noticeable. We still have a very high SNR; the quality will be excellent.
But, but, but we said that the cells are the same size!
Shouldn’t the SNR be the same for the cells of the two sensors? Yes.
Each of them collects the same number of photons in the two sensors at the cell level.
The signal-to-noise ratio at the cell level is identical (we will neglect any effect: we consider the same technology, etc.)
If we enlarge the computer images to 100% to see each pixel, we will see that their variability (noise) is very similar in the two images.
The difference is that one pixel in the small sensor image corresponds to a set of pixels in the large sensor image.
The large sensor has a much higher resolution (the condition that we had set is that the two sensors’ cells had the same size; therefore, there are more cells).
So, at a very local level in each small area of the image (pixel peeping), we find that the SNR is very similar in the two images. But the image from the large sensor contains more information about the scene, more dots.
More information about the same noise level: this already gives us a clue; it tells us that the image of the large sensor, even locally, looking at the image with a magnifying glass, has a higher quality, a better signal-to-noise ratio.
What if the smallest sensor has larger cells?
In all the combinations we make, in the end, the image of the large sensor will have a higher SNR (except in very, very specific cases).
At a local level, looking at each area of the image with a magnifying glass, it is possible that in certain areas, for example, the darkest places, the quality is a little better in the small sensor (due to the better signal-to-noise ratio of each cell).
But if the two images show the same frame, the large sensor will have built it from more photons in general: it will have more information and a better SNR than the image generated by the small sensor (if we cut one or the other, the comparison would not be worth it. , we would be cheating)
Scale to compare
Okay, that’s all theoretical.
Let’s get down to business. If you want to compare two images from two different cameras, you can do it, for example, by printing on paper at the same size.
Or, if you want to see it on the screen, you can scale the two images to have the same resolution.
In either case, the process of scaling an image to make it smaller has the effect of increasing the signal-to-noise ratio.
This is so because the scene information that appears in the image usually is spatially correlated. Simultaneously, noise is a random component that is neither spatially correlated nor related to the elements of the scene.
By rescaling down, we say that although we lose information (detail, resolution), we reinforce information from the scene and reduce noise.
Rescaling up to higher resolutions does not provide new information. SNR increases or remains constant (noise will probably become a little more visible).
Big sensor vs. small sensor
Is sensor size that important?
No (for what we are discussing here regarding noise and SNR / image quality)
When we use the camera with good lighting in the scene, there is plenty of light.
And all cameras, including mobile ones, offer excellent image quality (above those 30dB SNR that we mentioned)
Imagine that we progressively lower the lighting of the scene.
There will come a time, a level of light. For example, the camera with the smallest sensor will not be able to generate images with that ratio of 30dB, and the noise will begin to be visually noticeable.
Meanwhile, another camera with a 1-inch sensor will continue to offer images with SNR> 30dB.
We keep lowering the lighting.
Images from the 1-inch sensor will already start to drop below 30dB
Then it will be the turn of the Micro 4/3 sensor.
Then to the APS-C sensor
And finally to the Full Frame sensor
In other words, from this point of view, a larger sensor gives us a small additional margin for less light situations: dusk, interiors, museums, or situations in which we need a high shutter speed, and we have to raise ISO: sports photography (mainly indoor), children, pets, moving objects – when the light conditions are not perfect.
What margin or real difference is there between sensors of different sizes?
- Between a mid/high-end mobile and a 1-inch sensor camera: 2 light steps (approx. 2 EV)
- Between a 1-inch sensor and a Micro 4/3 sensor: slightly more than 1 step (approx. 1 EV)
- Between a Micro 4/3 sensor and an APS-C sensor: 2/3 step (approx. 0.7 EV)
- Between an APS-C sensor and a Full Frame sensor: About 1 1/3 steps ( approx. 1.3 EV )
These differences are only approximate and with cameras/sensors of similar technology.
A small current sensor may perform better than a larger but much older sensor.
It’s just to get a mental idea and to realize that cameras are not magical.
Note that a Micro 4/3 camera, an APS-C, and a Full Frame are all within about two light stops of difference.
First, we will see with real cameras if those differences that we have discussed are met.
There are many ways to estimate the differences, such as the DXOMARK data or the graphs from photonstophotos.net.
For example, here is a comparative graph based on the dynamic photographic range based on the size of the sensor:
But we are going to make our graph.
We are going to choose several more or less current cameras (at the time of writing this article) with different sensors:
- 1 inch: Canon G7 X, Sony RX100 VII
- Micro 4/3: Panasonic GX800, Panasonic GH5, Olympus E-M1 Mark II
- APS-C: Sony a6400, Nikon D3500
- Full Frame: Canon EOS RP, Sony a7S, Sony a7R 4
The models I have chosen random, the first that has occurred to me, and I have found in DXOMARK for each sensor type.
For example, within the micro 4/3, there are professional range cameras (E-M1 for a photo, GH5 for video) and entry range cameras (GX800). And in APS-C, the Sony a6400 would be an upper mid-range, while the Nikon D3500 is an entry range.
For each camera we look in DXOMARK, the value for Sports (Low-Light ISO)
What is that value?
The maximum ISO we can configure in the camera to achieve at least a signal-to-noise ratio of 30dB in the resulting image (excellent image quality).
Now we are going to try to analyze the data and draw conclusions.
The horizontal axis represents the sensor surface (in square millimeters)
The vertical axis represents the ISO value of the camera on a linear scale.
The red points correspond to each of the cameras; they represent that ISO value for which we have SNR = 30dB
We see that the dots are vertically aligned in their corresponding standard sizes: 1 inch (116mm2), micro 4/3 (225mm2), APS-C (350mm2), and Full Frame (860mm2)
Note that the jump between the ‘crop’ sensors and the Full Frame sensor in terms of the capture area is vast.
There is a specific variability for each sensor size: it may be due to the sensor technology, its age.
I have included in orange the lines corresponding to the typical ISO values (ISO 400, 800, 1600, and 3200) to have a more precise estimate of the differences between cameras in light steps.
The cameras with 1-inch, micro 4/3, and APS-C sensors that we have chosen in this example are all in a range of less than two light steps.
APS-C has a slight advantage over micro 4/3.
And micro 4/3 has a slight advantage over 1-inch sensors (the GX800 probably has a somewhat old sensor, an entry-end camera, while the G7 and RX100 are high-mid-range compacts).
We see that Full Frame sensors have more than one light step of advantage over APS-C.
In any case, it is seen that there is a fairly clear correlation between the size of the sensor and the signal-to-noise ratio that we can achieve.
Comparison in a real case
A situation where all cameras suffer:
Imagine that we want to take photos of our son/daughter who practices some sport in a sports hall.
In pavilions, especially in small ones, the lighting is usually quite bad.
On the other hand, we need a high shutter speed to 1) freeze the players’ movement and 2) avoid shake if we shoot freehand.
Let’s assume an initial configuration that gives us a correct exposure:
- A lens with f / 4
- Shutter speed 1 / 250s
- ISO: 800
1-inch sensor cameras would be a bit on edge, but overall, the resulting images would be entirely usable.
The Micro 4/3, except for the GX800 – seems to be slightly below average, would give a good image quality. The APS-C the same and, of course, the Full Frame.
Now we want to raise the shutter speed a bit to freeze the action more. Outdoors the recommended rates would be above 1 / 500s depending on the sport.
We go up to 1 / 500s, and to compensate, we have to go up an ISO step, up to ISO 1600.
At ISO 1600, the APS-C would be a bit on the limit but still give good quality images. Also, some micro 4/3, like the E-M1.
In cameras with a smaller sensor, you would already notice some graininess with the naked eye, especially in the darkest areas.
Cameras with a Full Frame sensor would still have a different light path. They could go up to ISO 3200 and 1 / 1000s, and the image would always be excellent.
Summary and conclusions
Remember that when we talk about image quality in this context, we are referring only to the level of digital / grain noise.
Key ideas we’ve seen:
- The exposure does not depend on the size of the sensor.
- It tells us the amount of light the sensor receives per unit area.
- The exposure we perceive in the image also depends on the ISO.
- Value. Although the actual exposure measures the light received, in most cases, we talk about the exposure triangle, including the aperture, the exposure time (shutter speed), and the ‘ sensitivity ‘ (ISO)
- Any camera with the same parameters: aperture, shutter, and ISO will produce a similar image in terms of brightness/luminosity.
- Larger sensors receive more light.
- The amount of total light (photons) depends on the exposure and the sensor’s full surface.
- The image quality depends on the signal-to-noise ratio.
- The higher the SNR, the better the image quality.
- It is considered that with SNR> 30dB, it is an image with excellent quality.
- Ultimately the perception of digital noise in the image is a subjective matter; each photographer has their criteria of what they consider an excellent, acceptable, and lousy image (not usable).
- Larger sensors – images with better SNR (higher quality).
- Sensors of the same size but different resolution: The sensor with lower resolution (larger cells) may perform better in terms of noise (less variability between points of similar tonality) and offers less detail of the scene.
- To compare the two images’ quality, we should do it with their final copies: paper copies or pictures in their actual publication size. Or at least we would have to rescale to compare to the same resolution.
- All cameras produce excellent images in a good light.
- In these situations, the SNR is high, and the noise level can be considered negligible. The differences would be more in the optical quality of the objectives or other external factors.
- The difference between sensors will only be noticeable in certain situations (low light, when raising ISO, when we raise the shadows in development/editing, scenes with very high dynamic range)
- The differences between sensors of different sizes are gradual. For example, the difference between an APS-C sensor and a Micro 4/3 sensor is minimal.
- A Full Frame sensor would give an advantage of just over one light step over an APS- sensor.