Smaller Sensors Don’t Absorb Light As Fast As Larger Sensors And Therefore Need More Exposure; Wrong! Let’s Settle This Once & For All

Relative digital camera sensor sizes.  Do smaller sensors need more exposure to equal the
luminance (brightness) of larger sensors?

Do cameras with smaller sensors require more light to equal the exposures on cameras with larger sensors?  Let’s settle this once and for all.  I really tire of seeing and hearing supposedly ‘professional’ photographers, podcasters, YouTube creators and bloggers—people who portend to know what they are talking about and give others advice—say, with (evidently false) authority, that larger sensors absorb light faster than smaller sensors.  It is simply not true.  There are differences but luminance isn’t one of them.

One of the most common debates in photography circles is whether a larger sensor—full frame versus APS-C or Micro Four Thirds—creates a brighter image when the photographer uses the exact same settings: ISO, aperture, and shutter speed. Some swear that big sensors “soak up more light,” while others insist that sensor size has nothing to do with exposure. It’s easy to see why beginners find the subject confusing.  So let’s clear it up once and for all, using two very simple analogies.

This old debate rose its ugly head for me when recently I was watching a YouTube video featuring a professional sports photographer who had tried out the Micro4/3 OM-1 Mark II with the newly released OM 50-200mm f/2.8 lens.  The photographer was impressed.  It was a glowing review.  This photographer, a die hard full frame Sony user, actually decided to purchase the camera and lens.  However...at one point in time when comparing an f/2.8 Sony lens to the OM 12-40mm f/2.8 lens, the photographer made this statement:  "...this is an f/2.8 and this is an f/2.8 (holding a lens in each hand).  They are not the same.  Because it is the micro4/3 sensor, basically half the size or literally half the size of a full frame sensor, it is capturing only half the light.  So, it really does become like a 5.6."  (That is very close to a direct quote.)

First, the Micro4/3 sensor is not one half the size of a full frame sensor (it is 1/4 the size) and second, a full frame sensor does not capture twice the light.  Hence, this blog post.

I'll try to explain this issue in basic terminology so anyone can understand it.

At an identical ISO, shutter speed, aperture, and scene luminance, a full-frame, APS-C, and Micro Four Thirds camera will produce images with the same brightness.  But they will NOT look identical due to noise, depth of field, and field-of-view differences.  Brightness is the same but other aspects of the image can be different.

Where the Confusion Comes From:

People confuse total light with light per unit area.  A full-frame sensor gathers more total photons because it’s larger.  A Micro Four Thirds sensor gathers fewer total photons due to smaller area.  But exposure is not about total photons — it’s about photon density on the sensor plane.

Here are a couple of simple analogies to illustrate the concept.

Analogy 1: The “Plates of Sugar” 

Imagine three plates:

• a large dinner plate (full frame)

• a medium plate (APS-C)

• a small dessert plate (Micro Four Thirds)

Now sprinkle powdered sugar from directly above using a sifter.  The sugar falls evenly, like light falling into a camera.

• The big plate catches more total sugar.

• The small plate catches less total sugar.

• But the thickness of the sugar layer is the same on all plates.

That “thickness” of the sugar is what exposure really is.  How much light is falling on each pixel or photosite.  

When you take a photo, the camera isn’t showing you the total amount of light the sensor caught—it’s showing you how bright the image appears, which matches the density of the light, not the total volume.  The thickness of the sugar.

Just like the sugar layer, the brightness is the same regardless of plate size.

Here's another one for those who still may disagree or be lost...

Analogy 2: The “Window and Bucket” 

Think of each sensor as a window letting light through, and your image as a bucket inside the room that catches the light.

If you open shutters for the same amount of time (shutter speed) and to the same width (aperture), the light entering through each square inch of window is identical.

A larger window lets more total light flood the room, but the quality or intensity of the light on the floor is the same.

Your bucket of water—the final picture—fills to the same level, because you’re measuring the depth (brightness), not the total gallons available in the room.

Again: more total light exists with a bigger window, but the final brightness does not change.

Analogy 3: “Sunshine on a Tabletop”

Imagine you place two objects outside on a sunny day:  A large cutting board and a small cutting board.  They’re made of the same material.  They sit in the same sunlight.  They’re side by side.  Does the large board look brighter?  No.  Does the small board look darker?  No.  Both have exactly the same brightness, because the sunlight falling on them is equally intense everywhere.

The only difference?  The large board receives more total sunlight (because it has more area).  The small board receives less total sunlight.  But the brightness per square inch is identical on both boards.  This is exactly how camera sensors work:

A full-frame sensor is like the large board—it collects more total light.

A Micro Four Thirds sensor is like the small board—it collects less total light.

But the brightness (the exposure) is the same because light intensity does not change with size.

And…if you STILL don’t believe that the images will have the same brightness, I decided to conduct a real world test with my cameras.  As many of you know, I shoot with full frame Nikon, APS-C Fujifilm and Ricoh as well as with Micro4/3 OM cameras.  I use both zooms and primes so I thought I would test each kind of lens as well to see if lens selection has any bearing on final image brightness. 

I went out on a recent morning.  Blue sky.  No clouds, full sun.  Sun at my back.  I made the same photograph from the same position with the same exact manual exposure (1/1000th second @ f/5.6 at ISO 250) with the six different cameras and five different lenses, both primes and zooms, representing the three different sensor sizes.  I'm not sure how more varied I could have made the test with the gear I own.  I'm sure some scientists could say this test is not very scientific and I can somewhat agree, but it is the best I could do to make all things as equal as possible.  I’ll call it a real world practical exercise.

All images were shot in raw format and all were imported into Lightroom Classic using the Adobe Standard profile to eliminate any differences among camera manufacturers' profiles.  Also, after reading lens reviews for the past 54 years, I've learned that sometimes the manufacturers don’t accurately state the correct actual widest aperture (example is a lens marked f/2.8 might actually be f/3.0 or an f/4 marked lens might actually be an f/4.2 lens) so all images were made stopped down to ensure parity.  Taking into account small differences in light transmission due to the number of lens elements, lens coatings, etc. all of these images, no matter what sensor was used, have essentially the same luminance.

Here are the images.  All images were made using the raw format, Daylight White Balance, Manual Exposure, 1/1000th second @ f/5.6, ISO 250 at a 40mm full frame or equivalent field of view.  They are all straight out of the camera.  No adjustments were made.

45.7mp full frame Nikon Z8; Nikon 24-120mm f/4 lens; 

45.7mp full frame Nikon Z8; Nikon 40mm f/2 lens;

24mp full frame Nikon Zf; 24-120mm f/4 lens

24mp full frame Nikon Zf; 40mm f/2 lens

40.2mp APS-C Fujifilm X-T5; 16-55mm f/2.8 lens;  There is a 1/4 stop difference in this exposure
versus the Nikon Z8.  The difference could be attributed to light transmission through the lens'
numerous glass elements or lens coatings.  In essence, however, the luminance is the same.

24mp APS-C Ricoh GRIIIx; 26mm lens

20.2mp Micro4/3 OM-1 Mark II; 12-40mm f/2.8 lens

20.2mp Micro4/3 OM-1 Mark II; 20mm f/1.4 lens

20.2mp Micro4/3 OM-3; 12-40mm f/2.8 lens

20.2mp Micro4/3 OM-3; 20mm f/1.4 lens

As you clearly can see, the overall exposures for these 6 cameras using 5 different lenses, both primes and zooms, are essentially identical.  As mentioned above, the lens does not know what size sensor is behind it.  All things being equal in exposure, the luminance of the images will be the same.  F/8 is f/8 is f/8, no matter what the sensor size.  Aperture numbers are based upon a mathematical equation, not a single linear dimension.  Among sensor sizes, field of view changes and depth of field changes but luminance is constant.

I hope this settles this once and for all.  I'll get off my soapbox now....  Lol.

Join me over at my website, https://www.dennismook.com.  

Thanks for looking. Enjoy!  

Dennis A. Mook  

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