Aperture vs Sensor Size About Gathering Light

VYC Majoris · Feb 18, 2015

Hi,

I'm new here but I intend to be an active user because there is so much information I need as a beginner at photography. My question might be utterly illogical or nonsense and there might be other factors as well but I have to ask anyway because I've been researching about the basics and I couldn't find a satisfying answer to my question, or any answer at all.

My question is which one of aperture and size of the sensor has more effect on the amount of light absorbed?

I mean for example; both apertures and sensor sizes are close to each other like f/2 and 1/2.3" vs f/2.4 and 1/1.7"

Thanks in advance and sorry for my english.

photoguy99 · Feb 18, 2015

A sensor with twice the surface area will, all else being equal, absorb twice as much light. But it will also need twice as much light to render the same image at the same ISO.

Light is usefully thought of not as an absolute, but as a "how much per square unit of area". The aperture changes that latter quantity, which is why f/2 is f/2 no matter what the sensor size.

TCampbell · Feb 18, 2015

The focal ratio determines how much light is gathered when the shutter is open. The sensor size does not matter.

This is because you have to think of the light gathering not as a "total" but as a "per unit area". Obviously if I have a space which is exactly 1" square and another area which is precisely 2" square and they are both equal distance from the same light bulb, the "area" which is 2" square will actually have twice as many photons land on that space. But if we ignore the total space and just pick a unit-area (say we only bother to measure the light falling on a sample piece which is 1 centimeter square in the middle of our 1" or 2" areas) the amount of light falling on those "per unit areas" will be identical (assuming they are located precisely the same distance from the same light bulb when we measure the light.)

Another way to think of this... suppose I have a movie projector and I'm projecting a movie onto a screen which measures 12' tall and 18' wide.
NOW... suppose I remove that 12x18' screen and replace the movie screen with a smaller screen which only measures 6' tall and 9' wide BUT I place that screen at PRECISELY the same distance from the movie projector and I do not touch the projector at all (no adjustments to how much I zoom in, etc. are changed.) The question is... will the image suddenly get dimmer on the small screen?

The answer is, of course, no... much of my projected movie may miss the screen entirely and spill off onto the wall in the background, but the brightness of a projection of an image onto a movie screen that's (let's just use an example) 10' away from the projector is the SAME amount of light that can make it 10' away from the projector... period. I could take a pair of scissors and cut half the movie screen away... and the part of the screen I did NOT cut away would look the same.

The properties of a lens which are significant is that it has some given focal "length" and it also has an aperture opening of some "diameter". I could have a lens which is 100mm long ... but a diameter of 25mm wide. 25mm divides into 100mm exactly four times. So that would be "f/4" as our focal "ratio". (the notion f/___ means focal-ratio with the following number telling you what the ratio of the length divided by diameter works out to be.)

I could then change that 100mm lens out for a 24mm lens... and let's suppose the 24mm lens has an aperture opening which is 6mm wide.

The "problem" with this scenario is that if I sell you a lens by listing out all its dimensions then it might not be obvious to you that the ratio of 100mm ÷ 25mm happens to be the SAME ratio as 24mm ÷ 6mm... even though they are. It turns out the amount of light conveyed through the lens per unit of time will be exactly the same (given the two cameras are in the same location with identical lighting.)

So rather than tell you the focal length and the physical diameter of the aperture opening... it's more useful to a photographer to just know the "focal ratio" (regardless of the actual length & diameter). I can use a light meter to check the available light under the shade of a tree, it might tell me that there's enough light to take a photo at ISO 100 with a 1/100th second exposure IF I set my aperture to f/8 (I'm making this up)... but notice what the light meter does NOT need to know... is the focal length of the lens I'll be using to take the photo. It also doesn't need to know the make, model, or sensor size of the camera I'll be using. This is because f/8 is f/8 is f/8... regardless of what camera you are using.

VYC Majoris · Feb 18, 2015

Wow! Thanks for amazing replies. I guess it comes down to simply using that "light per unit area" effectively.

So, we can expect better image quality (especially in low light?) and dynamic range with a larger sensor, assuming apertures are the same.

I think I understood entirely however I want to ask one more thing but I refrain from seeming to misapprehend.

Supposing that all other factors such as shutter speed, iso etc. are the same, a photo produced by smaller sensor and a wider aperture looks brighter than the one with bigger sensor and smaller aperture. Isn't it? But noise would be less or not as harsh with bigger sensor, I think.

photoguy99 · Feb 18, 2015

A bigger sensor often comes with a bigger physical "pixel" so the total amount of light each pixel receives can be more, for a given lighting condition.

So, at least sometimes, you get better low-light performance. There's a bunch of other details as well, and the rule is by no means absolute - a more modern small sensor may well outperform a previous generation larger one because, well, because engineers are clever fellows.