Sensor size + Aperture + ISO

[jasjb]

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I really don't understand how a smaller sensor needs less ISO to achieve equivalence to Full Frame.

Shouldn't it need more given the smaller size? How does a bigger sensor at a higher ISO gather equivalent light to a smaller sensor at a smaller ISO? Seems backwards to me?

If someone could paraphrase for me, I'd be appreciative!

I'm obviously missing understanding something

Here is the rough spot in the video (12m 40s)

https://www.youtube.com/watch?v=DtDotqLx6nA&list=PLBE338967F8DB7F2A#t=12m40s

Thanks!

 

[OzRay]

This has been done to death already and I reckon the only reason Northrup made this video was to raise his YouTube click count (and commensurate revenue).

 

[jasjb]

OK sorry.

So it's wrong?

 

[OzRay]

I haven't personally watched the video, but it has raised the ire of many people and I believe that there was also some changes/retractions made after the video was released. I remember one of the biggest push backs was to Northrup's assertion that camera manufacturers were lying/cheating about their figures. Nothing gets the blogosphere going harder than a controversy.

 

[jasjb]

OK well I guess I have a specific question, rather than anything controversial like that:

Why does a smaller sensor need less ISO to capture the same amount of light as a larger sensor?

 

[OzRay]

I don't think it's that simple. It all has to do with the number of pixels, the size of pixels, the nature of the of the sensor technology, the circuitry, software algorithms etc. The only way that you could properly compare what's claimed, is to do it with exactly the same sensor technology and circuitry, but in reduced sizes. Even then, do you compare a FF sensor with 36MP and a m4/3 sensor with 9MP or FF with 12MP sensor and m4/3 with 12MP sensor etc?

But at the end of the day, who really gives a fig? Can you take photos with what you have or not? What actually is your limiting factor?

 

[jasjb]

OK so talking all things equal, I'd like to get my head around how it is a smaller sensors need less ISO to capture equivalent light

All things being approximately equal.

Maybe someone who's watched the video could fill me in on the theory behind what he was saying.

 

[RnR]

OK so talking all things equal, I'd like to get my head around how it is a smaller sensors need less ISO to capture equivalent light

He is not talking about light. He is talking about noise.

 

[bassman]

It doesn't. In his examples, you need to operate a smaller sensor at a lower ISO to achieve a similar noise performance to a larger sensor of the same technology generation. My m43 E-M1 has better noise at a given ISO than a Nikon D300, which is older technology but a larger sensor.

What this video correctly implies without explicitly saying so is that the notion of "equivalence" across different sized sensors only addresses one aspect of the resulting image at a time - brightness, DoF, angle of view, noise, etc. in my view we confuse ourselves endlessly by speaking of these equivalences when they always make something else not equivalent.

 

[jasjb]

He is not talking about light. He is talking about noise.

He explicitly says about that that chart : "Instead of keeping ISO constant, we are keeping the total light gathered constant".

This is what is confusing me.

If the chart isn't about equivalence of total light captured, then I'm confused about what he is saying or what the point of the chart is.

 

[RnR]

If the chart isn't about equivalence of total light captured, then I'm confused about what he is saying or what the point of the chart is.

Go to 12:51 in the video... he specifically says 800 ISO on ff is about the same noise as 200 ISO on an m43 sensor.

But I can also see your source of confusion (hopefully :smile, because at 12:10 (and probably also earlier) he does talk about 'light'. He is strictly correct. However this is a moot point from an exposure point of view, as you only care about the light per unit area, which for a given aperture and shutter speed will always be the same regardless of the sensor size.

Think of a large water hose (firemans size) being aimed at a large board with cups glued to its surface. The board will capture a certain amount of water in a single second. Do the same to another board, but which is 1/4 the size. It will capture 1/4 the water, but each cup thats on the board will still capture the same amount of water as a cup on the bigger board. The exposure to water has remained the same regardless of the size of the board.

 

[christofp]

He is essentially right.

Two ways to justify it:

1. If you judge a camera by its capability to make a good picture from the same amount of light/photons, you have only two variables:
a. the area of the lens which lets the light in and
b. the time how long the sensor exposed.

The third variable you might have in mind , the ISO value, has no physical meaning, it is only a more or less constant value to help photographers getting the same exposure.
As the input area of a FF lens is 4 times higher for the same f-number, in order to keep the light gathering area and exposure time equal, you have to select a 4 times higher f-stop value.
With the higher f-stop, the FF camera has to use a 4 times higher ISO to get the same shutter speed.

2. Given the same amount of light and the same pixel number, the area of a FF pixel is 4 times bigger.
The same amount of photons are spread over a 4 times larger area. The electrical voltage from this charge is 4 times smaller because the electrical capacity of the FF pixel is 4 times larger.
To compensate the 4 times smaller signal, the FF camera has to apply 4 times more electrical amplification. As amplification is coupled with ISO vale, it has a 4 times higher ISO value.

Christof

 

[jasjb]

OK thanks for the explanation christofp, RnR, et al

I'll study some more to get my brain around it

 

[HarryS]

OK well I guess I have a specific question, rather than anything controversial like that:

Why does a smaller sensor need less ISO to capture the same amount of light as a larger sensor?

I believe that question has been answered. Let's reinforce it with a thought demonstration. Set up the M43 camera and FF camera in a studio with lenses having the same f-stop. Take a picture with the M43. Now reduce the lighting by 4X because FF sensor is 4X larger. The FF camera will need 4X more ISO.

See how the marketing speak can fool the viewer. Light doesn't fall off by 4X when you pull out the FF camera. It's the same. You can shoot both cameras at the same ISO. Noise performance will depend on sensor size, semiconductor technology, and hardware and software tricks used in the camera.

 

[Cruzan80]

OK thanks for the explanation christofp, RnR, et al

I'll study some more to get my brain around it

Here is what you should do. Take your camera (whatever size sensor it is), learn about the noise characteristics of it, and then use it where you feel comfortable. Otherwise, why bother comparing to things you aren't going to use?

 

[mattia]

It's a fool's argument, with just enough truth in it to confuse, and doesn't matter in the real world. A smaller sensor captures less light because it's smaller. They'll hit the same exposure if set to the same settings. Which image has more noise depends on the specific properties of the sensor, not merely the size.

I shoot FF and MFT. Yes, noise and detail is better on the A7r in perfectly controlled situations. Which is not what I shoot. Sometimes a fast lens with IBIS on a smaller sensor will be the much better low light camera, regardless of the photons captured (or not).

 

[yakky]

I don't have a full frame, but the APSC-M43-CX certainly holds fairly true for noise level equivalents for me. I should specify current CMOS sensor technology as well. I still have a D3000 that at ISO 100 is AMAZING as far as detail and sharpness.

 

[OzRay]

Anyone remember what the noise level at ISO800 was like on the E-1? I do and wouldn't compare it to the noise level in the E-M1. The same applies to all sensors as time has gone on. A FF sensor from 2000 would be noisier than a m4/3 sensor from 2014. Technology progresses and many things factor into noise production/reduction in modern sensors. It's only when you compare exactly the same sensor and camera technology, but at different sensor sizes, can you make definitive statements of the situation at that specific point in time. Northrup didn't do this, he compared apples, oranges and pears.

 

[lightmonkey]

I have a FF cam contemporaneous (=same gen sensor) with the EM1.

I have to shoot it stopped down due to smaller allowance for misfocus. The higher MP demands steadier handhold. And no stabilization either.

In practice this makes the EM1 the superior night time camera. (Unless I'm shooting fixed scene on a tripod).

The rest of that drivel is for people to argue over on the internet

 

[barry]

a. the area of the lens which lets the light in and
...
2. Given the same amount of light and the same pixel number, the area of a FF pixel is 4 times bigger.
The same amount of photons are spread over a 4 times larger area. The electrical voltage from this charge is 4 times smaller because the electrical capacity of the FF pixel is 4 times larger.
To compensate the 4 times smaller signal, the FF camera has to apply 4 times more electrical amplification. As amplification is coupled with ISO vale, it has a 4 times higher ISO value.

Hi,

I don't think this is right, unless you are talking about different lenses (different FL) on each camera.
I do agree that a FF 16MP sensor will have bigger pixels than a m43 16MP sensor.

However, if the lenses are exactly the same, a lot of light will be lost (cropped off) the smaller sensor, so the total light hitting a single area will be the same.
The FF gets MORE light per pixel if the pixels are bigger, so it needs LESS amplification

Barry