A reminder for some memebers as to what the F stop is all about.

Started Jan 26, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 62,178
Re: A simple Yes or No.

CollBaxter wrote:

bobn2 wrote:

CollBaxter wrote:

Great Bustard wrote:

CollBaxter wrote:

Great Bustard wrote:

gsergei wrote:

Or why the light meter doesn't care.

Hello, again.

Let me repeat my point:

Given the same light source conditions and ISO setting the illumination, measured in lumens, received by any size of sensor is equal across all formats (no Ifs , no buts) , when cameras are set to the same A , S, ISO values. So, in practical terms this means, that the illumination received by my tiny c7070 at 125/5.6 and ISO 100 is exactly the same as on the so called FF sensor camera set to the same values. This will result in the same picture brightness/darkness , subject to DR and noise differences.

No one that I'm aware of claims that f/2 will not result in the same exposure regardless of format or focal length for a given scene luminance and shutter speed.

Please, do not feed me things like "amount of light", "sensor area", "light power" , "bigger lens opening" etc. and try to operate , using known physical terms and concepts.

If the difference between the amount of light per area that falls on the sensor (exposure), and the total amount of light the sensor records is difficult to understand, well...

Is the amount/intensity of light on a say a square cm at the center 4/3 , APS-C , APS-H , 1 inch , FF , 36X49 sensors at say f/2.8 not equal ?

A simple Yes or No.

Yes. Now, if you would return the favor:

For a given perspective, framing, and shutter speed will f/2 on 4/3 result in a photo with the same DOF as f/4 on FF as well as project the same total amount of light on the sensor, resulting in the same noise if the sensors are equally efficient?

A simple Yes or No.

Now, if I could trouble you for one more yes/no question:

We take a pic of the same scene from the same position at:

  • 50mm f/2 1/200 ISO 400 on 4/3
  • 100mm f/2 1/200 ISO 400 on FF
  • 100mm f/4 1/200 ISO 1600 on FF

Thanks for the answer . So a F/2.8 lens is a F/2.8 lens as far as light transmission , no mater what the sensor size is the same.

That depends what you mean by 'light transmission'. Think of it this way - if take a room with a single 10cm square window, now call the glazier in and replace the window with a 1m square one. Will the room be brighter or darker? Has the light passing per unit area of the window changed?

Lest keep things equal.

The depends on which things are being kept 'equal'. In this case, we're talking about the window, and the light transmission through it.

Firstly you are assuming the rooms are the same size and what size room do we want to light.

The rooms are the same size.

So lets keep things equal as your example used a factor of 100 where as with 4/3 we a using a factor or 4.IN your example the room would have to be 100 times smaller. ( I don't think there is a sensor that small)

I don't think that the number matters so much, make it 4, 2 or whatever, the principle is the same

Just a question . With a pinhole camera assuming film of the same ISO and one box is 4X4" and the other is 8X8" with the film the same size. What would be the ratio of the hole size to get the same exposure using the same time on both boxes ?

That is a completely different question. I say the room is the same size, because you will view the final image the same size.

As for 'same ISO', that's only as sensible constraint if you know what it means. What do you think it means?

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