[beholder3]

I should know, but I don't.
When people talk of the deference between FF & APS-C regarding the f-stop they mostly cover the bokeh.
My question is:
When using my SMC Pentax 55mm F1.8 @ 1.8 on a APS-C camera does that sensor get approximately the same amount of light as using it on a FF @ the same f:stop? There is a lot of talk about loosing almost a full f: when using lenses on crop body's but can you please clarify if this is mostly about the bokeh or does it also play a part in exposure?
Sample
I use a light meter and it says shoot the scene @ f2.4 Would I get approximately the same expo with a FF & APS-C body? Or would I need to open up to f:2 or f:1.8 on the APS-C to get the equivalent exposure as on a FF.

As I said I should know this, but I do not 😞

Thanks to anyone that can set me straight. 🙌

The key to understanding is the concept of „magnification“.

If you compare a roughly half area (of FF) apsc sensor to FF using images displayed same size on a screen then the apsc has been magnified twice as much.

Exposure parameters on the other side only defines how much light hits a specific same area of the sensor. Obviously if you set 1/100 second F2 ISO 100 one square centimeter of sensor of both a small and a big sensor get the same amount of light, generating the same „brightness“ in the image - and its not getting darker just because your print is magnified more or less.
That is mostly due to the fact that the fnumber is a relative term ignoring the absolute image circle of a lens. An f4 large format lens and a f4 phone lens allow the same amount of light per square centimeter, even though the total (!) light gathered by the large format lens will be much much more.

So *on the sensor* per same area the results are the same.

But: The „light gathering abilities“ are usually referred to in the context of noise, dynamic range and background blur. Those three absolutely depend on magnification, not only on what happens on the sensor. If you watch your images from farther away you reduce magnification and you will see less noise for example. Even if on pixel level on the sensor there is still the exact same amount.

[Fogel70]

Sure, that's all that's in the RAW files.

Anything afterwards is software manipulation. Like noise reduction or downsampling in making a JPG.

Even that ignores sensor size, it's based *entirely* on the number of pixels.

I can show you DxO's theoretical formula if you actually want to learn how it's done.

Perhaps you don't want to know.

No I do not want you to show DXOmark data, because you will only use the screen data, not knowing what it represents.
So many people on PF have tried to prove that you are wwrong on this over the years, but you do not listen to reason.

[slartibartfast01]

I have to say this the funniest thread I have read in a long long time.

[GUB]

No I do not want you to show DXOmark data, because you will only use the screen data, not knowing what it represents.
So many people on PF have tried to prove that you are wwrong on this over the years, but you do not listen to reason.

Still waiting for you to answer the OP question Fogel70 -- What fstop should the apsc camera be set to?

Sample
I use a light meter and it says shoot the scene @ f2.4 Would I get approximately the same expo with a FF & APS-C body? Or would I need to open up to f:2 or f:1.8 on the APS-C to get the equivalent exposure as on a FF.

[Fogel70]

Still waiting for you to answer the OP question Fogel70 -- What fstop should the apsc camera be set to?

Sample
I use a light meter and it says shoot the scene @ f2.4 Would I get approximately the same expo with a FF & APS-C body? Or would I need to open up to f:2 or f:1.8 on the APS-C to get the equivalent exposure as on a FF.

How hard is it to read the woule of my first reply? https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/391373-len...ml#post4706544

- Exposure is the same on all sensor sizes as exposure is measured in light per area unit. So 1sq mm of the sensor capture the same amount of light on a APS-C camera as on a FF camera (or any other sensor size), using same exposure settings on both.

[GUB]

How hard is it to read the woule of my first reply? lens light-gathering abilities - Page 2 - PentaxForums.com

- Exposure is the same on all sensor sizes as exposure is measured in light per area unit. So 1sq mm of the sensor capture the same amount of light on a APS-C camera as on a FF camera (or any other sensor size), using same exposure settings on both.

Great - your reasoning here is sound -- now supply the answer -- (hint - f2.4) then your job is done. No need for the other confusing stuff for a simple question.

[Fogel70]

Great - your reasoning here is sound -- now supply the answer -- (hint - f2.4) then your job is done. No need for the other confusing stuff for a simple question.

The core of the question was not on which f-stop to use, it was whether f-stop should be the same or not for same exposure between formats.

[GUB]

The core of the question was not on which f-stop to use, it was whether f-stop should be the same or not for same exposure between formats.

Splitting hairs.
Look at the situation from the OPers perspective - He has asked a genuine question - as such you can take for granted his knowledge is limited on the subject or he wouldn't have asked. In your first post you have given him "yes and no" reasoning with terminology you should not expect him to know. All you needed to do was give him the clarity of a direct answer.
But it is up to you - you can help your fellow photographers or you can show off your ability to use complicated terminology.

[Gimbal]

There are two questions:
“I use a light meter and it says shoot the scene @ f2.4 Would I get approximately the same expo with a FF & APS-C body?”

The answer is yes.


And the other question:
“When using my SMC Pentax 55mm F1.8 @ 1.8 on a APS-C camera does that sensor get approximately the same amount of light as using it on a FF @ the same f:stop?”

The answer is no, the lens projects an evenly lit image (vignette aside) but the APS-C sensor only captures the light in the center while the FF sensor being larger pics up more of the projected image.

So it’s the large bucket vs the small bucket again (note that the large bucket isn’t necessarily deeper, but it has a larger opening). More photons are picked up by the larger sensor, simply because it is larger. And more photons equals better SNR and better image quality.

[Fogel70]

Splitting hairs.
Look at the situation from the OPers perspective - He has asked a genuine question - as such you can take for granted his knowledge is limited on the subject or he wouldn't have asked. In your first post you have given him "yes and no" reasoning with terminology you should not expect him to know. All you needed to do was give him the clarity of a direct answer.
But it is up to you - you can help your fellow photographers or you can show off your ability to use complicated terminology.

The OP had two questions. One was to get clarification on the one f-stop difference between APS-C and FF he seen a lot of talk about, and the second question was whether exposure was affected by this.

I tried to give short, simple and accurate answers on both.

[Dartmoor Dave]

And the other question:
“When using my SMC Pentax 55mm F1.8 @ 1.8 on a APS-C camera does that sensor get approximately the same amount of light as using it on a FF @ the same f:stop?”

The answer is no, the lens projects an evenly lit image (vignette aside) but the APS-C sensor only captures the light in the center while the FF sensor being larger pics up more of the projected image.

So it’s the large bucket vs the small bucket again (note that the large bucket isn’t necessarily deeper, but it has a larger opening). More photons are picked up by the larger sensor, simply because it is larger. And more photons equals better SNR and better image quality.

That would only be true if the APS-C and FF sensors had a resolution of one giant pixel each. Which would make for kinda boring photos.

Camera sensors have millions of pixels, and the signal-to-noise ratio is dependent on multiple factors. The surface area of the photodetector per pixel is one factor, as is the design of the readout electronics (and remember that each pixel has its own readout transistor on a CMOS sensor).

It would be entirely possible to make an APS-C sensor with a lower megapixel count but a higher surface area per photodetector, and an FF sensor with a much higher megapixel count but a lower surface area per photodetector, in which case there's no reason why the APS-C sensor couldn't have a better SNR.

Talking about "total light" over the entire surface area of the sensor is irrelevant, and I really wish people would stop doing it. It's saddening that this thread has become a bit bad-tempered, but perhaps that's inevitable when equivalentists keep coming back again and again and just will not shut up.

[Gimbal]

Talking about "total light" over the entire surface area of the sensor is irrelevant, and I really wish people would stop doing it.

It is far from irrelevant, when creating a photo you better collect as much light as possible, thus total amount of light used to create the image is very relevant.

But of course if the FF sensor is inferior to the APS-C sensor then that might even the playing field but one has to assume "all else equal" when comparing, there is no point comparing if you change every parameter at once.

[GUB]

Talking about "total light" over the entire surface area of the sensor is irrelevant, and I really wish people would stop doing it. It's saddening that this thread has become a bit bad-tempered, but perhaps that's inevitable when equivalentists keep coming back again and again and just will not shut up.

Yes. And the OP really only asked one question and provided a "sample" to present it. The sample quite clearly was a question of exposure and given the OP level of experience in the subject is it reasonable to presume that was the base of the OP's question.

[clackers]

- Exposure is the same on all sensor sizes as exposure is measured in light per area unit. So 1sq mm of the sensor capture the same amount of light on a APS-C camera as on a FF camera (or any other sensor size), using same exposure settings on both.

So, let's get this right, Fogel, in all your ducking, weaving and evading … are you now agreeing that in the question put to you above, the values of the four pixels in the APS-C camera will be 127,127,127,127?

It was such a simple question, it seems to have baffled you. Everybody else understood it.

[clackers]

No I do not want you to show DXOmark data

I will show you DxOmark's *equation* if you like.

They only measure the screen data. That's the real value from examining the RAW file, it's what they do for a living, make a RAW converter.

Here's some news for you: the print data on the other hand is fictional. Made up. Derived from that formula, which doesn't even mention sensor format.

All those years, still no understanding, Fogel?