[GUB]

If we are using a 50mm @ f/2.8 lens on FF that iris is 50/2.8=17.86mm, to capture the same FOV as that 50mm lens on FF you would need to use 35 on your cropped body. Now if we use that lens at f/2.8 you are not using an iris of equal size as the one on your FF body 35/2.8= 12.5mm dia.

To use the same size of iris as that 50mm @f/2.8 on your cropped body you would need to use f/1.96 that 35lens divided by 1.95 = 17.9mm dia …… same size of iris same amount of light( but with a larger exposure value) will be projected onto the that smaller sensor.

I was responding to your quote here.
I was commenting on your flawed logic that the two iris had to be equal size. You are confusing iris diameter with fstop. fstop is as much a function of focal length as it is of iris diameter. (fstop = FL / iris diameter).
The point is a 35mm lens at say f2.8 has an iris diameter of 12.5mm. And a 50mm @ 2.8 is 17.9mm. Now the 35mm is about 35mm from the sensor and the 50mm is about 50mm from the sensor so it makes sense that for a given fstop (2.8) the exposure they impart is the same. (with one having a bigger iris diameter than the other)
Deciding like you did that they had to have the same iris diameter was wrong and inevitably created overexposure in the short lens. This is why I said you was talking in circles.
Now that you understand our differing opinions here can you understand that at no point did I say that two lenses at the same fstop imparted differing exposure. That is what I mean about being misquoted.

[stevebrot]

The crazy thing is all you have to do is put two different format cameras beside each other and take roughly the same image with the same settings to see how wrong all this is.

The empirical approach is so very passé.

It may seem old-school, but all I have to do is take the same composition in the same light using the same settings (shutter, relative aperture) with 35mm, 6x6, 6x7, and 4x5 on the same film and develop all three the same and note by visual examination that the density is the same on all four frames. No pixels...no problem.


Steve

(...wishing I had not misplaced my popcorn gif...)

[stevebrot]

I find this reflects more with reality

Orthogonal to the discussion.


Steve

[clackers]

Depends on your meaning of what is a correctly exposed image I guess

One of the problems with this line of thinking is that for raw or jpg you can have vastly different size of exposures when deciding what is correctly exposed for that scene and the file formats jpg or raw when you are writing them to your card.

The problem with your line of thinking is that it just doesn't work, Ian.

If f8, 1/200s, ISO 200 is chosen for a Pentax Q, for a K-3, for a K-1, and for a 645Z, guess what?

They get the same brightness picture.

'Total light equivalence' espousers are frauds, they're the flat earthers and anti-vaxxers of photography, IMHO.

[Ian Stuart Forsyth]

I was commenting on your flawed logic that the two iris had to be equal size.

They need to if you are discussing lenses being used on different size sensor and how much light gathering as the op asked

You are confusing iris diameter with fstop. fstop is as much a function of focal length as it is of iris diameter. (fstop = FL / iris diameter).

I am not confusing ƒfstop and the iris dia ( I used the wrong term when used iris it should be entrance pupil) I agree that ƒstop is = to FL/entrance pupil

The point is a 35mm lens at say f2.8 has an iris diameter of 12.5mm. And a 50mm @ 2.8 is 17.9mm. Now the 35mm is about 35mm from the sensor and the 50mm is about 50mm from the sensor so it makes sense that for a given ffstop (2.8) the exposure they impart is the same. (with one having a bigger iris diameter than the other)

But the problem is that we have 2 different size sensors and those 2 lenses focus very different amounts of light onto those 2 different size sensors at ƒ2.8. We can see this because they are shooting the same FOV but one of the lenses is using a larger entrance pupil.

Deciding like you did that they had to have the same iris diameter was wrong and inevitably created overexposure in the short lens. This is why I said you was talking in circles

Deciding that the needed to have the same entrance pupil was not wrong it was to show that when both image are shot using the same Shutter speed and the same entrance pupil that the small sensor could not collect the same amount of light as the larger sensor and just as you said will be overexposed.

---------- Post added 08-18-2019 at 09:16 PM ----------

But a meter only gives one reading.

What the meter reads and what is the correct exposure can be very different things, a correct exposure is as varied as the people shooting the camera

The question to what is a correct exposure? the most common answer most give is the one that does not clip the data you are try to store in the record in the data file of your choosing. The camera metering system places it into a target area for an outgoing jpg.

With you cameras metering system the jpg output and the metering system they are calibrated together to give you a output image of a certain lightness in a color space.

If you store your data to raw there really is not any outgoing lightness its just data in a raw format, So if we look at a raw file you will find that 1-2 stops of that space contained within that raw file is not being used. So if you are going to use that raw file in a converter outside pentax's they need to know what is the BLE baseline exposure so that when they go to process that raw data into a image that it falls with the tonal range as it appeared on your cameras display. A simple thing you can do is override this BLE value and you will get a better representation of what you see within the raw (minus the signal multiplication being done to the 3 RGB channels).
The BLE value is what some engineer in japan felt was the best level of headroom to leave within the raw file as a buffer.

So now the question is now what is the correct exposure for that raw file? If you are still using the more common answer of the one that does not clip then you windup with 2 very different optimum exposures for raw and for jpg as for raw will have a 1-2 stop difference in the maximum it can store over that of the jpg image lightness

---------- Post added 08-18-2019 at 09:22 PM ----------

They get the same brightness picture.

What is that brightness an image other than what someone in another county has decided what it should look like. Remove or override that value that person has entered into your raw data file and your image brightness is very different.

[clackers]

What is that brightness an image other than what someone in another county has decided what it should look like. Remove or override that value that person has entered into your raw data file and your image brightness is very different.

That it's all due to a formula somewhere, like some accounting trick, Ian?

People here on Pentax Forums are not as gullible as the brand jumpers and other hillbillies at DPR, you realise that, don't you?

Maybe you actually believe this stuff. I shrug my shoulders, if that's the case. That's not a reason for anyone else to.

Now, there are many species of Equivalence fanatics. Many of the cultists admit, unlike you, that Equivalence has always failed to explain exposure, so it's interesting to see you still staking your credibility on that claim. At a workshop if I meter a flash and tell the rest of the group 'It's f11', it works for everybody from the architect with his Hassy to the college student with their little Olympus, and whether they shoot RAW or JPG.

Some also grudgingly admit that the focal length of a lens is never changed by the size of the sensor behind it.

And a tiny number will concede that depth of field is the same between all formats: Understanding How Sensor Size Affects Depth of Field | Fstoppers

All that's left for these people to say is that the degrees of view of the scene has changed - but everyone knew that already, and have always done. Baby level knowledge, anyone can see through the viewfinders that putting their plastic fantastic DA 50 on a K-1 looks just like the DA35 did on their K-3.

[Fogel70]

That it's all due to a formula somewhere, like some accounting trick, Ian?

People here on Pentax Forums are not as gullible as the brand jumpers and other hillbillies at DPR, you realise that, don't you?

Maybe you actually believe this stuff. I shrug my shoulders, if that's the case. That's not a reason for anyone else to.

Now, there are many species of Equivalence fanatics. Many of the cultists admit, unlike you, that Equivalence has always failed to explain exposure, so it's interesting to see you still staking your credibility on that claim. At a workshop if I meter a flash and tell the rest of the group 'It's f11', it works for everybody from the architect with his Hassy to the college student with their little Olympus, and whether they shoot RAW or JPG.

I think equivalence is good in explaining exposure, and especially what effect exposure have on images from different formats. That exposure isn't universal between formats when it comes to resulting images using same perspective, FOV and DOF.

Some also grudgingly admit that the focal length of a lens is never changed by the size of the sensor behind it.

I have even heard stange things like people believing that it is only pixel noise that detiermines total image noise.

And a tiny number will concede that depth of field is the same between all formats: Understanding How Sensor Size Affects Depth of Field | Fstoppers

All that's left for these people to say is that the degrees of view of the scene has changed - but everyone knew that already, and have always done. Baby level knowledge, anyone can see through the viewfinders that putting their plastic fantastic DA 50 on a K-1 looks just like the DA35 did on their K-3.

Yes, f-stoppers has some nice articles about equivalence.

[pschlute]

What the meter reads and what is the correct exposure can be very different things, a correct exposure is as varied as the people shooting the camera

You are confusing the issue. My point is that a aps-c camera meter, a FF meter, a MF camera meter, and a handheld incident meter will all give the same exposure reading and all expose a scene identically. Yet you continue to claim that exposure is dependent on sensor size.

I will ask again. Show me an incident meter that has a setting to change format ?

[Dartmoor Dave]

Up until about twenty years ago, a person wanting to learn about photography would do so via an object called a book. These useful items could be purchased from establishments known as bookshops, or even borrowed temporarily from places called libraries -- the public library being perhaps the highest possible achievement of any human civilisation.

Having obtained a book about photography, the would-be photographer could then discover factual information that he or she could trust as reliable, on the basis that only somebody with authoritative knowledge would usually be able to get a book through the editorial process and into print.

Sadly those days seem to be gone, and nascent photographers nowadays prefer to suckle from the internet instead. On the internet, people who don't know much to begin with are able to speculate publicly about how they imagine cameras might work, then other people who know even less can come along and read that and repeat it. The more people who repeat the uninformed speculation, the higher it rises on the search results shown to those who are looking for photographic information. Ultimately, utter nonsense such as equivalence becomes accepted as fact by many, simply because of how often it has been repeated online.

I'd like to issue a challenge. Could a believer in equivalence please give us a page reference in just one book about photography published before the year 2000 that describes it in the way that they seem to believe it works?

[Trickortreat]

But the real question you should be asking is which one absorbed more light and converted it to heat energy? and not which one got hotter

Well - yeah, I was just putting it short

[GUB]

Originally posted by GUB*
I was commenting on your flawed logic that the two iris had to be equal size.
They need to if you are discussing lenses being used on different size sensor and how much light gathering as the op asked

Originally posted by GUB*
You are confusing iris diameter with fstop. fstop is as much a function of focal length as it is of iris diameter. (fstop = FL / iris diameter).
I am not confusing ƒfstop and the iris dia ( I used the wrong term when used iris it should be entrance pupil) I agree that ƒstop is = to FL/entrance pupil

Originally posted by GUB*
The point is a 35mm lens at say f2.8 has an iris diameter of 12.5mm. And a 50mm @ 2.8 is 17.9mm. Now the 35mm is about 35mm from the sensor and the 50mm is about 50mm from the sensor so it makes sense that for a given ffstop (2.8) the exposure they impart is the same. (with one having a bigger iris diameter than the other)
But the problem is that we have 2 different size sensors and those 2 lenses focus very different amounts of light onto those 2 different size sensors at ƒ2.8. We can see this because they are shooting the same FOV but one of the lenses is using a larger entrance pupil.


Originally posted by GUB*
Deciding like you did that they had to have the same iris diameter was wrong and inevitably created overexposure in the short lens. This is why I said you was talking in circles
Deciding that the needed to have the same entrance pupil was not wrong it was to show that when both image are shot using the same Shutter speed and the same entrance pupil that the small sensor could not collect the same amount of light as the larger sensor and just as you said will be overexposed.

What a load of nonsense!
And if you made the effort to simply understand and answer the clear and simple example question from the OP then you may well realise how silly you are being.

" 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. "
Please answer it. And note the word exposure - not the term gathered light.

[GUB]

That exposure isn't universal between formats when it comes to resulting images using same perspective, FOV and DOF.

But exposure is universal between formats when it comes to resulting images using the same fstop. And if you bothered to read the OP you would see this is the basis of his question - not some space cadet dream world.

[swanlefitte]

It would be strange if I put a 100mm at 2.8 on a k1 and needed a shutter of 180 then put a 31mm at 2.8 and needed a shutter of 240. Then put the camera in crop mode and needed a shutter of 300. In fact at f/2.8 the shutter stays the same. The fact one doesn't have to think about this is nice.

[pschlute]

Many, many moons ago when I would develop and print my own film, I would uses various sizes of paper, 5x4 or 10x8. I don't recall having to give a longer timed exposure on the 10x8 prints

[Fogel70]

But exposure is universal between formats when it comes to resulting images using the same fstop. And if you bothered to read the OP you would see this is the basis of his question - not some space cadet dream world.

This in an incomplete statement. I can easily get different exposures in resulting images using same f-stop, even on the same camera and even though exposure in the same on sensor level for all shots.

The OP question I have already answered.