[Ian Stuart Forsyth]

If you halve the amount of light on a FF camera with the mistaken belief it has double the gathering capability, congratulations, you just got an image underexposed by a stop.

I haven't halved the amount of light on FF, I have halved the exposure ( light per unit area) and doubled the size of the medium capturing the light. this balances it out and has captured the same amount of light.

[Photobill]

I dislike using water and buck analogy as it can be even as confusing to some.

Think of exposure in its context as the amount of light per unit area, we have 2 image capturing devices with 2 very different areas. if we have 2 very different areas that exposure (light per unit area) will put very different amounts of light into your final image.

Probably the best way to describe the difference is to think of exposure as a uniform layer of paint that you are applying to a surface, instead of thinking of light as per unit area you are using paint per unit area. Think of the shutter speed as the duration that you are spraying the paint onto the surface and the aperture as the density at which your spray gun applies paint.

Now if we are applying paint to both sensors using the same duration(shutter speed) and same density ( f-stop) both sensors will have the same thickness of paint applied to their surfaces but because there is a difference in size there is a difference in how much paint was collected and the larger surface area will have much more paint on it.

Now for you to place the same amount of paint onto the surface of the smaller sensor and you are using the same duration (shutter speed) you will need to adjust the density (f-stop) of paint being applied, this will put a thicker layer of paint ( larger exposure {light per unit area}) giving you the same net paint being applied to that smaller surface.

For the comparison between FF and cropped you will have to increase that density of light/paint by the size of their differences, for FF to cropped that is about 2.25 times, so to adjust the f/stop you will need to apply a 1.5 crop factor. For a f/2.8 on FF you will need 2.8/1.5=f/1.87 on cropped to put the same amount of light into that image.

Now with this knowledge you may think well why can I not capture the same amount of light with my cropped camera as my FF camera, the answer is yes but up to a point. There are 2 main factors 1. does the format have access to a lens with a large enough aperture to do this and if the answer is yes than you can. 2 is if your are not exceeding the maximum size of the exposure the camera can store then yes you can.

Think of iso as an exposure index and for iso 100 you can have very different settings of f-stop and shutter speed that will give you the same exposure size (they all will give you the same light per unit area). Now both camera formats that can capture the same size exposure at base iso 100 because of their size differences the smaller format would be unable to capture the same total amount of light/paint.

We can show this by going back to the spray paint . Just as shown above you will need to use different F/stop(density of paint) to apply the same amount of paint/light to the smaller sensor. But here lies the problem in order to do this you will be changing the size of the exposure and if you are unable to lower the iso in the cropped camera to a lower iso then you will be over exposing your image.

Think of it this way if I am shooting my FF camera at iso 100 150mm f/2.8 1/100sec and I am going to try and capture the same amount of light with the cropped sensor I will need to shoot using 100mm f/1.87 1/100sec. here is the problem at f/1.87 1/100 sec the image captured by the smaller sensor will be overexposed for the same scene. This will demonstrate why a smaller sensor is unable to capture the same amount of light as the larger sensor, If we are going to try and shoot that cropped body at f/1.87 we will have to change either the iso or the shutter speed as not to overexpose. Often times we are unable to lower the iso so we are stuck to using smaller duration on our shutter speed.

Now to keep the smaller sensor from being overexposed using f/1.87 we will need to decrease the shutter speed down to 1/200 sec cutting in half the amount of light reaching the sensor as not to overexpose. Half the duration half the amount of light collected.

Thanks for you in depth post!!!
I was able to fallow the first part about the size of the sensors (FF would grab more paint than a aps-c would in a single exposure, based solely on the size of the sensor) wouldn't it be like a solar penal aimed at the sun the larger the panel the more rays it could absorb in total volume but a smaller one would absorb exactly the same volume based on per inch/cm

---------- Post added 08-16-19 at 11:50 PM ----------

Seeing you mentioned bokeh in your OP rather than I suspect you meant DoF. Are you aware apsc is just about as bokeh capable as FF?

👍 absolutely 👌 i feel you just need to work a little harder for it but its still😁

[Ian Stuart Forsyth]

Thanks for you in depth post!!!
I was able to fallow the first part about the size of the sensors (FF would grab more paint than a aps-c would in a single exposure, based solely on the size of the sensor) wouldn't it be like a solar penal aimed at the sun the larger it is the more rays it could absorb in total volume but a smaller one would absorb exactly the same volume per inch/cm

Yup you got it
But most of the time in photography its not that simple as we have lenses that focus light, and just like photography they are now playing around with mirrors and lenses to focus the same amount of light but onto a smaller solar panel.
Concentrating Photovoltaics | Solar Power


Most important to understanding how much light is passing thru the lens you have to understand its the entrance pupil, this is seen through the front of the lens system and not just the iris as you would measure it if it was taken outside of the lens. The F/stop is just as it looks a formula the focal length divided by the 2.8 will give you the entrance pupil, The same size of entrance pupil between lens on different formats with the same FOV they will gather the same amount of light and also give you the same DOF. A lens and format with a larger entrance pupil will put more light onto your sensor for the same shutter speed.

DOF and low light capabilities are closely tied together, its only when one format allows you to shoot at a shallower DOF will you have better lowlight capabilities unless one sensor drastically better performance.

[GUB]

If it was as you said that because the 50mm lens iris is further away from the sensor than the 35mm lens then we should see a difference in the brightness of the final image using a 50mm lens at f/2.8 and a 35mm lens at f/2.8 shooting a uniform wall at the same distance with the same camera.

Please do not misquote me to try and reinforce your argument.
All lens at a given fstop will impart the same exposure to any format sensor.
There is small variations due to light transference inefficiencies (tstop) or in the situation of extension due to macro work.

[clackers]

, I have halved the exposure ( light per unit area) and doubled the size of the medium capturing the light.

Now your picture is incorrectly exposed.

This is the sort of bad science and bad photography found at DPR.

Are you a member there, Ian?

[Kerrowdown]

lens light-gathering abilities

That all used to be down to the size of the front element lump of glass...

[Trickortreat]

The illumination per mm2 ia the same bit the bigger sensor will gather more light.
Think od it this way - you have 2 black pieces of metal and leave them on open sun light. One is bigger and one is smaller which one will get hotter?

[pschlute]

Lots of analogies in this thread. I am going to put one of my own in.

2" of rain is forecast. You put out a small dish and an old bathtub. When the rain has stopped you check the two containers. The bathtub obviously has a lot more water in it, but both methods recorded 2" of rain.

The OP asked a question about whether the same f-stop gives the same result for both formats. By the generally understood term "exposure" the answer is yes.

Evenly lit, single shade of colour wall, FF and aps-c camera, same lens. The test is not difficult.

[GUB]

2" of rain is forecast. You put out a small dish and an old bathtub. When the rain has stopped you check the two containers. The bathtub obviously has a lot more water in it, but both methods recorded 2" of rain.

Depends whether you put the plug in it!

Evenly lit, single shade of colour wall, FF and aps-c camera, same lens. The test is not difficult.

Unfortunately reality is merely an inconvenience for some.

[StiffLegged]

My head hurts, or, it would IF I took much of this at face value. Whichever one of you lot is Tony Northrup in mufti, please leave now. (good grief...!)

Why do these spurious arguments keep recurring?

Please, please, please, STOP THE INSANITY!

[GUB]

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.

The question was straight forward.

Evenly lit, single shade of colour wall, FF and aps-c camera, same lens. The test is not difficult.

This test was suggested several times. The answer already known by anyone who uses different format cameras. There is no change in exposure.
The problem is does one defend the truth in these situations or bow out for peace and quiet.
I have learn'd a heck of a lot from PF and I see this site as a vehicle for learning for all. A thread like this is easily accessed via Google and is a valuable question and answer discussion. Until someone fills it with pseudobabble.
What would you have us do?Leave it unchallenged and damage PF reputation as a reputable source of information?

[Photobill]

2" of rain is forecast. You put out a small dish and an old bathtub. When the rain has stopped you check the two containers. The bathtub obviously has a lot more water in it, but both methods recorded 2" of rain.

Great analogy!!!!

the OP asked a question about whether the same f-stop gives the same result for both formats. By the generally understood term "exposure" the answer is yes.

👍 thanks I have already collected the the cold drink from my buddy

This post has been very informative and a bit over my head at times😵. BUT GOOD READING 📷

[Ian Stuart Forsyth]

The flaw to this is those two iris are not the same distance from the sensor - the 50mm one is further away and the light has dissipated more - you are essentially talking in circles.

So I don't miss quote you. If the iris are not the same distance and that the 50mm is the one that is further away and the light has dissipated more light then we should see that the 35mm lens as you say if it is closer should (then no matter what size of sensor that used behind it) should show a difference in the recorded image brightness as any other lens shot at f/2... I have not seen as yet this happen in real life where a 35mm f/2.8 lens show up as a brighter image over using a 50mm lens photographing the same grey card or any other object.

I am only interested in finding out you mean with the comment that you stated above.

---------- Post added 08-17-2019 at 08:59 PM ----------

he illumination per mm2 ia the same bit the bigger sensor will gather more light.
Think od it this way - you have 2 black pieces of metal and leave them on open sun light. One is bigger and one is smaller which one will get hotter?

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

Dump those 2 pieces of metal into 2 equal containers that contain equal amounts of water at the same room temperature and record that temperature. Place one of those pieces of metal into each of those containers of water and then measure how much of a temperature change you see in both containers of water. You will see that one of those 2 pieces of metal absorbed more light energy and stored that energy as heat and raised the temperature much greater than the one that absorbed less light energy.

---------- Post added 08-17-2019 at 09:08 PM ----------

Now your picture is incorrectly exposed.
This is the sort of bad science and bad photography found at DPR.
Are you a member there, Ian?

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.

[pschlute]

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

For the purposes of answering the question the OP posed it makes sense only to look at what a metered result of an average or uniform scene would be. Otherwise you are getting into the area of subjectivity.

In that respect it is clear for anyone to see that whatever format you use the metered exposure is the same. If it is not, perhaps you can point me to an incident light meter that has a setting for different formats.

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

But a meter only gives one reading.