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08-16-2019, 05:13 PM   #16
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QuoteOriginally posted by aslyfox Quote
the facts as I understand them:

1 I am easily confused

2 the sensor, whether large " full frame " or small " ASP-C " is an actual rectangle of a known size

3 the aperture allows a set amount of light [ brightness ] reflected from the target into the sensor and any light which falls outside of the sensor isn't seen

4 with the larger sensor, the reflected light hits a larger rectangle and you get more of the reflected light than with the smaller sensor,
in other words, you lose some of the top, bottom and sides using the smaller sensor

5 the brightness of reflected light doesn't change

isn't this what GUB is explaining ??
Yes. Imagine if your image was a grey featureless wall. So your focused image circle is a featureless grey circle. Why would the smaller sensor sitting on that image circle give a different value to the bigger sensor?

08-16-2019, 06:38 PM   #17
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QuoteOriginally posted by Ian Stuart Forsyth Quote
You have to remember that how much light is gathered in an image and the size of the exposure are different things when comparing them to 2 different sensor sizes.

If we are using the same f-stop and shutter speed for both sensors then the exposure value will be the same but the size of the sensor recording the light is smaller so it captures less light. Exposure is really the amount of light pre unit of area value, so that we can expose 2 different size sensor the same but because we have different sizes of sensors less light will be captured on the smaller format.

Now we can set both cameras to capture the same amount of light if we select lenses with the same dia of iris for both cameras.

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.
Thanks for the info Ian!
Tell me if I'm wrong on my interpretation. If light was a liquid and you wanted to know how long it would take to fill a gallon jug you would need to apply this formula. But this does not apply when exposing an image?
08-16-2019, 06:47 PM   #18
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QuoteOriginally posted by Ian Stuart Forsyth Quote
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.
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.
08-16-2019, 06:59 PM - 1 Like   #19
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Awesome input from all you Pentaxians
YOU HAVE JUST WON ME A COLD DRINK OF MY CHOICE!!!!
I think I have a firm grasp in how the x1.5 magnification works on the affective focus range & how it affects the DOF.
My buddy has been debating me saying that his Canon 24-70 2.8L is just as fast on his FF camera as my Sigma 18-35mm F1.8 Art is on my K3II. I had thought not, but didn't have the information to back it up until now.
Nothing quit like a drink at your best friends (over 40 years) expense!!!

08-16-2019, 07:13 PM - 2 Likes   #20
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QuoteOriginally posted by Photobill Quote
Awesome input from all you Pentaxians
YOU HAVE JUST WON ME A COLD DRINK OF MY CHOICE!!!!
I think I have a firm grasp in how the x1.5 magnification works on the affective focus range & how it affects the DOF.
My buddy has been debating me saying that his Canon 24-70 2.8L is just as fast on his FF camera as my Sigma 18-35mm F1.8 Art is on my K3II. I had thought not, but didn't have the information to back it up until now.
Nothing quit like a drink at your best friends (over 40 years) expense!!!
Having to listen to us you have earned it man!
08-16-2019, 07:25 PM   #21
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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? This is despite when comparing DoF, the FF has much shallower DoF. I started this thread when I first became aware. K1 bokeh vs K10 apsc bokeh - PentaxForums.com
08-16-2019, 08:00 PM   #22
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Worth mentioning in this discussion that not all lenses may pass the same level of light through to a camera sensor, even if a lens may have the same focal length, aperture and optical design.

The effect of the type of glass used, the lens coatings employed, ageing of the lens elements, optical design etc may simply make some comparable lenses 'darker' or 'brighter' than others.

Lens T-stop values indicate the light transmission of a lens. Commonly seen on cinema lenses.
08-16-2019, 08:29 PM   #23
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QuoteOriginally posted by GUB Quote
the 50mm one is further away and the light has dissipated more
If this was true then using a 50mm & 35mm lens on the same camera shooting the same grey card at different distance would show that in one image the grey card being darker and we know this not to be true.

08-16-2019, 10:02 PM   #24
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QuoteOriginally posted by Photobill Quote
But this does not apply when exposing an image?
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.
08-16-2019, 10:17 PM - 2 Likes   #25
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QuoteOriginally posted by Ian Stuart Forsyth Quote
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.
I'm afraid this is tosh.

The game is to fill each pixel with light, not the whole sensor.

A RAW file is the result, simply the value of each pixel and nothing else.

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.

08-16-2019, 10:27 PM   #26
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QuoteOriginally posted by Ian Stuart Forsyth Quote
If this was true then using a 50mm & 35mm lens on the same camera shooting the same grey card at different distance would show that in one image the grey card being darker and we know this not to be true.
I am talking iris to sensor distance not iris to subject. And a given lens doing a bellows macro shot, because of the bellows extension making the iris get further away from the sensor, does indeed get dimmer than a standard shot.
08-16-2019, 10:34 PM   #27
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QuoteOriginally posted by Ian Stuart Forsyth Quote
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.
Tosh indeed
08-16-2019, 10:36 PM   #28
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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.
08-16-2019, 10:56 PM   #29
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QuoteOriginally posted by GUB Quote
I am talking iris to sensor distance not iris to subject. And a given lens doing a bellows macro shot, because of the bellows extension making the iris get further away from the sensor, does indeed get dimmer than a standard shot.
Indeed it does but with the 50mm nor the 35 did we ever add a bellows changing the iris to sensor distance or the flange distance, they have remained the same. 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. Even now I have never seen this if you could show this to me maybe I will learn something new.

---------- Post added 08-16-2019 at 11:18 PM ----------

QuoteOriginally posted by clackers Quote
The game is to fill each pixel with light, not the whole sensor.
A RAW file is the result, simply the value of each pixel and nothing else.
What is more important is knowing the values stored in that pixel and knowing what that pixels occupies within your final image.

If you ignore this then you could be lead to believe anything or even something like this that the k10 delivers the same snr at iso100 as the 645z


But for those of us that understand that you need to know how much that data represents in your final image then we get a more real representation of how things are captured


I find this reflects more with reality

Last edited by Ian Stuart Forsyth; 08-16-2019 at 11:19 PM.
08-16-2019, 11:27 PM   #30
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QuoteOriginally posted by Photobill Quote
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. ��
I belive you mix up two things here. Exposure and total light captured by the sensor.
- 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.
- A larger sensor capture more light as it has larger light capturing area. A FF camera have a sensor that is 2.25 times bigger than APS-C. So using same exposure settings on both, FF captures 2.25 time as much light as APS-C. Which is why a FF camera like K1 II have lower noise at the same ISO as on KP or K70.

Last edited by Fogel70; 08-16-2019 at 11:33 PM.
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