# illuminance, image circle, cropfaktor question :)



## PicaPica (Jul 8, 2014)

i made a little graphic to illustrate my question.

lets say i have a 100mm f2, 62mm f2 and a 62mm f1.2 fullframe lens (ignore rounding errors).

when i attach these lenses to a crop sensor then only a part of the FF image circle will be used.

i attach the 62mm f2 lens to the crop sensor to get the same FOV as with the 100mm f2 on the fullframe sensor.
the illuminance is the same because both are f2 lenses.
the aperture is different (50mm vs. 31mm)

but the crop sensor will get less TOTAL light, because the sensor area is smaller.

now my questions.

when i use a 62mm f1.2 lens on the crop sensor i increase the illuminance.
but will the total light gathered with the crop sensor be the same as with the 100mm f2 on a FF sensor? i guess so?

when the crop sensor and the 62mm f2 lens gather less total ligth but the exposure for the images is the same. doesn´t has the crop camera to amplify the signal more? does this mean crop sensors internally use a higher ISO (amplification)?

i mean the numbers are the same (base ISO 100, lens f2, exposure time 100/s) but does it mean the signal is amplified the same amount?

i know what ISO means in the great scheme of things.
but is ISO 100 a fixed value, the same from camera to camera?
is there a standard what ISO 100 means for digital cameras (like : ISO 100= signal from the sensor is amplified by the factor 2).

or is the base ISO amplification factor calculated by the camera manufacturer so that the exposure comes out right. that the correct EV (exposure value) is reached. 
means amplification for ISO 100 on a crop factor is higher than for ISO 100 on a fullframe camera.

is this the reason why the same ISO looks different on different digital cameras?

my father says (im not sure he is right) that ASA/ISO 200 grain looked the same no matter which brand of film you used. but ISO does not look the same on different cameras. 

so it seems for film ASA/ISO has some real meaning.
as you can compare the grain (noise) for different films (no matter which camera you use).
for digital this meaning is nearly gone.

i hope you get what i mean. my english is not that good.


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## Sporgon (Jul 8, 2014)

I think what you are asking is 'can increased luminance make up for less total light ?'. I am not an expert on this, but I think the answer is 'no'. 

Regarding your question of ISO for different cameras and formats: in the days of film I believe that the emulsion used in say Kodachrome 110, 135 or 120 was all exactly the same. Someone correct me if I am wrong. This works because exposure works on luminance intensity, not total area of light. 

So I'm not sure if different digital formats use a given ISO differently.

However different cameras DO use a given ISO rating differently, the 'ISO sensitivity. For instance with the cameras that I have owned in recent years, for ISO 100 the actual ISOs are as follows: Nikon D200 - 98, 5D - 92, 5DII - 73. 1100D - 78, 6D - 80. Both the D200 and 1100D are aps so there is no pattern for the smaller sensor there.

You might say 'well who cares about this. The meter in the camera deals with it'. Well it is very important to those of us that use a separate meter to know where to set it. For instance if I set my Sekonic incident light meter to 100 ASA for the 5DII it will give me under exposure. In this case I find best results setting the Sekonic to ISO 64. 

This info is another of the good, accurate things DxO mark offer. ( Why DxO lets itself down with its ridiculous camera and lens 'scores' I do not know. )

So there is variation in individual model's ISO senitivity, but not in the way you were asking about.


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## 3kramd5 (Jul 8, 2014)

PicaPica said:


> or is the base ISO amplification factor calculated by the camera manufacturer so that the exposure comes out right.



Yes, the amount of gain applied to the signal is targeted at a net brightness according to ISO setting, all else being equal. The smaller the sensor, the more gain.

See neuro's post here:



neuroanatomist said:


> The answer lies in what ISO is...and isn't. Many people have a poor understanding of ISO, incorrectly assuming that a given ISO setting means a fixed amount of gain applied to the signal. ISO is a _standard_ (that's the 'S' in ISO, ISO 12232 is the relevant standard in this case), and *that standard effectively means that for a given exposure setting in terms of aperture and shutter speed, the resulting image will have a defined brightness*. How does an image taken at f/2, 1/100 s, ISO 200 on a PowerShot S100 have the same brightness as an image at f/2, 1/100 s, ISO 200 on a FF sensor, even though the FF sensor is over 20 times larger? *More amplification (gain) must be applied to the lower total signal from the smaller sensor*. More amplification means more noise. Obviously, the same is true for m4/3 and APS-C relative to FF, to a progressively lesser degree. Likewise, a medium format sensor needs less amplification than a FF sensor to achieve the necessary brightness for a given ISO according to the standard, and therefore has less noise than FF.


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