Please explain EV100 with different sensor sizes...

Started Jul 17, 2013 | Discussions thread
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 Please explain EV100 with different sensor sizes... Jul 17, 2013

The light meter says that EV100 = 5 in my home, and the exposure table suggests that at f/2.8 I should set the exposure to 1/4 sec.

a) My understanding is that this is true irrespective of the size of my sensor of my camera. So a fast lens is a fast lens, no matter what sensor I have. Given the EV value of the surroundings, I will need to set the same exposure.

b) My understanding is that if I use a 80 mm lens on FF on a subject that is 10 m away the DOF will be 2.68 m, while using a 50 mm lens on a 1.6 crop body will give me a 4.48 m DOF. So if I photograph the *same frame* by two different sensors, then on the cropped body the DOF will be larger at the same aperture.  If I want the same DOF, I would need to use a larger aperture on a FF.

This is why many times you can read that a 'faster' lens on a cropped body equals to a 'slower' lens an a FF.... Yes, as far as the DOF is concerned when photographing the same frame with two different lenses it is true, but as far as the required exposure is concerned EV100 is the same for both cameras regardless of their sensor sizes, so with a faster lens I can have a faster exposure. Don't I?

BTW, I expected here a 1.6x factor, and 4.48/2,68 = 1.67! Can you explain it to me why? Does this factor depend on the distance of the subject?

d) My understanding is that if I do not switch lenses (or focal lengths) and I use 50 mm lens with f/2.8 on a FF and a cropped body from 10 m, then the frame will be different, and the FOV will be 7.62 m and 4.48 m respectively, so the *same lens* will have a larger (frame and) DOF on a FF body. In this case it sis not true that 'smaller sensors have larger DOF', this common statement is only true when you photograph the same frame... Isn't it?

d) My understanding is that using a larger sensor has the advantage of having less noise, and the noise is also proportional to the sensor size. On the smaller sensor the noise will be more. On small sensor at ISO 100 there will be the same noise than on the FF at ISO 160. So apart from the FOV and the DOF the noise also 'needs to be multiplied' by the 1.6x crop factor.

e) BTW - what else needs to be multiplied by the crop factor?

I am pretty sure that I have made some mistakes when thinking in the above way, please bare with me, and help me to understand the whole picture...

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Thanks, Imre

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