# Crop Factor. Something to consider when buying a lens when you are shooting other than FF.

Started Jun 6, 2014 | Discussions thread
Re: Definition of f stop?
1

forpetessake wrote:

I haven't read all the comments here so my apologies if this has already been addressed. I was under the impression that f stop number was a mathematical calculation where the focal length is divided by the diameter of the lens.

Yes, f_stop = effective_focal_length / effective_diameter.

But f-stop is only meaningful when you compare the same size sensors. It's meaningless for comparison of different sized sensors, and that's where the equivalence comes.

I understand that the actual quantity of light would me more on a FF as opposed to aps-c.

Yes, you need to integrate the illuminance (in lux) over the surface to get the flux (in lm).

However, will there be an intensity difference per mm2 on the sensor of say a aps-c 23mm f1.4 against a ff 35mm f1.4?

No, illuminance will be the same (disregarding the losses, t-stop is actual measure). But illuminance on the sensor doesn't matter, the amount of light translated per unit area of the final image (print, display) that what determines the noise -- you need to magnify the smaller image more to get the same size final image. The two same size pictures taken with different format cameras will look the same when they collect the same amount of light, and since the smaller sensor must be magnified more it must have proportionally higher illuminance, i.e. brighter lens to produce the same image.

I am more confused than before. You answered 'No' and then said, 'illuminance will be the same'

Let me ask the question in another way. One aps-c lens at f1.4 and one FF at f1.4. If I took two photos wide open at f1.4 at the same shutter speed would the photos be exposed the same or would the FF need to be stopped down?

For your example, if you choose the same f-stop, shutter speed and ISO, the brightness of the image will be the same.  The FF will have less noise though and a shallower DOF (assuming you chose equivalent focal lengths like in your original post).

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