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

Started Jun 6, 2014 | Discussions thread
RicksAstro Veteran Member • Posts: 3,842
Re: Definition of f stop?

Roadrunner123 wrote:

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

Assuming the same "T-stop", which measures true light transmissivity through the lens, the light intensity per unit area on the sensor will be the same on an APSC vs FF thus, as you say, the quantity of light gathered would be more on FF.

THe FOV would be similar between the 2 as well.

The FF would have shallower DOF since it's a longer focal length.   If you stopped the FF down to match DOF and use the same shutter speed, then the would have similar FOV, DOF and overall light gathering on the sensor (you would use a lower ISO on APSC to yield the same image brightness)

Assuming FF and APS-C can give you your desired DOF and FOV, then generally FF doesn't have any advantage since you need to stop it down more to get to the DOF.    But FF opens up shallow DOF options or better performance at the same ISO.

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