# Thread: Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses

1. ## Crop Factor: Why you multiply the aperture by the crop factor when comparing lenses

Just sharing with everyone. When we 'calculate' the 35mm-equivalent focal length for APSC cameras, we usually multiply the focal length by it's crop factor. But we often forget about the equivalent aperture value.

Example:
50mm F1.8 lens on a Canon APSC camera is actually:
1.6 (crop factor) x 50mm = 80mm, 1.6 (crop factor) x F1.8 = F2.88 [on a full frame camera].

Hope everyone find this useful.

2. ## Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

I am pretty sure you don't just multiply the F stop value by crop factor, based on an app I downloaded.

It is true that you will have greater DOF (and reduced DOF control) with smaller sensor size of course, that is a well known fact.

3. ## Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

if my exposure on crop f8 and 1/100
will I have same shutter on FF using f8
shoot same things and same lighting conditions

4. If one use a 50mm on FF to shoot a scene, and then use a crop with the same lens to shoot the same scene, he will need to move backwards in order to get approximate the same field of view. Because u move back, the distance between camera n point of focus is increased, hence less DOF.

Exposure will still be the same, assuming the scene is uniformly illuminated.

To get the same dof as the 50mm (e.g at f/1.8) on the crop camera, the 50mm on the FF will have to move forward (to get same dof) and stop down the aperture by a factor of the crop factor, to get same dof (but reduced exposure). Everything is approximate values, but works well most of the times.

Hope this helps.

5. ## Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

Originally Posted by Shizuma
if my exposure on crop f8 and 1/100
will I have same shutter on FF using f8
shoot same things and same lighting conditions
Yes with same ISO.

6. U never heard before full frame can achieve shallower depth of field? How come shallower? Because the aperture is bigger

7. ## Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

Originally Posted by sin77
U never heard before full frame can achieve shallower depth of field? How come shallower? Because the aperture is bigger
Erm, you seem to be saying that:

Full frame (sensor size) can achieve shallower depth of field because aperture (lens characteristic) is bigger.

It's more of crop frame sensors taking a "crop" from the centre of the image circle.. Nothing to do with aperture size. FF and CF, same aperture, FF still have shallower DOF.

8. ## Re: Crop Factor: Why you multiply the aperture by the crop factor when comparing lens

Originally Posted by konmin
Just sharing with everyone. When we 'calculate' the 35mm-equivalent focal length for APSC cameras, we usually multiply the focal length by it's crop factor. But we often forget about the equivalent aperture value.

Example:
50mm F1.8 lens on a Canon APSC camera is actually:
1.6 (crop factor) x 50mm = 80mm, 1.6 (crop factor) x F1.8 = F2.88 [on a full frame camera].

Hope everyone find this useful.