Crop Factor affects a lens' max Aperture?

[Ado Wolf]

I just watched a Youtube video from Tony Northrup, where he claims that the maximum Aperture of a lens is also affected by the crop factor.
For example, the 300 mm f4 on Full-frame is equivalent to 450 mm f6 on an APC camera (in 35 mm terms). Is this really true?
If so, does it mean that the 300 PF is brighter on a Z6 than on a Z50 (for example) under the same settings?
If so this will definitely swing me in the direction of Full-frame for my next upgrade..

 

[Steve]

That is possibly the worst video he has ever made.

No, it's not true.

What he's trying to say is that if you put a 300mm PF on your camera, you'll have the field of view of a 450mm lens on a full frame camera and the depth of field will look like it was at F/6. This is because you would need to be 1.5 times closer to get that same framing with a full frame camera as you are with the DX camera. (remember, a 300mm is still a 300mm regardless of the sensor behind it). So, in short, you get 1.5X more DoF with the same framing using the same lens on DX vs FX.

However, it does NOT affect exposure one bit. If you're at 1/500th @ F/4@ ISO 800 with a full frame camera and attach the lens to a DX camera, you'll still be at 1/500th @ F/4 @ ISO 800 with the DX camera. After all, hand-held exposure meters don't have special exposure settings for DX, M4/3, FX, medium format etc. It's all the same exposure, regardless of the sensor size.

 

[Woodpecker]

What he's trying to say is that if you put a 300mm PF on your camera, you'll have the field of view of a 450mm lens on a full frame camera and the depth of field will look like it was at F/6. This is because you would need to be 1.5 times closer to get that same framing with a full frame camera as you are with the DX camera. (remember, a 300mm is still a 300mm regardless of the sensor behind it). So, in short, you get 1.5X more DoF with the same framing using the same lens on DX vs FX.

However, it does NOT affect exposure one bit. If you're at 1/500th @ F/4@ ISO 800 with a full frame camera and attach the lens to a DX camera, you'll still be at 1/500th @ F/4 @ ISO 800 with the DX camera. After all, hand-held exposure meters don't have special exposure settings for DX, M4/3, FX, medium format etc. It's all the same exposure, regardless of the sensor size.

Something probably worthwhile mentioning is - and I have been asked that a while ago because of the same missunderstanding - that this "aperture change" of a lens being mounted on a DX body also does not chase you in AF limitations because hitting the f8 wall !
If I put my 500 f5.6 PF on my D7200 I get something like a 750mm f8,4 from the photographic perspective, i.e. FOV and DOF. Technically the lens remains a f5.6 lens, the camera recognizes it as a f5.6 lens and thus all AF sensors are working as they should, instead of having just one or a hand full left because going smaller than f8.

 

[Ado Wolf]

@Steve Thank you for the confirmation. I always considered the DX as a mere "subsection" or "cut-out" of the FX image., that is why the focal change was obvious, as well as the DOF (considering the corners are being cut). That is why I had a hard time believing what Tony said.

@Woodpecker Thank you for this point. It did not cross my mind (to be honest.).

 

[Flyboy]

Great answers so far, particularly Steve's!
There is no magic in crop sensor cameras.
What happens with FX lenses on a DX format is: the FX lens's designed coverage circle covers the 24X36 mm area of the FX sensor, or the film area of a 35mm camera. Put it on a DX camera, where the sensor area is less, and uses the central portion of the FX lens coverage, which is also the most highly corrected and sharpest portion of any lens. It's all good!