Rf 100-400mm f5.6-8 + 1.4 extender

tkbslc wrote:

John Sheehy wrote:

tkbslc wrote:

R5 is still 18MP in 1.6x crop mode. Have you considered using that? Gives 640mm for zero additional weight.

Crop mode does not increase resolution of small and/or distant subjects. Using equivalent focal length as a proxy for subject resolution is not meaningful.

Using 400mm in 1.6x on R5 is like using 400mm on a very advanced 18MP APS-C camera.

Yes, but what does that have to do with the fact that the full sensor mode captures all the same detail as the 1.6x crop mode, and you don't get any more by enabling it. It was that very lack of desired subject resolution that is there in both full sensor mode and crop mode that likely created the desire for more focal length.

We would call it 640mm equivalent if it were a native 1.6x crop sensor.

I used 400mm lenses with APS-C cameras for 17 years. I never thought of the crop factor as some optical "power" or "magnification", and always though in terms of the actual focal length. The only thing "640-equivalent" in your example above that meant anything to me was the fact that the angle of view was narrower than if the lens was used on a FF camera, eliminating more of the image circle. I used the concept more with wide-angle lenses, where the crop factor was a clear limitation. It is the pixel density and the real focal length that primarily determine maximum detail. There are resolution reasons to use crop-sensor cameras when they have more pixel density than your FF sensor, but using crop mode is inferior to using an extender on a FF, as long as the optical quality of the TC is good, and AF works as well as needed. There are some practical reasons to use crop mode, but "focal-length times 1.6" is not one of them, IME, unless you're talking about the viewfinder magnification, which does not increase subject detail.

Feel free to continue your pedantry if you wish.

One person's clarity is another person's pedantry.