R5+100-500+1.4x or R7+100-500?

Steve Balcombe wrote:

Robbey TC wrote:

The 1.4 TC reduces the amount of light hitting the sensor by half, or one full stop. (1.4^2 approximates 2)

The 1.4 TC on a 100-500 f4.5 to f7.1 means that you will be either shooting between f6.3 @ 100mm to f10 @ 500mm, or have to double the ISO or halve the shutter speed for each stop foregone (or a combination to achieve correct exposure).

When the R7's FOV is at 700mm (actually between 580mm to 755mm), with the 100-500, the maximum f-stop is f6.3 . This is 1.3 stops better than the R5 with a 1.4 TC will achieve for 700mm (f10). (The R7 moves to 7.1 above 755mm FOV and the R5 moves to f10 above 660mm FOV ...(1.6 vs 1.4 comes into play))

You've omitted one mission-critical fact, which is that a 1.4x crop also reduces the amount of light collected, by exactly the same amount as a 1.4x TC. (Canon APS-C is a 1.6x crop of course, which is even worse.) This is why it is so important to use equivalent f-numbers as well as equivalent focal lengths.

If you do the comparison correctly, the R7 without a TC and the R5 with a TC are equivalent to within a third of a stop. Then, if you also trim the 700 mm (actual) R5 image to match the FoV of the R7's 800 mm (equivalent) image, they become exactly the same with regard to light collected.

Going on from that, the trimmed R5 image is now 7168 pixels wide, compared with the R7's 6960 - 3% difference, which is so close as to be negligible.

This is why the OP and I are so keen to see an actual comparison, without which it is too close to call.

So in the end it's a choice between reducing the $5,000 lens's capabilities (IQ, fstop, speed and ISO) to gain 700mm reach. While 800mm reach is achievable, with better IQ, ISO, fstop and shutter speed, on the R7 for an extra $1,200 above the TC's cost.

Choices... choices

Rob

Exactly!