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

Robbey TC wrote:

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

Hello Steve,

What you observe is interesting indeeed .. and I think I followed your line OK.

What I need to clarrify is that at a FOV equivalent to 700mm ( the maximun reach of R5 plusTC) there is a difference of 1.3 f-stops which is purely a factor of the R5 being in the f7.1 zone plus 1 fstop for the TC ... and the R7 being at the f6.3 zone of the 100-500.

You could summarise what I'm saying like this: the consequences for both DoF and light collected by the sensor are the same regardless of whether you use a TC, or crop to the same framing without the TC.

Apart from f-stop, SS and ISO the amount of light capable of actual capture by each piixel is influenced by the design and size of the pixels.

And indeed the number of pixels, but for the comparison which is the subject of this thread, the two sensors are very similar technology (in particular, neither is BSI) and the pixel count is very similar too. This is a big part of what makes this so intriguing.

The larger pixels and better sensor design the R5 is an advantage in light recording capability per pixel ... but remember that there is also 1.3 stops less light available.

This is where a good understanding of equivalence is invaluable. That 1.3 stops of brightness is compensated for by the larger area of the r5 sensor.

With only a finite amout of light available, the smaller pixels on the R7 mean each each R7 pixels has a lower share of the available light ... and each may therfore become susceptable to more noise.

The IQ relationship between f10 on a FF 45MP sensor and f6.1 on an APC 32.5MP sensor is unknown to me.

I really, really recommend reading up on equivalence. Forum discussions often contain too much misinformation, but you could worse than to start here as it's a trusted source: https://www.dpreview.com/articles/2666934640/what-is-equivalence-and-why-should-i-care

How these factors balance out in the real world will interesting to see.

Can't wait!