RF100-500 with TC at 420mm vs without TC at 428mm and 300mm scaled

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Karl_Guttag Senior Member • Posts: 1,029
RF100-500 with TC at 420mm vs without TC at 428mm and 300mm scaled
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I think this has been a burning question for those with the RF100-500 and considering a TC.

Below are back-to-back shots using the RF100-500 with and without the TC at about the same focal length. I shot with the TC at 300mm plus the 1.4x or 420mm. I then removed the TC and moved the camera back by about the length of the TC (maybe a mistake). I then adjusted the zoom without the TC until the framing was identical. The EXIF said it was at 428mm (close enough to make a comparison). All the pictures below are out of the camera JPGs.  Finally, I zoomed the RF100-500 back to 300mm without the TC.

I "picked" 420mm as it was about all I could fit in the room where I test lenses with my 42-inch (~1 meter) wide test pattern that I have used with all my other RF lenses.

As I noted in a recent thread (https://www.dpreview.com/forums/post/65499338), the 100-500 is excellent wide open.

I overlayed the six images below in Photoshop and looked at various sub-targets. The pictures are at 300 x 1.4 = 420mm at f8, f11, f16, and at 428mm without TC at f6.3 and f8.

I noticed a loss in contrast in the center and the corners with the TC, and I see only a very slight loss in resolution. About half of the contrast loss comes back at f11, and the image is slightly sharper at f11 with the TC. By f16, diffraction is becoming significant, and the resolution is lower at f8.

Comparing the 1.4x TC at net 420mm to the 300mm at f8 and then scaled by 1.375x (to match the same size in Photoshop), using the 1.4x TC has higher resolution, and I would estimate by about 1.3x (very approximately).

I have looked at many RF lenses with these test patterns. Subjectively, I think it is holding up reasonably well with the TC. Assuming my results would be the same at other focal lengths with the TC, it would suggest the optimum sharpness is about one-stop down with the TC.

420mm f8 using 1.4xTC

420mm f11 using 1.4x TC

420mm f16 using 1.4x TC

Shot at 300mm f8 from the same location without TC and scaled by 1.375x to match the size of TC image.

428 F6.3 (no TC)

428 f8 (no TC)

 Karl_Guttag's gear list:Karl_Guttag's gear list
Canon EOS R5 Canon RF 15-35mm F2.8L IS USM Canon RF 24-70mm F2.8L IS USM Canon RF 70-200mm F2.8L IS USM Canon RF 24-240mm F4-6.3 +13 more
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