Below are thumbnails for four image crops in two pairs that show at 100% view (pixel-level) the results of using my Tamron-F AF 1.4X C-AF1 MC4 Teleconverter (TC) on two different telephoto zoom lenses - the consumer-grade Canon EF 70-300mm f/4-5.6 IS USM lens and the professional-grade Canon EF 100-400mm f/4.5-5.6L IS USM lens. Each pair shows an image taken with the TC attached and without it and with that non-TC shot then being scaled up in Photoshop by 140% to get it to approximately the same size as the TC shot.

The first pair shows that the teleconverter on the 70-300mm IS lens adds only very minimally more detail, if any, compared to the scaled-up non-TC shot, the only advantage of the TC shot being that it has less apparent noise since scaling up the non-TC shot correspondingly increases the size of the noise by 140%, therefore making it more noticeable. Also, the camera’s automatic focusing system (AF) worked with the TC only under the most ideal situations (i.e. bright light and good subject contrast) and even then not very well, so since the cost of using it (loss of one stop of exposure that all 1.4X TCs take and very poor AF) was greater than dealing with more obvious image noise, the TC rarely saw the light of day until I bought the 100-400mm IS lens.

The last pair shows that the teleconverter on the 100-400mm IS lens adds noticeably more detail when viewed at pixel level compared to the scaled-up non-TC shot. With this combination of lens and TC, the camera’s AF takes a small hit but hardly noticeable in typical situations and not so much that it can’t focus on birds in flight and not so much that it outweighs the advantages of gaining some detail and having no increase in apparent image noise, so I don’t hesitate to use this combination of lens and TC and I seldom leave home without it.

(It should be noted, I learned a few months ago (mid-2012) that while the the Tamron's AF works pretty well with a Canon T1i, it didn't with the higher-end Canon 60D! Go figure.)