We lab tested an early version of the Tamron on a Canon mount earlier in the year and did a direct comparison against the Sigma.

At 500mm wide open the Tamron started to go soft; not significantly but quite clearly. At 600mm wide open the softness was definite but again not what you would call a major problem.

At 500mm wide open of the two the Sigma was sharper.

Sharpness improved in both lenses as you decrease aperture size. At F11 the Tamron was at it's best; the Sigma at F8.

This was shooting a calibrated card at 10 and 15 metres in lab conditions.

Using the Tamron in field tests it proved to be very versatile, light in weight for what it is, with a good OS that was comparable with the Sigma.

Emphasising that this was an early Canon mount Tamron and there have apparently been some changes since we had the test unit the decision was made to stick with the Sigma as on the Tamron high quality above 500mm was difficult to get.

This was a lab test based on a specific set of requirements which it could be argued has little to do with the real world. I know people with both lenses; both have very happy users. Like all these long zooms there is a 'sweet spot' - you just have to find it.

While trying to decide on the purchase of a Tamron 150-600mm versus the Sigma 150-500mm lens, I wanted to estimate the apparent difference in size of an object--the blue heron--when taken with a full frame camera and the lenses extended to their respective longest focal length.

According to several published reviews, both lenses exhibit some sharpness degradation at focal lengths greater than 300-400mm; most other features and specifications seem quite comparable. So it all boils down to weight, size, price, and most importantly image size on the film/sensor plane.

The two images below are intended to depict the difference between a field-of-view of 5.0 deg for the 500mm (left image) and 4.1 deg for the 600mm (right image) as viewed at a distance of approximately 30 feet.

It seems that a small amount of cropping, especially when using a high resolution camera, could bring the left image to the equivalent of that obtained with the 600mm Tammy lens. So, is it worth the $200.00 difference in price between the two lenses? The size? The weight?

I would like to hear recommendations from those who have had first-hand experience with either or both of these two lenses. Thank you.