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- Stuart or just Stoo!
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I have a 5D3 and 70D and use them both with a Sigma 150-500 OS for wildlife and aviation shots. The Sigma is very sharp and works great with the 5D3 but even with MFA it's not as sharp with the 70D (maybe something to do with the dual pixel tech with an older lens) and a lot of examples are on various threads. However, I've recently been toying with the idea of changing the Sigma for a Canon 300 f4L IS and 2x and 1.4x MkIII converters. Would the 300mm and converters be sharper than the Sigma and give me a better IQ overall and does anyone think that would be a good idea. I could also swap the 150-500 OS for the wifes 120-400 OS and exchange that for the Canon 300 so I'd still have access to the 150-500 OS that I know is great on the 5D3.
With the 5D3 able to AF at f8 I could use the 2x and lens to get 600mm (I'd also be able to get 300mm and 420mm with the 5D3) whereas with the 150-500 OS I only get around 500mm and I usually use it at f8 anyway as it's a bit sharper at that rather than wide open at f6.3. With the 70D I could get 300mm (equivalent to 480 on FF) and 420mm with the 1.4x converter (equivalent to 672 on FF) so I'd end up losing a bit of reach on the 70D but gain a bit on the 5D3.
With the 5D3 able to AF at f8 I could use the 2x and lens to get 600mm (I'd also be able to get 300mm and 420mm with the 5D3) whereas with the 150-500 OS I only get around 500mm and I usually use it at f8 anyway as it's a bit sharper at that rather than wide open at f6.3. With the 70D I could get 300mm (equivalent to 480 on FF) and 420mm with the 1.4x converter (equivalent to 672 on FF) so I'd end up losing a bit of reach on the 70D but gain a bit on the 5D3.