Do you think that keeping my Sigma back a little at the 400mm level and increasing the ISO ( like 400-800) and aperture to f8 or narrower will offer me better clarity on the 70D body?
Nope, the lens is already outresolved esp. on crop, the tiny sensor difference will be lost. You'll get 2mp more of blur though :-p ... face it - there are no good and inexpensive 400-500mm tele zooms, it's better to crop a good 300mm. Btw all I can look at are the charts, and I don't see any reasons why the conclusion shouldn't be valid.
Sigh. The message just doesn't seem to sink in.
There is no such thing as sensors outresolving lenses or lenses outresolving sensors. Output resolution, the measurable resolution of your images
, is the RMS of the resolutions of the components involved in producing the image. Lenses, additionally, are non-linear. At some apertures their intrinsic resolving power may be less than the sensor, and at other apertures, it may be more (sometimes MUCH more) than the sensor. By increasing sensor or lens resolution, regardless of which one is doing better, will still increase output resolution. (And we are still quite far away from diminishing returns yet, so increasing sensor resolution is still the cheapest way to increase output resolution.)
As for the comment about there not being any good inexpensive 400-500mm zooms, I beg to differ. The Tamron 150-600mm has been tested and demonstrated to be quite good for it's class. It's no EF 600mm f/4 L II, but it is the closest thing your going to get, and there is really no alternative for good optical reach. Even at f/8, a 600mm lens is going to increase subject area by 2.25x relative to a 400mm lens. It would increase subject area by a full factor of 4x relative to a 300mm lens. The fact that the lens is diffraction limited at that point is irrelevant. There is absolutely no way that an upsampled 300mm crop is ever going to compare to an uncropped, unscaled 600mm image. Simply not going to happen. You can't overcome four times as many pixels on subject.
Thanks for the comments guys, I like the Digital Picture comparison charts, but they just don’t drum in “real life” comparisons that I can draw from.
Do you think that keeping my Sigma back a little at the 400mm level and increasing the ISO ( like 400-800) and aperture to f8 or narrower will offer me better clarity on the 70D body? Just a thought as I think I’ll be able to get away with those settings in most cases as it’s full on sunshine here 365 days a year! Ultimately I’d love to own a 300mmL prime with a 1.4X and a 2X convertor, but that’ll be next years challenge. I think you’ve answered my question regarding the 70D's IQ, I’ve set my heart on it and will be buying one next month, hopefully before I go to Berlin!! I’ve also managed to bag myself a mint condition 24-105L so I’ll be in a good place in a couple of months
If you have the 300mm f/2.8 L, then you already have a superb lens. Using a 2x TC is easy, and at 600mm you have four times the detail on your subject. Regarding aperture, use the aperture you need to get the necessary DOF. Don't worry too much about ISO, especially at ISO 400-800. The 70D should do quite well up to ISO 1600. It is only when you get beyond ISO 1600 that your IQ may start to degrade enough that you might need to be concerned, however the 70D is sharper than the 7D, and sharpness eats noise for breakfast. (Background blur, on the otherhand, tends to be eaten by noise for breakfast, lunch, and dinner, but blurry backgrounds are super easy to clean up, so it really doesn't matter much.)
If you need reach (i.e. you shoot birds or wildlife), then there is really no substitute for optical magnification. Raw focal length is your best friend. Backing off your focal length from longer to shorter is actually a bad idea. Instead of thinking about upsampling a crop from a shorter lens, think about downsampling the full image from a longer lens. No matter how you slice it, a downsampled image from a longer focal length will have more detail and less or equivalent noise to any image shot at a shorter focal length.