Why more people choose D300 over 40D as I can see in the review the pic quality/sharpness of D300 are bettered by A700.
Re: Picture quality between D300 and Sony A700: Picture quality is subjective. For some, sharpness is primordial; for others it might be the way colors are rendered, or the way that the camera allows them to be. Also, picture quality is a result of how easy it was to take that picture in the first place. Here, ergonomics and ease of use would come into play. In other words, picture quality would be hard to qualify because everyone looks for different things in a picture.
Re: Sharpness difference between pics taken with D300 and A700: two factors here:
1. The camera.
I can assure you that you can get tack sharp pictures with the D300. The default setting might not please everyone - how can it? - but you can easily boost sharpness within the D300 functions. Reviews normally test cameras with their default settings to even out the playing field for comparison-purposes, so if camera's default settings are more conservatively set than another camera, then you will get differing results. I think you should consider what you can actually do with the settings of the camera, i.e. whether you can get the sharpness you are after.
2. The lens
. the lens plays a huge factor in the sharpness of a picture. Some people sometimes buy a particular brand of camera because said brand has an amazing lens that no other has. For instance, the new Nikon 14-24mm, albeit expensive and not fully tested yet, is said to be absolutely awesome in its sharpness across the focal range
. This is an important point, because - unless you're dealing with a non-zoom lens (called a "prime" lens), say, a 50mm f2.8 lens - then you will find that a lens' performance will vary depending at which aperture setting you are testing it. A lens might be very sharp at f5.6-8.0, but might be compromised at its extremes. Besides sharpness (or resolution), there are also other factors that will come into play, such as darkening of corners at some aperture settings (called "vignetting") or other unwanted artifacts such as CA aberrations that might compromise the overall picture quality. To come back to my example of the Nikon 14-24mm, it looks like it's an amazing lens in the way that it's sharp across most of its aperture range and is relatively distortion-free.
Thus, when you are asking whether you can get sharp pictures with the D300, or whether the Canon 40D or Sony A700 would be better, I would say that all three are capable of taking sharp pictures. You just have to tweak the camera's settings to your liking, and use a sharp lens. for example, setting the D300's controls to additional sharpening, and with a 14-24mm, for example, you would probably be quite blown away (read an early test of the 14-24mm here, if you're interested: http://www.16-9.net/lens_tests/nikon_14_24mm_1/nikon14_24mm_a.html
). The same probably applies to Canon (although I'm not sure which lens exactly would be the sharpest. A great place to have a look at is on this site under the "Lenses" tab, as well as here: http://www.photozone.de/8Reviews/index.html
Lens-wise, I think that Canon and Nikon would have an advantage of the Sony, which has a more limited range of lenses at this point in time. However, it's really about which manufacturer has the lenses that YOU want. If you're looking for sharpness, than, as I said, the Nikon 14-24mm seems great. I think Canon has a similarly-sharp lens in that focal range, but I'm not sure which one exactly it is. As far as other brands go, I don't claim to have enough knowledge, so if others want to chime in on this one?... The point is that most manufacturers will have good lenses, but a few of them will have some outstanding (and expensive!) ones. They might, or might not, be the lenses you are after, though. For example, the Nikon 14-24mm might be great, but what if you're after a lens with a different zoom range? Maybe another manufacturer has exactly what you're after.
So, unfortunately, there is no easy and straight answer to your question as to which camera model to get in order to be able to take great/sharp pictures. Personally, I would stay with either Canon or Nikon, simply because lenses play such a huge part in the photo-taking equation, and both Canon and Nikon have not only a wide range of lenses (wider range than Sony) but also come out regularly with new ones as well, so there seems to be more supply with those two.
I think that both the 40D and the D300 are great cameras, and both will be able to give you great/sharp pictures. So look at which one feels better to you, and which one falls within your budget - the Canon 40D being cheaper by a comfortable margin. Then, look a their lenses and see whether one manufacturer has a lens in the range you're after, which gets good reviews for the features you're after (sharpness, for example).
A final word about sharpness. I am a beginner. I just did a thorough research in order to get myself tools that are as good and future-proof as possible for my investment, so I look for sharpness as well. However, as I progress, I wonder sometimes about whether sharpness is that
important... Certainly, for some types of pictures, sharpness is a real standout feature. Just look at the pictures of one of this forum's member - Zorro - on the Flikr website where he has his pictures posted. But I also think back at the pictures that I have seen over time, and which have actually made me stop doing whatever I was doing a the time, because I just had
to look, and look again
, at them. Some of those black&whites 1930s Paris scenes that seem to exist all over the word in all cafés would be a prime example! Was I looking a them, and did I think they were great because they were the sharpest? Nope. Artistic composition, colors, whatever tickles your bone, but sure wasn't sharpness...So, sharpness - or, more correctly, the pursuit of it - might be a "disease" affecting beginners (like me), those who put more emphasis on technicality over maybe more relevant factors. A bit of honest mea culpa
and something to think about...