Originally posted by Nicolas06 I am not silly I am practical. Somebody here say wonderfull I can get a 1000mm equivalent framing with my Q for less than $1000 and showed some quite blury image wiht very few details. My APSC photo cropped to 900mm looked signifcantly sharper meaning that my system was able to record more details. That's just how it is. I didn't spend thousands to get that 55-300. Most people here already have some K-mount body, one can be get used for something like $200 and the previous version of the 55-300 can be get on the bay for $150-200... This isn't more expensive. And you get AF as a noticable improvement.
A sigma 70-300 is completely outresolved by a 24MP APSC sensor anyway so making the pixels smaller doesn't help.
This will be my last comment on this subject unless something radically different comes up.
The fact is that I am extremely practical. My undergraduate degree was in Math with a minor in Physics; I have a graduate degree in Computer Science and a minor in math tools. I mention my background only to demonstrate that I am adequately equipped to consider these questions.
Several years ago I was totally frustrated with the pictures I was getting of birds and other wildlife, so I started looking at options.
Considering the possibility of a birding bag that contained mirror lenses of several different focal lengths, I bought a 500mm mirror lens and took pictures using it. I purchased a TC and took pictures using it. I took some of my pictures where "I wasn't close enough", and similar pictures from other sources, and tried cropping them. None of these produced what I wanted.
So, just about two years ago, I put together a spread sheet comparing all characteristics anyone suggested might be helpful, and compared the Canon SX-50, Pentax Q-7/Q-S1, Nikon 1, and MFT cameras. I asked lots of "devil's advocate" questions here, and seemingly offended some people. I looked at lots of images displayed on-line which had been taken using the various cameras in question. Halloween 2014, around the time Canon released the SX-60, and SX-50 prices plummeted, I almost purchased an SX-50, but I decided that its 1/2.3" sensor just plain had too many short-comings; the SX-50 was clearly second to the Pentax products with their 1/1.7" sensor, and in December 2014 I purchased my Q-7. I am mildly amused that you have never addressed the SX-50 in this thread, even though I have repeatedly referenced it, and the Nikon P-900, as other contenders. I don't know what your real thinking is, so I won't attribute motives to you.
Once I had chosen the body, I considered various lenses. I could not depend on tests using an APS-C sensor, because the quality of image produced by a lens depends partly on how much of the lens {radius} is used. Since it is meaningless to compare images of different subjects, I set up a paper target outside and took pictures using various lenses at various apertures. I was extremely surprised that the Pentax-DA 55-300 was outresolved by the Sigma 70-300, but that is what my practical results were {but the Pentax-DA did come in second out of six lenses tested}; I was also surprised that my tests showed the Sigma lens to work best on the Q-7 in the range f/8 to f/11, but that is what my practical results were. Just to be sure of my choice of the Q-7, I also used the same lenses on my K-30 and established that the Q-7 images were significantly sharper than cropped images from the K-30. So the Sigma became part of my birding bag {essentially permanently coupled to a Pentax K-to-Q adapter}, the DA went back into my K-30 bag, and the other lenses, ... well, someday I will do something with them.
The bottom line is that my decision was entirely practical, based on lots of fact gathering, including looking at lots of images; frankly I haven't seen much change in the situation since then. I am 68 right now. I am guessing that under judicious use, I will be able to use a Q-7 / Q-S1 for birding / wildlife as long as I will be able to use any camera; if that is not true, and the facts then are essentially the same as the facts are now, based on my practical research, I would purchase a Canon bridge camera long before I would follow any of the paths you have advocated. And the bottom line is, looking back at the sentence I bolded above, for the very first time,
today I am totally satisfied with the pictures I am getting of birds and other wildlife.
I am not convinced you actually do much photography of this sort. That is not my concern, but as long as you're satisfied with what you are doing, that is wonderful.