I was with you until the "enlarged more" thing. Native resolution is huge on most APS-c cameras, At 100 dpi like your computer screen a K-5 image is 50 inches wide, a K-3 image is 60 inches wide, a K-1 image is 70 inches wide. In actual fact, 99.9999999% of images are reduced to a smaller size for out put. In the old days, you had to enlarge the negative more for the smaller format image to be the same size, also enlarging the grain twice as much and often creating visible grain. With digital the smaller format in this case APS-c has to be reduced less. The difference being with film, you're grain just got larger and larger. With digital the noise gets smaller and smaller and in many cases disappears completely, as you reduce an image.
The point at which a K-1 is the same as a K-3 image is probably at about 5000 pixels wide. You don't have an output device that wide. Printing at 200 dpi, you'd still have to print over 25 inches wide to see a difference, as a guess, it may not even be apparent then.
So essentially, when shooting a 24 MP full frame, and a 300mm lens ƒ2.8 , and a 24 MP APS-c with a 200mm ƒ 2.8 lens, you will have the same field of view and same resolution, in essence, identical images. Both will suffer the same amount of enlargement to print them big. That's why people say the APS-c gives you more reach. What's the difference You ask? Well it's the difference between this and this for the same image. We are talking a 3 pound total package against an 8 pound total package. One manageable one not.
This is the extreme example, showing how bad it can get. But most folks don't shoot either 200mm or 300mm so as a comparison it makes no difference. But on the whole APS-c lenses are lighter, and smaller for the same field of view.
You've got a 300mm equivalent lens for the price and weight of a 200mm with APS-c. And neither image is reduced or enlarged in size, any more than the other when viewed on a computer or printed. Once they are pixels, sensor size has nothing to do with it, (at 100 ISO, at least) in terms of enlarging or reducing.
But as Pentaxians, our FF is 36 mp, so it's a lot more complicated. You have to add Pixel Density to the equation. If there were a simple way to figure this out I'd have done it. And I haven't. But here's how much you have to crop a K-1 image before you could have taken a better image with a K-3.
The best measure of resolution for Digital Cameras is lw/ph, line width per picture height. It's the largest number of distinct lines the sensor could display if you took a picture of alternating black and white lines. It is in no way equivalent to MP. The K-1 is measured at around 3600 lw/ph tops with a great lens. A K-3 is measured at about 2700 lw/ph so the K-1 has roughly 33% more resolution, with 50% more pixels.
But a Panasonic Lumix DMZ1000 with a one inch ( 1/4 the size of APS-c) sensor can produce 2700 lw/ph just like a K-3. That little 1 inch sensor can take images equivalent to your APS-c camera at 100 ISO. (It loses lw/ph very quickly as the ISO goes up , but hang in with me, that's irrelevant.) That little sensor can produce images as good as any K-3 and better than a 22.3 MP Canon 5D mk3 FF costing 3 times as much.
So just to summarize. As has been pointed out above, the easiest thing to do, is look through the viewdifinder, take the picture you want with the camera you have, and do what you do with it. It is very unlikely that you are going to push the limits of your camera, no matter what you are shooting. And if someone is going to figure out a formula taking into account, sensor MP, real sensor resolution and sensor size more power to them, but bottom line, in digital, sensor size and MP are probably the least important things for understanding resolution or enlargeablilty.
The lw/ph is really the only thing that matters, and that exists independent of sensor size and MP, which at least IMHO opinion are at best unreliable measures of resolution.
Here's an example.
In this case, in terms of printing. the 1 inch 20 MP sensor will produce an image equivalent to a K-3 image, with a smaller file. There's a reverse magnification effect. The Lumix will give you the same resoluiton with a physically smaller image, based on how good it's sensor is at converting MP to lw/ph. The Lumix at 100 ISO will actually out resolve a Canon 5D mk 3 FF, 22 MP sensor.
You also need to take into account that different sensors in different systems lose lw/ph at different rates as the ISO increases. So, while you can might get the image you want with a smaller sensor, as the ISO climbs it will very quickly fall behind the larger sensor, as well as the Dynamic Range and noise levels.
If you don't have a headache by now tough, I do. :D
The only things you need to understand with crop factor is, the more crop, the shorter lens you use to achieve the same Field of View.
As general rule, the larger the sensor the better it will be in low light.
A full frame camera gives you narrower depth of field and smoother out of focus areas if you need that, and APS-c isn't good enough. (many of us find APS-c quite good for narrow depth of field images.) FF is the most overall versatile format in digital but only because it's too expensive to build faster than ƒ2 with ƒ2.8 being more common lenses in Medium Format, and 1.4 can be common in FF, and 1.2 is available. But MF will still produce the highest resolution.
Full frame is the format where it become physically prohibitive to build fast glass, for the next format up. It's in a physical sweet spot. But smaller systems can be just as useful to you if not better depending on what you do. Unfortunately landscape and wildlife are on two different ends of the spectrum. A slow snails pace camera like a K-1, D800 or 645z s great for landscape. Not so good for wildlife. K-3s, D7200s, D 750 or Canon 1DXs and many other 20-24 MP cameras are better for wildlife, with their faster frame rates and deeper buffers.
You really need one of each. :D
And you can know all this stuff, and still have trouble finding a camera you actually like. :D
Liking the camera and how it works is probably the most important thing. :D
Personally, APS-c is my favourite compromise, but my most used camera is the K-1, and i would buy them in that order. But I shoot a lot of small bird pictures and wildlife. If you don't or landscape is more important to you, you might go at it differently.
Aren't you glad you asked? :D
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