There are many ways you can do that. Some ideas:
- Get a nice big portable SSD, plug it into each computer in turn, and copy the files over.
- Use SD cards or USB flash drives to move the files over.
- Get a NAS (Network Attached Storage), hook each computer up to your LAN (as well as the NAS itself), copy the files over via the network.
- Hook all your computers up to your LAN and copy all the files onto the “main” one.
- Get one of those external HDD case thingies; these allow you to turn your old computers’ HDDs into external HDD drives with a USB port; now you can plug them into your main computer and copy the files over directly.
- Pay for cloud storage and upload all your pictures there. (Caveat: assuming that we’re talking hundreds of gigabytes of photos, this is going to cost a bit, and you’ll need a generous internet connection, both in terms of upload speeds and data allowance - if you have fiber or cable, probably no problem, but with DSL, better check beforehand).
- If you live in the early 2000s: burn the pictures onto one or more DVDs and copy them over that way.
- Probably a ton of other ways.
Here’s the simplest (and easiest to remember) formula:
Crop factor equals ratio of focal lengths.
That’s it. To simulate a 600mm lens by cropping an image shot on a 400mm, calculate the ratio of the focal lengths: 400mm/600mm = 2/3. Then multiply the image size by that factor (e.g., a 4500x3000 px image would become a 3000x2000 px image), and crop accordingly.
Obviously this ignores things like lens distortion, and you do lose a fair amount of pixels, but if your subject isn’t too close, and all you want is an idea of the kind of reach you’d get from a longer lens, then this is perfectly fine.
Oh, and calculating the effective focal length of a lens on a crop sensor works the same way, only in reverse: say you’re using a 400mm on an APS-C sensor with a crop factor of 1.5; c = f1/f2 means that f1 = c * f2, so you just multiply: 400mm x 1.5 = 600mm; that’s your effective focal length.
The only caveat is that it might not be immediately obvious which focal length goes where in the fraction, but since cropping always makes an image smaller (and the subject larger in the frame), it’s easy to figure out whether you got it right.