Still continuing my crash course on cameras with my (sorta) impending first SLR purchase, and I've figured out something which I HOPE is true.. Hoping someone might be able to confirm.

Probably the thing I'm most interested in is shooting pics of airplanes as they pass over, especially the in flight refueling activities. My current camera, an S1IS has an image sensor which is a 2.0x factor to a 35mm camera. I also have an adapter that permits me to add a 1.7 converter that I have. So my current 35mm is:

58mm(the camera's max lens)* 2 (the conv factor) * 1.7 (my converter) = 197.2mm

The T3i I'm looking at has 1.6 conversion factor and I'm hoping to get a 250mm lens so it'd be

250 * 1.6 = 400mm equivalent. Multiply that by 1.7 if I get a 58-52mm thread adapter and it's 680mm

So by this logic, if it's right, I'll actually be getting more zoom with my new camera than my old one, even with a just ok length lens of 250mm.

Is this true? I hadn't counted on getting more magnification with my new camera, but I'm hoping it is. Thanks

I don't know if all the math works, but there may be some technical issues that may keep things from working out the way you're thinking. I'm not sure what 1.7 adapter you're referring to, but it may not work on that lens/camera. Also, if it reduces the amount of light getting to the AF sensor, and it probably will, the AF may not work anymore.

Also, the 1.6 crop doesn't exactly increase zoom, it reduces the field of view, so it may not end up offering more detail, just a tighter view, which may still be advantageous.

The 1.7 will probably just be an attempt, I'll be holding it in front of the lens just for a test.. Not really practical to shoot like that all the time. It's nothing special anyway, it's an addon lens that you could buy for a mid range Olympus camera at one point. Certainly not the same quality as an actual teleconverter that goes between an SLR and lens. At any rate it sounds like I will still be happy with the zoom level on the new camera, which is good because it's one thing I like about my current camera, it's just obviously not exactly current any more.

Thanks for the help!

The 400mm math is correct. It does kind of "increase zoom" as the object you are looking at fills more pixels on the sensor.

Still, 400mm in total is not a whole lot for what you seem to have in mind.

Yeah I'm not expecting to be able to read registration codes or see Boom Operators waving, but I've looked at airliners.net for 400mm equivalent shots and they seem better than what I can shoot now. Plus cropping some will be an option too. I think I'll be fairly happy, just wish there was a less pricy option for big zoom without big bucks.

I just picked up a 75-300mm for $9.95 on eBay so you might want to be a little patient and shop around for a while.

Wow, that's scary cheap. Have you received it yet? If so, how well does it work? Are you sure it's not stolen?

Holy crap! That's a 200.00 lens even if it's not the IS one. Let us know how it is when it arrives please!

It arrived today. Here are a few using the close focus ability.

Wow. Is it a Canon lens? I found six 75-300 lenses last night under 50 bucks on eBay, will see where they end up at.