2 new lenses, or just 1?

One lens, the lens you deserve but not the one you need right now. You'll hunt it. Because it can take it. It is not your hero. It's a silent guardian, a watchful protector

The Dark Knight wrote in post #14922970
Thanks, but this doesn't meet my needs for a longer zoom lens for wildlife photography. The reason why I mentioned the 24-105 was because I was under the (maybe incorrect) assumption that 105 would be long enough to get some decent wildlife photos with some well done cropping.

If I get the 17-55 (a terrific lens I admit), I might still need to add the 55-250 to get the range of what I want to do with my photos, and that tacks on another 200 dollars. I know it's just another 200, but now I'm looking at almost double what I was originally thinking of spending with just the prime and the 55-250.

Another consideration is I do harbor dreams/ hopes of moving up to full-frame one day, so I wanted to stick to EF lenses if at all possible... I know the 55-250 is an EF-S, but that's only 200 bucks and probably fairly easy to sell or cheap enough to keep on a hypothetical back-up crop sensor camera.

I use the 55-250mm on crop and I think it works excellently. I think it is underrated. Even at 55mm, I think the 55-250mm is much better than the 18-55mm. It will serve as a "walk around" lens at places like the zoo where you have a mix of portraiture and distance shots. If you are patient, you can find them on sale for $150 routinely. Aside from that, lots of people also sell new or almost new copies for around $150, sometimes because they bought it as part of a bundle. I sold an extra one, brand new, for $150 shipped several months ago.

For the other half of your question, if I were in your position, I would have the two other lenses be primes, the 50mm and 28mm like you were mentioning or possibly the Sigma 30mm. If 50mm is the majority of your shots, you could also consider upgrading to a better 50mm instead of getting a wide prime at all unless you want it for indoor settings.

The Canon 35mm F/2 is a good, cheap option too but it is really close to 50mm. OTOH, it is just a little bit more wide than the 50mm so it might give you the room you need.

Personally, I'm not a fan of zoom lenses except for telephoto so that's what I'd do. But... I also mainly shoot people and I prefer 50mm and above, even on crop.

You have an 18-55 & a 50mm prime so have so you considered a 70-200?

SoCalTiger wrote in post #14923119
I use the 55-250mm on crop and I think it works excellently. I think it is underrated. Even at 55mm, I think the 55-250mm is much better than the 18-55mm. It will serve as a "walk around" lens at places like the zoo where you have a mix of portraiture and distance shots. If you are patient, you can find them on sale for $150 routinely. .

I agree - get the 55-250IS - nice sharp telephoto
only $169 at adorama - includes free shipping and warranty
http://www.adorama.com​/CA55250AFSR.html

The Dark Knight wrote in post #14922970
The reason why I mentioned the 24-105 was because I was under the (maybe incorrect) assumption that 105 would be long enough to get some decent wildlife photos with some well done cropping.

105mm won't be long enough...unless you're some sort of dr. doolittle that can get really really close to wildlife...

ean10775 wrote in post #14923019
I have to agree here. You're going to need quite a bit of light or a really high ISO to shoot this indoors without flash.

Indoors without flash is really a recipe for disaster no matter the lens. At least for the light levels in my house, my 2.8 zoom is no where near enough to forgo flash. My 30/1.4 is enough but the DOF gets so thin that either you get blurred ears or missed focuses much more often. Maybe the High ISO capabilities of a 5D3 would cut the mustard better at least for me, but no 1.6 camera yet can and keep in mind the OP is using an older XSi.

Until then I much prefer to set up a couple flashes around the room and control the light instead of using crap ambient light. Thus my two f4 lenses aren't disadvantaged over my 2.8.

24-105 is a great choice but if it's too expensive you can look at the 28-135 IS lens which is an awesome lens. I have both.

For wildlife, you can get the 70-300 (non L) used for $300.

Laramie wrote in post #14922959
I like the separation of a long zoom. But admittedly, I shoot more 4 legged or legless critters than I do anything else, so obviously my needs are different.

Certain leses are right at home on FF, but again, for me they are just a bit awkward on a crop in terms of focal length. But obviously just depends on the type of shooting you do.

Nice.........

FEChariot wrote in post #14923986
Indoors without flash is really a recipe for disaster no matter the lens. At least for the light levels in my house, my 2.8 zoom is no where near enough to forgo flash. My 30/1.4 is enough but the DOF gets so thin that either you get blurred ears or missed focuses much more often. Maybe the High ISO capabilities of a 5D3 would cut the mustard better at least for me, but no 1.6 camera yet can and keep in mind the OP is using an older XSi.

Until then I much prefer to set up a couple flashes around the room and control the light instead of using crap ambient light. Thus my two f4 lenses aren't disadvantaged over my 2.8.

I would add that more houses are like this than not. Mine is. Going to higher ISO is often not ideal- no where near as is adding more light - whether it be more lights[continuous], brighter lights, or flash. I'd add flash to even a kit lens before reaching out and buying a different lens- whether it be f/2.8, f/4, or whatever. One will get more out of a flash or two.

wayne.robbins wrote in post #14925606
I would add that more houses are like this than not. Mine is. Going to higher ISO is often not ideal- no where near as is adding more light - whether it be more lights[continuous], brighter lights, or flash. I'd add flash to even a kit lens before reaching out and buying a different lens- whether it be f/2.8, f/4, or whatever. One will get more out of a flash or two.

I disagree, most places indoors has enough lighting for non flash portraits. I shoot primarily available light, and unless you're going to a dimly lit restaurant, 1/60 ISO 3200-6400 can capture MOST indoor shots.

Flash is ideal for professional work, but I think a 2.8 lens is as well, since it gives you that much needed dof, especially when shooting crop.

wayne.robbins wrote in post #14925606
I would add that more houses are like this than not. Mine is. Going to higher ISO is often not ideal- no where near as is adding more light - whether it be more lights[continuous], brighter lights, or flash. I'd add flash to even a kit lens before reaching out and buying a different lens- whether it be f/2.8, f/4, or whatever. One will get more out of a flash or two.

Charlie wrote in post #14925808
I disagree, most places indoors has enough lighting for non flash portraits. I shoot primarily available light, and unless you're going to a dimly lit restaurant, 1/60 ISO 3200-6400 can capture MOST indoor shots.

Flash is ideal for professional work, but I think a 2.8 lens is as well, since it gives you that much needed dof, especially when shooting crop.

both statements seem to be completely arbitrary.

wouldn't where one lives, combined with the size of the windows, coupled with the exposure, compounded by window treatments and proximity of other structures, also impacted by landscape, all play a part in whether or not any given home has enough light to take portraits.... indoors without additional lighting irrelevant of f-Stop & iso capabilities?

Charlie wrote in post #14925808
I disagree, most places indoors has enough lighting for non flash portraits. I shoot primarily available light, and unless you're going to a dimly lit restaurant, 1/60 ISO 3200-6400 can capture MOST indoor shots.

Flash is ideal for professional work, but I think a 2.8 lens is as well, since it gives you that much needed dof, especially when shooting crop.

Keep in mind you are using a 5d2 which has at least one stop better ISO performance than the best performing 1.6 crop camera. I am not touching 6400 on my 7D unless flash is just not allowed. For when I can use flash, I would much rather have ISO 200 than 3200. Also, I have had too many 1/60" shots ruined by subject blur as I more often than not are shooting kids that don't sit still to want to use that slow SS so I prefer to shoot at 1/80" or 1/100" min.

Additionally, the OP has a Xsi which maxes out at what 1600? 3200? My Xt maxes out at 1600 and even 800 is pretty crappy with it.