I have a Canon Rebel T5i. I have two lenses: EFS 55-250mm f/4-5.6 IS II Lens and a EFS 18-55mm. When I take photos, it's very grainy. I want to get a new lens that's very good and sharp. I want one lense where I can zoom I pretty good and I can zoom out and possible take candids of people at conventions. Any suggestions would be appreciated
If these were taken with your 55-250mm lens and you were zoomed in, the aperature would at or close to f/5.6. If shutter was also near at least 1/250s or faster, ISO would then be rather high.
Do you happen to know the settings used for each of the images above?
Low lighting can be challenging. But say you use a f/2.8 zoom lens. That will allow in 4 times the amount of light (2 stops) and thus ISO could be lowered by 2 stops as well. Sometimes that's a nice jump to really increase quality. e.g. lowering from ISO 6400 to ISO 1600.
Thanks for the metadata. As suspected, ISO was very high. And the shutter speed was quite low (1/17s) for the second image as well.
Unless you can add lighting, there are really only two things you can do:
As an example, I started out with a Rebel T4i with an 18-135mm f/3.5 to f/5.6 lens. Then moved to a 6D with a 50mm f/1.2 lens. Keeping the field-of-view the same for an example, the T4i setup would have been around 31mm at f/4. Then for the 6D setup, if at f/1.2, that's just under 3 stops of more light gathering. I also estimated at the time just under 2 stops of better ISO performance with the 6D. For a total of around 4 stops which really opened up low-light possibilities where adding in lighting wasn't possible.
Going back to your original question though... there won't be a single lens covering all your focal lengths and perform better in lower light. A really good, though very expensive set of lenses is Canon's trinity:
With the latter 70-200 probably being a good one to start with. But, I would recommend first renting it to see if it would suit your needs. You may find it too large and heavy of a lens.
Another alternative is if you can get close to your subjects, you could pick up the very inexpensive EF 50mm f/1.8
Also, for indoor shots at events, while it doesn't always have the best reach, I really enjoy the EF 135mm f/2. That extra stop of light gathering could be very useful over an f/2.8 lens. Though it does lack IS.
Ansel Adams couldn't get good photos with these settings.
f/5.0, SS 1/166, ISO 6400, FL 131mm. f/5.0, SS 1/17, ISO 12800, FL 100mm
Also, you won't either even if you buy an f2.8 lens as suggested above. You are just too many stops away from what it takes for proper exposure and good IQ. All photographic gear has its limitations. To add to the f-stop challenge the T5i is not a stellar performer at these very high ISO numbers. It's actually amazing you got as good as you did!
If you need to shoot in this poor lighting situation a whole new camera and lens combo is probably the only out for you. The newest cameras have very much better high ISO performance, even beyond ISO 12,800, and coupled with that f2.8 or f2 lens will get you as close to good IQ as possible.
What can you do right now with what you have? A couple of things. First get a monopod. Set the T5i to Av mode and select the most open aperture, say f5.6. Let the T5i handle the SS. Use One shot AF not Ai-servo, never use Ai-servo for this. Set the ISO to auto. You may want to set an upper and lower limit to the ISO. That's fine. Select Raw as the file format not jpg. Never, ever, jpg for this. You will need a post editor perhaps you already have one. Canon has one for free that you can d/l if you don't. It is DPP4 but there are others out there for free, also. Photoshop Elements is very good and not too expensive editor. Using Raw files will let you adjust exposure up to 4 stops, sometimes even more so this is mandatory.
All of the above plus using a good AI-based noise reduction image processor like Topaz DeNoise AI 3 or DXO PureRAW-2. The current (AI) generations can work wonders on noise reduction (graininess) with very little effect on sharpness reduction.
Haven't checked Topaz lately, but they used to have a free trial period, so you might give it a try on your current images.
Although Topaz DeNoise can be or is a solution to noise reduction it does nothing Adobe Camera Raw or the Detail Panel in Lightroom doesn't already do without any additional software add-on. Plus Topaz DeNoise isn't a photo editor so you need something like PS or LR or DPP4 to use after running Topaz DeNoise. Just learn how to use one of them.