RMT I've read the rest of your posts and I apologise for assuming what I did.
However I would still maintain that you should not be shooting a wedding - especially without the right gear to do so. The reason I say this is not for you but for the couple who are getting married. It's their special day and although it might sound simple to take a few shots you really need to have a real grasp of why that max aperture and variable max apertures are bad when shooting a wedding.
To answer a little more of your question, f4.5 is not that small an aperture IF you are shooting in bright light outside or in the studio. At a wedding though, the important shots are generally indoors in a lowly lit church or registry office and then f4.5 is very VERYslow..... you will require to use flash and that's sometimes not permitted during ceremonies and even if it is allowed it can ruin the atmosphere of a shot unless the user knows all about using flash. i know my flash work is not the greatest so I use it generally just for fill.
Above, Mike stated f4.5 is only 1 stop faster than f2.8 which is correct but if I'm shooting at f2.8 ISO1600 and getting shutter speeds of around 1/30th to 1/60th sec that leaves you with shutter speeds around 1/15th to 1/30th sec at f4.5!!! Not ideal for sharp shots and especially not of people moving
Weddings are unlike any other type of photography. You absolutely must have fast glass with constant apertures. Another scenario. I'm shooting at f2.8 with 1/60th sec. i can zoom out and in and maintain that maximum aperture. Lets say with a 3.5-4.5 lens you have 1/40th therefore at f3.5 (which is very slow) and then you zoom out to 70mm suddenly you are now at f4.5 (because you r max aperture when zoomed has changed) and therefore and you've lost any chance of a sharp imnage as your shutter speed is now down to around 1/25th!! Exposure changes are bad when shooting weddings.
If you are looking at a lens with a variable max aperture, in the main these are therefore consumer grade lenses not very suitable for shooting weddings.
look at the Tamron 17-50 or 28-75. Both have a constant max aperture of f2.8 which is a minimu requirement for a wedding. It's also the fastest aperture of any EF fit zoom lens. Faster lenses are prime lenses - and it';s always handy to have one in your bag, especially the 50f1.8 which is cheap and does a reasonable job although not for pro's who will prefer the f1.4 or the new f1.2L.
Also what if your camera breaks do you have a backup? Do you have a backup flash and backup lenses? Ask any wedding pro and 100% will say you need backups (and some will have backups to their backups)!!
A friend of mine whjo was shooting a wedding called me the morning of it as her camera broke at the bride's house!! She had no backups.
RMT - I hope i'm more of a help than a hindrance. I want you to be the best prepared you can be. i know what it's like as I started out like you 9although i waited till i had the right equipment before comitting to a wedding).
Good luck and if I'm out of order again please let me know.
best regards
JD