I've only had the Rebel T7 for about a month. I've got a 18-55mm lens. When I am on AF and zoom in on anything and slightly hold down the shoot button to focus it will, sometimes, go ahead and focus, but even when it does once it actually takes the picture it just zooms all the way back out and takes it. So I cannot get any pictures with any level of zoom in focus at all.
Any advice would be much appreciated.
What might be happening is that the camera is not achieving focus lock. If that happens the lens will hunt for a moment and then, if it still doesn't get a lock, will refuse to take the shot. This usually happens when there is not enough light or contrast in the image area for it to do so. Also, if you are using the default focus setting, it will focus on the closest object by default, and that may not be what you are actually trying to get a lock on.
Still, to confirm that it is this, I would recommend, setting the camera up to aim at an object with distinct sharp edges and contrast - perhaps a structure of some kind. Try shooting at this from different focal lengths and see if it gains a lock and focuses. If it does, then the camera is working ok, but you need to look at your settings for focusing.
Whatever you do, do NOT try to manually focus the lens when it is set to Autofocus mode if the lens does not have the letters STM written on the front of the lens barrel. If it does not, and you try to manually focus with AF mode on, then you will screw up the gear system and it will refuse to focus properly at all. STM lenses do not use gears - they use a stepping direct drive motor so do not have this issue.
In addition to what Trevor explained, I would point out that the lens changes length as it refocuses on subjects. If that is what you mean by “zoom out”, that might be what you are seeing. The lens cannot “auto zoom”, meaning change its focal length on its own.
Also, your lens has an MFD, minimum focus distance. The lens can only focus on subjects that are outside of that minimum distance. Every lens has its MFD printed somewhere on the lens.
Be aware that minimum focus distance usually only applies at the shortest focal lengths. At longer focal lengths the MFD tends to increase as you manually zoom to longer focal lengths.
Bill makes a good point - if you are focusing on a close object, you will not get a focus lock. Try the experiment I suggested at some distance - say 10m (33ft).
What OP might be seeing when their lens focusing. The lens elements are moving. All version of the 18-55mm lenses are external focusing lenses. Except the STM version which is an internal focusing.
40D, 5D IV, EF 16-35mm F/2.8L III, EF 24-70mm F/2.8L II, EF 28-135mm F/3.5-5.6 IS USM, EF 50 F/1.8 STM
430EX III-RT & 600EX II-RT
Just as long as one notes the difference between zooming and focusing. All of these lenses extend to zoom (i.e. change focal length) and for the uninitiated that can be confusing.
"Any advice would be much appreciated."
OK jhillne, here is what you need to do.
First reset the T7. Menus>Tools>Clear all settings>Clear all custom settings. Check your manual.
(this is to just make sure you didn't accidentally set something incorrectly)
Now set the T7 to P mode. Set ISO 200. Set Average WB. Make sure the AF/MF switch on the lens is set to AF.
Go outside on a nice day and take some random shots. Everything, people, trees, dogs, buildings, landscapes, etc just random. Maybe go to a park. Take lots of photos. Go home and u/l to your computer, hopefully you have already d/l the free from Canon DPP4 image editor. It will u/l the photos from your T7 to your computer almost seamlessly.
Check them out. If they all look great or most all, there is nothing wrong with the T7 or the lens. If they don't you need to contact Canon 1 (800) 652-2666
jhillne it is very important to do the reset before you do the test.