What does THIS button do... ?

Now, not to sound snarky Dan ... but did you get the owner's manual with the camera? Very specific "what that is, how to use it, and when/why to use it" on each of those features.

As for whether those features are useful to you - impossible to know without knowing what sort of shooting you do, under what circumstances (and with what lenses/lighting), and how you'll be using the images - including some sense of your workflow. If you're too new at it to delve into each area for now, leave the features at their factory default settings, and explore them each as you have time to read that page in the manual, and stack each up against your shooting needs.

Hi Matt. Yes, I have the manual. It tells me how to activate these features, but not a good explanation of how they work, and of course not an objective opinion on how well they work.

As far as my shooting info... so far my limited experience has been NYC street photography, mostly daytime but I enjoy night photography as well. I have the 18-55 kit lens.

For best quality results shoot in RAW. Even in JPEG it is best to perform noise reduction in post processing. HTP and ALO are useful in settings where the exposure range of the image exceeds the dynamic range of the camera although combining multiple shots or using ND grad filters produces better results. Af assist beam is for dark shots where the AF would not normally work. Mirror lock up is very useful for tripod shooting - especially with long exposures. It locks the mirror up before you shoot resulting in a lot less vibration when you shoot. MLU is only for tripod use when the camera is fixed on the image you wish to shoot. Shooting in live view has a similar effect as the mirror is also locked up (depending on the Af setting you use). As the others have said read the manual. You should also be aware that many settings do not work in the full auto modes and that when shooting RAW a lot of image control settings have no impact (as the image is RAW).

1) either auto or on, it will take a second exposure with the shutter closed and subtract that noise from the previous, however it will not save the second and the first is saved after the noise reduction. Turn this off when you need to shoot without having to wait for the camera to do this, like with fireworks.

2) I'd just leave it on all the time unless you really figured out a reason not too. Yeah, that grainy look, but it also loses a bit of sharpness.

3) On my 40D I've found this basically just adds noise and makes the image ever so slightly softer. This may have improved with the XSi however. It uses the ISO's (film speeds) from above and bellow what you have the camera set too on dark and bright areas of the scene, trying to compress the tonal range. Take some test shots and see for yourself if you do or don't see a difference. By turning it on you'll lose the low and high ISO's, so if you need to shoot at 100 ISO turn it off.

4) ALO (I believe) is a setting to brighten the corners and edges of your images with certain lenses that naturally have dark corners or edges. It doesn't work with every lens so you'd have to read up to find out if your lens is supported.

5) AF-assist beam. When it's just on it uses the pop-up flash, you'll see it do a popcorn poppity pop, lens focuses, poppity pop, lens focusses... kind of confusing to the subject when they walk away before you even take the picture. > or if you have an external flash attached that has a focus assist beam it will use that instead.

6) Mirror lockup, leave this off until you get told to turn it on. It makes you press the shutter release, the mirror locks up, then you have to press the button again to take the picture. If your having problems with critical sharpness and suspect the mirror slap is moving the camera then use this. Otherwise leave it off.

7) this is for forensics type stuff (I believe). If you plan on using your images as legal evidence you can use this to prove that this image came from this camera or something to that effect. Most won't need it.

8) You didn't list it but the print button (the thing with a little squiggly blue line and a printer icon) doesn't do much of anything, but that's usually what people ask when they start a thread with something like the title that you've used.

Fantastic information. Thanks Dan.

Guys not all manuals do a good job explaining these features. Some tell almost nothing more than how to turn the function on or off.

Dan I found links that answer many of your questions using google and photo.net's own search function. Hopefully you will find the links helpful.

• Long exposure noise reduction(1). I would leave it on Auto. Long Exposure noise reduction takes two pictures. One with the set exposure and a second picture with the identical exposure but with the shutter closed during the exposure. The exposure with the shutter closed is used to measure the noise generated by the sensor which can very due to temperature and length of exposure. The two exposure are subtracted to generate the final image. The noise reduction methode is better than post processing noise reduction because it actually measures the noise generated by the sensor.
• High iso noise reduction(2). The linked Canon article goes into detial on this function. It is basically a software smoothing function which can also be done in post processing with third party software.
• 2)Highlight Tone Priority(3) and Auto Lighting Optimizer(4). These are software functions that Bob Atkins briefly discussed in his review of the 50D.
• AF assist beam(5) is nothing more than a special light beam that can help the camera focus in low light situation. Sometimes it is IR beam and not visible. In other case it is visible. Without knowing what camera you have no one can provide more information on specific camera you have we cannot go into details on that setting.
• Mirror lock up(6). I find this very helpful when using a tripod with long exposures.
• Add Original Decision Data(7). This function adds additional data to the picture file. An additional software package sold by Canon can then use this data to tell you if the picture has been changed since the picture was taken.

Thanks Steven!

Hi Dan
"Hi Matt. Yes, I have the manual. It tells me how to activate these features, but not a good explanation of how they work, and of course not an objective opinion on how well they work."

Its not your fault Dan. Most manuals that I own are written in a style called "Clear if understood", I.E. if you already understand how the camera works the manual makes perfect sense. If you don't, the manual is only of limited help. It doesn't help that the manufacturers seem determined to put every bell and whistle imaginable in every camera so as to be able to charge more for them. (the customer is getting more so we can charge more)
Paul

Hi Dan
"Hi Matt. Yes, I have the manual. It tells me how to activate these features, but not a good explanation of how they work, and of course not an objective opinion on how well they work."
Its not your fault Dan. Most manuals that I own are written in a style called "Clear if understood", I.E. if you already understand how the camera works the manual makes perfect sense. If you don't, the manual is only of limited help. It doesn't help that the manufacturers seem determined to put every bell and whistle imaginable in every camera so as to be able to charge more for them. (the customer is getting more so we can charge more)
Paul