Auto focus points

[Nathaniel]

Most of the DSLR's have numerous focus points which light ups in red showing the area of focus. That being the case is it worth while to set the focus point to, say, the centre for general,all purpose use, so that one can place this point exactly on the subject to be photographed. I find that if I set the focus point to cover all the focus areas, and I take a photo, the area of sharp focus is not what I had aimed for but something/somewhere else which the focus point picked up i.e like a tree and not a sheep in a farm yard for example.

[eeyore]

Re: Auto focus points

I would just select the centre point, however, on my camera, I manually select the focus points, using the cross keys function, (set up as a custom function). That being said, the centre point is the most often used.

[colin C]

Re: Auto focus points

Mine is set up fot the centre point and I manually shift it if I need to. Whilst leaving it up to the camera to decide can be useful while you are learning the ropes, there is no way the camera can decide what you think is important to focus on.

As I tend to use back button focus, it is easy to stab the rear button on what I want if focus, release the button, recompose and shoot.

Colin

[Nathaniel]

Re: Auto focus points

Mine is set up fot the centre point and I manually shift it if I need to. Whilst leaving it up to the camera to decide can be useful while you are learning the ropes, there is no way the camera can decide what you think is important to focus on.

As I tend to use back button focus, it is easy to stab the rear button on what I want if focus, release the button, recompose and shoot.

Colin

Thanks Richard/Colin,

I will also do the same and shift the focus manually when i want to.

[Guest]

Re: Auto focus points

s I tend to use back button focus, it is easy to stab the rear button on what I want if focus, release the button, recompose and shoot.
Colin

Colin, what is 'Back Button Focus' ?
John

[djguk47]

Re: Auto focus points

Colin, what is 'Back Button Focus' ?
John

John, Colin will come along soon with a great explanation but in the meantime have a look at this from the Canon Digital Learning Centre

http://www.usa.canon.com/dlc/control...articleID=2286

I used the back button focus to take my Vulture shots at the Hawk Conservancy.

David

[Guest]

Re: Auto focus points

......................but in the meantime have a look at this from the Canon Digital Learning Centre

http://www.usa.canon.com/dlc/controller?act=GetArticleAct&articleID=2286

David

WOW. I have saved this site as a favourite and will delve into its content ; it looks to be extremely informative. Thanks David.
John

[colin C]

Re: Auto focus points

A good article and David is correct, that it saved me a lot of explanation.

One aspect that they didn't major on was flying subjects. Whether aircraft or birds, they are moving around the sky with the light source ever changing against the subject. Using the front button to activate A1 Servo and the exposure with the first press of the button, the exposure is essentially locked when you first pressed the button.

By using the rear button to activate A1 Servo and track the subject around the sky and only using the front button when you want to initiate exposure and the shot, you separate tracking from exposure. Result, many more correctly exposed shots.

The downside is that it doesn't come natural to most people. I tried it a couple of times and it wasn't instinctive so I gave up. The third attempt, I stuck with it for a couple of months and now it is so natural that it seems strange using a camera where the back button isn't set up. So if you think there are benefits for your style of shooting, be prepared to persevere.

Colin

[Nathaniel]

Re: Auto focus points

Perhaps this should be another thread but I am raising it here. It is agreed that the centre focus point is the one that is most sensitive and the best to be used. Aligned with this, are there any ad hoc rules as to when to use the various metering modes (on the creative side)- evaluative,spot,partial etc etc. One can place the centre autofocus point(red light) on the subject & then which is the best metering mode to follow up with this? is it spot or evaluative etc? for portraits,architecture,landscape,wild life etc etc.

[NickJ]

Re: Auto focus points

Colin,

I tried to set up the AF-ON button on my 7D as per the Canon Digital Learning Centre tutorial but there doesn't appear to be the menu in C.FnIV as they decribe, I assume in the 7D they are doing it some other way?. But when on AI servo and half-pressing the shutter release the exposue values change depending on the lighting of the scene so it doesn't seem to be locked when using the shutter release when set up in the 'normal way'. So it appears to me not to be behaving as you describe in your second paragraph. Am I doing something wrong (probably!) or have Canon thought of this when designing the 7D?

[colin C]

Re: Auto focus points

They may well have Nick, I am not sure because I only played with the 7D and never actually owned one. Is your mode set on Av, Tv, or manual? I think it has to be in one of the advanced modes for some of the Custom Functions to "Kick-in". Can any 7D owners confirm this?

What I should have mentioned was that I use manual exposure, based upon an 18% grey card, or meter for the grass, or any other neutral tone, but that's a whole different subject in the context of back button focus.

Colin

[colin C]

Re: Auto focus points

Colin,

I tried to set up the AF-ON button on my 7D as per the Canon Digital Learning Centre tutorial but there doesn't appear to be the menu in C.FnIV as they decribe, I assume in the 7D they are doing it some other way?. But when on AI servo and half-pressing the shutter release the exposue values change depending on the lighting of the scene so it doesn't seem to be locked when using the shutter release when set up in the 'normal way'. So it appears to me not to be behaving as you describe in your second paragraph. Am I doing something wrong (probably!) or have Canon thought of this when designing the 7D?

Hello NickJ

On my camera, when I press the shutter button half way, assuming the exposure is correct, the meter scale in the right of the viewfinder shows an arrow in the middle of the scale. If I keep the half pressure on, but move the camera to a brighter portion of the scene, the arrow separates into two, with one part staying in the centre of the scale and another part moving towards the top of the scale. Similarly, if I point it to a darker area, the main arrow still stays in the middle and the secondary arrow moves towards the bottom of the scale. The digital readout of exposure doesn't change during this operation, as long as I keep the half pressure on the shutter button. If at any time, I take my finger off the shutter button when deviating from the original exposure reading and re-apply it, the digital readout of exposure changes to the new setting.

I understand that the arrow (actually its a small square) splits to provide an indication that the camera considers the exposure may not be correct. Does this happen the same on the 7D?

If that is correct, then the exposure is locked when you first apply half pressure to the shutter button and why back button tracking/focus can provide more accurately exposed shots of moving subjects. If it is not correct, what would be the logic of separating focus and exposure and providing an extra button the acheive it? Perhaps I am overlooking something obvious here and if so, I would welcome further information, as we are all here to learn and improve our techniques.

Colin

[Les McLean]

Re: Auto focus points

Perhaps this should be another thread but I am raising it here. It is agreed that the centre focus point is the one that is most sensitive and the best to be used. Aligned with this, are there any ad hoc rules as to when to use the various metering modes (on the creative side)- evaluative,spot,partial etc etc. One can place the centre autofocus point(red light) on the subject & then which is the best metering mode to follow up with this? is it spot or evaluative etc? for portraits,architecture,landscape,wild life etc etc.

It's a difficult one to answer Nathaniel, as a lot depends on the light.

If the scene has a fairly even light across it, then evaluative metering should work OK, but in a very contrasty scene, say a single person against a black background, I'd try spot metering on the person.

One way round this (and good practise) is to bracket exposures, so if you were shooting aperture priority on evaluative metering, using the back dial, you could take shots at the displayed reading, then under/over expose by 0.5 stop/1 stop etc. Once you get the images up on your computer, you can normally tell which looks the best.

Perhaps not for this discussion, in landscapes I generally use filters, particularly ND grads to balance the exposure between the ground/sky.

[Nathaniel]

Re: Auto focus points

Quite a lot of sense, Les. Thanks. Perhaps that explains why some of my shots come out a bit soft/dark/light even though via the view finder the image looks great.(this applies more to the focussing point etc, which I had got wrong). Anyway, I am sure I am getting there slowly!

[colin C]

Re: Auto focus points

Les is spot on as usual, but I would like to add a word of caution regarding spot metering:

They were originally the preserve of high level pro cameras and only metered the central 2.5%, or so of the picture area. If you fully understand how a meter works and what you want to meter with the spot facility, that’s fine.

However, I have come across a few photographers who have asked for exposure advice, because the exposure for their shots was all over the place and on further questioning, they were using spot metering. When I enquired why, they assumed it was better and more accurate, because it only used to be on Pro cameras. A quick lecture on how the meter works and for Canon users, how good evaluative is for 90% of general photography shots and they go away happy, a little wiser and the meter switched to evaluative.

Colin