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Thread: Single/Continuous Focus

  1. #1

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    Single/Continuous Focus

    I'll put this question as it relates to the camera (Canon G2) that I have got. There is an option on the menu to choose either single or continuous focus. As I understand the single mode only focus's once whereas the continuous option is checking for changes in the scene to focus on. What I am not quite understanding is how this is utilised. As I understand it on the G2 when you press the shutter button half-way down this locks the focus therefore whether it is in single or continuous doesn't seem to make much difference? Does it? One of the things I was thinking about was in terms of taking a picture of something that is in motion. I would be happy for someone to clear my confusion?

    Cheers for now

    Gary

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    Administrator Manfred M's Avatar
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    Re: Single/Continuous Focus

    While I don't know exactly how the G2 handles the functionality, the explanation on the two different focusing methods is fairly simple. If you are taking a picture of a relatively static scene; a landscape or a group portrait, locking in the focus when you press the shutter half-way down and making some minor adjustments in composition work just fine. The image is quite static and the focus point is unlikely to move on you.

    Change to an action scene, for instance a sports scene where the subject matter is moving around, this is a bit different. If the action goes side to side, the single mode focus might work out just fine, but if the subject is approaching you or moving away from you, the point of focus is changing continuously, and that split second where you hesitate before you press the shutter release all the way and take the picture could mean that your focus point is no longer sharp.

    This is where continuous focus comes in; the camera keeps adjusting the focus until you press the shutter release all the way down and take the picture. So ultimately, this is how I determine which mode to use; motion photography where the subject is moving toward me or away from me, I will use continuous, and subjects where motion is minimal or more or less parallel to the focal plane, I will use single.

  3. #3

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    Re: Single/Continuous Focus

    Thanks for your reply Manfred. I didn't appreciate that in continuous mode the camera continues to focus until the shutter is fully pressed. I will have to try this out and see what results I get. I have been trying to take sports/actions shots so this sounds like it is something worth trying for that.
    Thanks.

    Cheers for now

    Gary

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    Administrator Manfred M's Avatar
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    Re: Single/Continuous Focus

    This is why I say that I don't know how the G2 has implemented this functionality My D800 works this way, so I assume that this is likely similar to what the G2 does. As long as I have the shutter release pressed half-way down I can see the focus markers in the viewfinder changing and can physically feel the focus motor in the lens moving the lens elements around.

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    Re: Single/Continuous Focus

    Similar to AI Servo in 'newer and bigger cameras'.

    Works fine in theory and can produce good results for flying birds etc. But, providing you keep the camera pointing directly at the target and you don't keep going in and out of focus, etc.

    Worth experimenting. But not something which I used all the time.

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    William W's Avatar
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    Re: Single/Continuous Focus

    Quote Originally Posted by oldgreygary View Post
    I didn't appreciate that in continuous mode the camera continues to focus until the shutter is fully pressed.
    Caution - test it out: I believe this statement is incorrect.


    Quote Originally Posted by oldgreygary View Post
    I'll put this question as it relates to the camera (Canon G2) . . . There is an option on the menu to choose either single or continuous focus. As I understand the single mode only focus's once whereas the continuous option is checking for changes in the scene to focus on. What I am not quite understanding is how this is utilised. As I understand it on the G2 when you press the shutter button half-way down this locks the focus therefore whether it is in single or continuous doesn't seem to make much difference? Does it? One of the things I was thinking about was in terms of taking a picture of something that is in motion. I would be happy for someone to clear my confusion?
    Yes. Your understanding regarding the HALF DEPRESS is correct - it LOCKS FOCUS - and prima facie, the option of 'Continuous Focus' appears quite silly: I agree.

    ***

    I believe the Functionality of the G Series AF system is:

    Continuous Focus – Provided there is power the Lens is always focussing.
    Single Focus – The Lens will focus when the shutter is half depressed.

    Specifically for the G2 - the “Continuous Focus” Mode is NOT an AI Servo Mode:
    i.e. it does NOT track a Moving Subject


    For a G2 - the Half Depress Shutter position LOCKS Focus and this is confirmed by the Beep and the (selected) AF square becoming GREEN, if the (selected AF square FLASHES YELLOW the AF has NOT been achieved.) This is the method used to make ‘AF LOCK’ when using a G2, no matter which FOCUS MODE is selected.

    In later G series PowerShot Cameras, (such as my G10 and G11), Canon introduced “Servo AF” which is designed to track a moving Subject (or moving face in “Face Detect” Mode).


    ***

    Canon's reason for 'Continuous Focus' appears a mystery to many, but having had previous discussion on this matter, I have come to the conclusion that whilst 'continuous focus' just wastes power if the camera is slung over the shoulder, it is of some use if tracking a moving subject (and keeping the finger off the Shutter Release) - but then, of course when one moves through 'Half Depress' to 'Shoot', the AF is momentarily locked and then there is the shutter lag whilst confirmation is made.

    However having the focus 'almost correct' and with the intrinsic large DoF of a small format camera the Focus would probably be OK and if the movement can be predicted accurately enough, a good action shot could be made.

    ***

    You can confirm this understanding as correct (or incorrect) by using both ‘Single’ and ‘Continuous’ focussing MODES and acquiring AF on a stationary Subject and then recomposing the shot: if the Focus stays LOCKED after recomposing - then my understanding is correct.

    Confirmation of the above would be much appreciated as it is a long time since I used a G2 and I don’t have one handy to test.

    WW
    Last edited by William W; 19th June 2012 at 12:59 AM.

  7. #7

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    Re: Single/Continuous Focus

    Bill,

    Thanks for your reply. I tried using both single and continuous modes on a static subject with the Canon G2. The only difference between the two modes seems to be that when in continuous I can hear the focus motor(?) making adjustments. But, once the the shutter button is half depressed the focus is locked and re-composing in either mode has no effect on the focus. It stays the same. So, I can't see that much of advantage with the continuous mode. In fact more of a disadvantage as it is using up battery resources.

    One of the reasons I started to investigate this mode was that I read an article about sports/action photography. It suggested that having a continuous focus servo motor was a 'must have' for this type of photography. But, I guess that this is something that would only be available to me on a more expensive camera?

    Also, to be fair to the initial response from Manfred. He did qualify his reply by saying that he was speaking about his experience with his camera. So, I jumped the gun a bit in assuming that the Canon G2 would work in a similar fashion which when you think about it is a bit unrealistic for a camera of this type and age.

    Cheers for now

    Gary

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    William W's Avatar
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    Re: Single/Continuous Focus

    Quote Originally Posted by oldgreygary View Post
    I tried using both single and continuous modes on a static subject with the Canon G2. The only difference between the two modes seems to be that when in continuous I can hear the focus motor(?) making adjustments. But, once the shutter button is half depressed the focus is locked and re-composing in either mode has no effect on the focus. It stays the same. So, I can't see that much of advantage with the continuous mode. In fact more of a disadvantage as it is using up battery resources.
    Thank you for confirming.
    Correct: it is the AF motor which is working all the time.

    Quote Originally Posted by oldgreygary View Post
    One of the reasons I started to investigate this mode was that I read an article about sports/action photography. It suggested that having a continuous focus servo motor was a 'must have' for this type of photography. But, I guess that this is something that would only be available to me on a more expensive camera?
    Correct - Some ‘servo’ function in the terminology usually means the AF is used for ‘tracking’.

    As mentioned the later model G Series have ‘Servo AF’. I am not sure what model introduced this, but I do know it is the G10 and G11.

    IMO a ‘continuous focus servo motor’ is not a "must have" for action/sport photography: there are other means of securing action photos. Once upon a time there was not even Auto Focus.

    What is the 'subject in motion' you want to capture?

    ***

    But sure, I agree that if we have AF, then a (good) servo function can be useful for some shots; but with any automatic function, the driver’s knowledge of the function and knowledge of its shortcomings, bears heavily on the final results.

    But even so, it is my opinion that the G–Series cameras are limited for Action Photography when they are compared to what is now available as an ‘entry level’ DSLR: the shutter delay is one main issue and the other main issue is the variance of the AF acquisition – it occurs to me not as true/accurate/efficient as the centre AF point on (for example) a 600D.

    A third limitation is the Maximum Aperture (usually) available (combined with the ISO available) - which limits the use of a camera like the G-Series for Action Photography in low EV, (low light).

    On the later series like the G11 which I get to about F/4.5 at the telephoto end and can bump to ISO3200, if necessary: but on the G2 you only have ISO400 at about F/2.8 at the telephoto end, (I think, something like that anyway): so you are snookered by about two stops.

    Then compare that to the 600D (with the inexpensive lens mentioned below) - at 250mm we can get F/5.6 @ ISO6400 and maintain reasonable to good quality - which outdoes the G11 by another portion of a stop.

    AND the 600D (with that 55 to 250 lens) has longer reach, not reckoning the Digital Zoom of the G-series cameras – and that can be the fourth limitation of G-Series for Action Photography.

    Quote Originally Posted by oldgreygary View Post
    Also, to be fair to the initial response from Manfred. He did qualify his reply by saying that he was speaking about his experience with his camera. So, I jumped the gun a bit in assuming that the Canon G2 would work in a similar fashion which when you think about it is a bit unrealistic for a camera of this type and age.
    It is more about the camera type: not the age of it.

    So discussing the type of camera and referring to your comment about ‘only available in a more expensive camera’ - I don’t know off the top of my head comparative new prices for a DSLR and a G12 (latest model???) – but the 600D and the EF-S 55 to 250F/4~5.6, in skilled hands would make a reasonable fist of daytime Rugby or Soccer or Field Hockey – if one had access to the sideline. One could crop up to about 40% of the frame away in PP if necessary to ‘increase’ the reach of the 250mm.

    WW
    Last edited by William W; 19th June 2012 at 07:44 AM.

  9. #9

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    Re: Single/Continuous Focus

    Thanks Bill for more valuable information. I did have another post in which you also covered some points about action sports. Two events are coming up in the Olympics in the UK. One is the cycle road race and the other is road running (marathon) which I would like to try and get some shots from. Other sports that could also be possibilities are cricket and rugby union. The move to the DSLR such as the Canon 600D with EF-S 55 to 250F/4~5.6 suggested is something that I will have to find a way to get to. It does seem to becoming very clear that the Canon G2 will not be able to do what I envisage in my mind.

    This link London To Brighton Cycle Ride is some shots taken with the Canon G2 from a charuty cycling event I took recently.

    Cheers for now

    Gary
    Last edited by oldgreygary; 19th June 2012 at 02:40 PM.

  10. #10
    William W's Avatar
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    Re: Single/Continuous Focus

    Ah, OK Thanks,

    Good Luck,

    WW

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