Autofocus points - why so few

Discussion in 'Canon EOS' started by rodrigo_coutinho, Nov 5, 2010.

  1. Hi, is it just me or the number/location of autofocus points could be improved. I own a Canon 350D and it has 7 autofocus points in a cross pattern, other models (1 series) have more (45 or so, near the center). My point is:
    why do i always have to focus/recompose to shoot an offside subject?
    Couldn't there be some more autofocus points spread around in the frame?
    Better yet, couldn't we have i grid of (invisible) autofocus points that covered the entire frame?
    Let me alucinate a bit further, with Eye Control Focus also?
    What's the limiting factor here? Costs? Technology?
    Thank you in advance
     
  2. Canon has had the technology for a long, long time - even eye controlled focus (see EOS 3) - the reason 'lower' level cameras don't offer more focusing point is obvious: the must be differentiations among the different level cameras. If every camera offered the same focusing system, the same dust protection, metal body, etc... they would differ only in name, not in features.
    Since lower cameras are intended for those who are either casual shooters, beginners and/or who don't want/need to spend above a certain amount, it's clear that the included features have to reflect those limitations.
     
  3. I wish my camera had 5 autofocus points. 1 at each 1/3rds and one in the middle.
     
  4. "Couldn't there be some more autofocus points spread around in the frame?"
    I ask myself that question all the time, but I'm guessing that for composition and perspective purposes it would be better to have the main subject in focus and everything else a little out of focus. Or have the main subject falling under the imaginary "rule of thirds" grid with everything else consisting of background or foreground.
    Now I'm just guessing, but I think Canon figured out than in 95% of situations the main subject(s) are going to fall in that area, so this is where they concentrated the focusing points.
    As you know Canon placed the main focusing points inside their cameras around an eliptical grid that hardly covers the entire frame. But what about the other 5% ? I guess that's where you would need to lock the focus and recompose.
    Actually filling the entire frame with focusing points would probably slow down focusing since the camera would not go off until each focusing point reached focus. That's why allot of photographers including myself would rather choose one focusing point for speed and simplicity.
     
  5. [[What's the limiting factor here?]]
    Marketing departments.
     
  6. Newer EOS DSLRs and higher-line cameras do have more focus points. My 7D has 19 focus points, and I can select any one of those, or select a group toward one side, or high or low, and, of course, all.
    As others said, there have to be some things that make a 7D different from a 550D. Both are 18 MP APS-C cameras, but the 7D has 19 focus points, while the 550D has only 9. As contemporary cameras to your 350, the 20D has 9 AF points, and the 1D series have 45!
    And don't forget, time and technology march ever forward. At some point, we'll probably be able to buy a Rebel or XXXD EOS with 19 or more AF points, while the higher-end cameras get even more. Though, I imagine that there may be a practical limit to how many AF points one photographer can realistically deal with. A hundred AF points might sound interesting, until you have to scroll through them all to find the right one!
     
  7. Bill W.
    What happened to using the central focusing point, hold the shutter button down to hold the focus or slide the lens autofocus switch from autofocus to off, then, recomposing and taking the shot - I never take the time to dial in a particular AF point.
     
  8. That's all I need. More AF points to focus on what I'm not trying to photograph!
    Obviously, they could put in as many AF points in as many locations as they want, but it's going to cost more. And, as my statement above indicates, I think that would start to introduce problems as the camera focuses on what's on the periphery instead of the subject.
     
  9. The more focus points you have, the more likely it is that your camera will focus on some extraneous foreground object, instead of your intended subject. Do you want an irrelevant tree branch to be in focus, rather than your child? The optimal number of automatic focus point is one.
     
  10. Many people don't realize that the AF sensors are underneath the mirror... and to get to that, the mirror is partly silvered in the middle, and a secondary mirror is behind that one, sending light straight down to the AF system. See:
    http://media.the-digital-picture.com/Images/Other/Canon-EOS-1D-Mark-IV-Digital-SLR-Camera/AF-Optics-Cutaway.jpg
    When you do a bigger spread, you need a bigger secondary mirror (which gives more inertia to the main mirror, adds cost, weight, shorter life, etc) and a larger array of sensor points. All the other points noted above are valid, as well (cost, complexity, missed focus, etc...)
     
  11. why do i always have to focus/recompose to shoot an offside subject?​
    You don't have to, just turn the focusing ring until your point is sharp and release the shutter
     
  12. "You don't have to, just turn the focusing ring until your point is sharp and release the shutter" Spoken like someone under 40!
    Actually, I do find it a bit funny, but not useless, when I hear discussions like this. Obviously, not many shot in days when you had a central split screen and that was it-no AF at all, or how many have shot a modern MF camera, with only center point AF--we are a bit spoiled, no?
    Bernard hit upon one point regarding AF mechanism, but I also think there are optical issues the further you start to move out to the corners having to do with diffraction and such, that would test any AF system that could be built. If anyone has tried to focus out towards the edges with a loupe on a LF camera with a super wide lens on it, you know what I mean. Then, you might wait for ever for the camera to focus out there if you were shooting with many of the wide angle zooms, especially at the widest setting, they just don't focus in the corners!
     
  13. I only use the center focusing point, or focus manually as John has stated.
    If you have a camera that you can set a CF to use the thumb, I highly encourage you to explore that method. Has made a world of difference to me as far as autofocus goes.
     
  14. "not many shot in days when you had a central split screen and that was it-no AF at all"
    Remember it well. Try getting good focus doing candid shots in a near dark room (tip--focus on the hair). Actually, I almost wish my modern AF cameras had a split screen sometimes. Once you get use to it, it's really second nature.
     
  15. Actually, I almost wish my modern AF cameras had a split screen sometimes. Once you get use to it, it's really second nature.​
    If you miss it, order one and install it. There are companies that make view finders (including split screens) for most DSLRs. I personnally have one installed in my 5D.
    I don't know why but many people seem to think more focus points is better. If you look at photography web sites the cameras with many focus points tend to have more complaints about focus problems. With more focus points you tend to have more focus options in the cameras software which can make it more likely that a setting mistake could cause your problem. Furthermore when you let the camera sellect the autofocus point, it often doesn't sellect the right one. Cameras arn't smart. They cannot analyze the image and figure out what you want in focus and it can't read your mind.
    I learned photography with a manual focus camera and learned that you had to think about focus just as much as you have to think about exposure. Yes Auto everything can make it easy but it doesn't get it right all the time. With my DSLR I always use the central focus point and it generally gets it right all the time. The few times it hasn't was because I wansn't paying attention, or I made a mistake.
     
  16. "Split screen focussing in the dark": Ah yes...the good old days. Solution: Set focus ring at hyperfocal distance, use your feet and forget about it.
    In my old news days I used to tape the lens rings at f8 and midway between 2m and infinity and let the camera control the shutter....and just bang away. All you had to do was properly frame the shot. We were all on 35/2 lenses...no zooms back then. Before aperture priority made it a bit easier, we would still tape up the lens and just match the shutter speed to the ISO. 90% were in focus and either bright enough or capable of being pushed in developing. We used the iso setting to adjust if we has to. Overexposure was much more difficult to correct. And you soon got told if your shots were not very usable...so you attached yourself to a veteran and copied what they did. Pretty soon you realised that all the pros in the scrum from wherever, were all using the same settings. It only took me about a week to figure it out. We all got to know one another and the talk was like "I'm going up one or down one" and we all did the same. It was in our interests to all help one another, Because you never knew if you would be working with them one day. There was this mutual respect. Now, its a zoo.
    Sending the film cassettes back to the office, all we did was texta +1, +2 or -1 etc on each roll, and they would adjust the developing times.
    When they were available we used to always replace the split image screens with plain ones to remove the clutter.
     
  17. What do you need them for. I find that for action sports you tend to want the subject in the centre of the frame. Canon realizes this as their centre AF point has always been the best. Indeed the EOS 1N and 1NRS which were their sports bodies until the 1V in 2000 only had 5 AF points in a row. The extra AF points may be useful for subjects off centre but I find that I just use the centre and re-compose. Indeed I may just fine tune in MF (this is the way it was done with split screens). I do not like using multiple AF points at the same time on any of my bodies (1 series, 7D or 5DII) as I find that I want to know exactly where the point of focus is.
    The EOS 3 which had the 1 series AF but with eye control was a great camera. People either loved or hated it. It enabled you to change AF point without having to play with buttons. Some people are able to select an off centre AF point while composing the image. I can do this but find that it distracts me from the composition so I prefer just to re-focus.
    The old MF cameras had much brighter viewfinders than today's DSLRs. My 1DIIN and 5DII both have dimmer viewfinders than my old 1V film bodies. These EOS film bodies had darker screen than the old MF bodies like the New F1. The reason for this is the ned for a semi- transparent mirror in the AF bodies.
     
  18. The higher end camera need those AF points to assist with their insane autofocus tracking. They track focus based on which point the subject is in and where its going next, the more points the more accurate the AF tracking will be.
    I had the 20D and the 9 focus points was always enough. I mean how often is your subject in the extreme sides of the frame?
    Keep in mind focus and recompose only works if your depth of field is big enough to hide the eventual errors you'll introduce. With a fast or long lens the shallow depth of field with surely show some errors, very rarely is your subject exactly the same distance from one point to another.
     
  19. William Michael

    William Michael Moderator Staff Member

    "Keep in mind focus and recompose only works if your depth of field is big enough to hide the eventual errors you'll introduce."​
    If the circumstances permit, one can use different techniques, to minimize the potential errors:
    http://www.photo.net/photo/10963077&size=md
    WW
     
  20. If the circumstances permit, one can use different techniques, to minimize the potential errors:​
    Yes indeed you can! It all makes sense once you start thinking of your focus as a flat plane. I have a feeling this is not quite common practice though...
     
  21. William Michael

    William Michael Moderator Staff Member

    Hello Richard! - that was a very promt response . . . if you are still there . . .
    Probably not common practice at all - at least very few of my students have thought it - that's why I took that photo a few years ago as a teaching aid: (or maybe another one - I have done it several times to show the point).
    And I posted this sample in portfolio here to link to when this is discussed - A picture is sometimes worth a thousand words . . .
    But here are some words from another person’s view; he maintains that the focus errors induced by the focus/recompose technique are usually small enough as to not worry that much - but being a technique type person I like to know what technique to use and when to use it.
    There is an optical analysis, here: http://www.bobatkins.com/photography/technical/focus_recompose.html
    WW
     
  22. " "Split screen focussing in the dark": Ah yes...the good old days. Solution: Set focus ring at hyperfocal distance, use your feet and forget about it."
    Unfortunately, most of the venues we worked in were far too crowded to allow the distance needed for this technique. But, since the distance was limited so was the focus range. It wasn't too difficult once we learned it, but I wouldn't have turned down single point AF with IR assist if it had been available.
     
  23. Arie Vandervelden , Nov 05, 2010; 12:42 p.m.
    I wish my camera had 5 autofocus points. 1 at each 1/3rds and one in the middle.
    quite agree!all cross type points
     

Share This Page