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Centre AF point

willo500willo500 Member
edited July 2013 in portraits & people
Hi

I'm new here so first off, hello to everyone.

My question is this. I am mainly interested in portraiture and I often try to use shallow DoF to help add interest to my pictures. Typically I will use the centre focus point on my camera to focus on the appropriate area, usually an eye, then move the camera slightly to re-compose the shot as I prefer.

Quite often I like the eye in focus to be at the side of the picture to again add a little to the effect of the shot.

I have been with some friends tonight who told me that I should not do this as it leads to poor focus and that my results would be better by shifting the AF point as appropriate. I haven't compared the two methods and I should really have a good look at my photographs this week to see if there is any indication that what they say might be right.

I recall hearing many photographers saying at the time of the D800 left AF issue that they weren't bothered as they used the centre point AF technique all the time so it isn't just me.

I'd be interested to learn if this ""poor focus" thing is a theoretical issue or a reality. I did discuss this previously in another forum but the result was inconclusive. IIRC only very close - up product type shooters worried whereas wedding and portrait guys couldn't care less. Anyway, I forgot all about it until tonight.

Thank you so much for reading this and I'd love to hear your views

Comments

  • TrevTrev Moderator
    Called Focus Lock/Recompose is a great way for utilising the central point, however, on a shallow depth of field (ie f2.8 and larger aperture) this can lead to the focus to be out.

    This is evident in a large movement of the camera's sensor plane from initial focus point, then moving camera which will then alter the distance initially selected.

    (Hold up a flat ruler face on to you, then twist one edge away or towards you and that's what I mean by sensor plane being aligned to subject)

    If using a shallow depth field, it would ideally be better to select a point closer to the final composition, but you are then at the mercy of the quality of the camera/lens in those focusing points and the very outer ones may result in a softer image.

    I normally stick to center and the next outer ring but I am also careful with the camera orientation.

    eg: if you have a subject and you want to shoot in portrait [vertical] mode, but, you wanted the subject to be small in the frame and lots of dramatic sky say, then you zoom to the composition, focus with central point then dramatically move the composition to where you want it and tilt the camera up really high to include the sky, you will change the focus you initially did.

    By moving the subject to the bottom, then pushing the bottom of the camera [in vertical mode] in towards the subject and the top of the camera back towards you, the focus is now really out, so compose, and select a focus point near the bottom of the camera [vertical mode] and only a slight shift should result in a better image.

    Pick up your camera, put it in vertical position and push bottom towards subject, top back to you, and you will see what I mean, the sensor is out of plane of focus to the initial point you selected.

    The method you are using currently is fine on a lot of images if aperture f8 or shallower, it all depends on the subject size in frame, aperture, camera orientation (more noticeable in portrait mode then landscape), how much shift from central you then position the subject, etc.

    You simply need to test it. Preferably on a tripod, manual focus central point, then move camera to where you want final composition, shoot and check, trying all sorts of compositions, orientations, aperture settings.
  • MikeZMikeZ Member
    edited July 2013
    Here is an example of moving the focus point after orienting the camera to portrait. Her eye closest to the camera was actually on an outer focus point for my d700 with my 24-70 @ 2.8

    image
  • jhilgersjhilgers Member
    edited July 2013
    I agree with Trev on the use of the center AF point. I use back-button focus on my 1D-X and 5D Mark III a lot. I will attempt to use one of the outer focus points to start with first though. Using the center AF point really works the best when I need to nail focus in a low light situation.

    On a side note, I have noted that Canon has placed the 61 focus points (on the 5D Mark III and 1D-X) in a manner that makes complete sense when you compare their locations to the "rule of thirds" grid. I turn the grid display on to show up in my LCD screen which help helps me check the composition of all my shots after I take them.

    I found that if I keep my subjects face near the corner edge focal point in my display, my subject almost falls perfectly within the area of my frame that follows the "rule of thirds". In essence, I really do not need to recompose a whole lot at all if I simply follow these focus points closely.

    In my opinion this is another subtle "way-to-go!" point for Canon with their newest cameras. I feel dumb for not noticing this earlier. It "almost" makes common composition errors completely avoidable if you follow the points closely.
  • This is a calculation I made a while back when this discussion came up elsewhere. I'm not sure how valid it is and if someone with more knowledge could check my reasoning and conclusions I'd be grateful.

    Bear in mind, I'm not very scientific and these are a bit on the approximate side.

    I used 50mm and 85mm @ 86" in front of the subject. The distance between the point covered by the left-most AF point and the centre one were as follows:

    At 86"
    50mm 14.5"
    85mm 9.5"

    So for the 50mm if you make a triangle of 86" and 14.5" you get a hypotenuse of 87.21" a difference of only about 1.2"

    the DOF for 50mm at this range is even at f1.4 around 83-89" so plenty to play with.

    If you do the same for the 85mm and use the 85" and 9.5" you get 0.52"

    At 85mm DOF is around 2" so a bit less but sill as the difference is only about 0.5" it hardly seems to matter.

    Even for most close up shots I'd say it wouldn't matter because the distance covered by the focus points would be a lot smaller too.

    I appreciate for macro things might well change but certainly at portrait distances it seems fine. For landscape it wouldn't matter.

    Does this seem a reasonable conclusion?

    Thank you for reading.
  • TrevTrev Moderator

    Wow! that made my head hurt Willo :)

    Math and Physics not my forte.

    Me, I just shoot and check, nothing more than that.
  • LOL Trev sorry. I got a bit carried away maybe :-)

    My intention was to show that the DoF was sufficient to allow for center point focus and recompose for any portrait scenario even with large apertures of around f1.4 or f1.8.

    Not sure if there's a flaw in my reasoning

    Thanks
  • willo500 I am often a focus & recompose shooter too. The recomposing can be done with care and even slightly adjusting position to try to keep the distance the same. Sometimes, I think the higher accuracy of the center point along with slight adjustment in recomposing can be better than the outermost points and not recomposing. Especially in poor light.

    That said, one "flaw" in your reasoning might be that just because something is in the depth of field zone doesn't mean it is in perfect focus. DOF involves a subjective "acceptable sharpness" concept when a print of a certain size is viewed at a specific distance. The best focus is still always at the exact distance the lens is focused at, though. If you miss the subject's eye by 1" at f/1.4, while it might technically be considered within the zone of DOF and "acceptably sharp," it might have that certain fuzzyness to it that makes observant people think you missed focus by a little bit. This is especially true if you get into pixel peeping, or cropping the shot close in.
  • willo500willo500 Member
    edited July 2013
    Hi Nikonguy. I take your well made point about the DoF zone. What you say makes sense and must be true.

    OTOH when I check my outer AF points I do often notice a lack of accuracy and the point you make about the center point being more accurate is very valid too. I am still inclined to believe that this accuracy would be most important for portrait work. Or maybe it doesn't matter.

    I can't help feeling that photographers have used focus and recompose for some years ever since AF first appeared and so it is probably good enough for me. I'd personally love lenses that focus well manually along with focus screens on DSLRs which matched the one on my old Olympus OM1 of years ago.

  • I have always focus locked and recomposed, but have found at shallow depth of field there is a good chance my subject will be out of focus, during a wedding that's not good. I know either use a larger depth of field or another focus point unless I know I can nail the shot. One trick I have learnt from Neil is to not just shift the camera but your head and body to keep the focus plane the same.
  • Oh and anything less than 2.8 is critical!
  • this topic has previosly been discussed in this thread neilvn.com/forum/discussion/comment/4101#Comment_4101a
  • Thanks for your views people. I appreciate them. I will check out the link on the previous discussion, Naftoli. I hadn't noticed it when I did a quick search. Thanks
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