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12/25/2008 09:31:43 AM · #1 |
I have some very good comments on a current challenge entry, and all but one remark on the lack of focus. I missed the mark with respect to focus, but I did realize that going in, so this is not a complaint. It is, instead, a request for information and assistance, and perhaps a side of education.
I normally let the camera do my focusing (eyes aren't as good as they used to be) but I do think that glass and reflections will throw the camera off a bit - I really should have tried manually focusing. Is that a correct assumption? I also think glass tends to soften focus if reflections are involved, but I did get a few that were somewhat more sharp so I'm not entirely sure on that part.
Low light/low contrast also seems to lend to soft focus - I don't ever seem to get anything particularly sharp when shooting in those conditions, in part due to slower shutter speeds, but even when I crank the ISO to get shutter speed up. My theory is the lack of contrast prevents the sensor from seeing "edges" well enough for things to be crisp, but again, is that a correct assumption?
Finally, I went out to shoot at the local skating rink and was trying to catch this one cute guy (with the camera) as he went flying by. I set the camera to the AI Servo autofocus mode, but no luck whatsoever. I suspect this mode works better with side to side movement than straight at the camera movement, but am not sure what WOULD work in that situation. Any suggestions?
Oh, and I almost always use the center mark exclusively for focus. I find that trying to use any of the further points, particularly right or left edges, makes it harder for the camera to focus, especially when using a wide lens.
Thanks for any advice or enlightenment! |
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12/25/2008 09:57:12 AM · #2 |
| If you recompose after focusing and are using a large aperture (small number) then the change in position could be changing the camera to subject distance and putting your subject OOF. |
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12/25/2008 10:27:51 AM · #3 |
| With glass and reflections often a polarising filter or changing the angle you are shooting at can help. |
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12/25/2008 01:56:12 PM · #4 |
I never trust autofocus when shooting through glass. In fact, in general I don't trust autofocus for any critical task. As far as your eyes being not that good, neither are mine, and I can still manually focus very well. You do need to be sure your diopter is set properly at the eyepiece. An easy way to do it is to turn the camera on with the lens cap ON, depress the shutter button halfway and hold it, and focus the LEDs at the bottom of the screen with the diopter wheel.
You are correct that reflections in glass can fool the autofocus. As an aside, not many folks are aware that when you focus on a reflection you are focusing at twice the distance of the reflected object from the glass. Using a self-portrait in a mirror as an example, if you stand six feet in front of a mirror and focus on your reflection, the focus point will be at 12 feet.
You are correct that the AI servo autofocus is not especially useful in head-on motion; it is designed for motion that is traversing the screen.
When shooting in low light, sharpness can be illusory. It is true that autofocus has a hard time finding a lock in a low contrast scene (you can't autofocus on blue sky, for example, or on a white wall), but it is also true that what we PERCEIVE as sharpness is essentially an artifact comprised of two components; contrast and detail. So, for example, if you take a nice, crisp shot and process it to perfection in Photoshop, then dupe the image and reduce the contrast way down on the duped copy, then toggle between them, the lower-contrast version will APPEAR to be less sharp, as a rule.
Finally, on the 20D and probably on the 30D as well, the center autofocus point is of a different type than the others, and it is indeed faster and more accurate in marginal conditions. When you go up to professional Canons, then all of the autofocus points are of the more precise variety, and comsequently the 1d variants are much faster and more precise in their focusing, especially in marginal conditions and with off-center focus points. I'm not sure if the 5Ds share this trait or not. In fact I'm not 100% sure what i said about the 1d's is correct (it may be MORE of the points are the good ones, but not all of them, I'm not sure..).
R.
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12/25/2008 03:13:03 PM · #5 |
Thank you kindly for the information - very helpful indeed! If the 5D had more of the "good" focus points, I would be sorely tempted to upgrade for that reason alone. I do like to shoot wide, and I like to shoot "sneaky" by aiming the camera just away from the subject but still keeping them in the frame. I find that the side focus points just don't "catch" very often. I think the solution may be to work with manual focus more - to get more used to using it. I have the diopter set to work with my eyes when I'm wearing my glasses, which seems to be OK. I'll run some tests with my eyes versus the camera to make sure I've got it pretty good. Could come in handy for whenever my LB gets here, yes?
Definitely will work on manual focus when shooting through glass. Should have thought of that last week, but 'tis a good lesson learned! |
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12/25/2008 04:20:44 PM · #6 |
An ancient trick is to use a smaller aperture, so as to increase the depth of field. Though in a rapid movement situation, if there isn't enough light you then run into another problem. Last but not least, where the movement is rapid but repeatable (as in the cute guy is passing by at relatively the same distance each lap), you can focus on one pass and take the picture the next.
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12/25/2008 05:06:37 PM · #7 |
The center focus point looks for both horizontal or vertical lines with which to focus on. Most of the other points are only horizontal OR vertical. One of the differences in the prosumers (30Ds, etc) vs. the expensive pro cameras is that the high end pro cameras have horz/vert in most of the focus points instead of only a few.
AI Servo has a hard time in lower light, such as indoors and night (skating rink) I use AI Servo almost exclusively as I am an action photog. When I do the horses in motion, I try to keep the center focus point on some sort of identifiable line, like that saddle or on the mane or the edge of the horse. It can get lost when there is not some sort of a line for it to find.
some focus point info
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12/25/2008 05:26:23 PM · #8 |
| Very nice link, Cristy - thanks!! And Alex, I will try to watch my aperture, but I do run into the no light problem a lot this time of year here. If the sun isn't out (and that's frequently), ISO starts at about 800 and I go up from there. May have to go back and try again with the cute guy, though.... worthwhile exercise, I think. :-) |
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