Focus and recompose

[paulo_fonseca1]

<p>Hi folks! I have a custom mode of my camera (Canon 7D) set for portraits. In this mode, I have the autofocus mode set to single point AF and then I use the rear joystick to select the sensor over the subject's eye *after* framing. As convenient as this method is, it is sometimes just not fast enough for my kids. I guess it would be faster to just use the central sensor, lock focus and then adjust framing. I understand that, if one employs the latter technique, then the focal plane may move a bit off the intended position, resulting in OOF shots. Well, of course this will depend of the DOF and hence on the aperture, focal length and subject distance, however I am not sure whether this is a critical problem in practice. I may do some informal tests, but because I am a bit lazy, I would like to know which approach do you guys, much more experienced photographers than I, use :-p The question is of course vague so feel free to comment on any scenario you wish, but I am more thinking about half-body or head and shoulders portraits. In my case, I guess the more problematic situation would be with my primes (28mm f/1.8, 50mm f/1.4) although I do not often shoot above (wider than) f/2.8, except in poor lighting conditions. Thanks for any input!</p>

[hjoseph7]

<p>"I understand that, if one employs the latter technique, then the focal plane may move a bit off the intended position, resulting in OOF shots."<br>

========================================<br>

Not sure what you mean by "the focal plane might move a bit", but you can put your camera in AI Servo mode which is the continuous AF for Canon Cameras. </p>

[Matt Laur]

<p>I routinely use focus-and-recompose, but I know what the boundaries are (through practice, more than math). Obviously, the more acute the angles involved, or the more shallow the DoF, the more this is going to be a problem. With wide-open fast long lenses, you can notice it with even a minor change to the sensor plane angle after you focus. I can't think of any easy way to describe the cases where you should or shouldn't do this, but some simple tests (with your camera, your lenses, your typical working distances and subject type) will very quickly show you the way. And it's good practice anyway!</p>

[Brad_]

>>> Thanks for any input!<P>

 

OK, <a href= "http://www.bobatkins.com/photography/technical/focus_recompose.html">here ya go...</a>

[paulo_fonseca1]

<blockquote>

<p>OK, <a rel="nofollow" href="http://www.bobatkins.com/photography/technical/focus_recompose.html" target="_blank">here ya go...</a></p>

</blockquote>

<p>Yeah, I hadn't done the math yet but that's what I suspected this morning while reviewing some of my latest shots and thinking about this point. Moreover, I was thinking that the focusing errors I'd eventually get because of this focus distance change could still be less of an issue compared to the number of shots I miss for not being able to select the most appropriate focus sensor fast enough. I guess it will much easier to "track" my son using the centre AF point... I will give it a try. Thank you!</p>

 

[lachaine]

<p>I've always focused and recomposed, and I've never had a noticeable problem with it in the real world. If your subject is very close, macro range almost, and your lens is wide open, then it might be an issue.</p>

<p>Before multi-point autofocus came along, focus and recompose is pretty much what you did. without giving it a name. I suspect the inherent inaccuracy of autofocus itself is more than what you lose by focusing with a centre point and recomposing.</p>

<p>These things mattered a lot more in the days of large format camera because of the longer focal lengths.</p>

[bobatkins]

<p>99% of the time you can focus and recompose without loosing significant sharpness.</p>

<p>In the article cited above - <a href="http://www.bobatkins.com/photography/technical/focus_recompose.html">http://www.bobatkins.com/photography/technical/focus_recompose.html</a> - I show mathematically that there are very few cases where it will make a real difference (the worst case is a very fast wideangle lens focused at a very short distance). </p>

<p>Normally you'll be well within the DOF, though of course "within the DOF" doesn't mean "as sharp as possible". On the other hand AF accuracy for most consumer cameras and lenses is "within the DOF" and even on fact lenses with high precision AF sensors it's "within 1/3 of the DOF". Under most circumstances, focus and recompose will induce less error than uncertainty in focus!</p>

<p>In practical tests I've not found any difference between focus and recompose and focusing off center, even under pretty extreme circumstances. In fact since the center AF zone often gets focus better then the outer AF zones, my standard technique is to focus and recompose.</p>

<p>

Of course the best way to find out for yourself is to take a few test shots and compare them.

[bebu_lamar]

<p>Well back in the old manual focus days, if you use a rangefinder camera you would use the RF area which is only in the center of the frame. Many people with SLR also used the split image for focusing and it too is only at the center of the frame. So focus and recompose method has always been used very often and yet only when AF became available people started to question the technique. </p>

 

[jcoffin]

<p>Most of the sites that advise against focusing and re-composing use some pretty unrealistic assumptions. For example, <a href="http://www.mhohner.de/essays/recompose.php?lg=e">Michael Hohner</a> (who I generally respect and admire) does the math for a 50 f/1.4.</p>

<p>The problem isn't that he does the math wrong, but that he does the wrong math. He works the numbers for a 50mm lens focused at 2 meters. He assumes that since a 50mm lens covers a roughly 40 degree angle of view that the rotation is 20 degrees. This shows a 12 cm shift in the focus point (around 5 inches, for those who think in those terms). At f/1.4, that puts your focus point well outside of the depth of field.</p>

<p>The problem with that is that it's assuming you want to focus at the extreme corner of the frame, and could select a sensor at that extreme corner if you wanted to. At least in my experience, wanting to focus on the extreme corner is fairly unusual, and cameras that let you select focus there even more so (essentially the only possibility would be using focus peaking on something like a recent NEX). A typical SLR (for example) won't have an autofocus point at the very corner for you to select even if you want to.</p>

<p>For a more reasonable computation, let's assume you're going to put your focus at one of the "rule of thirds" points. In this case, instead of 20 degrees, your rotation is ~7 degrees. That brings your focus shift from recomposing down to ~1.5 cm (just over half an inch). Unless you're shooting from a tripod, you can't count on that accuracy on distance anyway. Just shifting your foot for a steadier stance could easily make a much bigger difference.</p>

<p> </p>

[john_tran14]

<p>How about focus and shoot at the center, then reframing in the computer, sacrficing some pixels?</p>

[robert_k1]

<p>For the majority of my shots, I focus and recompose. On a film body, using manual focus and manual spot metering, it is simple. I point and center the area for exposure, and set the exposure with spot metering (with or without exposure compensation). Then I point and center the area for focus, and lock the focus. If necessary, I would point again for the final composition and release the shutter. Whether for a static subject like landscape, or a moving subject like on the street, this process works well for me.</p>

<p>But on a dslr and using AF (Nikon D200), I have to change the process somewhat. And it took some doing to figure out how. Reasons:</p>

<p>- I can't accurately focus manually via the viewfinder, and have to rely upon AF.</p>

<p>- Locking the focus is far from simple. I have to use only the center AF sensor, disable a lot of default AF settings, and use an AF-ON button instead of half press the shutter to lock the focus.</p>

<p>- In AF, focus and exposure meter are tied at the same area by default. Decoupling them and releasing the shutter even when out of focus takes some doing.</p>

[bebu_lamar]

<p>On a manual focus SLR (I only have film camera) I would determine the exposure first and set the shutter speed and aperture accordingly and then I compose and focus at the same time. I use a full screen matte ground glass with no focusing aid (split image, micro prism etc..) thus I can focus on any part of the screen not only the center part.</p>

[luke_kaven]

<p>In one mode of autofocus on the Nikon bodies, and perhaps on the Canon bodies as well, you can acquire focus through one of the focus points, and it will track the subject as you recompose. </p>

<p>Recently I've ended up using manual focus exclusively, because I find that the focus points exert too much influence on my choice of composition -- and that's the worst thing that can happen from my point of view.</p>