Focus and Recompose Exposed
Focus and recompose is a frequently used technique by many photographers. It is often used when you want to compose your subject in an area of the frame that doesn’t have an autofocus point nearby. On many cameras which have just a few focus points, focus and recompose is sometimes the only strategy that you will have at your disposal. When shooting in bright light with small apertures, this strategy can work just fine. However, when you shoot with wide apertures, focus and recompose can lead to poor results, even failure.
Failure? Why should focus and recompose ever fail? After all, if you focus on somebody that is 10' away and then recompose your shot so that their face isn’t in the center of your viewfinder, that person is still 10' away and should still be in focus, right? Well, not so fast. You see, when you focus on a point 10' away, the the area in focus is actually a plane that is parallel to the sensor in your camera. Here’s a very simplistic two dimensional illustration of this:
This illustration shows the field of view of a normal 50mm lens mounted on a full-frame camera, around 45º. It also shows the approximate depth of field in front of and behind the focus plane at around f/2.0. On a full-frame camera, the depth of field is around a foot and a half in depth at 10'. Keep in mind, however, that this depth of field is the calculated range that is considered “acceptable” focus. The calculations behind this involve circles of confusion and the difference between the size of your imaging surface and your final use size. If your final use size is a 5 X 7 print, the calculated figures are appropriate to use. However, if you want to make large prints, say 12 X 18 or larger, you’ll effectively have less depth of field of acceptable focus to work with. The line between acceptable and not acceptable also varies between people. Personally, I go for tack sharp when I can and only settle for “acceptable” when I don’t have a choice in the matter.
In any case, we don’t need to get too pedantic about the math to illustrate the gotcha with the focus and recompose technique. Here’s what happens when you rotate the camera to the right so that your subject appears on the left side of your frame:
The red lines illustrate the recomposed frame of view, focal plane, and depth of field. As you can see, by rotating the camera, the focal plane is being rotated as well. Your subject is still 10' away from you, but the rotation effectively pushes the plane of focus behind where you want it to be. The result is that after recomposition, the area that you focused directly on is now somewhere just on the edge of the depth of field. When you take the photograph, your subject won’t be tack sharp and may not even be acceptably sharp, depending on your criteria for “acceptable”.
Furthermore, this illustration assumes a simple rotation of the camera. When you recompose out in the real world, you’re likely to introduce a bit more error into the process unless you use a tripod. If you lean forward while you recompose, you’ll throw things even further out of focus. On the other hand, if you happen to lean back, maybe you’ll bring your subject right back into the plane of focus if you’re lucky. Relying on luck, however, isn’t usually a good strategy for these kinds of things.
What about changing the distance to your subject? At 5', half the distance and not an uncomfortable working distance with a 50mm lens, the depth of field is less than a half foot deep. On the other hand, at 20', the depth of field is about 6' deep. As a rule of thumb, the further away you are from your subject, the more depth of field you have as well. This gives you more leeway to work. The closer you are to your subject, the less depth of field, and the less leeway. At very close distances, your margin for error gets quite small indeed.
What about if you use a longer lens than 50mm? Well, at 100mm, the depth of field at 10' for a full frame camera is around 5". Using a 100mm lens at 10' isn’t uncommon when making portraits. Having 5" of acceptable focus isn’t a lot. On the other hand, with a longer lens, you are working with a narrower angle of view and aren't shifting the focus plane to as great a degree when you recompose. This helps you out to some degree. Still, shifting things around a bit can still lead to poor results if you’re aiming for tack sharp eyes in that portrait.
This, in a nutshell, is what you have to be careful of when you use the focus and recompose technique. If you do need to focus and recompose, you should try to alleviate any potential issues by stopping down your aperture. For example, the depth of field in our setup above at f/8 would be a bit over 6'. Focus and recompose in this case would most likely produce decent results. The issue can also be alleviated by shooting with a wider lens. For example, using a 35mm lens in the above setup at f/2.0, the resulting depth of field would be around 3 feet. Of course, then you have a relatively wider angle of view and a different photograph entirely.
So, what should you do? Well, if you are using lens and aperture combinations that give you a lot of depth of field to work with, focus and recompose can work out fairly well. On the other hand, if you’re working with wide apertures or long lenses, then you’ll need to be more careful. In fact, you may want to practice focusing manually or at least overriding the focus decisions made by the camera to dial in a bit of compensation. This can be tough, however, as modern autofocus lenses don’t have the most forgiving of focus mechanisms for manual use. The best bet for most people, however is to manually select the focus point that is closest to your subject if you can and avoid recomposing altogether.
Finally, the desire to avoid recomposing is why photographers who own a camera with 45 or 51 focus points really dig their systems. With so many points, it’s more likely that they can manually select just the right one to use to make a photograph composed just the way they want it without recomposing. Then when their subject’s expression changes—say when something emotional happens—they can snap off the next picture immediately using the same focus point placed in the same place.
Related Posts:
Related Links:
- DOF Master has an online depth of field calculator for you to play around with the numbers.
- I’ve been playing around a lot with PhotoCalc, an iPhone application that lets you work with these numbers out in the field.
40 Comments
Thanks for informative post! Is there a published chart or easy way to remember what the depth of field is for given aperture, f-stop, lens?
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There are several charts and calculators available on the web, but lately I've been playing a lot with PhotoCalc. It could be a bit easier to use, but it does allow you to have the information with you wherever you go. http://www.adairsystems.com/photocalc/
Also, for an online calculator, check out http://www.dofmaster.com/dofjs.html and http://www.bobatkins.com/photography/technical/dofcalc.html
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Excellent illustration of something that I've been struggling with lately. That little shift in distance has messed me up way too often.
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Of course, if you've got all that time to recompose, you've probably got time to focus manually.
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One thing you're missing is what the angle of the "recompose" is. It sounds like this fact is making you believe the problem is worse for telephoto lenses focused close instead of wide-angle lenses, when the opposite is true (forgive me if I misread you above).
Sure, at 200mm and 10' you have a very thin depth of field, but the angle you shift by (and, thus, the variance in the subject to the plane of focus after the shift) is much smaller than with the 100mm or 50mm lenses.
Mathematically speaking, the ratio between the distance at the center of the frame and the distance after recomposing to the edge of the frame is the cosine of half the angle of view. Obviously, this is largest with a wide angle lens (cos(60 deg) == 0.5, so if you are well-focused on an object 10' away from you, then shift it to the edge of your 120-deg FOV, it will be focused similarly to an object 5' in front of you at center).
At the same time, the "front focus" of many lenses is better than the "back focus", so having the subject in front of the plane of focus isn't as bad as having it in back of the plane of focus.
For your 200mm lens on a full frame sensor camera, the angle of view is about 10.3 degrees, so the max shift in focus-recompose (meaning, if you put the subject right on the edge of the frame) is 5.15 degrees, which leads to an offset of 0.996. If you are focused at 10', that means the recomposed subject will act as though it were 9' 11.5" away instead of 10.0". At f2.8 and with a 0.03mm circle of confusion, that's pretty much just inside the DOF for the scenario.
I used the tools at http://www.tawbaware.com/maxlyons/calc.htm to calculate angle of view and depth of field for this.
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I've been a rangefinder and Leica shooter for a decade. Anyone that uses autofocus should really think about avoiding it. Learn to focus, and intuitively flick the focus ring based on recomposing, the new plane of focus, or as your subject is moving. Rangefinder photographers can't even see through the lens! Its all about skill, and seeing things differently than what you are taught in Photography 101.
Lastly, unless you are a journalist, focus is highly overrated. Some of the best and most iconic photographs ever shot were out of focus. In fact, taking the time to NOT carefully focus will tune you in on what really makes a good shot, and good art. Its about emotion and showing the world a different way to see things.
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You're missing the point. When you focus and recompose, the whole idea is to focus on something that is the same distance from the camera as the thing you want to be in focus in the final shot. In your example you're focusing on somethign a bit closer than what you want to be in focus in the final shot. What did you expect to happen? In this case, take a half step forward, focus, and step back to shoot.
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Tom, thanks for seeing that. I wasn't trying to go down the road you caught me on, but in the process of simplifying the explanation, I definitely said the wrong thing there. I've fixed that up now and am going to add a bit more to it. For the most part, even shooting long, I never end up with the focus I want when doing a recompose. Of course, I'm always after tack sharp if I can get it. :)
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Joey, manually focusing modern SLR lenses which are designed for AF is nowhere the same experience as focusing a Leica rangefinder. The amount of control that Leica gives you on that front is awesome. I'd love to have an M8, and hope to have a follow-on to that model at some point in the future when it's gone full frame and they've worked out some of the other kinks.
As far as whether focus as a concept is overrated, that's a judgement call. I shoot some things OOF because they deserve it. Other things deserve to be sharp. At least my own opinion. But, the primary thing is to know how your tools operate so that you can accomplish what you want to accomplish in a particular situation.
I did add a bit about manual focus to the end of the post, in part thanks to your comment. :)
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Mark, I get the point. The point I'm making is that when you say "focus on something that's the same distance", people don't tend to think about plane of focus. At least not until they realize that you have to think in terms of plane of focus.
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I always thought the focal plane was parallel to the lens, not the sensor or film plane, thus making the focal plane spherical. But after reading this and doing a little research I learned that lenses are specifically constructed to flatten the focal plane.
You learn something new every day.
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Hey James, Thank you for explaining this. It was a lot of info I have never really thought about. I will be focusing more carefully from now on. I'm glad to see you posting more now too.
Thanks for all the great information.
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Yes, in theory, focus and recompose can induce backfocusing because you're shifting the plane of focus.
But there are a number of drawbacks to the alternative of using the closest focus point. First, on most Canon's (including my 30D and the upcoming 5D MKII), the center focus point is far more accurate than any of the side points. On my 30D and Sigma 30mm f/1.4, I can usually hit the focus with the center point in marginal lighting (say 1/30s, f/1.4, ISO400), but the side points won't focus at all. It's hard to say whether in actual practice the error from focus and recompose is greater than the error from using a less accurate focus point.
Secondly, at least for the sort of street photography that I like to do, you're constantly switching your composition. Sometimes you'll want the subject more to the left (or bottom if a vertical orientation) or the right (or top). How do you switch focus points? You can either enable all of the focus points, which can be frustrating when it doesn't focus on the thing you want (in particular, if you're deliberately focusing on something further away and you want a closer object to be OOF.) Or you can be constantly switching which focus point to use, which again can cause you to fumble around as you miss that "decisive moment." Using the center point only and assigning the focus to the * button is by far the fastest way for me to get the point I want in focus sharp.
The idea of manual focusing any of Canon's 1.6x cameras with narrow DOF is a joke.
In other words, focus and recompose for me is the best, but less than ideal solution to the quandary of having sucky side AF points.
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Great explanation, thanks JDD. While I shoot with D3 and D700 (usually dialled down from 51 to 11 points) these days I still use F&R quite often, but got into trouble with my 35/f2 regulary.
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On my list of things to buy: Canon's Ef-s screen (for the 50D I'll be picking up soon), and Canon's Ee-s screen (for my current 5D). Reason? Exactly that outlined above: to aid in manually focusing in difficult conditions.
If there are other options that will make manual focusing a Canon DSLR even easier, I'm all ears.
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Nikon's D90, D300, D700, D3 all have a 3D tracking auto focus system that helps you in this case. It keeps your object in perfect focus when you're moving the camera. It's perfect for recomposing your shots.
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I find myself in wayne's position. I do shoot fast lenses and am finding that I am trying to investigate more with selecting focus points (D3 so 54 to play with but gathered in the centre). Problem is that in low light the outer ones are not as sensitive and are also not cross sensors so a lot of time focus and recompose is more accurate. Also I do a lot of v.low light work that requires AF assist (so needs focus and recompose as only some points support AF assist. End of the day, I am still stuck with focus and recompose other than for occasions when I have enough light. Trying to change habit for these instances (especially after a life of manual focus) is a challenge for me!
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My experience is that having lots of focus points (say 51 vs. 11 in the two D3 selection modes) doesn't help that much for this particular aspect of photography. Instead, I'd like to see the focus points spread over a larger fraction of the field of view (or at least an option to enable some focus points there). A small recomposition angle doesn't noticeably affect sharpness, but the lack of focusing points in the outer half of the field, forces a fairly large recomposition angle when the subject is near an edge of the image.
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It's funny, because I've always thought of a multi-point autofocus system for anything other than point and shoot to be kind of a joke. I've never met a multi-point system that had any clue which point I was hoping to focus with. I'll probably always focus and recompose with the center point, or manual focus, at least until they're able to make an autofocus system that can read your mind.
The big exception to this would be the AI Servo focus or something similar when doing action/sports shots. I've never found that to actually work worth anything, either, though.
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Interesting, I would have thought that the autofocus used a "radius" of focus rather than a plane, which would not introduce the problem in the first place. (It would, however, introduce the converse problem, depending on your expectation, in which two objects at the same distance from the plane on which you're standing are not similarly sharp).
Nice tip and great explanation, though!
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Regarding selecting focus points, I also find it too time-consuming to select the "right" focus point over what I want to have in focus (if indeed there is one, with the 9 points on my 40D), compared to putting the object dead center then flicking it back to "ideal" composition. I can't even imagine turning the dial until the right one of 45 points lights up! Are there better control schemes for such cameras?
The "ideal" system would allow me to directly set any focus point (I think it was Sony video cameras that had a touch screen to select the focus point where you just tapped on what you wanted in focus and it did the right thing), and would have high-quality cross-type focus points spread throughout the frame.
But, in the "real world" we actually live in, it's good to know the trade-offs we are making with focus/recompose. I'm going to make a more concerted effort to use the edge focus points, to see if I can get the interface working well enough with my fingers that it's more viable for me.
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Only macro lenses are corrected to have a focus plane that is truly a plane. The fact of the matter is, most lens designers (and Leica has always talked about this as being their philosophy) know that by allowing some curvature of field, they are able to correct for more important aberrations. One company, Minolta, even made a lens that allowed the user to vary the curvature of field. I believe it was a 24mm lens.
Anyway, the point being that since the plane of focus curves for most non-macro lenses, that it's going to be hard to figure out how to adjust for it, unless you know the amount of curvature. I suppose the best way is to conduct tests and focus manually and see how it adds up.
As for the comment about focus being highly overrated, that's bunk. There is a very narrow point of optimum focus, and no matter how much one stops down, that is the point where focus is best (tack sharp). And depth of field is calculated from there. When the out of focus circle of confusion reaches a certain point (for Leica it was 1/30 mm) that is determined to be out of focus.
Another interesting point is that depth of field is 1/3 between that point and the camera and 2/3 from that point away from the camera. Basic optics. So you have to take into consideration that the ideal point of focus is 1/3 the distance from the closes point of acceptable depth of field to the furthest point.
Anyway, the basic principle of the article is sound, but much more complex than one would think after reading it. Manual focus is often the fastest way to operate in low light, if the viewfinder is bright and contrasty enough to allow you to focus accurately.
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I have to try it but the D200 has a mode where, in continuous focus mode, it tracks your subject across the different focus points as it moves in the viewfinder. It's great when you're trying to shoot jet fighters with a long focal since they don't always want to stay centred, but it could also work in a focus and recompose situation.
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Pentax AF lenses have a nice auto-clutch mechanism where you can use the autofocus then immediately adjust it manually. No need to flick a switch or anything, it just works.
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Wayne, indeed. The side focus points on the 30D and both the original 5D as well as the new 5D Mark II are pretty weak. It's still a puzzle to me why Canon left these points so crippled, and is one of the reasons for why I wrote the Source of the 5D Mark II AF Angst post the other day.
As far as switching composition and moving focus point, I do so on all of my Canons with the joystick on the back. On the 1D, 5D, and (I think) the 30D, you have to enable this with a custom function. Once you set this up, it's a heck of a lot easier to select the focus point you want versus spinning the dials.
I definitely agree with you that focusing manually through a 1.6x crop viewfinder is really really tough. The 5D is better, but I do miss my old 1980's era film cameras for manual focusing.
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Jim, one of the things that Canon did right with the focus system in the 1D Mark III was to put some high precision cross-points out around the edges of the focus area. The focus system on the D3/D700 is quite nice indeed (and kudos to Nikon to pushing it down to the D300 and D700), but the one thing that one could definitely ask for is more cross-type sensors out on the edges.
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David, I agree 100% with you on that front. I'd love to see focus points as far out onto the edges as possible. I was talking with Erik Barzeski about this on chat the other day and he suggested even just a 5 point sensor with focus points on the thirds points would be nice. See http://nslog.com/2008/09/29/viewfinders_and_autofocus_points
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Joey, having a multi-point AF system doesn't mean you have to always let the camera choose what point to use. In fact, I never let the camera choose. I'm always in control of what point to use.
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Tom, try setting the custom function that enables the joystick on the back of your camera to be used to select focus points. On my 5D, it's Custom Function 13 and I set it to "1: Multi-controller direct". I don't have a 40D, but a quick search on Google brings up results that indicate it's Custom Function III-3.
We had the ideal system back on the Canon EOS 3 film camera. Eye control. It didn't work for some people, but it worked great for me. Just look through the point that you want to use and hit the button. Magic. That rocked.
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Also, Canon EF lenses with USM motors and Nikon lenses with SilentWave motors let you immediately adjust focus anytime without flipping switches. Quite nice indeed.
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I wonder if a future camera will be able to record the angle the camera is moved and then compensate. If they can build something like face/smile detection technology into a camera they should be able to lock on to the area of the focus point and track its new position relative to the sensor.
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That's great the way you write a whole article specifically replying to my comment on your last, but without ever allowing that comment, which sparked this response, to be published! Was it embarrassing to you or something? I bet I was not the only one you censored. It must be nice for you to squelch serious dissent and yet take advantage of your soapbox to reply to what you just erased from existence. Kind of like taking a double-move in Chess. Perhaps your ego is simply too brittle to actually allow your opponent a turn at the board?
Anyway, I would respond in detail to your arguments above (summary: the recomposition issue you are warning about is almost non-existent in practice), but what would be the point? Nobody's going to read it. You obviously don't understand the value of a true conversation with your readers.
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DBL, in point of fact, your comment was published. It wasn't on the last post (Source of the 5D Mark II AF Angst), but instead several posts back on Canon 5D Mark II Initial Thoughts. Maybe you didn't see the first post on the sidebar because it had already faded off.
In any case, here's the link to your comment.
You'll see that right after your comment is a reply from me yesterday. It was last night after your comment, and many other comments about focus and recompose, that I wrote this piece.I found your tone on the original comment a tad insulting, but approved its posting anyway. I find the tone on this comment to be quite insulting as you are quite obviously jumping to conclusions and wanting to get mad about it. I'm letting it go through because I want to publicly account for the accusations you level at me.
I do have a comment policy. It is linked to at the bottom of each page. As it is my site and I am the publisher, I will moderate where I feel it is necessary. However, you were not censored in this case, nor are you likely to be unless things get really ugly.
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Regarding DBL's post, I often shoot close subjects wide open with a normal or wide angle lens (24mm 1.8) and I can tell you this IS a problem in practice.
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Joystick control over the focus point ... smacks forehead ... If only Canon would compile a list of such features into book form and include it with each camera sold!
Thanks for the tip. I think I'm due for a manual reading session now :)
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Joysticks?? Focus points? Selection modes? "I wonder if a future camera will be able to record the angle the camera is moved and then compensate"
You guys are literally writing the recipe for how to create the most boring photographs possible.
Not to sound snobby, but this is exactly why I left the SLR (and DSLR) world for rangefinders. You don't need a Leica M8 - get a cheap Bessa and shoot film where you don't have a chance to even tell if you got your focus right until you get film back from the lab! I use Leica's 50mm f1.0 lens where focus on a pupil will throw eyelashes soft and I shoot street photography with it while walking with my subject. Its possible.
It sounds to me a lot of you modern AF shooters are slaves to your tool. You shouldn't have to base how you photograph on what options a computer program gives you.
Eric, you misinterpreted my point completely. I'm not speaking of scientific focus. I'm speaking about being an artist. I can show you hundreds of the world's most iconic, memorable, political, and emotional photographs where the subject is out of focus, grainy, or not "properly" exposed. Yes, you must learn the fundamentals, but unless you want to bore your audience to death, you must learn to shoot outside the box. Thats what the true art of creating memorable images is about. Being a true photographer, not a computer operator.
I say this because the world needs more great artists..
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I always wondered why focus and recompose gave varying results when working with wide apertures - now I know - thank you.
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I'm stumbling onto this article six months after the fact, as I'm looking at how others explain both the logic and the mechanics of the various focusing modes on the D300 and its siblings. James, I think you've done pretty well, but after having read various posts from all sorts of sources on this topic for about a year, I had gotten to about three-quarter way through your initial posting when I couldn't help expecting some form of "attack", and true enough, reading on, there was DBL's, which sort of completes the typical scenario!!!
To get back to the plane-of-focus issue, it would help if manufacturers published (as part of the kit supplied with the lens) a diagram indicating the shape of the plane at various distances, say, at minimum focus, 2metres, 5m, 10m because there's so much variation in the design philosophy between the brands. It's "common wisdom" that macro lenses are geared to a flat focal plane ... but at what distance? Where does the presumption of "flatness" break down?
With DSLRs, we are horribly handicapped with pathetic viewfinders, so having in the back of our minds as we're shooting the "shape" of the plane of focus, as we quickly predict DOF will help in estimating/visualizing.
Being able to get focus tack sharp IS an issue; whether or not we finally choose to use it is another matter altogether. I'd just like to have it as a stable, ready-to-use option when I pay so much for lenses and bodies!
We should also note that with today's zooms, in most cases there is a clear and significant shift in focal length as we zoom - giving a whole new meaning is given to "focus and recompose"!!
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hey.. that is really good info and i was wondering what the hell is happening when recompose the shot on my DSLR? now i know. as according to canon 450D book, it should stay focus under rule of third, but the practicality is different.
once again, thank you!
cheers
Faraz
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