Autofocus for Landscape

[RossK]

I have only had my first DSLR camera, a Canon 6D, for about 6 months.  I got it mainly for landscapes and whenever I've taken big views from the top of a mountain or of a mountain range I've thought I don't want to focus on a particular point, so rather than choose one of the little red square autofocus points, I've chosen the option where you highlight them all, thinking this would get the whole picture as in focus as possible.

However I've just watched this autofocus lecture by a Canon employee expert  http://www.youtube.com/watch?v=iAx86nblZ2g  and he says when you choose all the red squares, not only does the camera just pick out ones it thinks you want to focus on, perhaps 2 or 3, but it tends to pick the nearest ones - completely the opposite of focusing on a whole landscape that could stretch several miles into the distance.  It may therefore pick out some boring rock protrusion and a tree in the near & mid-distance, have them nicely in focus at the expense of the rest.

  What therefore should I auto focus on ?    Am I best to pick out one red square on the most ditstant part of the view?

And if I'm doing this, should I just go to Manual Focus, which as you turn the dial marks distances up to 20ft, but beyond that turn it a little more and it has infinity - so should I just use MF & turn it as far as it will go and focus on infinity?

(A similar dilemma occurred at the weekend when I was at the end of a long table of kids at a birthday party.   To get them all in the best possible focus, who should I focus on, as picking all the autofocus points would, it seems, focus on a few only, and probably it would be those at the nearest end of the table?   And even more confusing what if I just wanted the 8 kids in the middle, so I don't want to focus on the whole room, but if I choose the red dot on one of the 8, he might be in focus along with the guy opposite who is the same distance away, but the 6 sitting just alongside them would not be in focus presumably?

[hsbn]

You are confused between Focus points and Depth Of Field. By turning on all the focus points, the camera will just use any point that it can get focus on, which in most case may not be a desirable point where you want to focus. In order to bring everything in focus, you will need to grasp the concept Depth Of Field which is controlled by the aperture of your lens.

You can learn more about DOF here: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

[RossK]

Hi "hsbn",

thanks for the reply.   So that my question wouldn't be too long I omitted to say that I'm using f11 or higher for the landscapes to get as much depth of field as I can, as I realise f4 isn't going to work.   But even so, if I'm going to autofocus I still need to select something - either one of the points or all the points - so what is best?    Or do landscape photographers ignore autofocus and manual focus on infinity?

However, in my long table scenario f11 may not be good either if I don't want the front end or the far end of the table in sharp focus and I only want them middle.   Then I have to go lower, even if not f4, so presumably the focus point I select will have more impact, yet I want to get all the middle of the table area sharp

[cicopo]

You simply can't choose all the points. The lens can only focus accurately on a plane parallel to the camera bodies sensor plane. Everything closer or farther is less in focus & depends on the f stop to either add it to the in focus depth of field or blur it because it's outside the usable DOF. Generally speaking F8 & f11 will usually have the best DOF without major issues.

[hsbn]

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@RossK wrote:

Hi "hsbn",

thanks for the reply.   So that my question wouldn't be too long I omitted to say that I'm using f11 or higher for the landscapes to get as much depth of field as I can, as I realise f4 isn't going to work.   But even so, if I'm going to autofocus I still need to select something - either one of the points or all the points - so what is best?    Or do landscape photographers ignore autofocus and manual focus on infinity?

However, in my long table scenario f11 may not be good either if I don't want the front end or the far end of the table in sharp focus and I only want them middle.   Then I have to go lower, even if not f4, so presumably the focus point I select will have more impact, yet I want to get all the middle of the table area sharp

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You can use auto focus, but just choose single point for more control instead of enable all the points. Also, I recommend you to look up BACK BUTTON FOCUS, which will take the focus out of your shutter release button. The 6D doesn't have a lot of auto focus point, so most of the time you'll be dealing with focus and then recompose. It's easier to do this using BACK BUTTON FOCUS. It takes some practice to get use to it but you will never go back.

About where to focus, it depends. You can focus at HYPERFOCAL distant, it's the best way but it takes practices and calculation. For example, your camera is set at F11, and your lens is set at 24mm, the hyperfocal distance is 5.65ft from where your camera is. If you focus anything at this point, everything from 2.8ft in front of where you focus and infinity behind what you focus will be in acceptable sharpness. I know this is a lot of calculation. So in practice, most people will recommend to focus about 1/3 way into your scenes (through the viewfinder). It will be close to the hyperfocal distant. If you have smartphone, you can download free app to do this calculation for you.

Here is an online DOF calculator and Hyperfocal distant calculator, so you can play around with it to explore more about the concept: http://www.dofmaster.com/dofjs.html.

Since, the hyperfocal or where you want to focus may not lie at one of your camera focus point, that's why people use manual focus so they don't have to move/recompose the camera. But a lot of people still use auto focus for landscapes.

[RossK]

Thanks again.  I think I get the idea.

If I pick an AF point too far away, on the mountains say, I'd not have any  rivers and forests further forward in sufficient sharpness.  So MF on infinity is not a good idea either.

As I'd probably use f11 and 24mm as per your example, and 5.6ft is pretty close to me, I'd pretty much have everything pretty sharp (beyond 5.6ft)

So it seems if I'm not fussed about anything in the first 10ft  I'm pretty safe that I've got everything all the way from there to as far as I can see back as sharp as I can?

I could really focus on any object that's not really close, with any red box, and everything further back behind it would be ok?  

I could always pick the nearest thing I'm interested in be it 100ft, 1000ft or 5 miles away, such as the river say, and know everything beyond is as good as I can get it?   Or as you suggested focus on an object a third of the way to the mountains, as long as my river is at are beyond this third of the way.

[hsbn]

Everything is just a general guide, you can get out and shoot more. You'll notice yourself. Like it says, DOF is range of acceptable sharpness and it is different for different people.

[TCampbell]

You may want to learn about "hyperfocal distance".

Assuming you want to maximize focus (in some cases there be artistic reasons why you do not want to maximize focus -- but let's assume that you do), for any given combination of focal length and f-stop, there is a focused distance which maximizes the depth of field.  

I'll post an image because it'll make the example easier to understand.

This is my old Canon AE-1.  I use this because the DoF marks on the lens are easy to read and also the aperture (f-stop) ring is on the lens.  

In this first example, the camera is simply focused to "infinity".  But look at the depth of field range to see what happens here.

Modern zoom lenses no longer include depth of field marks and even some primes no longer include them, but they make it easy to visualize the depth of field without using a calculator.  You can see that the f-stop on this lens is set to the "22" mark indicating f/22 (which provides the highest possible depth of field for this lens).    That means everything between the pair of "22" values on the depth of field range markers will be in acceptable focus.

But since this lens was focused to infinity, and nothing is "beyond" infinity, we really only get the depth of field on the "left" side of the range... giving us focus from about 12' to infinity.

Now look at the next example:

In this example, the camera is not focused to "infinity" per se... what I do is align the "infinity" mark, with the depth of field indicator which matches the f-stop on the "far" side (right side) of the depth of field scale.  Since I'm using f/22, the "infinity" mark is aligned to the "22" value on the right.

But notice what happens to the depth of field as a whole... rather than having focus from 12' to infinity (as we did in the first example), this provides depth of field from roughly 6' to infinity.  We've increased the total depth of field by doing this.

The lens' focus mark in the middle looks like it's probably roughly at 12'.

This "maximizes" the depth of field.  This lens is focused to the distance will provides the greatest possible focus range for the given focal length and f-stop.  This is distance is referred to as the "hyper focal distance".

In landscape photography you can use this technique to maximize focus.  It's one case where focusing manually is better than focusing automatically.  

But there is a problem... modern "zoom" lenses will not have depth of field marks at all.  And many primes may not have them.  So while the "concept" of a hyper-focal distance still exists... you may not simply be able to align the "infinity" mark with the depth of field range marker like I did in the example above.

Fortunately there are depth of field calculators (and some of them are free.)   The website dofmaster.com has a free online calculator (and a downloadable table).  They "sell" a calculator that you can download and install on a smart phone.  But it turns out there are also free downloadable depth of field calculators for smartphones (even the non-free apps are very inexpensive.)

To use them, you indicate the camera body type you are using.  A Canon 6D is a "full frame" camera.  So you can set the body type to indicate it's a full frame camera (24mm x 36mm sensor).  If the app does not list your 6D as a camera on the list (since the 6D may have been released after the app was written) just use any other "full frame" body... such as a 5D series body.  You'll also need to indicate the focal length you are using (they don't care which "lens" per se... just the focal length you plan to use.  e.g. if you have a 24-105mm f/4 zoom... but you plan to use a 35mm focal length, then just set 35mm on the app.  Lastly... tell it the f-stop you plan to use.  Note that higher f-stop values (which are smaller aperture openings) provide the greater depth of field (you can see that plainly if you look at the DoF scale in the photos above)... but these will require longer shutter exposure times.  A tripod is highly desireable.

When you enter all that info, the apps usually have a button to tell you what the "hyper focal" distance is for that focal lenvth, f-stop, and body combination.   

When I enter this info into my app for Canon 6D, 50mm focal length, and f/22, the app tells me that the hyper-focal distance for that combination is actually 12.2' and this will provide focus from 6.11' to infinity.

You would switch the lens to manual focus mode and manually adjust the focus ring to that distance.  Since it's hard to know *precisely* when you're focused to 12.2' and it'd be tragic to focus a bit too close and allow distance objects to go soft... you should err on the side of focusing just slightly farther than the proper hyper-focal distance to be conservative.

[RossK]

Thanks Tim, that's brilliant - all that info and that great explanation.   It would make it so easy if the lenses still had depth of field marks - how daft to get rid of them!

 My lens just has 3 red marks as you will see in this photo of it.    http://www.ephotozine.com/article/canon-ef-24-105mm-f-4l-is-usm-interchangeable-lens-review-14847  .....and they appear to relate to the different possible focal lengths

(I'm not sure what they're used for, but my guess is once the subject is in focus, whatever distance lines up with the focal length you're using, is the distance you are from the subject?)

Anyway, regarding your example of 50mm & f22, this Manual Focus with the hyperfocal distance apps sounds great, although it seems that as long as I have no special interest in the very near foreground  ie stuff less than 6.11' away, everything beyond that will be sharp.  So I just need to manually focus on an object at or beyond 12.2' to be sharp to infinity really.

  So just one question, if I was to still use AutoFocus instead, does this mean I just need to pick a red focus box that is over any object which is more than 12.2' away to get the far distance sharp (even if I have no special interest in the object itself). And probably as you said, if I don't have a tape measure, it's best to err on safety and make sure the object is definitely over 12.2' away.    (although if I want as much foreground as possible to be sharp, ie max depth of field, I should try not to aim for an object that's  too much more than 12.2' away.   Maybe aim to guess for 20ft)

[MikeSowsun]

There are no Depth Of Field markings on your 24-105 lens.  Those red markings are for shooting with InfraRed film.