topic Re: Autofocus for Landscape in EOS DSLR & Mirrorless Cameras

Autofocus for Landscape

RossK — Tue, 18 Feb 2014 19:06:35 GMT

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?

Re: Autofocus for Landscape

hsbn — Tue, 18 Feb 2014 19:25:24 GMT

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

Re: Autofocus for Landscape

RossK — Tue, 18 Feb 2014 22:14:11 GMT

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

Re: Autofocus for Landscape

cicopo — Tue, 18 Feb 2014 22:27:13 GMT

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.

Re: Autofocus for Landscape

hsbn — Tue, 18 Feb 2014 23:05:14 GMT

@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

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.

Re: Autofocus for Landscape

RossK — Wed, 19 Feb 2014 00:09:44 GMT

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.

Re: Autofocus for Landscape

hsbn — Wed, 19 Feb 2014 00:46:49 GMT

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.

Re: Autofocus for Landscape

TCampbell — Wed, 19 Feb 2014 06:58:07 GMT

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.

Re: Autofocus for Landscape

heyjp — Thu, 20 Feb 2014 00:00:36 GMT

As hsbn and others have pointed out, the camera focuses on a single plane and some distance infront and behind that plane depending on the distance and your f/stop.

If you have "all points" on, it will pick one distance to focus and flash all the red squares that are in that same focal plane. So, it is not "bringing all the points into focus", rather, "here all all the points that are going to be in focus". And, the "all points" auto-focus does tend to find the closer object in the image... it assumes that anything close is the subject you are trying to focus on and lets the rest go to a nice background blur. (Or at f/11, it lets the rest go to a slight blur, which may be annoying).

I almost always use single point-of-focus so that I know EXACTLY and INTENTIONALLY what will be in focus. I don't let the camera decide. When shooting from the top of the mountain, I would either focus on the horizon or a distant object that brings the majority of the image into focus and slightly blurs the foreground... or conversely... focus on an object (tree, fence, animal) closer in the foreground, and let the background blur. As you already are doing, the smaller the aperture (f/8, f/11, f/16...) the more "around" your focus point is in-focus and the less blurred the out of range (too far, too close) objects are.

The beauty of your 6D is the instant feedback. Play with different focus points and f/stops and see what you get immediately. Or do it the lazy way, shoot 3 or 4 variations and pick the one you like over a glass of wine in your socks on the easy chair that night! 😉

Jim in Boulder

Re: Autofocus for Landscape

RossK — Thu, 20 Feb 2014 01:17:17 GMT

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)

Re: Autofocus for Landscape

MikeSowsun — Thu, 20 Feb 2014 13:42:33 GMT

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

Re: Autofocus for Landscape

TCampbell — Thu, 20 Feb 2014 15:41:14 GMT

@RossK wrote:
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)

Some lenses still have DoF marks but typically only on "prime" (non-zoom lenses) and not all primes have them. The depth of field changes for every foral length. That means the spread of the marks has to change if you change focal lengths. Push/pull zoom lenses had sweeping curves... most zooms use a rotating zoom ring... not the "push pull" type. But with a push-pull type, you would read the scale just at the edge where it meets the zoom ring. As you zoom in or out, the point where the ring meets that scale would change... causes the read of the depth of field marks to be narrower or wider.

As Mike pointed out, the marks you see on your 24-105 are the IR focus points. The amount that you can bend light by passing it through a prism depends on the wavelength of light. As light passes through each element, the shape of the element bends light by behaving like a kind of curved prism. IR will bend the least. This means the distance from the rear-most element to the point where the image comes to focus will actually be a little farther away. This also changes by focal length. When shooting iR, you have to do a final manual focus adjustment. Keep in mind the camera is going to focus based on visible wavelengths. After it focuses, you read whatever focus distance appears at the visible wavelength focus mark (the one in the middle) and then rotate the focus ring so that this same point is aligned with the IR focus scale appropriate for the focal length you happen to be using... and NOW you're focused for IR. Your camera can't shoot IR, but film cameras can -- and these EOS lenses can be used on Canon film cameras. It is possible to perform a permanent modification to the camera so that it can be used to shoot IR (you can also rent IR modified cameras) -- but this is a very invasive modification (the whole camera pretty much has to be disassembled from the back to access the IR filter in front of the sensor (and of course the sesnor has to be removed to get at it.) This would, of course, void your warranty (so it's better to "rent" a pre-modified IR body if you want to experiment with IR.)

Your assessment is correct in that by going to the "hyper-focal" distance for that f/22 example using a 50mm lens... we gained 6' of focus. There are times when you deliberately do not want max depth of field... more often you do, but not always.

You can let the camera focus automatically by picking something to focus on which you are confident will provide the focus coverage you want. In the f/22 example where the hyper-focal distance was at about 12'... as long as you focus on anything 13' away or farther, everything from your focus point and farther will be in acceptable focus... and of course MUCH of the space in front of that focus point will also be in acceptable focus.

But keep in mind that this example used f/22. If we used f/11 and focused at infinity, the near edge of the depth of field is about 23' away. At f/8 it's about 33' away. So now you're losing more of the foreground by using lower f-stop values.

There's one more nit-pick I'll point out (just for completeness... but please do not obsess over this). Light technically does not focus to a single point. It focuses to something called an "Airy disk" named for an astronomer who discovered this in the 1800's. This is due to the wave nature of light and how it behaves as it's pushed through smaller openings. Your camera sensor is covered with light sensitive points called "photo-sites". These are clustered in groups of 4 sites designed to be senstiive to green (mostly green), red, and blue wavelengths. When combined they create a "pixel". As you increase the f-stop value, you ALSO increase the size of the "Airy disk". At some point, this enlarges enough such that it is no longer possible for a single point of light to focus on just one "pixel" and it begins to overlap adjacent pixels. This technically "softens" the image -- an image which is focused as well as possible will not be perfectly sharp under close scrutiny. Of course you can only see this if you really zoom in to 100%. Most of the time you wont view or print an image that large.

An APS-C camera (your 6D is not APS-C... it's "full frame") is just starting to become slightly diffraction limited by f/8 and by f/11 it's fully diffraction limited. For this reason, some people do not like to shoot over f/11 IF they intend to blow up the image to 100% size. For a full frame camera (like your 6D) you gain about a stop... the camera is just starting to become diffraction limited at f/11 and by f/16 it has the full effect.

There's a mathematical way to calculate when a camera is diffrcation limited but it requires knowing the size of an individaul pixel -- so the info above is really just a general guideline for our cameras which are in the neihborhood of 18-22 megapixel sensors.) This is one of the reasons why I don't see the point of very high megapixel cameras (like the Nikon D800... that camera is diffraction limited at f/8).

Diffraction limits, by the way, are based on laws of physics which describe the behavior of light and these are based on an assumption of perfect optics. So it's not a question of just owning a good enough lens. If someone could produce a lens which was technically "perfect" (which would have some ludicrously high price tag), you would STILL have this problem.

So why bring this up? I only bring this up so you can file it away in the back of your head... if you plan to blow up an image really really big... don't use f/22 and you should probably avoid f/16 as well. But for most images you plan to share on sizes that can fit on a computer screen ... or print out to 8x10 size... go ahead and use any f-stop you want. If you don't *need* to push the depth of field to the max... consider backing off a bit to avoid the diffraction issue. Don't fall into the con of "pixel peeping" ... your images are not beautiful just because focus is sharp. There are SO MANY OTHER elements that go into making an image beautiful. Sometimes we deliberately do not want focus to be so tack sharp. True "macro" lenses tend to have _very_ high detail resolving capability. I find that if I shoot people with these lenses... I need to slightly "soften" their skin or it wont look good (so there is such a thing as "too sharp").

There is a general rule that when you focus at a point, 1/3rd of the focused area will be nearer than your focused point and 2/3rds will be behind it. This rule, isn't strictly true by the way (often times it's flat out wrong) -- but it's usually true enough that it's "safe" to follow the rule.

Re: Autofocus for Landscape

RossK — Sun, 23 Feb 2014 19:40:35 GMT

Heyjp,

Thanks for the very clear reply. Now I clearly understand that's what I've been doing wrong - picking all points whenever I had no particular point in mind, and thereby letting the camera pick its own point(s), which was one i probably didn't want. "All Points" seems a pretty useless concept.

The only thing with what you say about choosing a point on the horizon or the most distant object - which is what I was starting to think I should be doing - is that after reading some of the other replies, it appears that it's not necessary to pick something that far in order to still get that far away thing in focus. It seems like I could autofocus on an object in the middle ground and still that far object would be in focus. By aiming too far I might lose things in the middle ground, but if I aim for middle ground I will still get the far ground sharp, but also the middle will be sharp too.

ie If I just want the summits and ridges on the horizon in focus, yes autofocus on them, but if I want the summits and also the island and rivers that are perhaps only a quarter of the distance to those summits, it seems like I would be better off focusing on the island and getting the whole lot, islands and summits, sharp?

Thanks to everyone for their replies, they've certainly sorted a beginners' mistake out.