View Full Version : Focal length + apeture + focus point _ it dosent add up

26-10-2009, 8:04pm
I am told the wider the lens (10mm cf 100mm, for example) the deeper the depth of feild in the shot so I look at a landscape (say your standard seascape (land/foreground/ocean.skyline) and tell myself to go for a wide focal length lens and small apeture to ensure depth of feild and depth of focus too... right .. wrong.. well at least for me.

Assuming I have the notion right, that a wide angle lens (say a 10-22mm CF a 70-200mm) is going to give deeper depth of feild no matter what apeture you set on the camera, I go with the wide angle lens for landscapes. Then I tell myself okay set the Apeture at F 8 or F 11 or even F16 to promote end to end sharpness in the image, from foreground to background.. makes sense... so far so good right ?

Then I ask myself about focus points and thats where I get into confusion.

IF I use AUTOFOCUS on the Canon 50D the camera punches out 9 focus points ...BUT I am told in AUTOFOCUS mode the camera will reach for the NEAREST to the CAMERA focus point and focus on that.....leaving anything beyond that closest to the camera focus point NOT sharp, not focussed and crappy at the back.

So, I tell myself, use a manual focus point and either pick a focus point of the nine (the one in the middle mostly or if you want to be really picky a focus point covering your subject/centre of interest), but for landscapes there is no centre of interest, no specific individual one subject.... so then what ?

Am I confusing depth of feild with depth of focus. ..surely not.. thats the point of the DOF, to control the sharpness of the image and blur (deliberately) sections of the image (eg birding or portraits or in this case, lizards).

Ah someone says, for landscapes use a wide angle lens, camera apeture at F.8 - F16 and use Manual focus, setting the lens at infinity..hey presto, sharp back to front landscapes... really ? Is THAT it ?

26-10-2009, 8:19pm
David, check out THIS (http://www.ausphotography.net.au/forum/showthread.php?t=34246&highlight=hyperfocal) thread.

26-10-2009, 8:26pm
David, check out THIS (http://www.ausphotography.net.au/forum/showthread.php?t=34246&highlight=hyperfocal) thread.

Thanks Tom, I will go to it right now

26-10-2009, 11:45pm
I'll take a stab at this.
Your premise about shorter/wider focal length having greater dof with respect to longer focal lengths is correct. But u need to use this information in context of the scene. Not every landscape shot has a large range of "depth" and you can just as easily make lanscape shots with a 200mm lens vs 10mm lens.
Take for example a landscape taken with a 10mm lens set at f8. In this scene there is a rock in the foreground and detail all the way to infinty where there are mountains. So where do u focus?? Depending on how close that rock is, if u focus on infinity even with the large dof the rock may be out of focus. If u focus on the rock, your results tend to improve because u generally get more dof behind your point of focus than in front. But bear in mind those other things that effect dof, namely your focussing distance in this case. If the rock is very close to the lens then even with the large dof of the short focal length lens and a small aperture isn't enough to get sharpness from near to infinity.
Now consider a landscape shot of the mountain ranges in the distance with a telephoto lens. The closest object in this scene is the near mountains some 5km away and the furthest being the far mountains some 20km away. The first thing u'll notice is the scene appears compressed as if the mountains are stacked against one another. From a focus point of view, whether u focus on the near mountains or the far mountains, it is both essentially at infinity and I'll bet u that even with a rather wide aperture on the telephoto lens, u'll get good sharpness from near to far.
You can't think of dof as just focal length or just aperture etc. It is all interelated and scene specific. You choose your composition, focal length, aperture and point of focus to create what you want to achieve and not apply a single set of settings for everything cos u'll come unstuck sooner or later.

27-10-2009, 1:39am
Thanks Swifty.

I have a lot to learn --

27-10-2009, 10:02am
another thing you need to understand too David:

DOF (which is both depth of focus and depth of field!!.. same thing) is an illusion!
That is there is no exact definition of where the focus starts and ends..

it should really be called depth of acceptable focus. There's only really ever one plane of sharp focus, and any area fore/aft of that line will be subject to slight degradation of sharpness by varying degrees.

If you're viewing your images at 100% pixel view on your screen, to determine the depthness (:p) of the focus you achieved in your shots, you may be disappointed if you wanted an almost infinite amount of focus. the horizon may look acceptably good, but not always perfectly sharp if you focus near, even with the 10mm.

The other point to make is USM! Can have a significant impact on the sharpness of the background.. probably more so than on the point of focus.

From memory, diffraction will set in quite early on a 50D.. say at at least f/11.. so maybe f/8(or f/9 or f/10) may be a better bet.
Do a few standard boring 'brick wall' tests on something to determine where the lens operates at it's best, and use that info to produce the best results for your purposes.

I'd found that my Sig10-20 seems to work at it's best(clearest sharpest..etc) at f/11 on my D300, whereas on the D70s I used to use it more at f/16 for 'more' DOF. Still works well at f/16 on the D300, but definitely improved at f/11 with slightly more clarity of fine detail.

Generally though if your end result is print.. don't worry about it to much, as long as it looks good enough at a reasonable viewing distance, that's all that counts.

Finally, with the Siggy 10-20, what I sometimes do is to focus at say 1m(or 0.5m) and just stick with f/11. From about that 0.5-1 meter distance to infinity is usually sharp enough, but!!.. the horizon is not as sharp as the subject is going to be. The subject(or foreground, in a landscape) may have more importance in the image, and should really try to stand out a lil more anyhow.
Infinity on a 10mm is generally regarded to start at about 3m.