General guidelines for focusing?

[PekkaM]

I've been shooting with my EOS300D for few weeks now and some questions have risen:

When to use manual focus? I find it terribly hard to see if picture is in focus through the viewfinder, should I just check the distance from the focusing ring and estimate the distance or what? In contrast autofocused images allways seem sharp through the viewfinder even though the end result necessarily isn't.

What is "focus to infinity"? How far is that infinity actually? Should I use it when shooting distant landscapes in mf-mode (like closest point hundred meters away or so)?

Can the lens focus no matter how far the subject? I remember reading Sigma EX 12-24 review saying something about max focus distance being quite close actually.

With G1 it seemes that my depth of field was infinite, everything was usually in focus. How is this best approached with an SLR? Small aperture and far focus?

[Scottes]

Do a Google for "Hyperfocal Distance" which should give you a lot of info about this. Basically the hyperfocal distance is the distance to the focus point where everything from half that distance to infinity is in focus.

So let's say (I'm making up numbers) that a 50mm lens at f4 has a hyperfocal distance of 30 feet. So everything from 15' to infinity would be in focus. At f22, that same lens might have a hyperfocal of 6 feet, so 3 feet to infinity is in focus.

So "inifinite focus" is probably a different but slightly incorrect way of describing the hyperfocal distance. Focusing at a point where it seems that everything is in focus. Of course it could also mean that the lens is focused on infinity, which just means that it's focus point is far enough away that the lens has everything in focus (if it's not too close).


Your G3 probably had a fixed aperture of f8 or maybe f11, giving you seemingly infinite focus. I say probably but I don't really know the G3.

I'd be amazed if the Sigma 12-24 couldn't focus to infinity. It's possible - I know that adding diopters to a lens means you can't focus at infinity. But for a lens out of the box to not be able to focus at infinity... Seems rather odd. Especially a wide-angle lens that will be used for landscapes quite a lot.

[Scottes]

As to "when to use manual focus" - well, if you *need* to do so, do it, otherwise don't.

Use manual focus if::
...the minimum aperture of a lens is larger than f/5.6, which means a 10D/300D can't autofocus (though top cameras, like the EOS3 and 1D, can)
...shooting macro - AF can often be quite useless at 1:1.
...shooting moving objects, when AF can't keep up
...if AF chooses the wrong part of the object (like when you want eyes in focus, but AF choose the nose)

The 10D (assuming 300D too) isn't so great for manual focus because it's rather dim. Using a lens that can do f/2.8 does help though.

[PekkaM]

Here's a quote from the Sigma 12-24 review I was referring to:

"The HSM motor is silent and appears fairly fast. The lens focuses accurately up to about 2 meters: hyperfocal distance wide-open is about 1.6 meters, so much beyond that, there's no way for the AF to tell how far the subject is. In other words, for situationals with close-up subjects, use AF, but for landscapes and architecture, zone focus manually."

The article can be found here: http://194.100.88.243/petteri/pont/Reviews/a_Sigma_12-24_f4.5-5.6/a_Sigma_EX_12-24_f4.5-5.6.html

[Malaxos1]

I have D Rebel and thingk it focuses fast and accurate, then again I was using a 70-200L lens, so that should help. Anyway today I was photographing a bird in a tree and the auto focus was having a lot of trouble as it didn't know what I was trying to focus on. So, I switched to auto and all worked fine. You should also know that the G3 is designed to give sharp photos as the camera sharpens them for you. It isn't the same with a DSLR, I wish it was, but they expect you do the sharpening yourself. I hope I was able to help...Dean

[perfectpixel]

... How far is that infinity actually?

it's late at night, so hope it's ok to see a little humor in that question :wink: :D

Great answers from Scottes, and BTW I also find it hard to manually focus since the view finder is smaller than "traditional" 35mm cameras. Or maybe I'm just getting older...

Thanks for the chuckle though.

[PekkaM]

Ok. I'm trying to figure this thing out.

So what is the point of focusing beyond hyperfocal point (unless you want to get foreground blurred)? In the Sigma 12-24 review it says that it's hyperfocal point is only some meters away. So in essence you could just set the lens on MF, focus no HFpoint and snap away with everything in focus (except things less than few meters from you)? I have Sigma EX 17-35, how can I determine the hyperfocal point for it?

What I originally meant with focusing to infinity is that there is an infinity symbol on the focusin ring on my 17-35. I'd gess that it's far beyond the hyperfocal point of the lens since the AF seldom uses it.

Another observation: When set wide open ( 2.8 ) the said 17-35 has real problem focusing on landscape photos. It always focuses too close and image gets blurry. When I use MF and set it to said infinity symbol the pics come out just fine (not as sharp as stopped down to 8.0 which is really sharp IMO).

Forgive my newbieness but I'm trying to understand this thing from principles and up from there ;)

[Scottes]

So what is the point of focusing beyond hyperfocal point (unless you want to get foreground blurred)?

GREAT question. Remember that DoF is the part of the image that is has ACCEPTABLE sharpness. Setting at hyperfocal DOES NOT mean that something 215 feet away will be in focus while something 214 feet is not. You won't be able to tell the difference at all.

The HypDist is a mathematical formula - photography is art.

I use the HypDist when doing a landscape and I want as much as possible to be acceptably sharp. But if Something is 250 feet away, and HypDist is 480, that says that everything from 240 to infinity will be in focus. That's too close for me, and I'll adjust my focus point, or stop down another aperture setting to decrease the HypDist so that I can be assured that my subject is IN FOCUS, not just acceptably sharp.

But if my HypDist is 20 feet and the closest thing in the frame is 40 feet away I can just set it to the HypDist and go for gold.

In the Sigma 12-24 review it says that it's hyperfocal point is only some meters away.

At what aperture? HypDist is a function of lens length AND aperture.

[jkelley]

My understanding is that DOF is also a function of the final image size you plan to display your work at. The larger the display size, the shallower the DOF for a given aperature and focal length. This is because what "appears" to be in focus is a function of what the human eye is able to resolve in terms of lines per inch. Something referred to as "Circle of Confusion" feeds into the hyperfocal distance equation. The equation I have seen is as follow:

CoC = 1/(r x (D/d)

Where:
r = resolution desired in the enlargement (I use 30 lines per mm as what the human eye can resolve)

D = diagonal size of the enlarged image

d = diagonal size of the negative ( in the case of the 10D, I use the CMOS size as 15.1 mm x 22.7 mm) = 27.3 mm diagonal.

Hope this all doesn't "confuse" you. Bottom line is that DOF is a function of many things, one of which is the display image size. There is a lot of info on this topic all over the web.

[jkelley]

I need to revise my previous post. A more "reasonable" resolution for the enlarged image would be 5 lines per mm when viewed at about 250 mm. 30 lines per mm is too high for normal viewing distance. :oops: