how to get sharp pics with 30mm 1.4?

[wwwcjx]

I use sigma 30mm 1.4 with XSi (450D). if I shoot stuff, it looks focus well. however, if i take photos of my son with big aperture like 1.4 to 2.0, it is so hard to get sharp pictures focusing on his face. about 1 of 10 is sharp.

the lens probably focuses OK because about 3 of 10 tests are good using f1.4 and a chart.

my confusion is that when you use the central AF point to focus and recompose, doesn't it change a little bit. other word, is the fousing plane flat or an arch.?

I normally handhold the camera, what speed is safe?

when use F1.4, the DOF is so shallow. so the central AF point became a little too big for locating the focus point. also there is a small circle surrounding this point and a box outside. do those do some kind of focusing job as well? I normally put the AF on the middle of my son's eyes, but wonder is it possible the focus is moved to the eyes or lashes if those areas have more contrast.

[Collin85]

Tip #1: Don't focus and recompose. Especially at somewhere like f/1.4 - you're absolutely looking for trouble there.

Secondly, you should be aware of the extremely thin DoF at f/1.4. I pretty much NEVER take portraits wide-open on my 35L - I'm not a real fan of the whole one-eye in-focus, nose outta focus etc. yadda. Unless you have a genuine reason to shoot wide-open like that (e.g. extremely poor lighting), then just stop down the lens a little.

1/50th should be relatively safe, but 1/60-1/80th is even better.

[tunin]

I learned something here.

[Ade H]

....the central AF point became a little too big for locating the focus point. also there is a small circle surrounding this point and a box outside. do those do some kind of focusing job as well? I normally put the AF on the middle of my son's eyes, but wonder is it possible the focus is moved to the eyes or lashes if those areas have more contrast.

You can probably find diagrams of AF arrays on the 'net (possibly even in the manual) and they will show you how the markers in the finder do not precisely relate to the true size of the sensors. The markers indicate the centre of the cross formation of each sensor. Any point of increased contrast that falls under the catchment area of the active AF sensor will likely be favoured. If you had a parfocal lens, I suppose that you could zoom right in on the eyes for precision, but most lenses are not parfocal, unfortunately. Stop down a bit to introduce some margin for error or, if possible, focus manually if and when extremely shallow DoF is your aim. And, of course, any slight longitudinal movement by either you or your subject after focussing could throw the focus off.

[number six]

Originally Posted by wwwcjx

my confusion is that when you use the central AF point to focus and recompose, doesn't it change a little bit. other word, is the fousing plane flat or an arch.?

The focusing plane is *supposed* to be flat. Some lenses, especially macro lenses like my 60mm f/2.8 macro, are very flat, but many lenses are only almost-flat.

Focus/recompose is not a good technique with very large apertures, as Colin said. It'd work well if the focus "plane" were spherical, but it's not.

Here's a good treatise on the subject: http://visual-vacations.com/Photogra...pose_sucks.htm

-js

[timmyeatchips]

Originally Posted by number six

The focusing plane is *supposed* to be flat. Some lenses, especially macro lenses like my 60mm f/2.8 macro, are very flat, but many lenses are only almost-flat.

Really? I assumed the 'plane' of focus would be curved like the inside of a sphere, so it covered points of equal distance (equal distance from what exactly eludes my limited knowledge of optics, I guess the focal plane?). Making it flat would be quite an optical trick. Perhaps it just looks flatter in macro shots as you're looking at a smaller area of the sphere?

[shannyD]

i find that the lens just needs to take some getting used to.

i have used f/1.4 several times, at slow shutter speeds and no tripod and got some pretty sharp images.

it took a couple weeks for me to get used to it.. but i used it so frequently, its by far the best lens i own.

shan

[number six]

Originally Posted by timmyeatchips

Really? I assumed the 'plane' of focus would be curved like the inside of a sphere, so it covered points of equal distance (equal distance from what exactly eludes my limited knowledge of optics, I guess the focal plane?). Making it flat would be quite an optical trick.

Yes, the focus plane is flat. That's why it's called a plane. It's easy enough to prove it to yourself: shoot a brick wall with your lens wide open. A decent lens will give edges that are nearly as sharp as the center, but if you calculate distance off-axis to the wall you'll see the edges are significantly further away than the center.

And yes, it's difficult to get this fairly-flat plane, especially with wide angle lenses.

Perhaps it just looks flatter in macro shots as you're looking at a smaller area of the sphere?

Nope, my 60 mm macro is very flat when used at any distance. I use it for other things than macro also.

At high magnification, up to 1:1, the very thin depth of field would make any deviation from a flat plane more obvious, not less.

Flatness of field is most obvious when copying documents, not really at macro distances generally. The flatness of field is one reason macro lenses are more expensive than general purpose lenses. There are other unusual goodies in the 60 macro too, like a second diaphragm to control flare. It's a nice lens.

-js

[Ade H]

I visualise the plane of focus like this: Think of a letter T, the horizontal line of which represents the plane of focus and the vertical line of which represents the distance between the subject and the camera. Swing the camera around just a few degrees in adjusting your composition, which moves the T around by the same angle, and the horizontal line is now shifted away from the subject and is effectively passing behind it. That line corresponds to the brick wall in number six's analogy. If the DoF is shallow enough, the shift will be visible and your shot is ruined. The fact that there is less DoF in front of the plane of sharp focus than there this behind it does not exactly help either, though this is much less the case at close subject distances.

www.dofmaster.com is worth using to help you to visualise DoF at a given focal length, distance, aperture, etc.

[wwwcjx]

thank you all for those very helpful information.

now it's clear that the focus plane is perfectly flat generally.
the best contrast pin point fall in one of the set AF areas or 9 catchment areas will be set as focus point. (the manual doesn't mention it but indicates this on P19)

after more than one year possession of sigma 30mm 1.4, it's too late to know those basic knowledge but so helpful for afterward shots.

[yogestee]

When focus is critical especially when using large apertures I find focusing manually works best..

[wwwcjx]

i'm learning to use live view x10 and focusing manually. it looks it sorted my focusing difficulty no matter which f number used.

[number six]

Originally Posted by yogestee

When focus is critical especially when using large apertures I find focusing manually works best..

Absolutely. Especially with macros. Razor-thin DOF at high magnification will defeat any autofocus system. Gotta do it manually...

-js

[angryhampster]

Originally Posted by number six

Absolutely. Especially with macros. Razor-thin DOF at high magnification will defeat any autofocus system. Gotta do it manually...

-js

With macros I always just find a working distance I like and then rock my body back and forth for focus. This won't work if you're light-limited and need huge shutter speeds though.


OP: Since you mentioned focus&recompose, when you recompose, just flick the focus ring a little bit to bring it back into perfect focus. I never rely on autofocus with really fast lenses.

[wwwcjx]

Originally Posted by angryhampster

OP: Since you mentioned focus&recompose, when you recompose, just flick the focus ring a little bit to bring it back into perfect focus. I never rely on autofocus with really fast lenses.

this is very tricky if using big aperture, i guess.