How much do you use your ultra wide angle?

[adakshi]

BTW, do you guys find these ultra wide lenses sharp? When I compare it against Sony 18-250 or even 18-70 kit lens, I don't find it sharp. Maybe mine is a lemon.

See this full size image

http://farm4.static.flickr.com/3170/269 ... c50e_o.jpg

Thanks
Ayan

[David Kilpatrick]

They are very difficult to focus correctly. Even a tiny back focus or front focus error will lose the sharpness. Our 11-18mm for example doesn't nail focus well on the A350 or A100, but it is superb on the A200 and A700. So we got a second 11-18mm and we now have two. The new one (Sony branded not Tamron) focuses more accurately on the A350.

David

[Javelin]

well I know my 35-70 is pretty sharp across the frame and now it looks like this 17-35 is just as sharp if not maybe a little sharper. they are better than the kit lens anyway which is why I wanted these.

[01af]

"adakshi" wrote:
> Do you guys find these ultra wide lenses sharp?
> [...] I don't find it sharp. Maybe mine is a lemon.
> See this full-size image ...

But it's very sharp---just look at the skylines of the distant hills!

On the Dynax 7D, my AF-DT 11-18 (D) won't focus accurately, too. So I acquired the habit to estimate the subject distance and then to focus manually after the distance scale which improves results significantly. I tested the accuracy of the scale, and I found the infinity mark spot-on at all focal length settings.

-- Olaf

[adakshi]

I am getting a notion that the lens is front focussing a bit. But with such a huge DOF, should an inch of focus shift matter?

Let me try posting another pic.
http://farm3.static.flickr.com/2186/268 ... 9be5_o.jpg

EXIF
Exposure: 3.2 sec (16/5)
Aperture: f/9
Focal Length: 10 mm
ISO Speed: 320
Exposure Bias: 7/10 EV
Flash: Flash fired

Thanks
Ayan

[David Kilpatrick]

I can almost read the writing on the tea-towel or poster at the top left - Eagles, etc. I think the unsharpness is due to a mix of ISO 320 (this sort of detail always looks best at 100); f9 - diffraction may affect the results with the 11-18mm at this aperture, f8 is probably sharper, likely to be a minor effect only; 3.2 second exposure - if this was not on a tripod, but using SSS, then it would never be expected to be sharp; if it was on a tripod and SSS was left on, it could be blurred by fractional movement of the sensor; if on a tripod with SSS off, you would need to use the self-timer or a remote release to avoid some vibration.

The focus field of all lenses is curved, and it looks as if the field here may be in focus better close to the edges than the centre (floorboards, mentioned poster). The red colour of the interior would tend to make the lens back focus, not front focus, but curvature might be putting the edges into front focus (a dish shaped focus plane is most common). It is still sharp enough to make a very good print even at 12 x 18" anyway.

David

[01af]

"adakshi" wrote:
> But with such a huge DOF, should an inch of focus shift matter?

It's a common misconception to think that focusing accuracy didn't matter when depth-of-field is wide. Actually, the opposite is true. The degree of focus accuracy required to get the image sharp depends on depth-of-focus, not depth-of-field. And when depth-of-field is wide then depth-of-focus usually will be narrow, and vice versa.

That's why wide-angle lenses on SLR cameras are notoriously hard to focus, on manual-focus cameras and auto-focus cameras alike.

-- Olaf

[adakshi]

Hello,

Depth of focus as per wiki:
Depth of focus, however, is a measurement of how much distance exists behind the lens wherein the film plane will remain sharply in focus. It can be viewed as the flip side of depth of field, occurring on the opposite side of the lens.

Depth of field:
the depth of field (DOF) is the portion of a scene that appears sharp in the image.

So, wouldn't larger depth of field translate to larger depth of focus? or depth of focus is an independent characteristic of a lens? I am confused .

Thanks
Ayan

[David Kilpatrick]

Olaf will explain. A smaller aperture will produce both a greater depth of focus and a greater depth of field, but a shorter focal length lens gives a greater depth of field, less depth of focus.

The two are equal with any lens - of any length - when you are focused at 1:1 macro.

Basically if the camera is not set up to within microns of accuracy, depth of focus won't prevent focusing errors, no matter how much depth of field you add by stopping down. You just get a less unsharp blurred picture

David

[01af]

Ayan "adakshi" wrote:
> Depth of focus as per wiki:
> "Depth of focus, however, is a measurement of how much distance exists
> behind the lens wherein the film plane will remain sharply in focus. It can
> be viewed as the flip side of depth of field, occurring on the opposite side
> of the lens."

Wikipedia is not always right, especially in the field of photography ... but in this case it's perfectly correct.


Ayan further wrote:
> So, wouldn't larger depth of field translate to larger depth of focus?

With always the same lens at always the same focus distance, yes. With different lenses and/or different focus distances, no.

As David already has explained: Depth-of-field will increase with smaller aperture, longer focus distance, and shorter focal length. Depth-of-focus will increase with smaller aperture, shorter focus distance, and longer focal length. So wide-angle lenses have less depth-of-focus than standard or telephoto lenses and thus, will suffer more from an inaccurate or mis-adjusted focus mechanism.

When I said, "And when depth-of-field is wide then depth-of-focus usually will be narrow, and vice versa" in my previous post, it was not fully accurate. Because when stopping down in order to increase depth-of-field then of course depth-of-focus will also increase. When I made that statement I had the other parameters in mind, not the aperture (namely, focal length).


David Kilpatrick wrote:
> The two are equal with any lens - of any length - when you are focused at 1:1 macro.

That's right (provided the lens is a more or less symmetric design). And at magnifications larger than 1:1, depth-of-focus will be even wider than depth-of-field which leads to strange and uncommon focusing behaviour. Consider a bellows lens head on a bellows unit, set to a really large magnification like 5:1 or so. When moving the bellow's front standard back and forth, you will effectively travel through depth-of-focus (which is wide). When moving the whole rig back and forth on the focusing rail then you will effectively travel through depth-field (which is narrow). Note how the slightest touch on the focusing rail knob will instantly throw the image out of focus due to narrow depth-of-field, while at the same time you can turn the front standard's knob quite a long way without affecting focus too much due to wide depth-of-focus.

-- Olaf

[adakshi]

Thanks a lot Olaf and David

[01af]

Addendum to the previous post: I said moving a bellow unit's front standard would effectively travel through depth-of-focus. That's not perfectly correct. You should move the rear standard instead. Moving the front standard does rather complex things with regard to depth-of-focus and depth-of-field (and a few other things) ... however many bellows units have fixed rear standards and can move their front standard only.

-- Olaf

[harveyzone]

(Sticks head above parapet, and prepares to look like a fool)...

Call me mister thickie if you like, but I still do not get the difference between depth of field and depth of focus. I am fairly comfortable with the concepts of DoF, Circles of Confusion etc. I have been aware for a long time that wide angle lenses are not as easy to get reliable focus with, but never quite knew why. At last someone is trying to clarify it, but I still haven't 'got it'. Is it some complex physics concept that the average Joe will struggle to grasp, or should it be simple to understand? Can anyone point me towards some idiot guide?

[Winston]

Depth of focus is measured at the image; depth of field is measured at the subject.

[Dr. Harout]

I think I made a concept for myself to get it:
you turn the focus ring for accurate focus on a wide angle much less than a telephoto. Hence accept this as depth of focus. It's not the real explanation but it'll do (it did for me).
If they'll make a wide angle lens which when rotating the focus ring you make too much turns, then forget about what I said, but also do forget about any company making such a lens