How much do you use your ultra wide angle?

[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

[Javelin]

I think I understand (which could be dangerous) but correct me if i'm wrong

Depth Of Feild can be measured at the subject and has a certain thickness (if you will allow that term) Depth of Focus which is measured at the sensor and also has a thickness but because the distance from the front element to the sensor is shorter than the distance of the front element to the subject then the D/O-focus is necessarily thinner. and the shorter the focal length the thinner that gets as the front element gets closer to the sensor with pregressivly shorter focal lengths?

Or maybe I should be saying aperture instead of front element in this case?

In Magnified Macro photography then this equation is reversed? (if you go beyond 1:1)

Enter the COC: the COC would tend to make the D/O-Focus have a little leeway depending on the diameter of the COC? because the sensor can't detect diferences smaller than the COC

[harveyzone]

Bingo - I think I have got it...

One is the point (or field) of focus in front of the lens, and one is behind it. So if you have a very narrow depth of focus, but the plane onto which you are focusing (e.g. the sensor) is not within that depth then your image will always be out of focus for the entire image. Short lenses have a very narrow depth of focus compared to longer lenses so will show any focus problems more readily than on longer lenses.

Is that correct?

Thanks all.

[bonneville]

I will admit that it can be far too tempting just to put it at 10 for the drama, IDM

Hi Ian

Cracking wide shot of a refuelling spike. I recently bought the Sig 10-20 but confess to hardly ever using it yet. Now I've seen your shot I have been looking at the exhast pipes on my bonneville in a completely different way

[ianmiddy]

Cracking wide shot of a refuelling spike.

Cheers - it was taken probably not too far from you - Bruntingthorpe [just S. of Leciester] during the Lightning Preservation Groups photographers day earlier in the year - they have a taxi day on tomorrow, which I'd hoped to get to, but other commitments now prevent - oh well...



Cheers

IDM

[David Kilpatrick]

(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?

No need. There are two focal planes for any focused image - one is the subject plane (think of a test chart, a flat plane) and the other is the image plane (normally called the focal or film plane, because that is where the image from the subject plane is focused and where the film is). Depth of field is the tolerance you get in the subject plane, how much in front of and behind the exact point will still look acceptly sharp. Depth of focus is the tolerance you get in the image (sensor, film) plane. At 1:1 macro the lens itself is theroetically exactly midway between film plane and subject plane, so both are equal.

At infinity focus, the lens is exactly one focal length from the film plane, but the subject plane is an infinite multiple of this. Being realistic, a 50mm lens is 50mm from the film plane; infinity is anything beyond, say, 100m or 2000X that distance. In the film plane, depth of focus might be just 0.5mm, but in the subject plane at 100mm, it would of course be many tens of metres.

The AF system of a camera has to be placed in a position exactly matching the film (sensor) plane, and so does the focusing screen. The mirrors and the design of the darkchamber (camera mirror box, mount, film gate etc) ensure this. But tolerances in a system with just a 44mm cube to contain everything are such that errors of .001mm or even .0001mm are significant. With extreme wide angles like 11mm even the smallest error inside the camera can shift the focus by metres at landscape distances.

Your depth of field might conceal that slightly, but depth of field is always a gradual thing - a tolerance - and with digital imaging, we are able to spot the exact plane of focus in a shot so easily. So extreme accuracy is needed at the imaging plane and all associated planes (mirrors, AF module, screen) to ensure that a very short focal length lens is truly accurately focused. Depth of focus, in the camera body, is the tolerance at each aperture setting for positioning errors without the user ever being to say that the subject is not perfectly focused.

You don't need to think about depth of focus really, but the camera maker does - and if you find you have FF/BF errors, it helps to understand how tiny the adjustments involved inside the camera are, for an apparently big shift outside.

David

[Cogito]

Dear All,

Recently I bought a Sigma 10-20. Took a couple of interior shots and some landscapes, but I couldn't find too many occasions to go ultra wide. I think I am not able to compose a shot. So, here is my question, a stupid one , how much do you guys utilize your ultra wide lenses? often? Can you post some images to help me? Any regular shot

Not a lot. You will PROBABLY try to avoid the natural distortion generated by the lens. Unless you really want to shoot images as in Bakubo's first post (please don't read this as criticism, 'cos it isn't!).
Without entering the discussion on DOF/DOF, there's a lot to be said for a number of landscape opportunities. This taken with a 18mm lens on a film camera - 12mm on a Sony DSLR.

But leaving the lens on a camera can produce irritating images.....

I should have used a longer lens and stepped back a bit.....

I have seen a couple of creative shots with them in different forums but not many compared to normal/tele lenses. Also, do you correct horizon/barrel distortion, etc? If yes, can you please mention the workflow? I am sure many of us is using sigma 10-20 on their Alphas. Thanks in advance for your help.Ayan

No I don't. Those are the images I shot. I'll live with them!hiccup!
Oh! And here's using the Sony 10-20 on a Minolta 7D. Yep! At close proximity, not everything is in focus!hiccup!!


There's been an excellent discussion on depth of field/focus. Something worth noting!