Qu 2 - Manual Focus When Visually Impaired

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April 2008
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Attempted some 'smoke art' last night, and as a result I now have a head full of questions.

Question number #2...

Manual focus. How do you do it if you can't see the subject very well. I have read that you can use markings on the lens to manual focus a prime. But what about zooms... something like my Sigma 24-70. It has some markings on it in metres and feet that rotate, and also has some static markings that I thought might be aperture values, but they go 22 16 11 5.6 | R 11 16 22. Which makes no sense. And nothing changes at all when you adjust the zoom.

So is it possible to manually focus to a given point without setting up something that you can see and focusing on that?
 
The static markings are a way of telling how much depth of field a given aperture should give you, the movable one is the distance away that you are focused.

If the scale is reading your focused 20ft away f11 the two values that f11 fall at should be roughly the limits of your depth of field.

For that type of photography I usually just get a torch or similar object and place it where I want the focal point to be then autofocus on it and lock the focus in place there. The other way is measure/estimate the distance to your subject and set the lens focus in position based on that.
 
Cheers, that makes sense. :)

Except... on my lens in question (Sigma 24-70 f/2.8) none of this changes as you adjust the zoom. And would the DoF not be greater at 24mm than at 70mm, if everything else stayed the same?

Or is this another of those sciencey things that I've misunderstood?
 
Focal length changes inherent depth of field not actual depth of field. Basically it's magnification so the background is compressed more which can make it look more blurred in some cases.

This is demonstrated here:
Luminious Landscape
 
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