Question about pre-focusing the scene (understanding exposure book)

[Gazman]

Hi, i asked a question simialr to this a little while back, but i dont think i put it accross correctly, either way i still don`t really understand it.

In the book "Understanding Exposure" the author goes on about prefocusing the scene via the distance setting, when taking photos at small apettures , f/22 for example.

To quote the book:
"With my camera on a tripod, i set my aperture to f/22. I then prefocused the scene via the distance setting and adjusted my shutter speed for the light reflecting off the blue sky until 1/60 sec. indicated a correct exposure. The scene didnt look in focus when i shot it, but due to my aperture choice it was rendered sharp on film"

Now i understand the whole f/22 = large depth of field. Its purely the prefocusing via the distance setting i just cant grasp.

Anybody understand this and can explain?

Sorry for large post.

 

[reg]

If you have to ask what a distance scale is, you don't have a lens with one.

 

[ksmattfish]

Fixed focal length lenses have DOF scales built in. Variable focal length lenses don't allow for simple, built in DOF scales, although if you google "dof calculator" you can find lots of online help. Here's a link that sort of explains using a DOF scale on a fixed focal length lens.

http://www.fortunecity.com/marina/marine/569/rusrngfdrs/focusing.html

 

[reg]

Variable focal length lenses don't allow for simple, built in DOF scales,

O rly??

http://static.photo.net/attachments/bboard/00L/00Lldo-37307584.jpg

 

[ksmattfish]

You're right. The lens manufacturers exclude them just to spite us. Or maybe the consensus among photo enthusiasts and pros was that those 80s zooms just sucked, and their DOF scales were worthless?

 

[usayit]

My lenses have DOF scales....

Honestly... The reduced quality of the viewfinder, the lack of DOF scales, the lack of well damped focusing rings all point to the same thing... that today's AF cameras were meant to be used with autofocus.

 

[reg]

Or maybe the consensus among photo enthusiasts and pros was that those 80s zooms just sucked, and their DOF scales were worthless?

Yes, almost all 80s zooms sucked, a couple Viv S1s are decent but only as good as a consumer lens today.

I don't, however, recall that being a qualifier of

Variable focal length lenses don't allow for simple, built in DOF scales,

.

 

[prodigy2k7]

Also, when not snapping a pic, your aperture is wide open, so your DoF when looking through the view finder is at its widest, unless you press the DoF preview button if you have it...

 

[Gazman]

yes, it does, canon 400d.

Yea, i understand how aperture is wide open till you take the shot, only then it moves to aperture you set. I think its just the terminology thats getting me at the moment.
So, is it to do with your focal length as to where your focus should be? i.e if im at 35mm , i should focus x" into the frame, wheras if i was at 18mm i should focus at y" ? Or am i way out?