>I had a question pop into my mind today, probably simple, but I
>couldn't explain it.
>
>Wide aperture lenses have shallow DOF, this I know. My 2.8L and
>1.4/50 both produce a striking blur and I probably shoot the vast
>majority of my pics at 3.5 and below.
>
>Why, then when you are looking through the camera to focus these
>lenses do you see the whole field in focus?
You cannot see all the field in focus in a 50mm f/1.4
> Isn't the lens operating
>at wide open aperture when focusing in EOS cameras?
Yes
> I thought so, to
>aid in the auto focus speed (which is why 1.4/2.8 type lenses focus
>faster than 3.5-4.5 zoom, right [heavier glass aside]?)
Yes, but to take full use of the 2.8 aperture you need an profesional
camera as the EOS 500n, for instance, is optimized for f/5.6
>That is to say, how come only the film exposed has the DOF effect.
That's not possible. You can see the DOF effect on the focusing screen.
But there is something interesting about that:
I have a 50mm f/1.4 Pentax Super Takumar screw lens that I use in my 500n
by means of a M42 adapter. I think that DOF is greater on the focusing
screen that on film.
That coincides with the fact that open wide (at 1.4) the camera overexposes
1.5 stops.
In fact, if I stop from f/1.4 to f/2 I see no change on the viewfinder
brightness, that diference is quite evident from f/2.8 to f/4.
Perhaps that's some kind of mirror vigneting, because stopping down at
f/2.8 the mettering is ok, but I think 1.5 stops is too much vigneting.
So I think it's possible that DOF is greater on the focusing screen than on
film, but it's not possible that you can get infinite DOF on screen (all
the field in focus).
>I thought about this as I"m contemplating a new body purchase - wife
>wants EOS elan 7 due to smaller size and built in flash, I want the
>EOS3 for pure speed and because I hate that built in flash :)
What camera you are using now?
>I'm sure I've overlooked something obvious here, please be gentle. :)
Best regards
Vicente
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