Kelvin Ang <[EMAIL PROTECTED]> writes:
> What I meant to ask in the original post is that yes
> I'll be composing the shot similarly, but in one case
> I'll be using a 50mm f1.4 with TC and another case
> without. Of course, with the TC the lens is
> effectively 100mm f2.8, although the lens is wide
> opened at f1.4. Am I right here?
>
> My impression of a TC (I don't know if I'm right), is
> that it takes the middle portion of the image formed
> by whatever lens it's attached to, and magnifies that
> portion. The f-stops drop in light is due to the lens
> opening being further away now that it would have
> been. It sounds a little too silly and simple for
> something consisting of 7 elements, but I don't think
> it should get more complicated than this.
>
> That being the case, wouldn't the "100mm f2.8" lens
> have the same DOF as the original 50mm f1.4?
I share your impression that TCs take out the middle portion of the
original image and magnify it appropriately.¹ Among other things this
means that the notion of a 2x TC doubling the focal length is only an
/approximation/. But when magnifying the image, it also magnifies the
circles of confusion. So when I think about DOF and TCs I leave the
focal length f and aperture N (or effective aperture Ne) untouched but
only half the allowed circle of confusion COC. Also, if the formula I
am using uses magnification M I will use half the desired magnification,
as the 2x TC will do the rest.
But here these details aren't needed, as we can use a good approximation
for the total DOF in this discussion. For focusing distances short with
respect to the hyperfocal distance, the total DOF is approximately
DOF = COC * Ne / M^2
So we can either half both COC and M (my point of view) or double Ne
(common point of view) and end up with th e same result that the total
DOF doubles. So your 50mm set to f/1.4 with 2x TC should have twice the
DOF of your 50mm set to f/1.4.
I did find this result a bit surprising, so I just tested it with my
f/1.4 50mm and my 2x TC, and indeed, there seems to be an increase in
DOF when the total magnification is kept constant.
cheerio
ralf
1 Simply put a TC is just a diverging or concave lens, in the same way
that a normal lens is just a converging or convex lens. Of course, in
both cases one needs more elements to correct the different
abberations.
--
Ralf Stubner <mailto:[EMAIL PROTECTED]>
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