How does adding extension affect macro lenses?

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

Belgos - 28 Apr 2005 17:30 GMT

Hello,

Regarding extension and magnification, I understand that the level of
magnification is defined by the calculation focal length/extension used, so
for a 100mm lens, if I add 50mm of extensions I get a 1:2 (half life size).

Now, what happens if this lens is a macro lens already offering 1:1 life
size ? John Shaw didn't mention anything about that. With 50mm extension,
does the life-size Canon EF 100/2.8 macro become 1:2 ? Or 1,5:1 ? Is it's
'macro' function cancelled ?

Thanks in advance for your helpful answers.

Kostas

Reply to this Message

Joseph Meehan - 28 Apr 2005 18:24 GMT

It is the total extension from the infinity setting that counts. A real
macro lens has more built in range (distance) so the extension will just
extend that some more, but it is not linear.

BTW without knowing what specific 100mm lens you added 50mm to I could
not say what they resulting max size might be, as they all don't focus to
the same original distance.

Here are a couple of formulas:

f = Lens focal length
h = Subject size
v = lens to subject distance
h' = Image size

Image Size =

f x h
h' = --------
v

h' x v
h = ----------
f

As I recall the distance to subject is from the optical center of the
lens - often marked on macro lenses.

Signature

Joseph Meehan

Dia duit

> Hello,
>
[quoted text clipped - 12 lines]
>
> Kostas

Reply to this Message

Nick Zentena - 28 Apr 2005 18:29 GMT

> Hello,
>
[quoted text clipped - 6 lines]
> does the life-size Canon EF 100/2.8 macro become 1:2 ? Or 1,5:1 ? Is it's
> 'macro' function cancelled ?

Speaking in general. When the lens is focussed at infinity it's got
extension equal about it's focal length. At 1:1 you've got twice the focal
length. If you add an extension tube then you won't be able to focus at
infinity. You will be able to focus closer then you did before. How much
closer depends on how much extension and the focal length of the lens.

Nick

Reply to this Message