New to it all - DSLR crop factor

[John Weaver]

Hi all
I have a canon 450d and i have been thinking about buying an ef 85mm f1.8 lens. My question is does this have 1.6 mag factor and turns it into a 135mm f2.8 as not quite sure how it works.

[Zuts]

Hi,

No, I dont believe so. Rather than think of it as a magnification factor think of it as a crop factor. For example if you took a shot of the moon through say a EOS 5D with a full frame sensor and your EOS 450 D with an APS C size sensor using your 85mm F1.8 then the image formed on both chips of the moon would be the same size. There would be less sky around the photo taken with the 450 D though.

The FL in both cases and F number would be the same. This is because the lens is focussing the light on the sensor in exactly the same way with exactly the same strength; it's just that one sensor has less area.

That's my take on it anyway.

Cheers
Paul

[RB]

That's right Paul.

Here's a link to an example: http://en.wikipedia.org/wiki/File:Crop_Factor.JPG

The Blue box shows what a crop body sees and the Red box, what a Full Frame body sees.
The f ratio is unchanged.

There's no real magnification taking place, it's just that a crop camera sees less field of view.

BTW I've edited the thread title to clarify the topic of the thread.

[matt]

Thanks RB and Paul...

This is also useful for me, having just bought a new DLSR with the APS-C size sensor:thumbsup:

[DJDD]

hi!

there is also this recent thread:
http://www.iceinspace.com.au/forum/showthread.php?t=38091&highlight=crop+factor

i found "crop factor" confusing until i read that thread.

cheers,
DJDD