Lens question

[Warren_L]

Actually Canon has three crop factors - Full Frame - 5D and 1Ds MK II
1.3x - 1D MK IIn and ID MK II
1.6 - All others

And don't forget that Nikon's D2x has two crop factors - 1.5 and 2x in the high speed cropped image mode.

It sure was simpler when a 20mm was a 20mm in conversation. Lord, I sound old.

Yes, you're right Andy. I didn't want to complicate matters too much.

 

[victor]

Warren on diopters
Yes this is helping, I can understand the magnifying glass principle.

But now I am not sure why some people say you have to add a diopter if you are using such and such a lens behind such and such a port.

Surely the port is as optically neutral as possible. With or without the Diopter the lens should still work.
Is the problem to do with the autofocus, another device that I don't understand how it works.
Sorry if I am asking the obvious.

 

[Warren_L]

When you are using a dome port, the dome port creates a virtual image of your subject that is closer than the actual subject. This is what your camera lens focuses on, rather than the actual subject itself! Seems hard to believe. As a result, some lenses (depends on the lens) cannot focus close enough to focus on the virtual image and you need the addition of a diopter to allow it to achieve focus.

Here is a link I use often to help explain how dome ports work. The mathematics behind it might be complicated, but the diagram explains a lot about how the virtual images are formed.

 

[Jamdiver]

Marc, have a look on this tutorial explaining lens nomenclature and the effect that different focal lengths have on portraits.
It's from Canon but still it's very helpful .

http://www.canon.co.jp/Imaging/enjoydslr/p_2_001.html

 

[Jeff_S]

One more little piece on diopters - a number 1 diopter places infinity at 1 meter, ie if your camera is focused on infinity, with a number 1 diopter whatever is 39 inches in front of the camera will be in focus. A number 2 diopter places infinity at 1/2 meter, etc. There are also fractions of diopters, ie a number 1/2 diopter places infinity at 2 meters, and so on. We use diopters underwater, as was mentioned earlier, to allow a standard lens to focus closer than it is designed to so the virtual image from a dome port can be in focus. Not needed for flat ports.

 

[Fastmarc]

Thanks Jam. I'll give it a look.