Is this correct.....

Is the following correct:

A 70-200 F2.8 on a 7d gives...

200 x 1.6 (Crop) = 320mm

Then if I add the following teleconvertors I get....

320mm x 2 (x2 TC) = 640mm
320mm x 1.4 (x1.4 TC) = 448mm

My question is how much does the F2.8 drop down by adding the two convertors is it just a couple of stops?

mick

To be clear, it's your equivalent field of view that changes; the actual focal length does not change on a crop.

But yes, your math is correct in that those are the equivalent fields of view that you'd get in those situations.

As for your apertures:

1.4 > f/4 (1 stop)
2.0 > f/5.6 (2 stops)
1.4 + 2.0 > f/8 (3 stops)

Thanks Eric.

Snydremark wrote in post #13240862
To be clear, it's your equivalent field of view that changes; the actual focal length does not change on a crop.

But yes, your math is correct in that those are the equivalent fields of view that you'd get in those situations.

To be more correct, the bold terms above should be stated as: "35 mm equivalent field of view". That refers to the 35 mm film format as the reference for the equivalency.

The 35 mm film camera format (24 mm by 36 mm) is not, never has been, and never will be the camera format against which all others are measured and thus it still needs to be referenced. There really is no such thing as a "equivalent field of view" or an "equivalent focal length" without stating the references.

To the OP: I realize that you're currently using a Canon 7D. Do you have enough experience with a 35 mm film format camera to have a gut feel for what the field of view with a 200 mm lens mounted on one would look like? If not, then I suggest that you totally forget the "crop factor" multiplication as it will do nothing for you unless you are directly comparing two different format cameras.

Thanks for that, Skip! Yes, what he said.