Again non-photographers go onto the next message.

Disclaimer: I am not a bird photographer but I do understand optics. And I don't have a Nikon/Canon preference either.

On Apr 4, 2011, at 2:57 PM, cgluckman at aol.com wrote:

> I have a Nikon 1.4 TC which is the best Nikon makes and have not been satisfied with the results. It loses 1/2 to 1 stop of light (and speed)

All 1.4x teleconvertors "loose" exactly one stop because they're (essentially) diverging lenses. They extend the focal length of the lens in front of them by 1.4x. It still has the same aperture so the longer effective focal length reduces the focal ratio by one stop.

Or another way of looking at it is you increase the magnification (focal length) of the lens so the same light is spread out over a circle 1.4x larger. The area of that circle is 1.4 x 1.4 ~ 2x larger than the circle without the TC. So the brightness (the light per unit area) drops by a factor of 2. That's the definition of a stop (and why TCs are 1.4x to make the calculations easier ... one stop less).

It's the physics of optics that "looses the stop".

On Apr 4, 2011, at 2:57 PM, cgluckman at aol.com wrote:

> Three years ago I photographed a Short-eared Owl that was sitting on a tree stump 30 feet away (yes it does happen occasionally) with my Nikon 600mm lens and D300 at Rawlins Rd. with a photographer from Canada at the same distance who was shooting the exact same camera and lens I was using (including the same tripod) except he was using a 1.4 TC at one f/stop number larger. The light was good and the owl was sitting still. He later sent me a raw copy of his image which I compared with a raw copy of mine, cropped to the same size as his. Mine was noticeably sharper. This is not a real vaid test. I might have had better telephoto technique, my focus could have been better, etc. I have been looking for a study that actually takes pictures with the same camera, same settings, with and without the TC, under the same light conditions of a static subject, at the same distance, etc. and so far no one has produced one that came even close to covering the possible variables.

That's last sentence is not really true. There are a lot of "measurebaters" out there who do this. And some of them even work for the camera/lens companies so they can do it properly.

The only way to really resolve the question is to make accurate measurements of the MTF with and without the TC with identically the same setup looking at MTF test targets (not lifelike targets).

Of course it also requires you understand MTFs.

http://www.pbase.com/samirkharusi/canon_mtf_curves

The guy doing these tests is an astrophotographer (his targets are points so he should be able to see the difference). Again he sees pixel sharp stars with and without the TC on a Canon 600mm for example. That's a tough test.

Canon even publishes this data for their TC with their lenses as the scan here show.

http://www.canon-europe.com/Support/Documents/digital_slr_educational_tools/en/ef_lens_work_iii_en.asp

But you may have to stop the lens down more to get to that sharpness you want so you end up with the usual aperture/speed/ISO conflicts if you don't have enough light which is really why you spend the "$2000" on a "big honking lens".

Another example showing no apparent degradation with a (not even name brand) TC.

http://www.clarkvision.com/articles/relative-lens-sharpness/

Of course YMMV and a bigger lens is clearly better but if you don't have the bigger lens is the TC "good enough" for a particular shot. Only the individual photographer will know the answer to that.
--
Kevin Purcell (Capitol Hill, Seattle, WA)
kevinpurcell at pobox.com
http://kevinpurcell.posterous.com
http://twitter.com/kevinpurcell