DPReview.com is closing April 10th - Find out more

F11 lens with 2x TC = F22?

Started Aug 5, 2020 | Questions
makofoto Contributing Member • Posts: 534
Re: F11 lens with 2x TC = F22?

The Dp ones are terrible! 

ratman42 Junior Member • Posts: 31
Re: F11 lens with 2x TC = F22?

Sittatunga wrote:

makofoto wrote:

F11 is not a physically small opening in a 800mm lens. I find it hard to believe that Diffraction plays any part in this?

It does. The radius of the Airy disc on the sensor (the brightest image from a point light source through a lens with a circular aperture) is exactly proportional to the f/number. See https://www.dpreview.com/forums/post/64229267 for the algebra.

Indeed. This is well-known in the astrophotography community. A lot of discussion is held in those circles regarding the proper match of pixel size to Airy Disk size. I've created a Javascript calculator and populated it with a few cameras and lenses so you can play with various parameters. I've added the known specifications of the EOS R5 and EOS R6 along with the Canon 600- and 800-mm f/11 lenses.

One thing you will note is that these f/11 lenses are pretty well-suited to use with the EOS R6 with its larger pixel sizes.

https://rcunniff.com/res_calc/res_calc.html

 ratman42's gear list:ratman42's gear list
Sony RX10 IV Canon EOS R5 Canon EOS R7 Canon Extender EF 1.4x III Canon Extender EF 2x III +5 more
Keyboard shortcuts:
FForum MMy threads