topic f stop reading: f00 when using 1.4 extender between EOS R3 and 400mm f/2.8 in EOS DSLR & Mirrorless Cameras

f stop reading: f00 when using 1.4 extender between EOS R3 and 400mm f/2.8

rehoopes — Tue, 23 Jun 2026 13:27:29 GMT

Using Canon R3 with 2.0 extender and 400mm f/2.8 lens, works fine. When switching to 1.4 extender, f stop reading is f00 and nothing works. No camera functions work. This problem did NOT occur previously when using the 1.4 extender. Any thoughts on why this is happening.

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

jrhoffman75 — Sun, 21 Jun 2026 23:22:10 GMT

@rehoopes wrote:

Using Canon R3 with 2.0 extender and 400mm f/2.8 lens, works fine. When switching to 1.4 extender, f stop reading is f00 and nothing works. No camera functions work. This problem did NOT occur previously when using the 1.4 extender. Any thoughts on why this is happening.

Look for any damage to the contacts (or sticking of the spring-loaded contacts) on the 1.4X Extender. There is a communication problem.

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

deebatman316 — Sun, 21 Jun 2026 23:30:56 GMT

Hi @rehoopes is this the older EF Mount version or the native RF Mount. If the EF Version is the Canon brand Mount adapter being used. If NOT that could be the problem right there. 3rd Party Mount adapters cause all sorts of problems. Also are you using the Canon brand Teleconverters or 3rd Party. Stick with the Canon brand version.

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

March411 — Sun, 21 Jun 2026 23:48:00 GMT

Hey @rehoopes, as @deebatman316 asked in his post additional information would helpful to assist you in possibly resolving your challenge. Having used the R3 for a long time and trying third party hardware on it I gave up as I had intermittent problems with the gear. I am by no means an all Canon guy as I own third part gear but the adapters and extenders failed for me to often in the field when I needed them most.

Did you try the 1.4 extender after the failure?

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

rehoopes — Mon, 22 Jun 2026 00:52:27 GMT

I agree it is a communication problem that just occurred today. Will check as you suggest. Thanks.

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

rehoopes — Mon, 22 Jun 2026 00:53:58 GMT

All Canon RF equipment new last year.

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

rehoopes — Mon, 22 Jun 2026 00:55:06 GMT

All Canon RF equipment knew last year. Thanks.

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

shadowsports — Mon, 22 Jun 2026 11:58:52 GMT

This is probably a unnecessary question question, but you're supporting the camera by the lens and not allowing it to hang off the front of the camera in any way, right? I know the chance of this is less than zero, but I just want to confirm there isn't any type of undue stress on the mount.

Have you tried testing the 2.0 extender with another lens?

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

SignifDigits — Mon, 22 Jun 2026 14:19:59 GMT

I think something is seriously wrong with that lens and you should offload it to me and get a new one to replace it. ;P)

Seriously, I am very sorry to hear about your frustrating situation, and I hope you get the comms problem sorted. Please let us know what you learn and how you get it resolved.

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

March411 — Mon, 22 Jun 2026 16:16:55 GMT

A couple things. Have you tried to clean the extender and lens contact points with 90–99% isopropyl alcohol?

Did you pick up your gear from an authorized Canon dealer? If it's been under a year you are still under warranty and can contact Canon for support but you will need to register the products first. Here's the contact number if you would like to give them a call: 800-652-2666

Re: f stop reading: f00 when attaching 1.4 extender Using Canonbetween R3 and 400mm f/2.8

rehoopes — Mon, 22 Jun 2026 21:03:40 GMT

2x extender works fine. Weight of system is well balanced with no bad stress on the camera/lens connection. Thanks for your input. Will try cleaning the contacts with alcohol as suggested by another.