90D and Canon 300mm f4 L IS USM inconsistencies in exposure

[Wandalynn]

I wanted to see if (1) anyone else has had this problem and (2) if there are any fixes. First, let me say that my Canon 300mm lens has worked perfectly on my 70D, my 6Dii, and my R cameras. Add to that that all of my other Canon lenses perform perfectly with the 90D. I almost exclusively shoot in AV mode, mostly nature closeups, and mostly use evaluative metering, sometimes spot.  Taking pictures with the 90D + 300 through the VF, almost all of them are severely underexposed, almost black. Taking pictures with same lens using Live View, the exposure is fine. Again, that never happens with my other lenses on this camera, and I have no problems with this lens on my other cameras. Input appreciated.

[Chris_N]

The latest 90D firmware is 1.1.1. I checked and there is no available firmware update for the 300mm f4 L IS USM. (Sorry, my mistake)

https://www.canon.co.uk/support/consumer_products/products/cameras/ef_lenses/is/ef_300mm_f4l_is_usm....

The only other thing to check is if the contacts are good and the aperture is working properly. You might also test the lens on another 90D body if you can get hold of one. Maybe, you could also have both the body and the lens checked at a camera service centre. 

I hope the programe you found can help to tune the equipment and you can solve the problem. It can be very frustrating when expensive, state of the art equipment doesn't behave as it should. 

Please let us know how you get on.

Kind regards

Chris

[timmyleeturner]

I will post back, pretty much have a problem with over exposure and tak focus..  But if you set this camera to auto, you have to edit the picture quite a bit, if you can even use it.

[Wandalynn]

@timmyleeturner Updating the firmware is very easy because Canon gives you complete instructions in the zipped folder that you download from the Canon site. I prefer to use the SD card method. Go to the 90D page on Canon and then to the Firmware tab. There are all sorts of instructional videos out there about updating firmware but I HIGHLY recommend using Canon's instructions, they are EASY TO FOLLOW, with pictures of the screens, etc.