T90 & Canon Extenders Odd Exposure Compensation

[loccdor]

I plan to go out with the T90 and teleconverter this weekend, so I was flipping through the manual. Good thing I did, I had no idea they required adjustments to exposure beyond the normal 1 or 2 stops of light loss.

Does anyone know the reason for it?

 

[loccdor]

I haven't gotten a response on this but I think it must be because the optical path to the light meter has some important differences with the optical path to the film. This could also have implications that light meter readings are inaccurate with certain lenses even without the teleconverter, as I've experienced with my FL 58mm f/1.2 lens.

 

[koraks]

I think you may be right. Note the metering light path here: https://www.mir.com.my/rb/photography/hardwares/classics/canont90/htmls/metering.htm
Between angles, reflections and otherwise imperfect transmission I can very well imagine that there can be an effect of the teleconverters on metering.
I had not expected it to be so pronounced and in particular so seemingly erratic.

 

[loccdor]

Thank you! That image is extremely helpful. I'll add it here in case it comes up in anyone else's search.

 

[loccdor]

I ended up printing out that page of the manual on a laminated card to put in my camera bag for reference. Though, planning to just stick to spot metering which will mean setting it on -2/3 the whole time.

When I get my 58mm f/1.2 lens back from servicing, I'll have to experiment and come up with an exposure compensation chart for that as well.

 

[koraks]

my 58mm f/1.2 lens

A monster!

Honestly, I still find the T90's multi-spot metering function one of the greatest innovations in (quasi-)modern camera technology (although I'm not quite sure if Minolta was earlier than Canon with their version).

 

[loccdor]

Indeed. That, and the hand-contoured grip which was also new, though a person wonders why it took so long to come up with it. Now it's standard on almost everything.

 

[koraks]

Funny, isn't it? Although I once had a Mamiya ZM (35mm) which also had a somewhat ergonomic body shape. There was a bit of an evolution occurring in the direction that culminated in the flower-bulb/wine-barrel like designs of the mid-1990s.

 

[reddesert]

I think the brief answer to this is that each teleconverter changes the location of the exit pupil, and that affects the angles at which the light beams are incident on the different metering systems, and probably the amount of vignetting within the optics in front of the meter cell (for the spot meter at least). The teleconverters also may vignette fast lenses, so that the effect on fast lenses is different from slow lenses.

 

[MattKing]

Indeed. That, and the hand-contoured grip which was also new, though a person wonders why it took so long to come up with it. Now it's standard on almost everything.

Great for right handed users - and a scourge for those of us who need/prefer to use our left hand.

 

[loccdor]

Great for right handed users - and a scourge for those of us who need/prefer to use our left hand.

Thanks for letting me know, I hadn't heard of people who need to use cameras left-handed, it must be slim pickings.

I'm left-eyed but it doesn't present a big obstacle.

 

[loccdor]

I think the brief answer to this is that each teleconverter changes the location of the exit pupil, and that affects the angles at which the light beams are incident on the different metering systems, and probably the amount of vignetting within the optics in front of the meter cell (for the spot meter at least). The teleconverters also may vignette fast lenses, so that the effect on fast lenses is different from slow lenses.

Makes sense but I think this would also apply to different lenses generally without teleconverters, no? I'll have to test it when I get time.