T3i metering with preset long telephoto

[lin]

I have a Canon T31 and a very good preset 500mm preset lens. In "live view" (mirror up) it exposes correctly. In normal (mirror down) overexposes by 1 or 2 stops. Metering mode is set to "evaluative" and everything else is equal. [Tripod and wired remote shutter release].

Any ideas?

[Ray-uk]

With a manual aperture lens the camera doesn't know the aperture and doesn't need to, it just measures the amount of light coming in and adjusts the shutter speed to give the right exposure. (Unless the lens is fitted with an AF confirm chip then it will be read electronically by the camera to determine the maximum aperture, but that can be a totally different minefield.)

Old manual lenses on modern cameras will usually give inconsistent exposures throughout their aperture range and the only way you can overcome this is to either shoot in manual exposure mode while making your own exposure corrections or shoot in AV mode and apply exposure compensation.

Early manual SLRs that had closed aperture metering had to have the lens stopped down before exposure was measured, cameras that had open aperture metering measured with the lens wide open but had a mechanical linkage between the lens and the camera so that the camera knew what aperture was going to be used, this enabled the in-camera exposure meter to make the right adjustments. Later SLRs had electronic contacts between the lens and the camera which served the same purpose.

[Waddizzle]

---

@kvbarkley wrote:

This raises a question:

I always thought that the exosure was set with the lens wide open, but how does the camera know the maximum aperture of the lens? Nowadays that is easy since the lens tells the camera the maximum aperture. But lenses have *always* worked this way, even before electronics. How did the all manual SLR's know that you had installed an F/1.8 vs an F/1.4?

---

On some mounts, there was a mechanical lever connection between the lens and the camera that controlled the aperture.

[lin]

Thanks everyone for all of the responses, even the ones that missunderstood the question (my fault for giving the whole question in little chunks).

I am shooting in AV (aperture priority) mode with a 500mm telephoto lens that has no automatic functions; the camera doesn't know nor care what the lens is or what it is is doing, since it reads the light coming in and sets the shutter speed that in its estimate will produce the proper exposure.  The available apertures on the lens have no bearing on that estimate, and the AV system will work even shooting through a Coke bottle.

What prompted my question was that I get a different exposure in normal (mirror down) mode than I get in "live view" (mirror up) mode.

It seems that there are 3 possible reasons for this:

1- it is normal, due to the 2 different metering systems being used for the two different 'view' configurations and I need to pick the configuration that is best for my purposes.

2- I have overlooked some feature/setting in the camera that would account for this & can bring them into agreement by changing the appropriate setting.

3- the camera needs to be serviced to get these two configurations to produce the same exposure(s).

I'd like to know which of these I might be advised to pursue.

[ebiggs1]

lin,

Set a fixed ISO and try again.  Av is not manual mode.  It is a semi manual mode. Auto ISO just adds another adjustment the camera is allowed to make.

[RobertTheFat]

Since this discussion has already gone off in several directions, I might as well weigh in with a wild guess: This lens is so "manual" that it doesn't know how to maintain full aperture until the shutter button is pressed, like most modern lenses do. (That's tied in with the correct definition of the term "preset", but I won't pry open that can of worms. I did in a previous thread, but nobody took the bait.) When you're in viewfinder mode, the metering system thinks it's looking through the lens at full aperture (f/8), so it adds a stop of exposure to account for the fact that the picture will be taken at f/11. But in live view mode, the camera knows it's already seeing the scene at f/11, so it doesn't add an unnecessary stop to the exposure.

OK, I promised a wild guess, and that's what it is. It's probably wrong, because the metering system probably doesn't work that way. But it's about as plausible as most of what has already been said.

EDIT: Looking back, I now see that Tom said almost the same thing last night. So maybe it isn't so far-fetched after all.

[lin]

RobertTheFat

That seems to be the closest to the actual explanation, and it may be exactly correct, though I don't know why the camera assumes anything in AV mode (added to all of the other stuff I don't know).

I did succeed in eliminating option 3, I believe, by doing the same test with the same lens mounted on my Canon Xsi with the exact same results, so either both cameras are performing properly, or they both have the identical malfunction. Incidentally they were both set to a fixed ISO of 100.

FWIW, I will likely not incur the nuisance of setting both aperture and shutter speed in M mode since the rig is mounted on a sturdy tripod and the subject is essentially stationary, so I don't care how long the shutter is open and the camera can as they say, take its best shot.

FWIW2, I use live view to minimize camera shake which often (always?) results from mirror movement or having one's face planted against the viewfinder or both.

Thanks again for all of the help, I'll accomodate the camera since it won't accomodate me.

[Waddizzle]

"EDIT: Looking back, I now see that Tom said almost the same thing last night. So maybe it isn't so far-fetched after all."

Tom, is correct.  The one part that does not make sense is saying that the camera's aperture setting can be adjusted.  On a lens that is truly manual, the camera's aperture will read as "F00", and you cannot adjust it all. 

If a mount adapter with focus confirmation is being used, then that might explain why the camera does not show aperture as "F00", because it fools the camera into thinking that it is adjusting the aperture, when in fact it is not.  This will also fool the metering system, too.

With a variable aperture , manual lens, the more you stop down the lens, then the greater the difference in exposure readings between the viewfinder and Live View. 

[ebiggs1]

bob from boston and lin,

"It's probably wrong, ..."

Right, it is wrong!

I use that lens. It doesn't and can't change its aperturte unless you do it.

[RobertTheFat]

---

ebiggs1 wrote:

bob from boston and lin,

"It's probably wrong, ..."

Right, it is wrong!

I use that lens. It doesn't and can't change its aperturte unless you do it.

---

Go back and read what I wrote. It may or may not be wrong, but it's based on exactly what you're saying. The camera thinks the lens switches itself between its full aperture and the preset aperture, but it can't.

[Waddizzle]

---

@RobertTheFat wrote:

---

@ebiggs1 wrote:

bob from boston and lin,

"It's probably wrong, ..."

Right, it is wrong!

I use that lens. It doesn't and can't change its aperturte unless you do it.

---

Go back and read what I wrote. It may or may not be wrong, but it's based on exactly what you're saying. The camera thinks the lens switches itself between its full aperture and the preset aperture, but it can't.

---

Both of you guys have a valid point.  The aperture size does seem to fool the metering system, which the camera reads as "F00" on a a fully manual lens.  All of that stuff you mention about the camera setting an aperture at f/11 doesn't apply. 

I am not sure what the camera is thinking when it shows an "F00"  aperture.  One thing is certain, though.  The narrower the aperture becomes, the greater the difference in exposure between the viewfinder and Live View.  You get the least differences in exposure at the widest aperture settings on the lens.

But, it could work the way you describe on some mount adapters that have a focus confirmation chip.  I have used one and I have to set the camera's aperture to match the lens aperture to get accurate exposures.  Doing so also causes the Live View exposure to nearly match that taken through the viewfinder.