Could someone explain this to me.

First off, pardon the horrible diagram. My tablet is not working anymore and I suck with the mouse.

What I woud like to know is. When we look through our SLR's and focus on a subject., we are focusing using the center point on the mirror correct?
If the Digital Sensor or Film Plane is slightly behind the mirror, shoulden't our photos be less sharp or blury.

Sorry if this is a stupid question but I'm really tired and I can't stop thinking about it.

HTTP response: NOT FOUND | MIME changed to 'text/html'

I dont 100% know the answer but the point of focus liies on the film/sensor plane and not the mirror. I believe the optics in the viewfinder correects for the eyes view rather than the other way around.

What he said.

Here is an example i just drew up-sorry about the general crapiness-i plead the same excuse as you

HTTP response: 404 | Byte size: ZERO

Yes....

The incoming image is reflected by the mirror, but it does not ***focus*** on the mirror.. It focuses on the ground glass at the top of the camera..

Despite the fact in one case the light is reflected, the distance it has to travel to the ground glass or the sensor are the same.. We couldn't focus manually otherwise...

Remember, the system is exactly the same that has been used since the first SLR and one would have thought it might have been noticed in the last 50 years. Your sensor is in the same place a film is.

heh
Now that it has been explained to me I feel pretty stupid. It makes perfect sence. I appreciate you all helping me.
Never underestimate the power of sleep.
thanks again

pj

Here's your diagram with a little extra stuff:

HTTP response: NOT FOUND | MIME changed to 'text/html'

Note that autofocus is done from under the mirror as in the drawing i linked above.
Metering is usually done by a sensor above the focus screen in front of the pentaprism,this reads directly off the focus screen

Also i have done a very simple rendition of how light bounces around inside the pentaprism.A pentaprism is the weird shape it is because it has to turn the focus screen image both upside down and back to front for our eyes.It was invented in germany in the 30's i believe,and finally made it's debut on a production camera once the issues of dimness and image quality were sorted.When all the japanese got into camera making ,they thought so much of the pentaprism feature that alot od brands used it in their name -eg.pentax (or considered doing so in the case of nikon)

Actually, I was told be several different Reps that the sensor is ahead of the normal film plane, hence the reason for having to use the EX series flashes.

The older EZ series would not sinc up properly because they were designed and calibrated to the film plane. Using the EZ series flash with a digital body would cuase the picture to be underexposed because the flash fires after the image sensor captures the image.

I think the "TTL" and "E-TTL" have something to do with this as well. That's why you can still use an EZ series flash with a digital body only when you have both the flash and body in full manual mode.

You're correct - AF is done separately. In an effort to simplify, I drew the focus screen as the sole source of autofocus (it is the primary tool for manual focus).

I think that there are some more sophisticated drawings out there that illustrate the scheme well, but the mirror has characteristics where it reflects a proportion of the light up to the viewfinder, while a smaller portion of the light passes through to the secondary mirror which directs the image to the focusing sensor. The distance from the lens to the focus sensor is either equal to the distance from the lens to the sensor or is corrected with a small lens ahead of the focus sensor. Hopefully, I can dig up an image somewhere on the net.

Wow,that is amazing-one of the strangest stories i've heard
The odd thing is it could actually be true.
I'm going to see if i can find out by testing it.

Not sure about the flash theory-too tired to think right now,lol

If the sensor plane were a different distance from the lens, you would not acheive proper focus. Think of how an extension tube works - it eliminates infinity focus as it moves the lens farther from the focus plane. The inverse is true also - if the sensor plane is moved closer to the lens, close-focus will be lost.

There are other design reasons that make an EZ series flash incompatible with digital bodies. It sounds like the rep was discussing two different issues and ended up crossing them a bit.

OK, I just measured (roughly) the distance from the focus plane symbol on the camera to the flat plane surface of the lens mount on my Elan II (film) and my 1D-II. Both are approximately 1.70 inches. I confess that this is an eyeball measurement, subject to parallax error, but I did it as carefully as I could.

A little more thought on the older flash systems (TTL) - there were a few systems prior to E-TTL that used light reflected off the film surface to meter the flash output. The problem there is that digital sensors have drastically different reflection characteristics that don't lend themselves to off-axis metering. Thus, E-TTL with its preflash scheme needs to be used to achieve proper metering.

That may change in the future as technology changes, but its what we've got now, unless you want to use full manual flash (set the aperture based on the distance) or thyristor-controlled flash units.

Tom W, I don't doubt for one minute that you are correct and the Canon Reps are wrong.

If you've seen the last few reps we've had come through here, (3 different ones in 4 years!), you would know what I mean. I pretty much take what they say with a grain of salt. Even if their stories agreed with eachother, that could have been the fault of whoever told them their information at their Rep meeting.

Unfortunately, living in Florida, we often get the "I don't know, I don't care" type mentality from some of these reps, not just Canon reps mind you, but most reps in general.

It kind of sucks, if you know what I mean. That's why I am now trying to get better information on my own through these forums and from knowledgeable photographers like you and others here!