Canon Elan 7E, please explain the f5.6 limit on AF

greenspun.com : LUSENET : Camera Equipment : One Thread

I am a raw beginner with a Canon Elan 7E on the way. I have been reading as much as possible in the last few days to be prepared when it arrives. I read where the 7E only autofocuses "down to f5.6" when using a teleconverter, therefore it will not work AF if the effective lens + TC minimum f is f8. This doesn't make sense to me since I assume that the AF will work at any f stop if a lens (without TC) is stopped down to say f2.4, f8, f16 or more. ie won't it work AF through all of the range of a given lens?? Is this something unique to the use of the TC? Perhaps this is the dumbest question ever but I'm sure confused..!

-- Venado Loco (deertex@swbell.net), January 24, 2001

Answers

I just thought I would comment on Kurt's post. I don't own any TCs, but understand the operation and limitations. From what others have said, some third party (Sigma, Kenko, whoever) TCs don't communicate the new effective aperture. Since the camera still thinks the maximum aperture is f5.6 (or larger), it will attempt to autofocus. It can be slow and unreliable though. I also think the ability to focus has as much to do with deapth of field as it does light. Autofocus cameras need lines to focus on, and most focus sensors aren't sensitive enough at f8 due to deapth of field and contrast. Of course, more light allows for greater contrast, and improved focusing sensitivity, so both are important.

-- Brad (bhutcheson@iname.com), January 25, 2001.

Autofocus will work as long as the WIDEST aperture you can achieve is 5.6 or wider. When you use a teleconverter, the widest you can go may be f/8, in your example.

Modern cameras autofocus and meter with the lens wide open, then stop down the iris to your selected aperture after you press the shutter release and before opening the shutter.

The reason for this is that metering and AF can be more accurate with more light. The camera adjusts the meter reading accordingly by taking into account the difference between the lens's widest aperture and what you've selected.

If you use a lens with a widest aperture smaller than f/5.6, enough light won't get to the AF sensors. Higher-end cameras can do slightly better by having larger or more sensitive sensors that cost more, but even some high-end cameras have this limitation.

-- Mark Wilkins (mark_wilkins@yahoo.com), January 25, 2001.


When I say "widest aperture smaller than f/5.6" I mean a widest aperture like f/8 or f/11 -- a smaller aperture, not a smaller f-number.

-- Mark Wilkins (mark_wilkins@yahoo.com), January 25, 2001.

Mark, thank you so much. It is now clear. The fact that the autofocus takes place with the "largest opening" and then stops down to the selected "opening" was the key I needed for it to make sense...!!

-- Venado Loco (deertex@swbell.net), January 25, 2001.

I seem to recall reading somewhere that Canon actually designed into these systems a type of shut-off mechanism which will disable AF when TCs are combined with certain EOS lenses, when combined with those EOS cameras which are not designed to reliably auto-focus at an effective maximum aperture of f8. (The same type of mechanism might also be operative when, for example, the 600/4 is mated with the 1.4x and 2.0x TCs *in tandem*). In other words, it may be that if this automatic shut-off mechanism was absent, these less "sensitive" cameras might occasionally be able to achieve AF with f8 lenses, but Canon engineers deemed it to be too unreliable, and took steps to simply dis-allow AF altogether with certain combinations. I believe this type of information is communicated to a given EOS camera's microprocessor via the electrical contacts of the EOS lens mount, and I seem to also recall that some after-market TCs may circumvent this shut-off mechanism. Hopefully, someone with more knowledge about this issue will chime in and clarify.

-- kurt heintzelman (heintzelman.1@osu.edu), January 25, 2001.


The AF does indeed work on the max effective aperture. I'm not sure exactly how the system works, but it is not possible to "dodge" the system with a smaller aperture. The problem with the third party TCs trying to get around the AF thing by not reporting the change in effective aperture is that the metering records the supposed full aperture. While it will not be inaccurate in it's results, the number it reports is. EG, a Canon 300F4L IS used with a third party 1.4X and with a Canon 1.4X. With the third party, it will report the aperture as being F4, but with the canon, it will report 5.6. Both will expose accurately, because the light falling on the sensor is the same. However, it becomes an annoyance to those who record exposure information, or to a camera that does (1V). Also is a pain if you're using flashes (although admittedly it is not often that one will use a flash with a TC). Coming back to the AF limit, it has nothing to do with TCs. If you were to go out and buy one of those bottle-glass 800- 1200 zooms, which are something hideous like F11-F27, you'll find they're manual focus because there's no point putting AF in. The aperture is not big enough. The only confusion arises because of the EFFECTIVE aperture of the lens with a TC, which the canon TCs report to the camera, and third party TCs don't. For the newbies, 1.4X TC effective aperture is lens aperture -1 stop. 2X TC is -2 stops.

Many people are confused by this non-linear relationship. "Surely the 2x TC is a better deal of magnification vs light loss?" They are in fact the same. Stack 2 1.4TCs and you will loose 2 stops. 1.4x1.4 is....2!

-- Isaac Sibson (isibson@hotmail.com), July 09, 2001.


Moderation questions? read the FAQ