Question regarding lens apperture

[OviV]

I purchased a Sigma 170-500 5-6.3 APO from Bob Gross. I am very pleased with it so this is not a complaint. I have a question that neither Bob nor I know the answer to. I took some pictures with the lens yesterday and I am getting a max apperture of 5.6 at 500MM. I was under the impression that my max app at this focal length would be 6.3. I was able to stop the lens down so lens is functioning correctly. Does anyone have this lens that has experienced this? Here is the picture with exif data:

http://www.shareapicture.com/vsalbum/getpic.aspx?fld=\OviV\Misc&pic=CRW_4549.jpg&OV=0

http://www.shareapicture.com/vsalbum/getpic.aspx?fld=\OviV\Misc&pic=exif.jpg&OV=0

Ovi

[CyberDyneSystems]

Because the standard Canon SLR bodies will refuse to even try to autofocus with an aperture smaller than f/5.6 Sigma (and other manufacturers) have lenses that "trick" the camera into beleiveing the lens is f/5.6 at the long end of the zoom.

This does not however adversely effect exposure.

Thus what is displayed to you is what the lens is "lying" to the Camera about. :)

This is further evidence that the "tape trick" with a t-con is allways worth a shot.. as these lenses pretty much allways work just fine while fooling the camera that the lens is within it's specs.

[OviV]

Ahh! I knew that there was a logical answer. Cool.

Thanks CDS.

[eric1]

hi Ovi,
CDS is right, and it will work fine, it may just AF a little slower at maximum aperature. also you might have to bump the shutter twice to get a good focus, sometimes the camera gets fooled when the light isn't so good.

[vfilby]

OT:

I presume that you can autofocus with the aperture smaller than 5.6 because it leaves the aperture wide open until you click the shutter. Is this correct?

Thanks,

Vince

[FrenchAmateur]

OT:

I presume that you can autofocus with the aperture smaller than 5.6 because it leaves the aperture wide open until you click the shutter. Is this correct?

Thanks,

Vince

Not really, all the camera with all the lenses do so.
It's only because the sensitivity of the camera allows it, like it can to at certain level in low lights condition. The problem for the camera is the same than with low light I think.

[Jesper]

But if the lens is lying about the aperture, how come it doesn't affect exposure??

[MarkH]

But if the lens is lying about the aperture, how come it doesn't affect exposure??

Obvious answer:
At f6.3 you have about 1/3 stop less light, under exposing by 1/3 is no big deal so the photo comes out fine.

[OviV]

I think the reason it does not affect exposure is because the camera calculates exposure based on the available light that it "sees" not the apperture settings. So I would imagine that the camera adjusts shutter speed to compensate. I could also be completely wrong. :)

[CyberDyneSystems]

It doesn't under-expose unless you shoot in Maual mode and do your own exposure calculation using the incorrect aperture (ie: f/5.6 instead of f/6.3) ....and why would you do that?



1. Canon and any modern SLR that Sigma is making this lens for uses TTL metering.. so as long as you use the cameras meter it will make the correct exposure based on light coming through the lens.. which is light coming through at f/6.3

So the shutter speed is still be used to make correct exposure with the light available at f/6.3 even if the lens says it's f/5.6... the camera does not use the leses reorted aperture to make exposure.. it uses what the meter tells it.

2. I'm not sure about your lens in particular.. but most designs "fake" the f/5.6 only when AF takes place. With all SLRs the lens is "normally" at it's widest aperture ALLWAYS for AF and only stops down to the desired settings after AF has occured. In this way the lens only lies during AF and can then treat the f/6.3 as being stopped down... just like takeing a pic with an f/2.8 lens stopped down to f/8 for example.