Sigma 170-500mm f/5-6.3 DG APO concerns

[DSLR noob]

If you point me to some reading on this, I would appreciate it. I have read two books on photography recently and have been doing a lot of reading on the internet and have never ran across this issue. It's also funny that the guy at the camera store (manager) wouldn't have told me about this.

Another question. Why is it that I get about the same results with manual focus. This is an autofocus issue, isn't it?

IS your di-opter correct for your eyes? also, the Rebel series uses a very smally viewfinder that is not very good for manual focusing maybe it's just too hard for you OR your camera to focus (not an insult to you, I personally wouldn't be able to MF on a rebel @ f/6.3, I never did when I had mine). The lens could be soft but I doubt it would be THAT soft.

 

[davebmck]

Yes, my di-opter is adjusted. I would agree at this focal length I can't do a better job than the camera by manual focusing. The point is that the camera is focusing, not hunting and is getting very close if that is the problem with my images.

I still do not understand the principle here. I know you need light to focus and that this lens will have problems focusing if there is insufficient light. What I don't understand is why I can't focus at a smaller aperature if the light is adequate, regardless of the maximum aperture of the lens. I sense that the answer lies in your statement "the camera is programmed to give up with what it's given after passing 5.6", but I don't know what that means. It seems you are telling me I can only use my camera at apertures of 5.6 or larger, but I know that is not right, so I must be misunderstanding what you are saying.

 

[Rhubarb]

DSLR Noob makes a very valid point. Let me try and fill in what appears to be the missing logic step.

When an SLR meters, auto focuses and when you look through the view finder, the lens is at its widest aperture; regardless of what aperture you have set it to. It is not until you press the shutter release button that the lens stops down to the selected aperture and triggers the shutter to make the exposure.

For example let’s say you are using a 400mm f4 lens. You want to make an exposure of f8 1/500th. You set your aperture to f8 and set your shutter speed. You look down the lens to compose and focus the shot. What you are seeing is the lens at f4 (its widest aperture) the camera uses this light and aperture to auto focus. You press the shutter release button, the lens stops down to f8 (the set aperture) the shutter releases and the exposure is made.

As DSLR Noob points out a lot of DSLR auto focusing systems are designed to work with the light from f5.6 or wider.

At 500mm your lens' widest aperture is f6.3, smaller than what your auto focusing system is designed to work with. This could be the cause of your problem.

The fact that you are shooting at f8 with this or any other lens is irrelevant, as the auto focus happens with the lens at its widest aperture.

I hope this helps.

I'm not sure exactly how you would test to see if this is your exact problem or not (it sounds like you may also have a tripod issue). I would also be of the mind that if there wasn't enough light for the AF system that it would hunt around, rather than consistently being slightly out of focus. That said I have never used a lens that slow before so I have no experience on how they should behave.

 

[JerryPH]

focusing at the fully zoomed in length isnm't possible. Canon EOS bodies are designed to focus at apertures of 5.6 or larger. A canon EOS 1 series body can manage f/8 or larger, but simply put, if you know the specs of your camera, you know why it doesn't autofocus at 500mm (because it maxes out at 6.3)

I do not know Canon cameras, but your post makes absolutely no sense.

Any dSLR camera's ability to focus is NOT based on the F/stop limitations of the lens, its based on available light and focus mechanism inside the camera. The camera controls the lens... not the other way around.

 

[Rhubarb]

I do not know Canon cameras, but your post makes absolutely no sense.

Any dSLR camera's ability to focus is NOT based on the F/stop limitations of the lens, its based on available light and focus mechanism inside the camera. The camera controls the lens... not the other way around.

But the f/stop has a direct affect on availble light, does it not?

 

[davebmck]

When an SLR meters, auto focuses and when you look through the view finder, the lens is at its widest aperture; regardless of what aperture you have set it to. It is not until you press the shutter release button that the lens stops down to the selected aperture and triggers the shutter to make the exposure.

Rhubarb, thanks. That is the piece of information I was missing to understand this characteristic of the camera. I'm still not sure this is my problem, but it is clear this would not be an optimum lens for this camera. It does hunt sometime when I don't expect it to. Any yes, my tripod is definately part of the problem. I will fix that this weekend.

Thanks again for everyone's help.
Dave