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Old Mar 14, 2004, 2:01 AM   #21
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NHL

I agree with you and you disagree with yourself. You say "it's not about the light" and then you said that you need light to help with contrast detection. What (I think) you should say is that "it's not only about the light."

I just had a thought, can someone correct my thinking? If I put a TC on my lens, I haven't actually changed the aperture, I've just reduced the amount of light getting through it. So I have made the lens act like it has a smaller aperture, but it doesn't really have one. I don't argue your logic, because... well, I don't see the flaw in it. But I also think you're driving your point home too strongly. My 100-400 might have the equivalent physical aperture to give it the amount of light required to be considered f5.6 without a TC, but it sure doesn't AF well (in exactly the same situation) with the TC inbetween.... but the physical aperture hasn't changed. So the physical size of the aperture isn't enough... it's also gotta be about the light too (and how much the AF and prism require to work correctly.)

I've never tried to use the 50mm f1.8 in almost no light, but I'm going to try it just to see what you're talking about.

Quote:
...there's less and less of a difference in info where the AF sensors can use
I am not sure what you are trying to say here. I would have interpreted this to mean "there is not enough light for it to detect the contrast to enough detail to detect focus well." So it tries, failes, readjusts until it finally convinces its self it can't do better. But that doesn't seem to be what you are saying, as you keep saying its not about the light.

I fully agree that the 1Ds has a better prism & AF system. That is why it can work at f8 (if it's the amount of light that gives or the physical aperture size... either way, it works much better than the weaker AF/prism on the 10D.)

I'm kicking myself because at one time I read a really good description of why/how the aperture effects how well the AF system works. Of course, I can't find it now. I thought it was in the dpreview forums, but searching isn't finding it.

Eric
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Old Mar 14, 2004, 7:03 AM   #22
NHL
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Eric

The best way to illustrate this point is to get your hand on an old film SLR with a good split-screen for manual focusing. On a standard lens (or an f/2.8 ) lens with wide aperture, you can see the two 1/2 of the picture misaligned on the split prism area even in the dark indoor, so you can turn the focusing ring and bring the two halves into alignment...

Now when you put a lens with a smaller aperture on, ie an f/5.6 or slower on the same camera one of the 1/2 turn black even when there is more light than the above scenario, such as sunny outdoor instead of dimly lit indoor! This is why I said it's not about how much light was available: without the other split-screen 1/2 on small aperture lens, it's is very hard to focus even manually... and having to swith to the polished matte areas! Come on someone must have used a manual film SLR before in this forum?

This is what I'm talking about when you put on a TC which effectively reduces the aperture (or the effective diameter) of the lens barrel. IMO this info is toward the outer edge of the lens and is slowly lost as the aperture is closed down to a pinhole... the amount of light (ie scene brightness) only plays only a small part -> the effective opening of the lens is more critical!
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Old Mar 14, 2004, 12:17 PM   #23
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normcar - what is a 15-55 lens? Didn't you mean an 18-55?
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Old Mar 14, 2004, 3:42 PM   #24
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I actually have used a split screen manual focus canon before. I kinda liked that screen and miss it some times. It wasn't mine, though, so I only messed around with it. I might go see my brother this evening, though, so I'll try your experiment there.

Are you saying that putting in the TC, actually physically reduces the aperture? I'm very surprised by this, I thought all it did was focus a smaller portion of the available light on to the 35-mm/sensor frame. But, truly, I have no idea how it really works, that was just my mental image.

Eric
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Old Mar 14, 2004, 11:07 PM   #25
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Railfire...

I stand corrected. However, in the midst of Eric and NHL's wonderful diatribe, what's a few mms? You two amaze me with your knowledge!

I luv yu guyz!
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Old Mar 19, 2004, 3:27 AM   #26
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Gandalf,
How is the lens at the moment, allready some good shots with it.

On DPR the lens is destroyed in some threads which hurts because it's for the wrong reasons, everything is based on the slower AF, well ok that's an issue but most important for me is the outcome of the shots offcourse.

Greetings,
Frank
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Old Mar 19, 2004, 10:52 AM   #27
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Frank,
I am loving this lens! The AF is actually much more responsive than I originally gave it credit for. The times when the lens hurts are when something moves in between you and your subject, and the lens 'hunts' a bunch. As far as quality, I am very impressed. Of course I am also not the kind of person with laser lens testing equipment and a propenisty for foaming at the mouth!
If the weather hadn't gotten so dang cold about the day after I got the lens, I am sure I would have gotten some good bird shots. So far, I have excellent shots of a fox's tail, blank sky, squirrel's foot, ect... :lol:
I need more practice! But its snowing outside!
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