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Old Feb 23, 2007, 5:58 PM   #21
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JohnG wrote:
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I have had the EF 1X4 for only a couple of days but I can recommend it. I use it on my 70-200 F4 L and it focuses all the way from F5.6 to F16 without a problem.
Just a point of clarity - a lens ALWAYS uses thewidest availableaperture to focus and closes down to take the photo. So, even though you set the aperture to f16 the camera/lens still focuses at 5.6. You may be aware of this but others reading this may not be
MMM I may be a fool but I really dont understand that LOL
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Old Feb 23, 2007, 6:29 PM   #22
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aladyforty wrote:
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JohnG wrote:
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I have had the EF 1X4 for only a couple of days but I can recommend it. I use it on my 70-200 F4 L and it focuses all the way from F5.6 to F16 without a problem.
Just a point of clarity - a lens ALWAYS uses thewidest availableaperture to focus and closes down to take the photo. So, even though you set the aperture to f16 the camera/lens still focuses at 5.6. You may be aware of this but others reading this may not be
MMM I may be a fool but I really dont understand that LOL
OK, the aperture is like an iris of an eye - it's an opening. the lower the f-stop the wider the opening. f 2.8 is wider than f16 - which I believe you get. Basically, even though you set the camera to have an aperture value of f16, that opening doesn't immediately change. Until the shutter actually fires, the aperture stays as wide as it can. On a 70-200 f4 lens that means it stays at f4. With a TC, it means the opening is 5.6. At the split second that the picture is taken, the opening closes down from it's widest setting to the setting the camera tells it to. So, the focusing system of the camera always gains the benefit of the maximum available aperture. In the case of this lens it will always focus at f4 (or 5.6 with a TC) and allow the maximum amount of light in. The addition of a TC and it's glass eats up some of that light that's why the affect is a one stop loss. The lens itself is still 4.0, but it loses some light as it passes through the TC. So, the net affect is just like you had a lens whose maximum opening equated to f5.6.

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Old Feb 23, 2007, 7:24 PM   #23
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Thanks, that explains why the 2.8 is more expensive than the F4 then
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Old Feb 24, 2007, 5:04 AM   #24
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aladyforty wrote:
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Thanks, that explains why the 2.8 is more expensive than the F4 then
-> It also explains why an f/2.8 is heavier - the same lens area has to be twice as large to let in twice the light as an f/4!

So bigger optics also require more metal to keep the whole thing tight... :lol: :-) :G
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Old Feb 24, 2007, 7:00 AM   #25
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Well, I have learned a ton since I posted this question...thanks to everyone. donna
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Old Feb 24, 2007, 7:40 AM   #26
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... and the lesson continues as it relates to the focal lenght!

A Tedious Explanation of the f/stop: " the f/stop is a ratio. The ratio is between the diameter of the aperture in the lens and the focal length of the lens"
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Old Feb 24, 2007, 7:24 PM   #27
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I play around with F stops a fair bit. The way I read the article, if you stop down the lens you get less light. So why is it that if I shoot a photo of say a bird at F11 it appears sharper than the same shot at F8? I would have thought that if the camera is set to AV F11 that the shutter speed that compensates would be slower than if I set the camera to F8 so I would imagine the photo should be less sharp at least handheld which is what I do most of the time.??????
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Old Feb 24, 2007, 8:39 PM   #28
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less light but greater depth of field (so more area before and behind your focus point is in focus).

http://www.dofmaster.com/dofjs.html

lack of sharpness could be any one or combination of:

camera shake, motion blur, too shallow DOF, missed focus. More often than not it's simply a miss focus. Just because your focus point is on the subject doesn't necessarily mean the camera accurately figures out the distance. light levels and contrast play a part. With a greater DOF, mis-focus isn't as noticable but more often than not it's the culprit (unless you're shooting at shutter speeds of like 1/60 with a 200mm plus lens)
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Old Feb 24, 2007, 11:56 PM   #29
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so would less light equal more noise?
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Old Feb 25, 2007, 5:45 AM   #30
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aladyforty wrote:
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so would less light equal more noise?
NO - It depends on where you started with:
-> less light with smaller aperture (@ f/16) in full sunshine is not the same as more light at larger aperture (@ f/4) in a dark gymnasium...

http://en.wikipedia.org/wiki/Exposure_value



For example (see chart below) - f/11 and f/8 in a typical full sunlight @ EV-15 you can use 1/250 and 1/500s shutter for the same exposure and the noise level would be about the same (or just about any other apertures with the corresponding shutter speed)

However at the same f/11 but at night sports the EV value is now only at EV-9 - The shutter would have been set to 1/4s for the same exposure, but the noise would have increased quite a bit... because the overall light has dimmed a lot (and you would have upped the ISO to get a satisfactory shutter speed for handheld even @ f/4 which now requires 1/30s)

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