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Old Jul 19, 2005, 6:39 PM   #1
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Is there any versions sigma 180mm f/3.5 macro an HSM and non-HSM version.

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Old Jul 19, 2005, 7:08 PM   #2
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pretty sure there is only a 180mm 3.5 ex HSM.. and thats the only version..

there is also a 150mm 2.8 EX HSM..

to go non-hsm you have to go down to 105mm 2.8...
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Old Jul 20, 2005, 1:34 AM   #3
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It may sound silly, but why go non-HSM ... price?
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Old Jul 20, 2005, 1:43 AM   #4
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yes, price would be the only reason...

but if you were only going to use this lens for macro.. HSM is not really a big issue as you are eventually going to learn that you need to manual focus for real macros, and if not, slower AF is not a big issue..

but if you plan on using your macro for other uses, the HSM becomes more important..
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Old Jul 20, 2005, 3:23 AM   #5
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I've noticed AF gets crazy for macro... and it is also well documented in this forum. Anybody knows why (the deep mathematical/physical reason(s), I mean )?
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Old Jul 20, 2005, 10:32 AM   #6
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Well, I'll take another guess attempt with why Macro has a hard time focusing.

First you need to know how AF on a dSLR works. If you don't know, you can read the next three paragraphs.

Typically, on a P & S, the AF is relatively simple. It uses a infrared waves to bounce off the subject, return to the camera, and do the necessary calculations (pretty much how a speedometer works). (FYI- this is why you can focus in the dark). On a typical dSLR, though, there are horizontal and vertical sensors to "look" at an image, itself - The sensors being large as pixels. I made a quick sketch below to help me explain. Now let's take a gander at the image in the example; it's an AF point on your camera looking at a vertical stripped wall.

Now the computer in your camera will have a basis: continue adjusting the focus until the vertical and horizontal lines of pixels have the highest contrast. What this means, for the example below, is that in every sensor, there should only be black or white, that's a high contrast because you can see one pixel being black and then the next one being white - very different colours for two pixels beside each other. Now if there was some blend of black and white (grey), the contrast would be low, which generally leads to the computer to thinking the image is not in focus, and will continue to search for the highest contrast.

Note that in this case the vertical line of pixels are useless because the image is a vertical strip of either black or white. This also explains why dSLRs cannot focus in the dark or at blank, uniformly coloured spaces- there is no contrast in those scenarios.

Ok, now that everyone knows how the AF works, lets take a look at MACRO. Extremely shallow DoF is mentioned as being one nasty nemisis of dSLR AF. Why? The blurred areas causes LOW contrast. Gasp! But the computer wants HIGH contrast!

Another reason (and this one is pure guessing on my part and is probably not right) might be the computer has a tough time picking a good scenario. There could be two reasons that the AF will take so long. Either there are a ton of low contrast scenarios (the one mentioned above) or there's a lot of HIGH contrast scenarios. If you're that close to a subject, (say a colourful butterfly), the computer might have a hard time with DOF, but lets say that there's a part of the colourful wing in focus- that's a lot of detail to cope with. Maybe there are so many focus adjustments that CONTINUE to be in high contrast.

And remember that this is averaged. The computer won't know which high contrast you'd prefer.

Again, that last part was guessing, but the DoF- issue would be the main reason.
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Old Jul 21, 2005, 3:04 AM   #7
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Thanks BoYFrMSpC

The autofocus is well explained! I also like the text you can find here

http://electronics.howstuffworks.com/

I would agree the shallow DoF is the key, and your "pure guessing"sounds like a typical multipleminima issue in a minimization loop which makes sense (to me at least...).

I am sure NHL and/or LBoy must know the answer...?
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Old Jul 21, 2005, 7:26 AM   #8
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oh by the way, if its manual focus you want just flick the switch on the lens fron AF to M and you now have complete control over it (as turn off the HSM). i myself prefer the 150 2.8 sigma macro over the 180 3.5. i use it FF with a 1Ds2 and crop frame with a 1D2.

this of course allows complete focus/DOF placementcontrol when using a good macro rail set up like the RRS one.
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Old Jul 21, 2005, 7:53 AM   #9
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lol, that's where I learned about the AF in the first place.

howstuffworks.com = best
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