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Old Mar 7, 2008, 6:57 AM   #1
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is it worth the money to switch to a sigma lens? my minolta is really slow to focus and dark indoors. what do you guys think?



sorry meant to say sigma 70-300
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Old Mar 7, 2008, 7:35 AM   #2
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Get a brighter lens for indoor use. Long, dim zooms and indoor lighting are not a good match. Indoor lighting is much brighter to the human eye that it will be to a camera wearing this type of lens. You'll want a brighter lens (larger available apertures, represented by smaller f/stop numbers) so that the camera's AF sensors can "see" better when indoors. Ditto for your viewfinder image.

There is only 1/3 stop of difference in brightness between these two lenses on the wide end (f/4 versus f/4.5), and they're the same on the long end (f/5.6). I doubt there is much difference in the focus gearing either. Neither one of them are going to be very good choices for low light use (and indoors is low light to a camera).



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Old Mar 7, 2008, 12:13 PM   #3
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the slow focusing was my real problem with it.
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Old Mar 7, 2008, 12:30 PM   #4
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The main reason you're seeing slow focusing is that you're using a long, dim lens. The AF sensors can't see well enough indoors with dimmer lenses.

Both the Minolta 75-300mm f/4.5-5.6 and Sigma 70-300mm f/4-5.6 lenses fall into this category.

You're better off sticking to a shorter focal length lens indoors if you don't want a brighter lens, so that the AF mechanism has lighter optical elements (and usually faster gearing), with the AF sensors still seeing sufficient contrast so that it's not too bad (especially when you add camera shake into it, which gets magnified at longer focal lengths). The AF sensors need to see to Autofocus, and long, dim lenses are not what you want to use if you want faster AF indoors.

If you plan on needing a longer lens in lower light, look at a 70-200mm f/2.8 AF lens of some flavor if you need a zoom.

Tamron has a new 70-200mm f/2.8 AF lens coming out in Sony/Minolta Mount for $699 (and http://www.adorama.com is taking preorders for it now). Most f/2.8 zooms in this focal range are also geared to focus much faster. Also, Sigma will be offering an HSM (Hypersonic Motor focusing) version of it's 70-200mm f/2.8 EX DG Macro lens in Minolta/Sony mount (and you can preorder it now for around $799).

Even if you don't need a longer focal length, I'd still stick to brighter lenses if you want faster AF in low light. One reason primes are popular is because they're so much brighter than most zooms. This helps both focus speed and shutter speed. I use a Minolta 28mm f/2, 50mm f/1.7, 100mm f/2 and 135mm f/2.8 with my KM 5D and Sony A700 in lower light. For example, my 100mm f/2 is 8 times as bright as many consumer level zooms are by the time you zoom in much (because their largest available aperture is usually f/5.6, and f/2 is 8 times as bright as f/5.6).

Even without using a prime (fixed focal length), a high quality zoom that can maintain f/2.8 throughout it's focal range is still 4 times as bright as most consumer grade zooms by the time they're zoomed in much (f/2.8 is 4 times as bright as f/5.6).

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