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05-27-2016, 01:26 AM - 1 Like   #31
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QuoteOriginally posted by wfullerton Quote
A Concise Explanation of How Crop Factor Affects Both Focal Length AND Aperture[/url]
Depth of field yes...exposure, no.

05-27-2016, 09:57 AM   #32
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QuoteOriginally posted by wfullerton Quote
I was reading reviews of the 50-135 on B&H and Amazon from 2014 and 2015 where people said they were still having the SDM problem. Also a lot of people say its really slow to focus. Those are two pretty major issues that are keeping me from this at the moment. What do you all think?
We've already been telling you what we think. SDM is slow and quiet and used to be failure prone. Screw drive is fast and not so quiet and rock solid. A lot of people don't realize how easy it is to change to screw drive (it doesn't help that the original instructions were way, way, way more involved than they needed to be, talked about using software you didn't actually need to use, etc - the process is something like "write some text to a text file, copy it to your SD card, turn on the camera with the card door open, page over to debug mode, write the lens data to the card, pop the card out, open the data file in a decent text editor - not a word processor - change 2 characters to 2 other characters in 3 or 4 locations, save the file, shove the card back into the camera, write the updated lens data back to the lens, and close the sd card door. - instructions may vary by body) and back (same thing, but reversing the change in the two characters).

There's a whole section of lens reviews on here that are considerably more comprehensive than Amazon or B&H, if you need to read 100+ reviews of a lens to make up your mind. Seriously, unless you're planning to photograph live music recording inside a sound booth or something, SDM isn't particularly important. I just wish I could turn it off on my 60-250, but no one seems to have figured that out yet.
SMC Pentax-DA* 50-135mm F2.8 ED [IF] SDM Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database

if you're worried about it failing, just buy one used that's already failed and been converted to screw drive. It'll be cheaper and you won't have to convert it yourself. Or if you desperately need silent lenses, buy one used that's already had the repair done & documented.
05-27-2016, 01:52 PM   #33
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Aperture does not change. An F2.8 lens still opens to F2.8 and still gives an exposure of an F2.8 lens. If a user set the exposure thinking an F2.8 lens was an F4 lens it would be overexposed.

What does change is depth of field and equivalent focal length.
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