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01-25-2013, 05:29 PM   #1
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Question on Pentax F50mm f1.7

I have recently acquired this lens. So far everything is spot on. Clean, clear, sharp, nice.

It only has one small quirk and I thought I would ask about it here.

When I turn the rear control dial to the right my aperture reads 1.7 then it jumps to 2.0. When I turn it to the left though it goes from 2.0 to 1.8 then to 1.7...

There is that one extra little step in there. The lens seems to function just fine, just curious if anyone had any thoughts on this phenomenon.

I had an A series lens that would actually let me adjust the aperture 1.7 1.8.1.9 2.0 etc etc...that one would let me go in .1 increments.

Any thoughts?

01-25-2013, 06:06 PM   #2
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Mine does the same thing.

Also in Av mode f/2.0 yields 1/100 shutter speed, f/1.8 yields 1/125 and f/1.7 yields 1/100. Pictures taken at those settings indicate an underexposure @ f/1.8 but correct exposure @ f/2 and f/1.7 even though there is a 1/3 stop difference.

Maybe someone with an SLR can chip in and let us know if it worked that way on film or if this is just on the new bodies.
01-25-2013, 07:26 PM   #3
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At least I know I am not alone. So far the lens is sharp wide open. I like it. I wonder if the FA version is any more of an improvement over the F version?
01-25-2013, 07:27 PM   #4
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QuoteOriginally posted by alamo5000 Quote
At least I know I am not alone.
I'm pretty sure that mine behaves the same way.

01-25-2013, 07:37 PM   #5
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The FA works the same way.
01-25-2013, 07:37 PM   #6
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Mine is just like this as well, a little quirky but fantastic all the same.
01-25-2013, 07:54 PM   #7
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I wonder really how that new f1.8 stacks up compared to this lens?


I have also heard that the f1.4 (that is still in production) is a bit soft wide open.
01-26-2013, 12:54 AM   #8
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QuoteOriginally posted by alamo5000 Quote
I wonder really how that new f1.8 stacks up compared to this lens?


I have also heard that the f1.4 (that is still in production) is a bit soft wide open.
I'd always been under the impression that the 1.7 lenses were superior to the 1.4 in most scenarios.

01-26-2013, 02:34 AM   #9
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My FA f/1.4 is underexposed over a stop at 1.4, almost no difference at 2.0 (bokeh looks the same, same exposure, same sharpness etc) and 2.8 looks perfect. It works out as being a 2/3 stop difference between 1.4 and 2.8 in terms of light gathering, but with way better bokeh and IQ.

I wish I'd have gotten the f/1.7 now, but it's far from being worth switching to. The 1.4 is very sharp wide open, but aberrations stop it looking crisp. It resolves a lot of detail.
01-26-2013, 05:29 AM   #10
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I started a whole thread on what is indeed the same topic some time ago, which has, I think, a good explanation of why this behavior is ocurring:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/193855-f2-5-vs...ed-coding.html
01-28-2013, 03:25 AM - 1 Like   #11
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If you set the EV step to 1/2 stops the f-stop setting will go from 1.7 to 2.0 and 2.0 to 1.7. Why? 1.7 is one half stop between f1.4 and 2.0 while f1.8 is on a one third increment. Adding one-third of a stop to f1.7 would be f2.1 so it rounds down to f2.0.

One half stop (full stops bold)
1.4 - 1.7 - 2.0 - 2.4 - 2.8
One third stop:
1.4 - 1.6 - 1.8 - 2.0 - 2.2 - 2.5 - 2.8
One quarter stop
1.4 - 1.5 - 1.7 - 1.8 - 2.0 - 2.2 - 2.4 - 2.6 - 2.8
01-28-2013, 05:45 AM   #12
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Not a Number beat me to it. There's nothing more to it than that...
01-28-2013, 07:12 PM   #13
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QuoteOriginally posted by Not a Number Quote
If you set the EV step to 1/2 stops the f-stop setting will go from 1.7 to 2.0 and 2.0 to 1.7. Why? 1.7 is one half stop between f1.4 and 2.0 while f1.8 is on a one third increment. Adding one-third of a stop to f1.7 would be f2.1 so it rounds down to f2.0.

One half stop (full stops bold)
1.4 - 1.7 - 2.0 - 2.4 - 2.8
One third stop:
1.4 - 1.6 - 1.8 - 2.0 - 2.2 - 2.5 - 2.8
One quarter stop
1.4 - 1.5 - 1.7 - 1.8 - 2.0 - 2.2 - 2.4 - 2.6 - 2.8

On a K-5 what menu would this be on? I had a Pentax A series that went 1.7 1.8 1.9 etc etc...if I could make this F series do that it would be great.
01-28-2013, 07:21 PM   #14
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QuoteOriginally posted by alamo5000 Quote
On a K-5 what menu would this be on? I had a Pentax A series that went 1.7 1.8 1.9 etc etc...if I could make this F series do that it would be great.
The K-5 doesn't have 1/4 stop EV steps, only 1/3 or 1/2.
01-30-2013, 05:12 AM   #15
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QuoteOriginally posted by alamo5000 Quote
I wonder really how that new f1.8 stacks up compared to this lens?


I have also heard that the f1.4 (that is still in production) is a bit soft wide open.
There's a recent thread about all of these lenses and any comparisons between them.
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