Question on Pentax F50mm f1.7

alamo5000 · 01-25-2013, 05:29 PM

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?

**jatrax** · 01-25-2013, 06:06 PM

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.

alamo5000 · 01-25-2013, 07:26 PM

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?

alphanerd · 01-25-2013, 07:27 PM

Originally posted by alamo5000

At least I know I am not alone.

I'm pretty sure that mine behaves the same way.

**boriscleto** · 01-25-2013, 07:37 PM

The FA works the same way.

Franky2step · 01-25-2013, 07:37 PM

Mine is just like this as well, a little quirky but fantastic all the same.

alamo5000 · 01-25-2013, 07:54 PM

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.

Eulogy · 01-26-2013, 12:54 AM

Originally posted by alamo5000

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.

Mareket · 01-26-2013, 02:34 AM

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.

macTak · 01-26-2013, 05:29 AM

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

Not a Number · 01-28-2013, 03:25 AM - **1 Like**

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

bdery · 01-28-2013, 05:45 AM

Not a Number beat me to it. There's nothing more to it than that...

alamo5000 · 01-28-2013, 07:12 PM

Originally posted by Not a Number

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.

**boriscleto** · 01-28-2013, 07:21 PM

Originally posted by alamo5000

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.

snake · 01-30-2013, 05:12 AM

Originally posted by alamo5000

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.

01-25-2013, 05:29 PM	#1
alamo5000 Pentaxian Join Date: May 2012 Posts: 2,962	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?

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Pentax F50mm f1.7 infinity focus.	David&karen	Pentax SLR Lens Discussion	5	07-16-2012 05:42 AM
For Sale - Sold: Pentax F50mm f1.7 , K28mm f3.5	Gr8erimage	Sold Items	5	12-22-2009 10:43 AM
For Sale - Sold: Pentax F50mm f1.7	ivoire	Sold Items	5	09-26-2009 10:11 PM
Pentax smc F50mm f1.7	RussellW	Pentax SLR Lens Discussion	9	05-26-2008 11:27 PM

01-25-2013, 06:06 PM	#2
jatrax Site Supporter Join Date: May 2010 Location: Washington Cascades Photos: Gallery \| Albums Posts: 12,991	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
alamo5000 Pentaxian Join Date: May 2012 Posts: 2,962 Original Poster	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
alphanerd Senior Member Join Date: Mar 2011 Posts: 119	Originally posted by alamo5000 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
boriscleto Loyal Site Supporter Join Date: Dec 2009 Location: North Syracuse, NY Photos: Gallery Posts: 16,475	The FA works the same way.

01-25-2013, 07:37 PM	#6
Franky2step Veteran Member Join Date: Aug 2011 Location: Niagara Photos: Gallery \| Albums Posts: 793	Mine is just like this as well, a little quirky but fantastic all the same.

01-25-2013, 07:54 PM	#7
alamo5000 Pentaxian Join Date: May 2012 Posts: 2,962 Original Poster	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
Eulogy Veteran Member Join Date: Feb 2012 Location: Fredericton, New Brunswick Photos: Gallery Posts: 632	Originally posted by alamo5000 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
Mareket Veteran Member Join Date: Jun 2011 Location: Chester Photos: Gallery \| Albums Posts: 719	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
macTak Veteran Member Join Date: Apr 2010 Photos: Gallery Posts: 759	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
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	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
bdery Pentaxian Join Date: Mar 2008 Location: Quebec city, Canada Photos: Gallery Posts: 9,360	Not a Number beat me to it. There's nothing more to it than that...

01-28-2013, 07:12 PM	#13
alamo5000 Pentaxian Join Date: May 2012 Posts: 2,962 Original Poster	Originally posted by Not a Number 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
boriscleto Loyal Site Supporter Join Date: Dec 2009 Location: North Syracuse, NY Photos: Gallery Posts: 16,475	Originally posted by alamo5000 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
snake Banned Join Date: Jun 2012 Location: NY/Germany Posts: 1,183	Originally posted by alamo5000 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.