Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
03-29-2021, 07:41 AM   #1
Closed Account
Michael Piziak's Avatar

Join Date: Dec 2013
Location: West Virginia
Posts: 2,815
Finding the sweet spot of my lens question

I've been reading about the sweet spot or sharpest f stop of a lens. One general rule of thumb is 2 to 3 stops from wide open (with 1.4 being equal to one stop).

I thought I'd put a sticker on each of my 3 most used lens as to where this would be. Please tell me if the calculations are correct and/or also if you've used any of these lens what you're experience is.


For the SMC Pentax-M 100mm F4 Macro Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
F4 being wide open, the calculated sweet spot would be between f6.8 & f8.2

For the SMC Pentax-DA 55-300mm F4-5.8 ED Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database
At 300mm zoom, F5.8 being wide open, the calculated sweet spot would be between f8.6 & f10

And for the SMC Pentax-M 50mm F4 Macro Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
Again, F4 being wide open, the calculated sweet spot would be between f6.8 & f8.2

03-29-2021, 07:49 AM   #2
Loyal Site Supporter
Loyal Site Supporter




Join Date: Oct 2018
Location: Quebec City, Quebec
Posts: 6,493
Sharpest openings are usually f/8 to f/11 on K mount lenses. Images start degrading past f/16 on these lenses. Medium format optics tolerate f/22. Fast or highly-corrected lenses may perform very well starting at f/4 (FF) or f/5.6 (MF). My new DFA* 70-200 mm f/2.8 peaks @ f/6.3 for example.

P645 FA 33-55 mm f/4.5 @ f/22

DFA 100 mm f/2.8 @ f/8

DA 55-300 mm @ f/11

DFA* 70-200 mm @ f/6.3

Last edited by RICHARD L.; 03-30-2021 at 05:18 PM.
03-29-2021, 08:00 AM   #3
Closed Account
Michael Piziak's Avatar

Join Date: Dec 2013
Location: West Virginia
Posts: 2,815
Original Poster
Thanks for the reply. Perhaps I should have added that my camera is APS-C
03-29-2021, 08:09 AM   #4
Loyal Site Supporter
Loyal Site Supporter




Join Date: Oct 2018
Location: Quebec City, Quebec
Posts: 6,493
I knew, since you are using a DA 55-300 mm. Death Valley sand dunes were shot with a K5, the rainy leaves with a K3 and the MF "fence" image with a 645Z.

Regards


03-29-2021, 09:27 AM   #5
Site Supporter
Site Supporter




Join Date: Sep 2014
Location: SF Bay Area, CA
Photos: Albums
Posts: 1,037
Depends on the lens, but 2 stops is a pretty good rule of thumb.

And it is not X + 1.4 + 1.4

it is (X * 1.4) * 1.4

So f/5.6 - f/8 is typically the sweet spot on APS-C
03-29-2021, 09:36 AM   #6
Pentaxian
Kozlok's Avatar

Join Date: Jun 2012
Location: Albuquerque
Photos: Gallery
Posts: 3,145
I just assume it's f8 for all lenses on APS-C. Mostly there is little difference between 5.6 and 8, so 8 has a little more DOF and usually better edges. If the light is dim, I may choose 5.6 just to keep shutter speed up and ISO down.

For slower lenses, it still seems to be 5.6 to 8, but it depends on if you are talking about in the center, across the frame, in the corner. That's all lens to lens dependent. Optical Limits has some charts for some lenses. For example, the 18-135 at 85mm : edges are better at f11, but center is worse. I think f8 gives a nice compromise between center and edge at 85mm for that lens.

Name:  18_135 at 85.JPG
Views: 451
Size:  40.5 KB

Last edited by Kozlok; 03-29-2021 at 09:43 AM.
03-29-2021, 09:45 AM   #7
Closed Account
Michael Piziak's Avatar

Join Date: Dec 2013
Location: West Virginia
Posts: 2,815
Original Poster
QuoteOriginally posted by caliscouser Quote
Depends on the lens, but 2 stops is a pretty good rule of thumb.

And it is not X + 1.4 + 1.4

it is (X * 1.4) * 1.4

So f/5.6 - f/8 is typically the sweet spot on APS-C
So would an f4 lens be f7.84 to 10.976 (or about f8 to f11)

and f5.8 lens be f11.368 to 15.9152 (or about f11 to f16)

?

---------- Post added 03-29-21 at 09:50 AM ----------

QuoteOriginally posted by Kozlok Quote
I just assume it's f8 for all lenses on APS-C. Mostly there is little difference between 5.6 and 8, so 8 has a little more DOF and usually better edges. If the light is dim, I may choose 5.6 just to keep shutter speed up and ISO down.

For slower lenses, it still seems to be 5.6 to 8, but it depends on if you are talking about in the center, across the frame, in the corner. That's all lens to lens dependent. Optical Limits has some charts for some lenses. For example, the 18-135 at 85mm : edges are better at f11, but center is worse. I think f8 gives a nice compromise between center and edge at 85mm for that lens.

Attachment 529374
Thanks for your reply - points well taken

03-29-2021, 10:17 AM   #8
Otis Memorial Pentaxian
stevebrot's Avatar

Join Date: Mar 2007
Location: Vancouver (USA)
Photos: Gallery | Albums
Posts: 42,007
The big question might be whether one can differentiate between missed focus and lower sharpness at wider apertures.

That said, and as stated above, conventional wisdom is stopped one or two from wide open for both resolution and contrast. For an f/4 lens, that would be f/5.6 or f/8. In practical terms, even if equally sharp wide open, narrower DOF works against the sharpness proposition such that one is usually working one or two stops done from maximum anyway.


Steve
03-29-2021, 10:33 AM - 1 Like   #9
Pentaxian
Fogel70's Avatar

Join Date: Sep 2009
Location: Stockholm, Sweden
Photos: Albums
Posts: 3,062
On high resolution cameras diffraction can start to limit resolution as early as f/5.6, if it is a fast and good lens.
Many fast prime lenses perform better wide open than at f/8, at least in the center of the image.

Sweet spot will differ depending on resolution of the sensor, where sweet spot comes at earlier f-stops with higher resolution (smaller pixels), if the lens is good enough.

On high resolution cameras I have also seen examples of zoom lenses performing best wide open.

Last edited by Fogel70; 03-29-2021 at 11:03 AM.
03-29-2021, 10:51 AM   #10
Closed Account
Michael Piziak's Avatar

Join Date: Dec 2013
Location: West Virginia
Posts: 2,815
Original Poster
QuoteOriginally posted by stevebrot Quote
The big question might be whether one can differentiate between missed focus and lower sharpness at wider apertures.

That said, and as stated above, conventional wisdom is stopped one or two from wide open for both resolution and contrast. For an f/4 lens, that would be f/5.6 or f/8. In practical terms, even if equally sharp wide open, narrower DOF works against the sharpness proposition such that one is usually working one or two stops done from maximum anyway.


Steve

Interesting. You say one to two stops, instead of two to three stops, and for the reasons you've stated.
03-29-2021, 11:26 AM - 2 Likes   #11
Master of the obvious
Loyal Site Supporter
savoche's Avatar

Join Date: Jun 2011
Location: Lowlands of Norway
Posts: 18,309
QuoteOriginally posted by Michael Piziak Quote
I've been reading about the sweet spot or sharpest f stop of a lens. One general rule of thumb is 2 to 3 stops from wide open (with 1.4 being equal to one stop).
Like Steve said, most would say 1-2 stops from wide open is generally where a lens is sharpest. However, this will vary from lens to lens (also from copy to copy).

QuoteOriginally posted by Michael Piziak Quote
For the SMC Pentax-M 100mm F4 Macro Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
F4 being wide open, the calculated sweet spot would be between f6.8 & f8.2

And for the SMC Pentax-M 50mm F4 Macro Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
Again, F4 being wide open, the calculated sweet spot would be between f6.8 & f8.2
I don't have these two, but other macro lenses I have are really very sharp already at its widest. I never bother with stopping down for increased sharpness, but I often stop down for increased depth of field.

QuoteOriginally posted by Michael Piziak Quote
For the SMC Pentax-DA 55-300mm F4-5.8 ED Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database
At 300mm zoom, F5.8 being wide open, the calculated sweet spot would be between f8.6 & f10
Copies are different, but testing my DA55-300 I have found very little difference in centre sharpness between f/5.6 and f/8. The corners will improve up to f/11, but beyond that diffraction will reduce sharpness (a bit at f/16, more from f/22). When shooting at 300mm I rarely care about the corners anyway, so I don't mind shooting at f/5.8 if I have to. And if the light is poor I'd say slow shutter speed (risking movement blur) or too high ISO will probably impact image quality more than what a very slight decrease in sharpness from the lens will.

In short, you have to get to know your lenses (as in your copies) and figure out what works and not - for you.

Technical comment on f-numbers:
An f-number is the relative diameter of the aperture to the focal length of the lens. If we look at a 50mm lens, f/1 would mean that the diameter of the aperture is 50mm. f/1.4 would mean a diameter of 35.34mm, f/2 is 25mm etc.

If the diameter of a circle is multiplied by the square root of 2 the area of the circle will double - and thus the amount of light let through the aperture will also double. So for each "stop" you will double (or halve) the amount of light let through the lens.

The reason for "stops" being the values they are (1 - 1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 22 - 32 - etc) is simply that they are all square roots (of 1 - 2 - 4 - 8 - 16 - etc).
03-29-2021, 11:44 AM   #12
Moderator
Loyal Site Supporter
Loyal Site Supporter
pschlute's Avatar

Join Date: Mar 2007
Location: Surrey, UK
Photos: Gallery
Posts: 8,113
QuoteOriginally posted by Michael Piziak Quote
I've been reading about the sweet spot or sharpest f stop of a lens. One general rule of thumb is 2 to 3 stops from wide open (with 1.4 being equal to one stop).

I thought I'd put a sticker on each of my 3 most used lens as to where this would be. Please tell me if the calculations are correct and/or also if you've used any of these lens what you're experience is.


For the SMC Pentax-M 100mm F4 Macro Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
F4 being wide open, the calculated sweet spot would be between f6.8 & f8.2

For the SMC Pentax-DA 55-300mm F4-5.8 ED Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database
At 300mm zoom, F5.8 being wide open, the calculated sweet spot would be between f8.6 & f10

And for the SMC Pentax-M 50mm F4 Macro Reviews - M Prime Lenses - Pentax Lens Reviews & Lens Database
Again, F4 being wide open, the calculated sweet spot would be between f6.8 & f8.2
May not be obtainable for the specified lenses, but you can look for lens tests done by folk with the right equipment.

Or simply use f4-f8 . All lenses are best there for overall sharpness.

Or is edge sharpness more important ? This can make a difference too.
03-29-2021, 11:54 AM - 1 Like   #13
Site Supporter
Site Supporter
Just1MoreDave's Avatar

Join Date: Feb 2007
Location: Aurora, CO
Photos: Gallery | Albums
Posts: 6,335
QuoteOriginally posted by savoche Quote
...In short, you have to get to know your lenses (as in your copies) and figure out what works and not - for you.
That'll be better than the rules or guessing. It's not that hard to put the camera on a tripod in front of a typical subject, focus carefully and find a good exposure at f8. Then turn off AF, use the 2 second delay and take some shots at different apertures but the same exposure. For example, if the f8 exposure is at 1/250, f5.6 is at 1/500, f4 at 1/1000, etc. With an A series or later lens, you can use the exposure lock button and Av mode, so you don't have to even figure it out yourself. Just lock the exposure and change the aperture; the shutter speed will automatically adjust. For M or older lenses, use M mode. Then look at the shots at different apertures side by side. If you are looking at sharpness, be very careful focusing, but for colors, vignetting, etc. it's not so critical.
03-29-2021, 12:01 PM   #14
Loyal Site Supporter
Loyal Site Supporter
UncleVanya's Avatar

Join Date: Jul 2014
Photos: Gallery | Albums
Posts: 28,171
Diffraction Calculator | PhotoPills

The way you judge diffraction depends on how you view things. At 100% crop on screen you will see diffraction earlier than printed images.

Use the above calculator and pick the appropriate camera. This leads to a claim of f7.1 < 8 being the smallest aperture that has no diffraction limitations using a k-3 on 100% crop and f11 < 16 being the limit on printed output.

Bear in mind other calculations may give different results.
03-29-2021, 12:04 PM - 2 Likes   #15
Senior Member
moggi1964's Avatar

Join Date: Feb 2021
Location: Manchester
Posts: 178
QuoteOriginally posted by stevebrot Quote
The big question might be whether one can differentiate between missed focus and lower sharpness at wider apertures.

That said, and as stated above, conventional wisdom is stopped one or two from wide open for both resolution and contrast. For an f/4 lens, that would be f/5.6 or f/8. In practical terms, even if equally sharp wide open, narrower DOF works against the sharpness proposition such that one is usually working one or two stops done from maximum anyway.


Steve
Missed focus? How dare you! Lenses are like people, some days they are sharp and some days they are not but I never miss focus
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
85mm, aps-c, center, database, edges, f11, f4, f8, k-mount, lens, lenses, pentax, pentax lens, reviews, slr lens, smc, spot
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Lens aperture sweet spot chart? eyespywithmyi Pentax SLR Lens Discussion 31 06-17-2020 08:36 AM
Where sweet spot on 18-135 lens? Bigdomino Pentax K-3 & K-3 II 14 02-10-2014 01:07 PM
lens "sweet spot"?... beaumont Pentax SLR Lens Discussion 7 04-22-2012 12:33 PM
Finally found a sweet spot with my K20 / 18-250mm MoparFreak69 Post Your Photos! 5 07-25-2009 11:38 PM
Silly "sweet spot" question?... beaumont Pentax SLR Lens Discussion 11 05-19-2009 04:52 PM



All times are GMT -7. The time now is 10:55 AM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top