Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
02-20-2010, 06:17 PM   #16
Veteran Member




Join Date: Oct 2008
Posts: 499
Cheers, Ben. I'm sure I wouldn't have been that concise

Am I right in that diffraction has to do with the actual non-relative aperture? The aperture diaphragm opening of a 90mm lens at f64 is about 1.4mm, same as an 22mm lens at f16. The larger your format, the longer your focal lengths and so bigger non-relative apertures.

02-21-2010, 05:16 AM   #17
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by brkl Quote
Cheers, Ben. I'm sure I wouldn't have been that concise

Am I right in that diffraction has to do with the actual non-relative aperture? The aperture diaphragm opening of a 90mm lens at f64 is about 1.4mm, same as an 22mm lens at f16. The larger your format, the longer your focal lengths and so bigger non-relative apertures.
You are correct. If we have a short look at how resolution of lenses is defined (simplified):

RESOLVING POWER RP = 1/1.22 lambda A ; where lambda is wavelength of the light an A the relative aperture (aka aperture number) ; If we use the formulae for the angular resolution of a lens, which I personally find much more practical, we get RES = 1.22 lambda/Diameter ; and that is fully independend of the f-stop/relative aperture, as it simply does not take fl into account.

As you see, resolving power or resolution goes down, when A gets bigger, i.e.the lens diameter D gets smaller. Indeed, the resolving power is only dependend on the real diameter of the open aperture (given in mm or whatever) and independent of focal length and thus of the relative aperture (f-stops).

With photographic lenses, the things get more complicated, as the entrance pupil gets the defining factor for resolving power (and its limits due to diffraction). The entrance pupil is only the projected image of the real aperture, as seen from the front of the lens. The diameter of that entrance pupil is not only dependent on the real size of the aperture opening (indeed it may be very different), but also from focal length and the construction of the lens. So, I think, that it is near impossible to calculate, what real resolution a specific modern lens (things are easier with older, simpler and often highly symmetrical lens designs) may achieve, just by knowing the aperture number and the fl.

Ben

Last edited by Ben_Edict; 02-22-2010 at 03:16 AM.
02-21-2010, 10:03 PM   #18
Junior Member




Join Date: Apr 2009
Location: Superior, Colorado
Posts: 26
It is a misperception that diffraction somehow manifests itself as soft edges only. Ben's response that diffraction is the same all over the image circle is spot on. I suspect that soft edge complaints among the small format crowd are more the result of optical aberrations at larger apertures, especially with some of the zooms.

To answer about f/22 in MF specifically, I will add my voice to the others in saying that it is a more than acceptable f/# for the format. When trying to maximize DOF for landscape shots I feel that I am quite good at f/22 and would probably go as high as f/32 when necessary (on the few lenses which actually stop down that far). Those f/#'s are about 2 stops slower than what I used for 35mm landscapes, yet the larger film real estate with MF seems to more than compensate. Of course if you are coming from the LF world already, then I am sure that extolling the virtues of larger formats is only preaching to the choir!
02-21-2010, 11:02 PM   #19
Pentaxian
Wheatfield's Avatar

Join Date: Apr 2008
Location: The wheatfields of Canada
Posts: 10,194
QuoteOriginally posted by Denverdad Quote
It is a misperception that diffraction somehow manifests itself as soft edges only. Ben's response that diffraction is the same all over the image circle is spot on. I suspect that soft edge complaints among the small format crowd are more the result of optical aberrations at larger apertures, especially with some of the zooms.

To answer about f/22 in MF specifically, I will add my voice to the others in saying that it is a more than acceptable f/# for the format. When trying to maximize DOF for landscape shots I feel that I am quite good at f/22 and would probably go as high as f/32 when necessary (on the few lenses which actually stop down that far). Those f/#'s are about 2 stops slower than what I used for 35mm landscapes, yet the larger film real estate with MF seems to more than compensate. Of course if you are coming from the LF world already, then I am sure that extolling the virtues of larger formats is only preaching to the choir!

One thing to remember is that in the real world of landscape photography, Depth of Field trumps all.
If the picture needs it, you stop down to the point that you have it.
If you get a little sharpness loss due to diffraction, you live with it, knowing that what you have attained is a sharper print overall.
You choose the aperture based on the picture, not based on some technical problem that puts up an imaginary wall.

02-22-2010, 03:13 AM   #20
Veteran Member
Ben_Edict's Avatar

Join Date: Jul 2007
Location: SouthWest "Regio"
Photos: Albums
Posts: 3,303
QuoteOriginally posted by Wheatfield Quote
One thing to remember is that in the real world of landscape photography, Depth of Field trumps all.
If the picture needs it, you stop down to the point that you have it.
If you get a little sharpness loss due to diffraction, you live with it, knowing that what you have attained is a sharper print overall.
You choose the aperture based on the picture, not based on some technical problem that puts up an imaginary wall.
That would also be my view. A slight loss in detail resolution isusually not a problem with landscapes, as you wouldn't scrutinize the image for single leaves… The overall impression, like colours, contrast and composition is much more decisive. So nobody should be afraid to use smaller f-stops if DOF is needed.

Ben
02-22-2010, 04:29 AM   #21
Veteran Member




Join Date: Oct 2008
Posts: 499
I ruined a ton of landscape shots thinking I should use the lens at its sharpest aperture, which didn't have enough DOF.
02-22-2010, 09:57 AM   #22
Pentaxian




Join Date: Jul 2009
Location: Seattle
Posts: 7,145
QuoteOriginally posted by brkl Quote
I ruined a ton of landscape shots thinking I should use the lens at its sharpest aperture, which didn't have enough DOF.
Sorry to hear that. But I guess that's part of learning. I wonder, though, why the "cheery aperture" was even instilled in peoples minds. When I started photography many years ago, before the internet and before lenses lived or died by a lens review, we knew lenses had a sweeter spot but that never influenced our picture taking. If you could shoot around f8, great, but if not, no big deal. Soft edges when a lens was wide open could be a feature, not a bug, when you took advantage of it.
02-22-2010, 10:33 AM   #23
Veteran Member




Join Date: Oct 2008
Posts: 499
It's almost certainly the capability to go look at the end result at a pixel level. It gives a sense of absolute resolution and performance, so that nothing less is acceptable (who knows when you'll need to project that photo on the side of a building!)

And then people mostly post the photos on Facebook.

02-22-2010, 12:56 PM   #24
Junior Member




Join Date: Apr 2009
Location: Superior, Colorado
Posts: 26
QuoteOriginally posted by tuco Quote
... I wonder, though, why the "cheery aperture" was even instilled in peoples minds.

I think it is still worth being aware of optimum apertures (in terms of sharpness, as well as color aberrations) of your lenses. It is easy to forget that the classic near-far landscape style is not the only genre of landscape/nature photography, and certainly there are inumerable other styles where DOF is not a driver for aperture choice. In such cases, opening up to a more optimum aperture can facilitate larger enlargements by improving the sharpness of the objects you care most about in the scene.

Even when large DOF is deemed important, sometimes there is enough margin that you can afford to go with a larger aperture to sharpen the parts of the scene of greatest interest. An example of that is when there are no near objects in the scene (imagine a panoramic landscape with no real foreground), or when the scene is "flat" (say for example a closeup of lichen growing on a rock face).

So overall there are times when knowing the "cherry aperture" is useful. But I would agree that it becomes less important as one moves to larger and larger formats.
02-22-2010, 04:23 PM   #25
Pentaxian




Join Date: Feb 2009
Location: Arizona
Posts: 916
Ben makes a really good point about diameter vs wavelength of light in regards to resolving power. This is one of the major equations in optical physics. The large diameter is the reason why the 400 EDIF (for the P67) has the potential to out resolve the 90mm f/2.8 lens. It's also a major reason why the best telescopes are large diameter.
02-23-2010, 12:37 PM   #26
Inactive Account




Join Date: Jan 2010
Location: Raleigh North Carolina
Posts: 18
Original Poster
Diffraction in Pentax lenses

After posting the original thread, I am happy to see all the conversation. My usual work is in 8x10 and 5x7, so no experience with the "smaller" MF. We never worry about diffraction with LF, as has been pointed out. I am quite satisfied with the results I get from my Pentax 67, especially like the AE meter. I use a tripod for most work, and have not seen any diffraction or softness in the lenses I use, mostly the 55mm and 75mm.
Thanks for all the posts.
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!
camera, diffraction, edges, lenses, medium format, pentax
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Hyperfocus and diffraction help. ladybug Troubleshooting and Beginner Help 14 05-13-2012 05:05 PM
K-5 possible diffraction limitations bymy141 Pentax K-5 37 10-15-2010 01:40 PM
Diffraction Limits revisited RioRico Pentax DSLR Discussion 48 05-25-2009 09:56 AM
Diffraction Article by Lloyd Chambers philbaum General Talk 28 01-16-2009 12:48 AM
Diffraction issue montecarlo Photographic Technique 12 09-10-2008 11:12 PM



All times are GMT -7. The time now is 03:49 PM. | 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