Forgot Password
Pentax Camera Forums Home
 

Showing results 1 to 25 of 300 Search:
Forum: Troubleshooting and Beginner Help 10-02-2012, 09:01 AM  
wide open lens sharpness
Posted By newarts
Replies: 23
Views: 2,409
Why with a grain of salt? You are perhaps too modest Sir! :) Or do you have reason to discount Takinami's data?

Dave in Iowa
Forum: Lens Clubs 10-01-2012, 09:08 AM  
The Raynox Macro club
Posted By newarts
Replies: 838
Views: 245,338
The 150 is superb with the DA55-300 in my experience. One seldom noted advantage is that the built-in flash works for almost all the magnification range (to a bit above 1:1). I also have the Tamron 28-75 and have seldom used it with the Raynox 150.

Dave in Iowa
Forum: Pentax DSLR Discussion 09-30-2012, 03:23 PM  
Current Cameras with No AA Filters (a running thread)
Posted By newarts
Replies: 22
Views: 10,182
It seems to me a 100% strength birefringent AA filter atop a Bayer sensor array must add at least one 50% bright pixel at the edge of a step function illumination pattern.

That's the result I get thinking it thru & if I simulate it numerically by averaging four adjacent pixels. I think maybe this edge broadening should add according to the Kodak rule with p=2.

Dave
Forum: Pentax DSLR Discussion 09-29-2012, 03:55 PM  
Current Cameras with No AA Filters (a running thread)
Posted By newarts
Replies: 22
Views: 10,182
Thanks, I was aware of that from Wikipedia Anti-aliasing filter - Wikipedia, the free encyclopedia.

Following up with Falk's work LumoLabs -- Nikon D800 AA filter vs. D800E -- Whitepaper it is clear that the net effect of the birefringent AA filter is to broaden an edge profile a bit. I don't yet understand his work well enough to compare his result to the corresponding effective ESF increase.

Dave
Forum: Pentax DSLR Discussion 09-29-2012, 09:44 AM  
Current Cameras with No AA Filters (a running thread)
Posted By newarts
Replies: 22
Views: 10,182
Are there any tabulated comparisons of AA filter strengths (like gaussian, sinc equivalent, etc) for various Pentax cameras?

Conversely, how might one test for a rough measure?

Like: I have a K-x camera - can I expect edge blur at best to be like that resulting from a Gaussian(1) filter, Gaussian(2) filter, ....?

Dave in Iowa
Forum: General Talk 09-28-2012, 08:41 AM  
Religion and the race
Posted By newarts
Replies: 68
Views: 3,856
If she was informed of her Cherokee ancestry by her family, she'd be a lair if she denied it. It is reasonable that such a topic would arise in casual conversation during the process of interviewing for a faculty position. It is also likely that a university would like to demonstrate its diversity by publicizing it.

The real question is whether or not she used the info to unwarranted advantage and did not properly qualify it as being family lore.
Forum: General Talk 09-26-2012, 05:26 PM  
Will an Obama victory in November set off a wave of right wing violence?
Posted By newarts
Replies: 9
Views: 1,175
I think it more probable that partisan attempts to limit/expand access at the polls will result in legal challenges after the election or violence during the election.

I won't be surprised to see conservative activists at polling places challenging voters to prove their eligibility and liberal activists trying to prevent such challenges. This has a high probability of creating real time conflicts.
Forum: Post Your Photos! 09-26-2012, 09:54 AM  
Landscape Calm and quiet - DOF+
Posted By newarts
Replies: 6
Views: 421
I like the image and appreciated the tutorial in how to do it in PP.

I think the resulting image would be MUCH better if the burned out highlights on the boat's stern (and the top of the last pole) were toned down to match the surrounding slightly greyer levels. As it is, my attention is strongly drawn to those overexposed areas.

Dave in Iowa
Forum: Troubleshooting and Beginner Help 09-24-2012, 09:22 AM  
Sharpness
Posted By newarts
Replies: 31
Views: 3,169
How so? The theory is well established & my application of it was conservative.

The K-5's high pixel count means that a 100% crop maps a very small sensor area to each display pixel.
Here's an illustration from Luminous Landscape of a Gaussian distribution (very similar to the Airy distribution expected from a lens) on a square grid of pixels. The Gaussian "spot" is about six pixels wide, just like op's case.


From LuminousLandscape - Deconvolution sharpening revisited

Clearly, what idealy be a single bright pixel is smeared out over quite a few of its neighbors.

The following is an attempt to give a reasonably useful approximation to what's going on in theory.. It ignores AA filters, etc..

Following Airy disk - Wikipedia, the free encyclopedia the Gaussian approximation to the Airy Disk is:

Relative brightness = exp(-(x^2+y^2)/(N/2)^2)/[(N/2)^2(Pi)] where x, y are in microns and N is the f-stop number.

The center brightness of a multi-pixel spot varies with 1/N^2, so an F:11 spot's center is about 4 times brighter than an F:22 spot's center.

Dave

PS I note your signature "Knowledge does not equal understanding." - true, but Ignorance is certainly a worse predictor of understanding.
Forum: Troubleshooting and Beginner Help 09-23-2012, 05:34 PM  
Sharpness
Posted By newarts
Replies: 31
Views: 3,169
Your K-5's specifications are 4928 x 3264 (16.3 MP) with a 23.4 x 15.6 mm CMOS sensor. That's 4.7 microns pixel spacing. The diffraction spot diameter for f:22 is:

d=2.44*N*(.55 microns)= 2.44*22*0.55=30 microns!

That means when you shot the photo, each infinitesimal spot of light was spread over a disk more than 5 pixels in diameter. When displayed at 100%, each point of light in the original scene is spread over 5 pixels on the display (almost 1mm). No wonder it looks fuzzy!

To prevent strong diffraction effects at 100% display, your f-number should be such that the diffraction spot radius is about the same as the pixel spacing. In your case,

N ~ 4.7(microns)/1.22*0.55(microns) ~ 8

Or, avoid high enlargements at high f-numbers.

Dave in Iowa
Forum: Pentax SLR Lens Discussion 09-22-2012, 07:44 AM  
Macro: Depth of Field
Posted By newarts
Replies: 16
Views: 1,966
Good point; the error you point out is in our sloppy use of assigning the name "2.8" to the number 2*SquareRoot(2) = 2.828427.....

Using the names of the f-stops in calculations is usually good enough; what the actually aperture opening a manufacturer calls F:2.8 on a particular 100mm lens is highly unlikely to be precisely 28.28427mm.

I suppose a reasonable thing to do for quick estimates is to use f-stop names in calculations then round up or down to the next common f-stop name; in this case f:3.9 rounds to f:4 (as would f:4.1).

Best wishes,
Dave
Forum: Pentax SLR Lens Discussion 09-21-2012, 10:29 AM  
Macro: Depth of Field
Posted By newarts
Replies: 16
Views: 1,966
No.



1mm x 11/2.8 = 3.9mm



Same magnification on the sensor.

Dave
Forum: Pentax DSLR and Camera Articles 09-18-2012, 11:06 AM  
A good lens sample?
Posted By newarts
Replies: 10
Views: 3,896
I think the statement must be modified to emphasize the stochastic nature of the beast. In aggregate we collectively get what we pay for, but may individually get better (or worse) than we paid for.

Dave in Iowa
Forum: Troubleshooting and Beginner Help 09-18-2012, 09:10 AM  
Correct Sequence of Teleconverters (Metering With Teleconverters)
Posted By newarts
Replies: 19
Views: 2,182
For any lens system there is at least one optimal combination of parameters. There is an aberration limit at the wide open end of the aperture range (if you open the aperture more the image gets worse) and a diffraction limit as the aperture gets smaller and smaller.

When you add auxiliary optics or reverse the lens (or go beyond the minimum focus distance) I don't think there is a way to predict how aperture changes affect the image. Experiment is the best way to find out.

Dave in Iowa
Forum: General Talk 09-09-2012, 12:30 PM  
Obama or Congress?
Posted By newarts
Replies: 2
Views: 435
The first few comments after the article were more disturbing than the article itself.

Dave in Iowa
Forum: Pentax SLR Lens Discussion 09-09-2012, 10:19 AM  
Rokinon 85mm f/1.4 vs SMC Pentax-M 85mm F2, which one to choose?
Posted By newarts
Replies: 110
Views: 33,390
A seldom mentioned benefit of the Samyang lens is that it has superb color transmission. See: Colour rendering in binoculars and lenses - Colours and transmission - Lenstip.com (a good article about coatings).

Dave in Iowa
Forum: General Talk 09-05-2012, 01:23 PM  
Need help asking a cute girl to be my partner in studio lighting class.
Posted By newarts
Replies: 80
Views: 5,829
You could say that you understand that women have different (and often more discriminating) views on color, composition, lighting, etc than men so you'd like to see that in action. Maybe a mixed gender team will be better than (or at least as good as) single gender; ie. you may learn from her & vis-versa.

Partnering with her would be a good way to see if there's any truth to that. Maybe your outlooks will be complementary - you won't know 'til you try. Treat her like a colleague or partner not a date.

Old Dave in Iowa
Forum: Pentax SLR Lens Discussion 09-05-2012, 09:08 AM  
pentax lens contacts - macro tube problem
Posted By newarts
Replies: 12
Views: 1,970
There should be no risk of electronic damage to the camera by short circuiting any of the camera mount pins. That's because the camera is designed to receive all metal mount K and M type lenses which always short circuit the camera pins.

Dave in Iowa
Forum: Pentax Camera and Field Accessories 09-03-2012, 09:48 AM  
Macro: edge quality - tubes vs achromat diopter
Posted By newarts
Replies: 27
Views: 3,928
No problem! Someday maybe.

Dave in Iowa
Forum: Pentax SLR Lens Discussion 09-02-2012, 08:52 PM  
not sure how to word this question..
Posted By newarts
Replies: 16
Views: 1,365
It is very close to exact if you are far from hyperfocal conditions (ie. the far background is noticeably out-of focus).
Forum: Do-It-Yourself 09-01-2012, 04:06 PM  
DIY Lens Hoods - no math required
Posted By newarts
Replies: 32
Views: 24,111
Awwwww, thanks [Blush].

Actually, you only need to put a tube /box on the lens & cut it back 'till it doesn't show any more. (As you point out!)

Dave in Iowa
Forum: Do-It-Yourself 09-01-2012, 10:23 AM  
DIY Lens Hoods - no math required
Posted By newarts
Replies: 32
Views: 24,111
In the equation Hood.diameter=Lens.diameter(Hood.depth/focal.length +1) the term in the parentheses must be bigger than 1 (because 1 is added to another positive number, so hood diameter must be greater than lens diameter - probably you used (hood.depth/(focal.length+1)) instead of
(hood.depth/(focal.length)+1).

Here's a better equation and a description of where it came from.


using your numbers and the above equation

Hood diameter > Hood.depth*sensor.width/focal.length + lens diameter = 21*24/28+20 =38

With film, sensor.width is 36, not 24 so the hood diameter should be greater than 21*36/28+20=47, so I'd expect vignetting at the corners.

Dave in Iowa
Forum: Pentax Camera and Field Accessories 08-31-2012, 07:55 AM  
Macro: edge quality - tubes vs achromat diopter
Posted By newarts
Replies: 27
Views: 3,928
Cropping the original image may be a good idea for comparison.

The best way to make sure the effective f-stop is the same is to use the same lighting, ISO, and exposure time - adjust the f-stop (not the time) so the exposures are the same. At first this seems to make no sense, but if +/-EV, ISO and exposure time, lighting, etc are the same, effective f-stop must be the same.

This method is better than calculating effective f-number I think.

Dave
Forum: Pentax Camera and Field Accessories 08-31-2012, 07:02 AM  
Macro: edge quality - tubes vs achromat diopter
Posted By newarts
Replies: 27
Views: 3,928
Thanks Lowell, you are right & the situation is clearly going to vary lens type to lens type.

Here's an image comparing the corners of good diopter lenses on the end of extended bellows. They looknabout the same to me and pretty good!


Dave in Iowa
Forum: Pentax Camera and Field Accessories 08-31-2012, 06:52 AM  
Macro: edge quality - tubes vs achromat diopter
Posted By newarts
Replies: 27
Views: 3,928
I think what you and Lowell say are both true. I have seen cases where tubes are better at the edges and where diopters are better at the edges.

However, I'm still seeking example photos of either case to discern which is more common and how to predict.

At this point I don't know what to tell someone who asks "Which is best; Tubes or Achromats?" Especially if edges are a concern.

Dave in Iowa
Search took 0.02 seconds | Showing results 1 to 25 of 300

 
Forum Jump


All times are GMT -7. The time now is 12:15 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