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Author Topic: Lets define 'Tack Sharp'  (Read 6420 times)

NormanBates

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Re: Lets define 'Tack Sharp'
« Reply #15 on: November 24, 2011, 05:32:27 AM »
I always run my own tests with all my lenses
http://www.similaar.com/foto/lenstests/lenstestsa.html

but if you want a database with more lenses, the place to go for sharpness tests is the-digital-picture; it is only a relative measure (lens A is sharper than lens B), but you get to actually see the issues with each lens:
http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=687&Camera=453&Sample=0&FLI=0&API=0&LensComp=771&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=2

of course, in any case, there are more things to consider apart from sharpness; e.g. bokeh:
http://www.similaar.com/foto/lenstests/bokehtests.html

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Re: Lets define 'Tack Sharp'
« Reply #15 on: November 24, 2011, 05:32:27 AM »

epsiloneri

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Re: Lets define 'Tack Sharp'
« Reply #16 on: November 24, 2011, 06:42:02 AM »
Sharpness can be well defined for focus at infinity. Typically, one can measure the so-called point-spread function, how well focused a point of light becomes in the image plane. I would call a lens 'tack sharp' on my camera body if stars in an image of the night sky remained unresolved by the sensor (e.g. FWHM <~ 2 pixels).

There are also more stringent conditions. For telescopes, 'tack sharp' would correspond to the diffraction limit, which is the smallest angle you can possibly resolve given your aperture and "perfect" optics. Typically, a 10 cm aperture corresponds roughly to 1 arcsecond diffraction-limited resolution. You can compute the diffraction limit for your lens by multiplying the f-number with 100 mm and then divide with the focal length to get the resolution in arcseconds, if you e.g. have a 400/2.8 lens the diffraction-limited resolution would be

2.8 * 100 / 400 = 0.7 arcsec

With a FF sensor the field of view would be ~5 degrees = 18000 arcsec, meaning we would need a 2 Gpix sensor to optimally sample a diffraction-limited 400/2.8 lens. Clearly the diffraction limit is way beyond most regular lenses and mostly relevant for the much longer focal lengths used in telescopes, which regularly do achieve diffraction-limited resolution.

PeterJ

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Re: Lets define 'Tack Sharp'
« Reply #17 on: November 24, 2011, 07:14:03 AM »
With a FF sensor the field of view would be ~5 degrees = 18000 arcsec, meaning we would need a 2 Gpix sensor to optimally sample a diffraction-limited 400/2.8 lens.
True but 2 Gpix is pretty ho-hum, soon my 5.5 Tpix will arrive, not sure if I should post a direct link but Google "5.5 terapixel camera". Build quality looks awesome and it doesn't look as intimidating and nasty as a 1DX ;).

neuroanatomist

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Re: Lets define 'Tack Sharp'
« Reply #18 on: November 24, 2011, 09:58:12 AM »
It would be helpful for the discussion if people would upload photos

Image on the left, 5DII + 70-200mm f/2.8L IS II = tack sharp.  Image on the right, 5DII + 24-105mm f/4L IS = not tack sharp.
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Jettatore

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Re: Lets define 'Tack Sharp'
« Reply #19 on: November 24, 2011, 10:53:05 AM »
Neuro, getting bad .jpg lossy compression on my end but I'm only able to see a thumbnail sized crop...  Because of at least that neither look anything like what I would describe as sharp.

mreco99

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Re: Lets define 'Tack Sharp'
« Reply #20 on: November 24, 2011, 11:36:30 AM »
clicked the lashes.jpg, and i only get an index.php file?

NormanBates

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Re: Lets define 'Tack Sharp'
« Reply #21 on: November 24, 2011, 12:18:15 PM »
clicking there, I got the jpg file, but it's still a thumbnail

in any case, his point is clear: "see each lash individually" vs "see a mash of dark stuff"

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Re: Lets define 'Tack Sharp'
« Reply #21 on: November 24, 2011, 12:18:15 PM »

Jettatore

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Re: Lets define 'Tack Sharp'
« Reply #22 on: November 24, 2011, 12:39:48 PM »
Has anyone else looked at this, or something like it yet.

http://www.phaseone.com/en/Downloads/Sample-images.aspx

Pain in the butts registration/log-in required for download.  But what it is, is an untouched sample from the highest end or close to it? Phase One medium format system.

If being able to identify individual eye lashes is a definition of tack sharp, then here we can identify individual mascara blobs on individual eye lashes in the small area of perfect focus.  It's so high resolution to an absurd degree that it would be easier to retouch and up-size a smaller resolution file or to throw out a lot of detail because it's capture gross microscopic qualities that only science wants to see...  Still pretty awesome.

neuroanatomist

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Re: Lets define 'Tack Sharp'
« Reply #23 on: November 24, 2011, 12:53:33 PM »
They're 200x200 pixel 100% crops, and Norman's spot on. Yes, the forum compresses attachments, so neither look as good as under the loupe in Aperture.
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Meh

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Re: Lets define 'Tack Sharp'
« Reply #24 on: November 24, 2011, 01:14:03 PM »
They're 200x200 pixel 100% crops, and Norman's spot on. Yes, the forum compresses attachments, so neither look as good as under the loupe in Aperture.

Is your 24-105mm just a bit softer or just a slightly OOF shot?  You've posted some pretty "tack sharp" shots from your 24-105mm in the past I seem to recall.

neuroanatomist

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Re: Lets define 'Tack Sharp'
« Reply #25 on: November 24, 2011, 02:27:57 PM »
They're 200x200 pixel 100% crops, and Norman's spot on. Yes, the forum compresses attachments, so neither look as good as under the loupe in Aperture.

Is your 24-105mm just a bit softer or just a slightly OOF shot?  You've posted some pretty "tack sharp" shots from your 24-105mm in the past I seem to recall.

The eye in each shot is at approximately the same relative position in the frame, but both are a bit away from the center. The 24-105mm can look sharp in scaled-down shots, but it's not nearly as sharp as the 70-200mm II. That does illustrate the point that much of this is splitting hairs (or eyelashes, as the case may be). Tack-sharpness is not the be-all-end-all, and a difference of 200-300 lw/ph on an Imatest result usually doesn't mean much in real-world shots. Although it's not 'tack sharp' the 24-105mm delivers good results, which is what matters.
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branden

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Re: Lets define 'Tack Sharp'
« Reply #26 on: November 24, 2011, 04:50:25 PM »
The sharpest photos I've ever taken have been illuminated by bright strobes, enough the point where I no longer worry whether the lens is sharp or not. If I want a sharp photos, I need bright lights.

Here's an example of what I mean, 100% crop of a photo taken with the nifty fifty:
« Last Edit: November 24, 2011, 04:54:02 PM by branden »

mreco99

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Re: Lets define 'Tack Sharp'
« Reply #27 on: November 24, 2011, 05:10:59 PM »
Why does a bright light make a sharper image, is it because of contrast is higher?

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Re: Lets define 'Tack Sharp'
« Reply #27 on: November 24, 2011, 05:10:59 PM »

branden

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Re: Lets define 'Tack Sharp'
« Reply #28 on: November 25, 2011, 12:34:03 AM »
I honestly don't know the reasons behind it -- it's just something I've experienced over the years, and I've assumed its similar to the reason that it's easier for the human eye to make out details in bright light than in dim.

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Re: Lets define 'Tack Sharp'
« Reply #28 on: November 25, 2011, 12:34:03 AM »