# Lets define 'Tack Sharp'



## mreco99 (Nov 24, 2011)

My example lens canon 70-200 mk2

Many people say tack sharp, but can we define tack sharp like this.
Take a good example of your image and open in something line photoshop, zoom to 100%, does your image look perfectly sharp where it should? mine do
Now zoom into 200%
At 200% i can definately start to just about notice pixels in some areas.
Does your image look tack sharp now? some of mine do, some dont. When the light was good, the images look sharper.
at less that 200% ie 190% i dont see pixelation.

This is just a stab at trying to quantify sharpness.
Can anyone confirm or alter what ive found?

thanks


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## Jettatore (Nov 24, 2011)

I take it to mean perfect, crisp focus, superb value contrast and color contrast as well as a total lack of any Chromatic Aberration (before or after RAW processing). So when looking at it at 100% on screen or in print you have 'tack sharp' quality even in the finest details and textures of the image, and any part that is intentionally out of focus just adds to the overall compositional contrast of the image.


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## mreco99 (Nov 24, 2011)

so tack sharp is only applies at 100%?


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## Mt Spokane Photography (Nov 24, 2011)

Expecting a image to be sharp at 200% is definitely going overboard. 

The type of lens testing done while on a given camera is only applicable for that camera. Thus, the tack sharp description only applies for that camera as well. 

A image taken with a low MP camera, will almost always look sharper at 100% for a given lens and aperture than a high MP camera just due to diffraction.

Other factors like CA and distortion also play a important part.

Thus, someone with a crop camera like a 40D might see a sharp image at 100% and f/11, while a 7D image would might be slightly fuzzy merely due to infraction. And, on a FF camera, the results might be better or worse.


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## Jettatore (Nov 24, 2011)

mreco99 said:


> so tack sharp is only applies at 100%?



I wouldn't necessarily say it only applies to 100% resolution. But anything less than 100% and you are having software interpolate the image for you for whatever scale you are displaying or printing it at. You could shrink even a somewhat noisy image down to the point where the noise is invisible and if the contrast and details still look good I guess you could rightly call it tack sharp. Call it whatever you want, language isn't reality, it's just a map we use to relate to our realities - "tack sharp"... just words and concept(s) for thinking and communication. It would be useful if we all have a similar working definition, at least if we want to talk to each other.


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## neuroanatomist (Nov 24, 2011)

As many of you will know, I like doing empirical testing. I have an ISO 12233-based chart that costs more than some L-series lenses. I use that expensive chart to test new lenses for flatness of field, centering defects, etc. But, my definition of tack-sharp is both simple and practical. I take a portrait-type shot of one of my daughters, with the lens wide open and the focus on one of her eyes, and if I can count each and every one of her eyelashes, I consider the lens to be sharp.


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## wickidwombat (Nov 24, 2011)

ISO Makes a big difference shoot the exact same thing off a tripod at iso 100 and iso 1600 and you will see a big difference in sharpness


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## briansquibb (Nov 24, 2011)

neuroanatomist said:


> As many of you will know, I like doing empirical testing. I have an ISO 12233-based chart that costs more than some L-series lenses. I use that expensive chart to test new lenses for flatness of field, centering defects, etc. But, my definition of tack-sharp is both simple and practical. I take a portrait-type shot of one of my daughters, with the lens wide open and the focus on one of her eyes, and if I can count each and every one of her eyelashes, I consider the lens to be sharp.



So only when wide open? What camera - ff or crop? 

FF would rule out several lens that if tested on a crop would be considered tack sharp - especially the 1.2 lens


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## distant.star (Nov 24, 2011)

"Focus is a bourgeois concept."

--H. Cartier Bresson


Why take an easy problem? 

Let's count the pieces of flyshit in the pepper. Let's finally get that number of angels that can dance on the head of a pin. Lets....

Seriously, the only measure I have a real sharpness is if the image makes things appear close to what I actually see with the naked eye. Obviously, no way to quantify that. One thing it suggests though, is that the image borders on approaching some 3D aspects. 

One of the most telling things for image sharpness is often the most nebulous -- clouds. With the naked eye, clouds can often be seen in distinct layers, and that's almost never seen in photos, at least without going to HDR or some kind of layering in post. If I take a picture that has layering storm clouds, and I can see distinct layering, I know it's "sharp." 

Okay, back to the kitchen for me.....


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## pedro (Nov 24, 2011)

When I saw Robert Frank's photographs in "The Americans" and Robert Capa's D-Day images (even though the film was damaged while still wet after processing-as the story goes) I forgot about tack sharpness forever. Or look at Al Wertheimer's Elvis Presley coverage from '56. Great photographers, great photographs. But I wouldn't say, that everything is tack sharp...So who cares?


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## AprilForever (Nov 24, 2011)

An image is only sharp if taken with an L lens on a 1D camera. Nothing else can ever be sharp, no matter what.

Actually, I am not terribly certain why many worry themselves with the mystical sharpness beast. With decent glass, you should't be too troubled... I have no idea why you are looking at your image at 200%, but that's what's amusing to you... ;D


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## candyman (Nov 24, 2011)

It would be helpful for the discussion if people would upload photos


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## lol (Nov 24, 2011)

Personally, things never look absolutely sharp at 100% without extra sharpening. Thanks to the bayer pattern sensor. Having said that, they of course can still look very good.


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## briansquibb (Nov 24, 2011)

candyman said:


> It would be helpful for the discussion if people would upload photos



+1

Unfortunately many of the online experts dont post - common across many forums


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## J. McCabe (Nov 24, 2011)

Why would a non-technical term like "tack sharp" be given a closed definition in measurable terms ?

If someone wants technical terms, optics supplies such and measurements can be found in several web sites.


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## NormanBates (Nov 24, 2011)

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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## epsiloneri (Nov 24, 2011)

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.


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## PeterJ (Nov 24, 2011)

epsiloneri said:


> 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 .


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## neuroanatomist (Nov 24, 2011)

candyman said:


> 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 (Nov 24, 2011)

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.


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## mreco99 (Nov 24, 2011)

clicked the lashes.jpg, and i only get an index.php file?


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## NormanBates (Nov 24, 2011)

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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## Jettatore (Nov 24, 2011)

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.


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## neuroanatomist (Nov 24, 2011)

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 (Nov 24, 2011)

neuroanatomist said:


> 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.


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## neuroanatomist (Nov 24, 2011)

Meh said:


> neuroanatomist said:
> 
> 
> > 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.
> ...



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 (Nov 24, 2011)

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:


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## mreco99 (Nov 24, 2011)

Why does a bright light make a sharper image, is it because of contrast is higher?


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## branden (Nov 25, 2011)

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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