Lets define 'Tack Sharp'

[mreco99]

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

[Jettatore]

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.

[mreco99]

so tack sharp is only applies at 100%?

[Mt Spokane Photography]

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.

[Jettatore]

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.

[neuroanatomist]

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.

[wickidwombat]

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

[briansquibb]

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

[distant.star]

"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.....

[pedro]

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?

[AprilForever]

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... 

[candyman]

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

[lol]

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.

[briansquibb]

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

[J. McCabe]

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.