ISO and Sharpness

Hi All,
I have done some research on websites and did not find clear information about relation between ISO and Sharpness.
I know that pushing the ISO will create noisy images. I just wonder if the ISO itself will affect the sharpness of the images!!
I assume some images are sharper with a slightly higher ISO rather than smallest ISO. ISO 100 or 50 will produce noise free images but I sometime think it makes the images or better say pixels a bit soft and therefore somehow reduces the sharpness!! Am I right???

And my first question again: Does ISO affect the image sharpness by itself at all or not and how?

Any information appreciated
Mo

Hi Mo,

I'm not aware of any direct relation between ISO and sharpness.... However.... ISO is part of the exposure triangle, and using an ISO of 100 instead of 400 (let's say) will have an impact on either shutter speed, aperture or both. Those changes may affect sharpness.

Changes in aperture will affect depth-of-field which may impact the perception of sharpness throughout the depth of the image.

Alternatively, reduced shutter speed can affect sharpness through the introduction of camera shake if you are hand-holding.

So, to the best of my knowledge, any impact of ISO on sharpness would be indirect: either through reduced depth-of field or introduced camera shake with longer shutter speeds at lower ISO settings (and also through additional noise at higher ISO settings).

Cheers
Karen

Karen is correct. ISO is simply an effect of the circuitry applied to the signal in the camera electronics. It doesn't do anything to change the actual sharpness of what's captured at the pixels. The effects on shutter speed and DOF that she refers to would be the most likely causes of what you're suggesting Mohammad.

Everyone is confused here. Higher ISO without a doubt will diminish sharpness. The build up of noise will remove fine details, and if the ISO is really high, it will remove large chunks of it. High ISO will dramatically reduce dynamic range, contrast and tonal depth. Some of the damage can be mitigated by shooting uncompressed raw images, but there is a limit to what can be done. Newer sensor technology has provided better dynamic range and tonal depth, so that you can now shoot the current crop of cameras at ISO settings that are as high as 3200 or higher, without an "appreciabel" loss of detail and sharpness. Check out any camera review on www.dpreview.com to see how badly high ISOs will damage images.

Here are some sites that explain what I just wrote:

http://www.apogeephoto.com/sept2010/bsharp92010.shtml

http://www.outdoorphoto.com/guides/basics_sharpness.php

and here is a real-life example of how high ISO robs detail and sharpness

http://www.techhive.com/article/2025509/balancing-iso-and-digital-noise-for-sharper-low-light-photos.html

I agree with Gene, I use high ISO with students and an image at lower ISO to show a clear differences in loss of colour saturation and sharpness, your edges are just not as crisp in higher ISO as well as the noise issue. Shoot an image at 200 ISO and one at 6400 and you will easily see the difference yourself.

If your camera base ISO is 100, and you shoot it at 50, the loss is pretty negligible and the benefits (shallow DOF, blurred motion) outweigh the slight loss in quality. Your base ISO is best, all things being equal, but there are trade-offs.

Most would agree that egregious excess of noise, consequent upon excessively increased ISO, unequivocally obscures detail and decreases image sharpness. However, there are also many of the opinion that slightly to modrately perceptible noise can, especially in high contrast images, result in the perception ( illusion) of increased sharpness. Close inspection will, indeed, reveal obfuscation of much fine detail by noise, but excess noise can , paradoxically, cause illusory enhancement of the more obvious, already more contrasy edges...Why? I don't know, but I entertain the possibility that perhaps it occurs similar to the manner of the effect of the classic unsharp mask. I suggest that those personally familiar with use of the unsharp mask "back in the day" of film will recall that a grainy negative masked with an unsharp positive mask of markedly less grain could be counted on to reveal best sharpening of already easily perceptible edges and little to no enhancement of contrast of the lower contrast detail edges.
Dave Graham
East River, SD

Thank you everyone for your inputs. It is great to to be here with NPN.
Mo

Higher ISO without a doubt will diminish sharpness.

That's true Gene as the noise breaks up the detail... but the OP seemed to indicate that he was asking if low-ISO, noise-free images are actually softer due to the ISO.