ISO vs. Aperture

NorBro

NorBro

Major Contributor
I want the sharpest image possible..I mostly use my 1.4 50mm...I heard that for the sharpest part of the lens you would use anything between 5 and 8..but I also want a clear image and sometimes I would have to pump up the ISO because of the lack of light..

Any thoughts about this? What would you guys do?
 
A lot of times it depends on how sharp the glass itself is. some lenses are just not as sharp as others, and you would absolutely have to give up sharpness if you are shooting at wider apertures, because of the shallow depth of field. inversely, if you are shooting at smaller apertures for sharpness, pumping up your ISO, decreasing your shutter speed (provided that your subject isn't moving) and using a lens with image stabilization will help as well.
 
I have a Nikon Nikkor 50mm F1.4 AI-S lens and from testing I have done, it is preceptably sharper stopped down to F2.8 or more. Cranking the ISO is a way to be able to stop down, but after a certain point I think that the increase in noise cancels out the benefits of the lens sharpness gains at the higher F-stops. It is a delicate dance and you have to make decisions based on the needs of the situation. Most of my lenses are F2.8 so I think about that as my base point of reference. Having the the F1.4 50mm is handy for the extra two stops when they are needed.
 
what do I do? test my lenses throughly, to know them in and out: http://www.similaar.com/foto/lenstests/lenstestsa.html - http://www.similaar.com/foto/lenstests/bokehtests.html

I understand not everybody has the time or patience to do that...

you can play with this to see how much sharper the lens is when stopped down, and in what part of the frame: http://www.the-digital-picture.com/...meraComp=453&SampleComp=0&FLIComp=0&APIComp=6
but it's not YOUR LENS, it's another copy of the same model (shouldn't be, but could be, far off - e.g. wide open my tamron 17-50 is sharp in the 17-24 range, soft in 35-50, while theirs is sharp at 24-35 but soft at 17 and 50)

and regarding ISO, same thing: test your camera
 
Sharpness is a bell curve generally. It will get sharper as you move from wide open to the 5.6 or 8 range than start to go downhill at 11 or so.
As noted it's a give and take between ISO and aperture depending on the situation. No hard and fast rules other than a max ISO where beyond the noise gets pretty nasty.
 
The general rule of thumb is to shoot a couple of stops smaller than wide open if you want your best sharpness.

The reason for this is that the effects of lens aberrations tend to decrease as you stop down. To a point. At some point you hit the diffraction limit, and then you start to loose sharpness as you stop down (but you gain DOF). IOW, if you want to maximize sharpness, you want to stop down some, but not too much. Typically a couple of stops down from wide open is a sweet spot.

For full frame 35mm sensors and glass, you typically hit the diffraction limit around f/8. So in the case of a 50mm f/1.4 lens on a full frame 35mm sensor, max sharpness is probably somewhere between f/2.8 and f/5.6.

A rule of thumb is by its nature not very specific. You have to run the experiments to actually find out what really works for you and your equipment. No way around doing that.
 
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