I have a few lenses. I've notice people saying that all lenses are sharpest at a certain aperture. How do I determine what that aperture is?
It very much depends on the given lens but if you can place a hi resolution target at least twice the minimum focus distance away, more is usually better, and then take some test shots at different apertures, you will have a pretty good idea of image quality when the images are viewed on a computer screen at 100 percent.
(All focus and resolution tests have to be tempered with some in-use shooting to really gauge the value and overall quality of a given lens.)
Typical targets are:
Newsprint
Shooting targets for rifle use.
Pretty much anything else that has high contrast and sharp edges.
It's also fairly safe to assume that a consensus of formal testing sites should be fairly accurate. For instance, looking at:
... if they all report the same or similar results, I can be pretty confident I will also have similar results, barring sample variations. (Some lenses are simply better than others within a sampling even from the same manufacturer and the same identical model.)
The answer is not simple. Any lens design involves some compromises. As you open the aperture up, more of the lens surface gets involved in creating the image and generally that makes those compromises more apparent with a loss in sharpness. On the other end, as you stop down diffraction caused by the aperture itself will soften the image. The sharpest aperture is the one just before diffraction softness starts to dominate the lens softness.
However this optimum aperture will typically vary across the frame. Typically (though not always), the lens design is sharpest in the center of the frame and gets softer toward the edges. However, diffraction softness is fairly constant across the frame. What that means is that the optimum apeture for center sharpness is going to be wider than the optimum aperture for corner sharpness.
As an example, my Canon 17-40/4L is at its optimum center sharpness somewhere between f/5.6 and f/8. However it doesn't achieve peak corner sharpness until around f/16. So in principle f/8, f/11, or f/16 could be considered the sharpest aperture depending on what part of the frame you are looking at. In practice, I end up picking an aperture based on how important corner sharpness is to the image.
Beyond that, you also need to take the camera into consideration. I shoot with a 5D which, with its relatively low pixel density only resolves diffraction at f/11 which means on my camera there is no practical difference in center sharpness between f/8 and f/11. However the higher pixel density of the 40D starts resolving diffraction around f/8, but the APS-C format crops out the corners making sharpness to the edges of the lens image irrelevant. So then, the best aperture for final image sharpness will be different depending on the camera you use.
The upshot is there is no substitue for testing your gear. MTF charts and reviews can give you an idea of what to look for, but to get the best results you need practical experience with your camera and your lenses.
Thanks for the links Ziggy. Found exactly what I needed.
My Fifty is sharpest @ f/4
My 200mm is sharpest @ f/8, 135mm @ f/8, and f/5.6 @ 55mm.
and the kit lense is sharpest @ f/5.6 @ 18mm, f/8 @ 24mm, f 5.6 @35mm, and f/8 at 55mm.
This information is critical when you want to produce the sharpest possible picture.
For instance, I'm shooting a portrait @ 50 mm, I want the sharpest image possible. Wouldn't make sense to use the 50 mm lense if I need a tighter aperture. So, if the light requires f/8 aperture, I would use the kit lense for that. Same shot in a darker setting that requires f/4, I would snap on my nifty fifty, bam, sharpest image possible.
I'm going to make a quick reference card for the MTF values for quick easy comparisons of my lenses. (The MTF value, for those who don't know, is the line widths per picture hieght. The higher the number, the better.)
The answer is not simple. Any lens design involves some compromises. As you open the aperture up, more of the lens surface gets involved in creating the image and generally that makes those compromises more apparent with a loss in sharpness. On the other end, as you stop down diffraction caused by the aperture itself will soften the image. The sharpest aperture is the one just before diffraction softness starts to dominate the lens softness.
However this optimum aperture will typically vary across the frame. Typically (though not always), the lens design is sharpest in the center of the frame and gets softer toward the edges. However, diffraction softness is fairly constant across the frame. What that means is that the optimum apeture for center sharpness is going to be wider than the optimum aperture for corner sharpness.
As an example, my Canon 17-40/4L is at its optimum center sharpness somewhere between f/5.6 and f/8. However it doesn't achieve peak corner sharpness until around f/16. So in principle f/8, f/11, or f/16 could be considered the sharpest aperture depending on what part of the frame you are looking at. In practice, I end up picking an aperture based on how important corner sharpness is to the image.
Beyond that, you also need to take the camera into consideration. I shoot with a 5D which, with its relatively low pixel density only resolves diffraction at f/11 which means on my camera there is no practical difference in center sharpness between f/8 and f/11. However the higher pixel density of the 40D starts resolving diffraction around f/8, but the APS-C format crops out the corners making sharpness to the edges of the lens image irrelevant. So then, the best aperture for final image sharpness will be different depending on the camera you use.
The upshot is there is no substitue for testing your gear. MTF charts and reviews can give you an idea of what to look for, but to get the best results you need practical experience with your camera and your lenses.
Liquid, you know I'm going blind. And I won't be able to tell the difference until I get the D300 later this year. But you are absolutely correct. It also depends on what part of the frame you need in sharpest focus. This is where the lense vigenetting and such come into play. And we all know that having the subject dead center, is just a snap shot. Glad that you chimed in. ;-)
The answer is not simple. Any lens design involves some compromises. As you open the aperture up, more of the lens surface gets involved in creating the image and generally that makes those compromises more apparent with a loss in sharpness.
Which is why people are so excited about the just announced Zeiss Distagon. They are known for edge to edge sharpness across different aperatures. But you will pay for it
"Don't ask me what I think of you, I might not give the answer that you want me to. Oh well."
-Fleetwood Mac
Comments
It very much depends on the given lens but if you can place a hi resolution target at least twice the minimum focus distance away, more is usually better, and then take some test shots at different apertures, you will have a pretty good idea of image quality when the images are viewed on a computer screen at 100 percent.
(All focus and resolution tests have to be tempered with some in-use shooting to really gauge the value and overall quality of a given lens.)
Typical targets are:
Newsprint
Shooting targets for rifle use.
Pretty much anything else that has high contrast and sharp edges.
It's also fairly safe to assume that a consensus of formal testing sites should be fairly accurate. For instance, looking at:
http://photozone.de/
http://www.dpreview.com/lensreviews/
http://www.the-digital-picture.com/Reviews/
... if they all report the same or similar results, I can be pretty confident I will also have similar results, barring sample variations. (Some lenses are simply better than others within a sampling even from the same manufacturer and the same identical model.)
Moderator of the Cameras and Accessories forums
Michiel de Brieder
http://www.digital-eye.nl
However this optimum aperture will typically vary across the frame. Typically (though not always), the lens design is sharpest in the center of the frame and gets softer toward the edges. However, diffraction softness is fairly constant across the frame. What that means is that the optimum apeture for center sharpness is going to be wider than the optimum aperture for corner sharpness.
As an example, my Canon 17-40/4L is at its optimum center sharpness somewhere between f/5.6 and f/8. However it doesn't achieve peak corner sharpness until around f/16. So in principle f/8, f/11, or f/16 could be considered the sharpest aperture depending on what part of the frame you are looking at. In practice, I end up picking an aperture based on how important corner sharpness is to the image.
Beyond that, you also need to take the camera into consideration. I shoot with a 5D which, with its relatively low pixel density only resolves diffraction at f/11 which means on my camera there is no practical difference in center sharpness between f/8 and f/11. However the higher pixel density of the 40D starts resolving diffraction around f/8, but the APS-C format crops out the corners making sharpness to the edges of the lens image irrelevant. So then, the best aperture for final image sharpness will be different depending on the camera you use.
The upshot is there is no substitue for testing your gear. MTF charts and reviews can give you an idea of what to look for, but to get the best results you need practical experience with your camera and your lenses.
My Fifty is sharpest @ f/4
My 200mm is sharpest @ f/8, 135mm @ f/8, and f/5.6 @ 55mm.
and the kit lense is sharpest @ f/5.6 @ 18mm, f/8 @ 24mm, f 5.6 @35mm, and f/8 at 55mm.
This information is critical when you want to produce the sharpest possible picture.
For instance, I'm shooting a portrait @ 50 mm, I want the sharpest image possible. Wouldn't make sense to use the 50 mm lense if I need a tighter aperture. So, if the light requires f/8 aperture, I would use the kit lense for that. Same shot in a darker setting that requires f/4, I would snap on my nifty fifty, bam, sharpest image possible.
I'm going to make a quick reference card for the MTF values for quick easy comparisons of my lenses. (The MTF value, for those who don't know, is the line widths per picture hieght. The higher the number, the better.)
Thanks again. :-)
Nikon Shooter
It's all about the moment...
Liquid, you know I'm going blind. And I won't be able to tell the difference until I get the D300 later this year. But you are absolutely correct. It also depends on what part of the frame you need in sharpest focus. This is where the lense vigenetting and such come into play. And we all know that having the subject dead center, is just a snap shot. Glad that you chimed in. ;-)
Nikon Shooter
It's all about the moment...
Which is why people are so excited about the just announced Zeiss Distagon. They are known for edge to edge sharpness across different aperatures. But you will pay for it
-Fleetwood Mac