Try f/8

2010-05-18

in Geek stuff, Photography, Science

The relative aperture of a photographic lens is really important, when it comes to the quality of the photos that arise in most situations.

If you have the sort of camera where you can specify an aperture – as is possible on all film SLR camera, all digital SLR cameras, and many point and shoot digital cameras – try taking some photos using f/8. Almost regardless of the lens being used, this will often generate rather lovely images.

If the shutter speed your camera picks when you set the aperture to f/8 is slower than one over the focal length of your lens, do something to keep the camera still. That is to say, if you are using a 50mm lens with a shutter speed of less than 1/50th of a second, you are likely to end up with a blurred shot. To avoid that, you can brace against something solid if you are just a bit below. If you are looking at really long exposures – say, more than half a second – either put your camera on a tripod or rest it on something solid and use a countdown timer.

f/8 is usually beautiful. It excludes stray photons that are problematic, and it doesn’t usually cause diffraction. Please give it a try.

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{ 12 comments… read them below or add one }

Dave May 18, 2010 at 9:08 am

I think that you may be mixing up a few things about diffraction, and F #’s.
I agree that you get some nicer pictures but it’s not really related to the diffraction. The aperture of your lens has to be around the wavelength of light for this to be a big problem (about 1 micron where a hair is about 100 microns in diameter; you can see it at 100 microns though). When the aperture is decreased the depth of field increases so and errors in your focus are masked, this is especially noticeable with landscapes where most auto focus lens mess up as they can focus beyond infinity plus the errors in the focus itself. You can see these effects easily if you need glasses (like me). Just make a small hole with your curled index finger a look through this without your glasses. You should see diffraction at really small apertures, but also you should see stuff in focus

Milan May 18, 2010 at 10:42 am

Diffraction is a problem with many lenses when using apertures smaller than f/8, such as f/16 or f/22.

You do get more depth of field with those apertures, but overall image quality can be lower.

. May 18, 2010 at 10:43 am

TUTORIALS: DIFFRACTION & PHOTOGRAPHY

Diffraction is an optical effect which can limit the total resolution of your photography– no matter how many megapixels your camera may have. Ordinarily light travels in straight lines through uniform air, however it begins to disperse or “diffract” when squeezed through a small hole (such as your camera’s aperture). This effect is normally negligible, but increases for very small apertures. Since photographers pursuing better sharpness use smaller apertures to achieve a greater depth of field, at some aperture the softening effects of diffraction offset any gain in sharpness due to better depth of field. When this occurs your camera optics are said to have become diffraction limited. Knowing this limit can help you to avoid any subsequent softening, and the unnecessarily long exposure time or high ISO speed required for such a small aperture.

. May 18, 2010 at 10:45 am

“Even when a camera system is near or just past its diffraction limit, other factors such as focus accuracy, motion blur and imperfect lenses are likely to be more significant. Softening due to diffraction only becomes a limiting factor for total sharpness when using a sturdy tripod, mirror lock-up and a very high quality lens.

Some diffraction is often ok if you are willing to sacrifice some sharpness at the focal plane, in exchange for a little better sharpness at the extremities of the depth of field. Alternatively, very small apertures may be required to achieve a long exposure where needed, such as to create motion blur in flowing water for waterfall photography.

This should not lead you to think that “larger apertures are better,” even though very small apertures create a soft image. Most lenses are also quite soft when used wide open (at the largest aperture available), and so there is an optimal aperture in between the largest and smallest settings– usually located right at or near the diffraction limit, depending on the lens. Alternatively, the optimum sharpness may even be below the diffraction limit for some lenses.”

magictofu May 18, 2010 at 12:07 pm

I always thought that different lenses had different sweet spots in terms of image quality; often about two stops above maximum aperture and changing with focal lenght with most zoom lenses.

Milan May 18, 2010 at 12:18 pm

For both my 24-70 f/2.8 and my 70-200 f/4, I think f/8 is the ideal aperture.

Same for the Canon 10-22. I am not sure about my 50mm prime, which the above photo (and yesterday’s) was taken with.

. May 18, 2010 at 12:24 pm
Tristan May 18, 2010 at 4:21 pm

I think ultimate sharpness has very little to do with the quality of pictures. My 70-210 is plenty sharp even at its “softist” – (210mm at F4).

Dave May 19, 2010 at 3:00 am

I’m was mistaken about the diffraction earlier as I was talking about
near field but I forgot that you need Fraunhofer diffraction if
you are talking about far field. So I looked into my old text books and
found a rough rule of thumb formula to do these calculations. So if we
talk about resolving power of a typical 50mm lens, using green light
(lets pretend we’re looking at lot of leaves) and a standard digital SLR

D = f/f#
50mm lens 1.4 f# D=3.5cm
limit of resolution ~ 1.22*f*wavelength/D = 0.9 microns
50mm lens 22 f # D = 0.2cm
limit of resolution ~ 1.22*f*wavelength/D = 17 microns
Canon 400D pixel size is around 5 microns
so best aperture is 1.22 * 50e-3 * 550e-9/5e-6 = .00671m
f# = f/D = 50e-3/0.00671 = 7.45 f# so pretty close to 8
So the sweet spot is a compromise between
the depth of field increase as the aperture goes down
(ie easier to focus) and the loss of resolving power so as you say around 8.

. May 19, 2010 at 6:36 am

“Sharpness is the most overrated aspect of lens performance.

Lens sharpness seems like it ought to be related to making sharp photos, but it isn’t.

Sales and marketing departments fuel this misconception because it scares people into buying new lenses. Sharpness is easy to test and analyze, so magazines oblige less experienced photographers with reams of colorful charts and graphs. People would make far better pictures if they spent time learning how to make great photos with what they already own instead of worrying about their tools.Sharpness is the most overrated aspect of lens performance.

Lens sharpness seems like it ought to be related to making sharp photos, but it isn’t.

Sales and marketing departments fuel this misconception because it scares people into buying new lenses. Sharpness is easy to test and analyze, so magazines oblige less experienced photographers with reams of colorful charts and graphs. People would make far better pictures if they spent time learning how to make great photos with what they already own instead of worrying about their tools.”

http://www.kenrockwell.com/tech/lens-sharpness.htm

Milan May 19, 2010 at 8:32 am

Dave,

Thanks for doing the calculations.

Tristan,

I am not telling people to buy new lenses, but rather to try setting their cameras to aperture priority mode and then a particular setting. It doesn’t cost a cent!

. July 18, 2010 at 7:10 pm

‘Your camera takes great photos.’

http://www.dannyst.com/your-camera-takes-great-photos/#more-1614

“So what makes a good photograph? Subjectively, it’s when the captured moment or subject invokes an emotional response from you… when the concept behind the photo engages you in a personal way.

Now on the technical side, the key to getting a good shot is determining the right quality & direction of light, the geometric composition, the correct exposure settings, the tedious post processing, determining the right lens for the situation, and finally using the right camera. It’s funny coz if you look at each of these elements I just mentioned, the camera is actually the least important. Why? If I replaced my Nikon D300 with a lower model D80 or even a entry-level D40 that’s 60% cheaper, but still retain all other elements like the light, the lens, and the composition, you can still get the exact same image quality. Conversely, if upgrade your camera to a Nikon D3x that’s 500% more expensive, but mess up with all the other elements, your image quality will still be crap.”

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