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Sharpest F stop

Jeff Nadler , May 19, 1999; 09:37 p.m.

I'm confused on the appropriate F stop for highest quality landscapes with sharp foreground to background. A survey would indicate F16 or even smaller as the only appropriate F stop to consider for DOF. Yet several publications including RRS catalog mention that F8 and F11 are indeed much sharper. So what's the best compromise, less sharp better DOF at F16 or sharper lens at F8-11?

Responses


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Matthew Francey , May 19, 1999; 09:52 p.m.

So what's the best compromise [...]

The red pill.

Or you make the selection based on what you want to achieve. We can't tell you what kind of photographs you like!

Jim Chow , May 19, 1999; 10:25 p.m.

BTW, each lens has its own sweetspot. To find it, you have to shoot at each aperture (on a tripod, of course) and take notes. Then examine the chromes under a high-quality loupe and compare the results to your notes. It maybe that although you can, say, shoot at f8 for enough DOF, your lens might be sharpest at f16, so I'd say, shoot at f16. Use a tripod and cable release, or any talk about sharpness/resolution is for naught.

Dave Wilson , May 19, 1999; 10:26 p.m.

Jeff, If its depth you want then shoot at f16 at the hyperfocal distance, if its sharp than f8 or so is the deal. Very small apertures cause a type of diffraction which can cause a loss of image sharpness, I think you actually answered your own question. I would shoot several and see what I liked in the end.

John Lehman , May 20, 1999; 12:11 a.m.

As mentioned above, lenses differ. Based on tests with a lens resolution chart (sorry to introduce empirical data), most of my medium format lenses have maximum center sharpness at f/8 and maximum edge sharpness at f/11; my large format lenses vary between f/8 and f/22 for maximum sharpness (of course, for some of them, f/8 is maximum aperture). So try your lenses and see. Then decide what compromises you want to make since the real world is not a flat chart at a fixed distance, and people's preferences vary.

Patrick Chase , May 20, 1999; 01:18 a.m.

I really shouldn't take this bait, since this is such an overdiscussed and contentious subject, but here goes anyway:

The first thing to note is that you need to decide what you are asking. The answers you've seen conflict because they're really addressing two different questions.

People who advise f/8 - f/11 are generally answering the question "What aperture gives the sharpest rendition of a subject _at the plane of sharpest focus_." For most lenses, f/8 is pretty close to the right answer. It's not a hard and fast rule, though: I've used lenses for which I thought that f/5.6 gave better results (e.g. the Canon 300/2.8 that I sometimes rent and regularly drool over), and other lenses for which f/11 was the best bet (most lower-end zooms I've used). In general, the "sharpest" aperture for any lens will be the one at which optical aberrations (which are worst at wide apertures) and diffraction (which is worst at narrow apertures) are roughly balanced in their effects. The optimum aperture for the center of a given lens may be wider than the optimum aperture for its corners, due to the fact that corners tend to be more severely aberrated to begin with. The oft-made statement (in the RRS catalog, in a previous response in this thread, and elsewhere) that lenses give the best resolution wide open is baloney; Bob Atkins has written an excellent explanation of why that is not the case. See

http://bobatkins.photo.net/info/rrs.html

The second question, and the one that I think you're really asking, is "what aperture will give the sharpest overall image for a near/far landscape composition". In such a composition, you're typically concerned about the rendering of objects which are well away from the plane of sharpest focus. The answer, of course, depends on just how far away from the plane of sharpest focus those objects are, and on the scene magnification. This is why people use hyperfocal distance charts, and why prime lenses with DOF scales are still very popular with landscape photographers.

My personal approach is as follows: I first use either the DOF scale on the lens, the DEP exposure mode on my body, or a hyperfocal distance chart to determine what aperture will yield adequate depth of field with a 0.03 mm permissible blur diameter (0.03 mm is the figure that most manufacturers, including Nikon and Canon, use to establish the DOF scales on their lenses and that Canon bodies use for DEP mode). If the resulting calculated aperture is f/8 or wider, I typically use f/11 to minimize lens aberrations and to reduce the defocus-induced blur diameter to below 0.03 mm. If it's between f/8 and f/11, I stop down 1 full stop, again to reduce the maximum blur diameter a bit. If it's between f/11 and f/16 I use f/16, because at apertures smaller than f/16 diffraction-induced blur starts to approach the 0.03 mm of defocus-induced blur upon which the initial calculation was based. If the initially selected apreture was smaller than f/16, I generally either back away from the foreground subject or switch to a wider lens. What I've described above is simply a system that works for me; Others will have different opinions.

One final comment: All of this talk about optimizing lens performance is pointless if you're not using a solid tripod and sharp film.

Dave Evans , May 20, 1999; 05:26 a.m.

Jeff,

You asked a good question and have received some very good answers.

If you want to learn more about lens performance, check out the lenses section of the following web site.

http://www.photodo.com/

At this site they have numerous lens MTF test results, as well as explanations of the tests and lens performance.

However, I agree that the only way to really determine where a given lens performs best, for a given type of photograph, is through your own testing. Take numerous pictures of the same scene using a tripod and low speed film, at different apertures.

Darron Spohn , May 20, 1999; 10:46 a.m.

If you want the sharpest landscapes from foreground to background, f/16 ain't gonna cut it. You'll need to stop down to at least f/32 (or maybe even f/64) and use some front tilt. What? You're shooting 35mm? Well then, use a tripod anyway, use Fuji Velvia or Kodak E100SW, and stop down to get the depth of field necessary. Don't worry about which aperture is sharpest. Think about it a minute. What good does it do to use a lens' sharpest aperture if you can't get the entire scene in the depth of field? So the better compromise is going for the depth of field.

Eielrt Anders , May 20, 1999; 12:23 p.m.

Through my testing of 35mm system lenses, I have found that uniform performance across the field for a flat surface (i.e., the board that held my resolution test targets) was at f11 for conventional lenses. At this aperture the effects of field curvature are essentially overcome by increased depth of focus. Macro lenses, on the other hand, display uniform performance from maximum aperture, and show optimal performance at f5.6 to f8 in terms of maximum resolution. At f11, diffraction effects limit resolution of highly corrected lenses to a value of just above 80 lpmm, which is plenty good with most films and applications. At f16, maximum resolution is about 68 lpmm. and at f22 it falls to 48 lpmm. What does this all mean? Go the fields and take your typical landscape photographs with your film/s of choice at different apertures, and look at the results as recommended in the earlier responses. It really comes down to what you will accept in resolution against what you need for depth of field. This is another reason why large formats have an advantage over 35mm. The enlargement factor is less, and field cameras can be adjusted to maximize depth of field without stopping down to a level where resolution is a problem.

byron medina , May 20, 1999; 01:28 p.m.

I shouldn't jump in, exceptional answers have already been given, but may I give a quick formula:

Slow Professional Film + Tripod + 2 stops minus your maximum aperture.


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