Which is sharper???

[Percy]

Just something to pick the brains of the gurus here...

Which would yield the highest resolving image? Let's say a brand X lens, best possible technology available making this lens. Would it be sharpest at either...

1. Stopped down two stops from maximum.
2. At f/16 for max depth of field.
3. Wide open.

Just to throw some numbers, let's say it's a 50mm f/1.4 (max aperture), while f/22 is the smallest it can go.

Guesses? You can get some hints at http://www.luminous-landscape.com check under the NEW headings. That'll give you some hints, but Mike isn't listing the correct reasons...

Percy

[bitkahuna]

Generally I'd think #1 is most likely to be true because lenses generally don't operate their best (color, sharpness, contrast) at the extremes.

Didn't have chance to read the link.

[Percy]

Bit,

Read the link...the answer might surprise you!

Good guess by the way.

Percy

[bitkahuna]

Found it here.

When I had the Canon 16-35/2.8 L lens for a while, I was totally disappointed though with it's performance wide open. It was BAD. I'm blown away though, by the 70-200/2.8 L lens though (yes, even though it's IS ) at wide open. I can leave it on that all day long and it's always sharp.

[Percy]

So the answer is... ?

[bitkahuna]

I guess I keep thinking zoom lenses which have additional issues over primes.

According to the article, a prime of the highest quality is likely to be limited only by diffraction caused by small apertures so wide open yields the sharpest images.

But FOCUS is a huge factor too and at wide open DOF is the smallest so focus becomes most critical. But assuming perfect focus on the perfect lens I guess wide open is best. But since mortals don't have the best glass or even perfect focusing, I'd still say for most people they're probably better off stopping down to say f/4 at least in most cases.

[Percy]

Bit,

EXCELLENT.

Wide open, for an ideally perfect lens, is the best. This isn't due to diffraction as the article mentions, but is a result of a simple thing called Dawes Law. It states that the larger the optic, all things being equal, the better the angular resolution. In other words, the larger diameter of the optic, the more you'll resolve. That's why the largest observatories are still going for bigger and bigger mirrors. If the Hubble space telescope used a mirror as large as the earth based ones, you'll see a tremendous, tremendous difference.

If you ever look through a small scope, say a 4 inch diameter at a tight cluster, it'll look like a small fuzzy cloud. But if you were to take a 8 inch diameter scope at the same object, you'll realize that the fuzzy cloud is actually about a million stars all clustered up together. Larger diameter for better resolution, all things being equal.

Stopping down to f/8 or two stops from max....here's what's going on. At f/8, it appears sharper, but there is a loss in resolution. You're gaining depth of field and apparent sharpness, but you're losing resolution since the virtual diameter of the lens is smaller. At two stops from max, since we're using a "practical" optics (commercially available camera lens), we're just using the "good" part of the lens. If the lens was made to a critically high precision with a perfect figure, then you would have the best performance wide open. Very very few lenses do this. High end t-scope optics, however, DO do this. Going from a 4 inch diameter to a 6 inch diameter yields highly noticable results.

Also, even with the highest available technology and glass, the fastest the t-scope guys can make a lens with a 4 inch diameter, with all the tricks thrown in and be nearly nearly perfect in color correction is f/5. Any faster and you WILL see CA. With a 5 inch, it's f/6. With a 6 inch, f/7. 7 inch, f/7. 8 inch, f/8. There's only so much that you can do while bending light.

Surprising answer?

Percy

[jmecbr900]

Dumb question, but......what do you mean "wide open"?

[Percy]

Wide open meaning a f/1.4 lens AT f/1.4, that is, maximum apeture setting.

Percy