Any interest in a line of Prime Landscape lenses?

Started May 13, 2013 | Discussions thread
brightcolours
Veteran MemberPosts: 9,447
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You are the one confused, not me.
In reply to Canyongazer, May 14, 2013

Canyongazer wrote:

"Whah?" back at ya.

You ask "Which lenses have best performance at f4?"  Answer: most of those w/ a maximum aperture of 1.4 to f 2.

No. That is not true. It depends on which body you test them on. Why? Because of diffraction, and of the sample frequency of sensors.

Again I will ask, which lenses perform best at f4, and then, which camera body are you referring to?

You state: "Lenses do not usually have the best performance. wide open."

No kiddin'.  That is why I wrote in the first sentence "...most lenses hit peak performance when stopped two or three stops down from max."  Fast lenses designed for all around use suffer from diffraction at small apertures, ...  that was kinda the point of my post...a series of slow, specialty lenses designed to minimize diffraction .  How successful might that effort be? I don't know, just speculating. I've never designed a lens. You?

You have to read my post again. You seem to have totally read past a crucial fact: diffraction is what limits resolution. Diffraction is linked to the size of the hole. That is what that f-number stands for, the size of the hole. And how much that diffraction spreads is linked to the distance that hole has to the sensor. So, that is where the focal length has an influence.

We really need to cut you some slack here, though. I see you have made over 7,000 (!) posts on DP. You can't be expected to do that plus read and comprehend the posts to which you are replying. Only so many hours in the day, no?

You still do not grasp the concept.

Slightly off topic, but your "...above f 4..."  You mean below f 4.

No, 8 is above 4. I did not say "an aperture bigger than" or "smaller than", just merely talking about f-numbers. Sorry to confuse you.

As you state, you are closing down the lens when going to a smaller aperture. There's enough confusion with beginning photographers as it is without this common error.  "Why is the number bigger as the hole gets smaller?" they ask.

F stops are not a measurement but rather a ratio...the ratio between the diameter of the aperture and focal length of the lens.

Thus, a 1" diameter aperture in a 2" (50mm) lens is f 2 (1:2)   A half inch hole is f 4(1:4 ratio)

That same half inch hole in a 100m (4" lens) will be f 8.

Again, read my post. You are totally off the mark with your thinking concerning f-values and resulting resolution. First, let me again apologize for confusing you, I will try to make it more clear:

Lenses usually do not have the best performance wide open. The more you close them down, less of many elements get used, and this results in increased resolution. However, at some point diffraction sets in. And diffraction is what limits resolution when you close down past that point.

What is diffraction? Diffraction is light "bending" past the edges of the aperture (or any other edge), and changing direction because of it. Diffracted light hits pixels next to the pixel it was intended for before it got diffracted.

There are tables which show how much a lens can resolve (in theory) for a given f-value. The tables give values for a single focal length of light, as different wave lengths of light diffract more or less. Lenses that are limited by diffraction at a certain f-value are called diffraction limited for that f-value.

When measuring resolution using a digital camera, something else influences the measurements: the sample frequency (or if you want, the sensor resolution). If the sample frequency is lower than the diffraction limit, you will not notice the diffraction hit. So, it depends on which camera you use when you will see the resolution drop due to diffraction.

For the Nikon D3x, the diffraction limit is past f5.6. If you look at for instance the photozone.de resolution measurements, you will notice that in general the resolution numbers start to drop at f8.

This means that if you have a D3x, if you want to get the highest resolution, you should shoot with f5.6 with every lens.

And that is what you have to understand. There is no way to get a lens to perform better at f11 or f16, because all get hit in the same way by diffraction. Your quest for a lens peaking at f11 or f16 can only be reached in two ways:

  1. You have to have an ABSURDLY bad lens, which has such a low resolving power that only at f11 or f16 the diffraction has a bigger effect than the bad resolving power of the lens itself.
  2. Or: you have to have such a low resolution sensor, that the sample frequency does not catch up with the diffraction until f11 or f16. For f16 that would be a 5mp FF sensor. For f11 a 10mp sensor.

For D800 with 36mp FF sensor, the diffraction limit lays arounf f5.6.

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