oldtigger Loc: Roanoke Virginia-USA

i would like to learn more about how closing a lens down introduces distortion.

Marionsho Loc: Kansas

I must have used the word distortion, when I should have used the word difraction.
selmslie can elaborate on this a lot better than I.

rmalarz Loc: Tempe, Arizona

Yeah, he's good at that.
--Bob

rmalarz Loc: Tempe, Arizona

Scotty, for the first part of this reply, let's take two known quantities and see how they combine to produce diffraction.

Visible light corresponds to a wavelength range of 400 - 700 nanometers (nm) and a color range of violet through red. That is a known quantity.

From the physics involved with diffraction, when the aperture size is larger than the wavelength, the wave passes through the opening and does not spread out much on the other side. When the aperture size is equal to the wavelength, maximum diffraction occurs and the waves spread out greatly.

Now, with these two facts in mind, I dare say any aperture of most any lens is considerably larger than 700nm.

Now, if you want to argue with known physics principles, go ahead.

Now, as far as f/22 being the same aperture for any lens, I beg to differ. The f-number is a ratio of focal length and diameter.

I have two lenses in particular, One is a 150mm, the other 210mm, that I have measured the aperture based on f-stop setting. The diameter of the aperture at f/22 for the 150 is 6.82mm, for the 210 it's 9.54mm. These are now known quantities, as well.

So, with a difference of several millimeters of dia. for those two lenses, how does the f/stop of 22 produce the same amount of diffusion for both, if any at all? Don't parrot either read the article or some BS like that. Since you said, "So far as diffraction is concerned, f/22 is independent of focal length when it comes to diffraction". It would seem that my measurements of two lenses of two different focal lengths shows f-stops of two different diameters. So, something must not be independent.

Explain how two different apertures produce the same amount of diffusion. You might also want to explain how the f-stop is independent of focal length when the f-stop is a ratio involving focal length.

Another thing you might want to explain is the, "22mm with a 1mm dia. aperture being 22mm from the sensor and the 220mm lens having a 10mm aperture 220mm from the sensor". I have a 20mm lens, sorry no 22mm, and the aperture is approx. 60mm from the focal plane. A 200mm lens, again sorry I don't have a 220mm, the aperture in this lens appears to be approx. 60mm from the focal plane. I also have a 28~300 which has an aperture that is fixed at about 60mm from the focal plane. So, based on your statement of the the aperture's location is equal to the focal length of the lens seems to have been violated by the people making Nikkor lenses. So, where did you come up with those numbers?
--Bob

selmslie Loc: Fernandina Beach, FL, USA

You have some fixed notion in your mind that is keeping you from understanding what I have said.

There is no point in my explaining it to you since you have not bothered to read the explanation at LENS DIFFRACTION & PHOTOGRAPHY at Cambridge in Colour or even the passage I quoted earlier at the bottom of their article. It answers your question:

"Technical Note: Independence of Focal Length Since the physical size of an aperture is larger for telephoto lenses (f/4 has a 50 mm diameter at 200 mm, but only a 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel farther before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size)."

Everything I have stated here is based on that article. It explains it all.

Don't shoot the messenger.

selmslie Loc: Fernandina Beach, FL, USA

Appearances can be deceiving. Where the aperture appears to be is not where it actually is physically located.

I have an old 80-200 f/4.5 zoom in which the aperture when looking from the rear of the lens appears to be over 6 inches (more than 150 mm) from the focal plane. It does not appear to move or change its diameter (at f/8) as it zooms from 80-200. However, looking from the front it not only appears to move but the diameter appears to get larger, which it has to in order to maintain the same ratio of focal length to diameter (f-stop). So which view of the aperture is more accurate? Probably the view from the front.

I also have a 75-300 f/4.5-5.6 zoom in which the aperture appears to move from about 4 to 5 inches from the focal plane as I go from 75-300 but the size of the aperture at f/8 does not appear to change. Looking from the front of the lens the aperture appears to move toward the rear of the lens and get larger as I move from 75-300, which logically it must to maintain the same f-stop. Once more, the view from the front of the lens says more about the aperture than the view from the rear.

From the rear of the lens, the aperture for every one of my prime lenses appears to be located at a different distance from the focal plane. However, there is no way I can measure the actual physical location or diameter of the aperture. But I know the diameter is D=f/N where D is the aperture diameter, f is the focal length and N is the f-stop number.

What you are seeing from either end of the lens is a virtual image of the aperture. You cannot determine the physical location or size of the aperture unless you disassemble the lens.

selmslie Loc: Fernandina Beach, FL, USA

That's not all that is deceiving your eyes.

For example, the flange to focal plane distance for Nikon is 46.5 mm, for Canon 42 mm, for Hasselblad 500 it's 74.9 mm and for Leica 27.8 mm. That does not prevent us from having wide angle lenses with shorter focal lengths. It's done with a retrofocus design. Likewise, a 1000 mm lens does not have to be 40 inches long when it uses a telephoto design.

The aperture to focal plane distance that the Cambridge article refers to is the effective distance for that focal length, not the physical distance. If it were the physical distance it would have to be located inside the body of the camera for any focal length shorter than the flange distance.