Is there such thing as an f/2.8 500mm lens?

[kevin_swan1]

I posed this question in another thread, but it was an older thread, and I think

it's gotten lost in the noise. However, I'm curious for an explanation, and I

thought others might be too, so I figured I'd make this its own thread to make

it easier to find.

 

In a discussion regarding f-stops in relation to focal lengths and aperture

diameters, Matt Laur said "the physical aperture on an f/2.8 50mm lens is

smaller than the physical aperture on an f/2.8 500mm lens." Mathematically, this

makes sense, but practically, how can there even be such thing as an f/2.8 500mm

lens?

 

Following the definition of f-stop, wouldn't that mean the aperture diameter

would have to be 500mm/2.8 = 179mm, or 7 inches? Isn't that correct? How is that

possible, given that the bayonette opening on most SLRs is much, much smaller

than that? At some point, doesn't the lens mount opening of the camera itself

limit the possible speed of lenses at varying focal lengths?

[Ed_Ingold]

The aperture is determined by the effective diameter at the entrance node, not the image circle size. A faster lens results in a brighter image, not a bigger one.

[evan_goulet]

<a href="http://www.popphoto.com/photonews/3930/pma-sigma-intros-200-500mm-f28-lens.html"> Sigma's Beast</a>.

[geoffs1]

As Edward points out, the short answer is "no", the size of the lens-mount doesn't limit the maximum speed of lenses (at least not in the straightforward way you describe).

 

To a first-order approximation, the maximum aperture of a lens is limited by the diameter of the first element. This is why the outside diameter of most large aperture telephotos (ex. the Canon 300mm f/2.8) steadily reduce from the front-element back to the camera. Other lenses do the same thing, but in many cases the mechanical construction hides the decreasing size of the optical elements with focusing and/or zooming mechanisms.

 

This is actually all a result of basic optics. I don't have a good reference handy (and it's been quite a few years since I took my last optics course), but I'll bet if you Google'ed around a bit you would find some good articles that describe the basics of lens design. I did find one page with a decent introduction to geometrical optics: http://www.answers.com/topic/geometrical-optics

 

Cheers,

 

Geoff S.

[kevin_swan1]

Thanks for the explanation, Edward, but I thought the "aperture" was measured at the point where the blades actually open and close? Isn't this typically at the rear element of the lens, i.e., right before the light enters the camera body?

 

Otherwise, how would the aperture ever change? That is, the aperture is variable because the lens can modify the diameter of the opening of the blades, right? But if the aperture is measured at the front element, which is constant, then shouldn't the lens' aperture also be constant? This is obviously not true, so I'm just trying to understand the relationship here.

[geoffs1]

"I thought the "aperture" was measured at the point where the blades actually open and close?"

 

No, the physical diameter of the aperture blades enters into the light-gathering equation in a complex way. The limiting factor isn't the diameter at the location of the blades, but the projection of the blades by the lens elements in front of them. If you look into the front of a lens that's been stopped down a bit, you should be able to see the image of the blades sitting pretty much at the front of the lens (sometimes they'll appear to float in front of the lens a little bit). Generally, the image of the blades is the same as the front element of the lens, and the maximum diameter of the aperture stop is the same as the diameter of the front of the lens.

 

The aperture blades are generally located at a point in the lens known as the "conjugate plane"; it's at this point that the image's spatial information is spread out uniformly across the light-beam. Because the spatial information (i.e. the image) isn't localized, the aperture just cuts down the amount of light going through the lens, and you don't see a shadow of the iris in the image.

 

There is an "image" formed at the conjugate plane (it's actually quite sharp and visible if you place screen at that point - that's a common lab exercise in every optics class). The conjugate-image is the Fourier Transform of the original image, and the spatial information is present as a series of ripples and phase modulations.

 

I hope this helps a little. You're actually asking some very fundamental questions about optics; I hope you're able to get some more complete answers.

 

Cheers,

 

Geoff S>

[alec_myers]

"Because the spatial information (i.e. the image) isn't localized, the aperture just cuts down the amount of light going through the lens, and you don't see a shadow of the iris in the image."

 

Kudos to Geoff for a really cool explanation. Question, though: if you block out part of the conjugate image (the outermost parts first) like the blades of the aperture do, don't you lose some of the spatial frequency information (high frequencies first) from around the edge of the conjugate image? Is this the reason/explanation behind the diffraction blurring of the lens which worsens as you stop down?

[geoffs1]

"if you block out part of the conjugate image (the outermost parts first) like the blades of the aperture do, don't you lose some of the spatial frequency information (high frequencies first) from around the edge of the conjugate image? Is this the reason/explanation behind the diffraction blurring of the lens which worsens as you stop down?"

 

Exactly!

 

In fact, the traditional "Airy Disk" blurring due to diffraction is really nothing more than the Fourier Transform (FT) of a sharp-edge circular filter (well, not exactly, but pretty close...). One of the cooler things you can do if you have a setup with the conjugate image plane available is to try different aperture shapes and look at the effect on the image that forms on the focal-plane of the lens.

 

Cheers,

 

Geoff S.

[steve_bingham]

Yes. But not until later this year. But at 35 pounds it is scary! One big lens. Sigma zoom.

[fred_c1]

You guys ain't seen nothin' yet: <a href=http://www.dpreview.com/news/0610/06100101zeiss1700f4.asp>http://www.dpreview.com/news/0610/06100101zeiss1700f4.asp</a><p>

 

<img src=http://www.dpreview.com/articles/photokina2006/Misc/IMG_0919.jpg>