f/57: Can anyone explain how an older (non-OS) Sigma 105, 2.8 macro lens, rated at f/22, can capture images at f/27, f/30, f/40, and even f/57? Thought f/22 was the minimum aperture...

4dogsken Loc: NE Ohio

Can anyone explain how an older (non-OS) Sigma 105, 2.8 macro lens, rated at f/22, can capture images at f/27, f/30, f/40, and even f/57? Thought f/22 was the minimum aperture, but data shows otherwise.

St3v3M Loc: 35,000 feet

http://en.wikipedia.org/wiki/Aperture
"In photography
...Accordingly, DSLR lens typically have minimum aperture of f/16, f/22, or f/32, while large format may go down to f/64, as reflected in the name of Group f/64. Depth of field is a significant concern in macro photography, however, and there one sees smaller apertures. For example, the Canon MP-E 65mm can have effective aperture (due to magnification) as small as f/96. The pinhole optic for Lensbaby creative lenses has an aperture of just f/177."

4dogsken Loc: NE Ohio

Thanks St. Then why does Sigma list the smallest aperture as f/22 if it's going at least down to f/57?

Nikonian72 Loc: Chico CA

4dogsken wrote:

Then why does Sigma list the smallest aperture as f/22 if it's going at least down to f/57?

The physical size of the minimum aperture is f/22, which means the aperture diameter is 1/22 the calculated focal length of lens. Many lenses change physical length when focusing close (such as macros), so the math changes. When extended, "apparent" aperture numbers are generated.

4dogsken Loc: NE Ohio

Nikonian72 wrote:

4dogsken wrote:

Then why does Sigma list the smallest aperture as f/22 if it's going at least down to f/57?

The physical size of the minimum aperture is f/22, which means the aperture diameter is 1/22 the calculated focal length of lens. Many lenses change physical length when focusing close (such as macros), so the math changes. When extended, "apparent" aperture numbers are generated.

But isn't a 105mm lens a 105mm lens no matter how it's focused? Thanks, Nikonian. Or is it that the actual size of where the light converges actually smaller than f/22 when focusing closer? And if so, how does the camera know this if the lens feedback says f/22?

Mpeter45 Loc: Springfield, Illinois

I used to have a Vivitar 135mm CF lens. It was a F 2.8. But while normal 135's focus to about 3 feet, this focused to 1 foot. The lens just kept getting longer as you close-focused. In that last two feet, it lost two stops. So even though the lens was 2.8-22, at full extension it was 5.5-45. This lens was not a true macro as it was not 1 to 1. The lens barrel indicated 1 to 2, which was why it was a Close Focus, not a Macro. I could easily understand a lens losing another stop to get to 1 to 1, which would give F64 minimum aperture.

4dogsken Loc: NE Ohio

Mpeter45 wrote:

I used to have a Vivitar 135mm CF lens. It was a F... (show quote)

Thanks Mpeter. Still don't get how the camera figures "apparent" apertures if the lens is set to f/22 though.

Bret Loc: Dayton Ohio

Think about this...say you set your 105mm lens on 1:1 at F22...now put an extension tube behind it...say a 50mm tube...now your at 155mm long lens... magnification goes up...your lens grew moving your aperture further away. Fold your index finger over so you can barley see through it...then move it away from your eye...you'll see the opening get smaller as you move it away.

4dogsken Loc: NE Ohio

Thanks Bret. That helps. Why do lens makers list f/22 as minimum if they can go "apparently" much smaller? Why not list the smallest possible aperture?

Bret Loc: Dayton Ohio

The lens maker cant tell how far away your going to move the lens....with a tube or anything. Move it to far away and you cant see anything. Works kinda the same way as a zoom lens..say my 28-105 3.5-4.5....at 28mm the biggest size will be F3.5...and then zoom in to 105mm and the largest size is going to be F4.5..I didn't change the size opening...I just moved it...which makes it appear smaller.

4dogsken Loc: NE Ohio

Bret wrote:

The lens maker cant tell how far away your going to move the lens....with a tube or anything. Move it to far away and you cant see anything. Works kinda the same way as a zoom lens..say my 28-105 3.5-4.5....at 28mm the biggest size will be F3.5...and then zoom in to 105mm and the largest size is going to be F4.5..I didn't change the size opening...I just moved it...which makes it appear smaller.

Thanks Bret. Think I got a handle on it now. Enjoy the weekend.

jeep_daddy Loc: Prescott AZ

4dogsken wrote:

Bret wrote:

The lens maker cant tell how far away your going to move the lens....with a tube or anything. Move it to far away and you cant see anything. Works kinda the same way as a zoom lens..say my 28-105 3.5-4.5....at 28mm the biggest size will be F3.5...and then zoom in to 105mm and the largest size is going to be F4.5..I didn't change the size opening...I just moved it...which makes it appear smaller.

Thanks Bret. Think I got a handle on it now. Enjoy the weekend.

I was going to disagree with all of this but since I don't know for sure I'll hold my tongue. I see the word "apparent" and "effective" thrown around here but it still doesn't make any sense to me. I've seen other threads about focal length when comparing 3rd party lenses to OEM lenses using the same focal length that used the same verbiage - "apparent" and "effective". But when someone posted 2 images exposed using the same focal length but two different lenses, the OEM lens made the subject apparently and or effectively appear closer. (I say "what?")) All I know is that I have all Canon lenses for my Canon bodies and if I use any of them at f22, or whatever f-stop, that is exactly what the metadata tells me. On the other hand, I don't use extension tubes so as far as I know, they may change effective or apparent data in the metadata of the image if tubes are used.

PhotoArtsLA Loc: Boynton Beach

These days, it would be fun to use the latest materials and technology to make a super high end pinhole camera. There really isn't a quality limit to that little hole, done right. F/stops on pinhole cameras are general in the hundreds, like f/634, and that is the ONLY f/stop short of adding neutral density. I mention this for those who think f/22 is the final stop. View cameras actually have good, even great performance at f/stops like f/64, f/96 and f/128. This is generally not true for DSLR shooters at f/22, where diffraction and other aspects of lens inadequacy can crop up. F/stop is merely a calculation, and is not limited at all, with the exception of the fast end. T/0.65, for example, (transmission stop, not f/stop, which is the "true f/stop" ) is about as fast as things get, though there may be oddball fun happening just about anywhere. The trick is, making the lens good.

Nikonian72 Loc: Chico CA

4dogsken wrote:

But isn't a 105mm lens a 105mm lens no matter how it's focused?

No! All focal lengths are measures at infinite focus, which every lens can accomplish (save one). As a lens focuses closer, it extends externally (old design) or internally (new design), technically changing the focal length.

All I need is basic understanding. I do not have to know the exact how or why, to be able to use any lens to its best potential.

4dogsken Loc: NE Ohio

Hey Jeep Daddy. No tubes involved. Just a D90 plus a Sigma 105. Thanks for the feedback.