Internal/rear focusing

[David Kilpatrick]

See this post:

http://forums.dpreview.com/forums/read. ... e=32389467

I can not reply here, but I think the reason the Sigma 50-150mm reports 120mm is because when it focused closer, the focal length changes and perhaps the chip is able to respond correctly to that and show the true focal length. This is a tyical effect of internal or rear focusing designs - they change the focal length in order to focus, instead of extending the back focal distance.

David

[Winston]

Interesting. I had assumed that my internal-focusing SAL70400 behaved that way but never looked. I just checked it at three focal lengths: 70, 135 and 400mm. The angle of view doesn't change at any of the focal lengths over the focusing range.

[pakodominguez]

That was fast!

http://forums.dpreview.com/forums/read. ... e=32399161

Certainly with many lenses the focal length does shorten considerably and it would be pretty cool if the chip can actually report the true focal length.

-Sonolta

Not just the focal length change, but the f stop cahnge too and you lose light, like on the 100mm f2.8 Macro and the50mm 2.8 Macro.

[David Kilpatrick]

Interesting. I had assumed that my internal-focusing SAL70400 behaved that way but never looked. I just checked it at three focal lengths: 70, 135 and 400mm. The angle of view doesn't change at any of the focal lengths over the focusing range.

Quite right. If the angle of view does not change as you focus, the focal length is changing. Here is why:

50mm lens - 44 degree angle of view.
50mm lens at 1:1 magnification - extension 50mm - 22 degree angle of view.
(figures are approx, I'm not looking up the exact angle of a 50mm just now...)

Any conventionally focused lens achieves closer focus by increasing the back focus distance, moving the lens away from the film/sensor plane. That is why it aperture also changes, so that a 50mm f2.8 at infinity becomes a 50mm (extended to 100mm back focus) f/5.6 at 1:1

Internal focusing lenses change the focal length instead. If the 50mm was a rear-focusing macro, it would have a rear group which changed the focal length to only 25mm at 1:1 if the front was to remain static (again, theory, this does not happen in practice due to multi-element designs, retrofocus and telephoto construction, asymmetrical triplet designs etc etc). But the aperture would stay a true f2.8.

This is what happens in zooms, which if you think about are approaching macro focus in normal ranges. A 400mm for example if focused on 80cm is at 1:1. They use internal and rear focus (putting the zoom mechanisms to a secondary use very often as well) and maintain the working aperture, so if you focus your 70-400mm at 400mm and f/5.6 on 1.5m it is still f/5.6 and not the f/7.1 it actually should be (approx) at 1.5m.

And its true focal length is probably around 280mm at 1.5m. Olaf is the expert at lens focus maths, I'm just guessing from experience - Olaf can probably tell you the exact focal length it must be, if an aperture of f/5.6 is to be maintained at 1:4 subject scale and 1.5m focus.

If you think this is complex, try large format studio photography - on a 10 x 8 camera even a full-face portrait is in the 'macro' range!

David

[pakodominguez]

...
Internal focusing lenses change the focal length instead. If the 50mm was a rear-focusing macro, it would have a rear group which changed the focal length to only 25mm at 1:1 if the front was to remain static (again, theory, this does not happen in practice due to multi-element designs, retrofocus and telephoto construction, asymmetrical triplet designs etc etc). But the aperture would stay a true f2.8.

David

You mean I was hidding my source of light?
Probably...

[Winston]

A quick caluculation says that the SAL70400 at closest focus and an indicated 400mm is actually 315mm.

[01af]

I think the reason the Sigma 50-150 mm reports 120 mm is because when it focused closer, the focal length changes and perhaps the chip is able to respond correctly to that and show the true focal length.

According to Sigma's website, the Sigma 50-150 mm 1:2.8 EX DC HSM II's minimum focus distance is 1 m, and the maximum magnification is 1:5.3. If at these settings the distance between the principal planes is approx. 100 mm---which to me seems a lot but not impossible---then the effective focal length indeed would be 120 mm. Still, it's unusual for a camera to report the lens' effective focal length at the current settings in the EXIF data rather than the nominal value, even though the camera sure does know the exact values---it must know them, for auto-focusing, auto-exposure, and image-stabilisation purposes.

I had assumed that my internal-focusing SAL70400G behaved that way but never looked. The angle of view doesn't change at any of the focal lengths over the focusing range.

But the angle of view would change over the focusing range when the focal length is constant. So if angle of view does not change then it inevitably means focal length changes.

A 400 mm for example if focused on 80 cm is at 1:1.

A 400 mm lens cannot cast a real image at a focus distance of 80 cm. The minimum focus distance is four times the focal length which for a 400 mm lens is 1.6 m. At this distance, magnification is 1:1. For greater magnifications, focus distance (i. e. the subject-to-image distance) increases again.

The SAL 70-400 mm G SSM has a minimum focus distance of 1.5 m. At the maximum focal length setting, magnification at this distance is 0.27×, or 1:3.7. So it's obvious that the effective focal length, at these settings, cannot be 400 mm. Actually it's something like 250 mm, or even less if the principal plane distance is positive.

To be clear: At infinity focus, the focal length at the long end of course is 400 mm. It will become shorter only at close range, and not in a linear way. At medium focus distance, focal length will still be almost 400 mm; it'll become significantly shorter only near the the minimum focus distance.

By the way, I am in no way an expert in optics or lens design. The math behind all this is trival. It mostly is just the thin-lens formula and some basic algebra ... that's junior high-school math.

A quick calculation says that the SAL-70400G at closest focus and an indicated 400 mm is actually 315 mm.

Winston, it seems you've erroneously taken the focus distance for the subject-to-lens distance. But at minimum focus, 1.5 m is the subject-to-image distance.

-- Olaf

[David Kilpatrick]

I failed junior high school math consistently. I'm still not comfortable with it. I had other abilities, and still have, and I am quite capable of understanding it all - rather the same way a dyslexic can be competent with language. There's no such thing as 'dysnumeracy' officially, but I seem to have had it, and it cost me a great deal of wasted effort during education, and a lot of hassle because it was always seen as laziness. I would still need to look up formulae and use a calculator for figures these days, even for stuff I should be able to remember and do in my head.

David

[Winston]

RESET

I made some measurements and a constrained CAD drawing. All in millimeters.
My lens focuses a little closer than spec: 1.346 meters
So, The magnification is a little bigger then spec: .277 vs .27
Effective FL at close focus: 229 mm

[01af]

I would still need to look up formulae and use a calculator for figures these days, even for stuff I should be able to remember and do in my head.

Well---I also have to look up the formula which translates distance and magnification into an effective focal length; I simply cannot memorize it. And I don't do the actual calculation in my head either.

I made some measurements and a constrained CAD drawing. All in millimeters.

Yes, you can derive the 'formula' the algebraic way or the geometric way, as you did here. Both ways are equally valid and lead to the same result. By the way, the algebraic formula is f = m * d / (m + 1)^2, with f = focal length, d = focus distance, m = magnification. So for d = 1,346 mm and m = 0.277, f is 229 mm indeed. Still that's just an approximation, as both my formula and your geometry don't take the lens' principal plane distance into account. After all, the SAL 70-400 mm G SSM clearly is not a 'thin lens.' Unfortunately we don't know the principal plane distance (and no, it's not equal to the physical length) so all we can do is neglect it in our calculations, i. e. assume it's zero which it surely isn't.

-- Olaf

[bossel]

Very interesting thread. I'd heard of the reduced FL before but not with so many scientific explanations.

Nevertheless, I suppose you're still all happy with your 70-400

For non-macro use, 1:3.7 should be ok (I barely zoom to 200mm when I stick someone my lens into his face at 1.5m distance!). The only question I would have, if the 1.5m min-foc-dist bothers you sometimes? It bothers me with my 28-135, but that's not a tele-zoom.

[Winston]

Very interesting thread. I'd heard of the reduced FL before but not with so many scientific explanations.
Nevertheless, I suppose you're still all happy with your 70-400

Yes, extremely.

For non-macro use, 1:3.7 should be ok (I barely zoom to 200mm when I stick someone my lens into his face at 1.5m distance!). The only question I would have, if the 1.5m min-foc-dist bothers you sometimes? It bothers me with my 28-135, but that's not a tele-zoom.

Almost all my lenses have magnifications in the range of 0.25 The exceptions are the 17-35 (0.185) and the 100 macro (1.0). It get's me close enough for chance, walk-about encounters.