Risk of focus shift issues in new 1.4/35?

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Archive 2011 · Risk of focus shift issues in new 1.4/35?

sebboh
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p.4 #1 · Risk of focus shift issues in new 1.4/35?

denoir wrote:
Lars Johnsson wrote:
I have 3 different Canon bodies and they can all AF very good at f/8.

And what f/8 AF lenses do you have?

does the 400/5.6 not autofocus with a 1.4x extender and the f/4 lenses not autofocus with a 2x extender? there's so much i miss not using autofocus.

Mar 13, 2011 at 05:56 AM

denoir
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p.4 #2 · Risk of focus shift issues in new 1.4/35?

From Canon:
http://cpn.canon-europe.com/content/education/infobank/lenses/extenders.do

"All earlier cameras have sensors that are only sensitive to light from a lens with a maximum aperture of f/5.6 or wider. The EOS 3, and some later models, allow autofocusing with a maximum aperture of f/8, though only with the centre focusing point.

All EF lenses have a maximum aperture of f/5.6 or wider, so all cameras will autofocus with any autofocus lens. The f/8 autofocusing ability is only needed when you are using a lens and Extender combination that reduces the maximum aperture of the lens to f/8 or smaller (see table). Some lens and Extender combinations mean that the maximum aperture of the lens is maintained at f/5.6 or wider and so autofocus remains unaffected."

Mar 13, 2011 at 06:18 AM

Lars Johnsson
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p.4 #3 · Risk of focus shift issues in new 1.4/35?

denoir wrote:
Lars Johnsson wrote:
I have 3 different Canon bodies and they can all AF very good at f/8.

And what f/8 AF lenses do you have?

I use my 800/5,6 with the 1,4x extender all the time (f/8). AF works excellent on all my 3 Canon bodies.

Mar 13, 2011 at 06:25 AM

denoir
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p.4 #4 · Risk of focus shift issues in new 1.4/35?

It looks like the 1D series can handle the center point at f/8, but not higher. The 5D series and even the 7D series with the improved AF system can't go above f/5.6. So your 800/5.6 AF would not for instance work with a 2x teleco.

Mar 13, 2011 at 06:26 AM

Lars Johnsson
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p.4 #5 · Risk of focus shift issues in new 1.4/35?

denoir wrote:
From Canon:
http://cpn.canon-europe.com/content/education/infobank/lenses/extenders.do

"All earlier cameras have sensors that are only sensitive to light from a lens with a maximum aperture of f/5.6 or wider. The EOS 3, and some later models, allow autofocusing with a maximum aperture of f/8, though only with the centre focusing point.

All EF lenses have a maximum aperture of f/5.6 or wider, so all cameras will autofocus with any autofocus lens. The f/8 autofocusing ability is only needed when you are using a lens and Extender combination that reduces the maximum aperture of the lens to f/8 or smaller (see table). Some lens and Extender combinations mean that the maximum aperture of the lens is maintained at f/5.6 or wider and so autofocus remains unaffected."

In other words, every 1 series body has AF at f/8

I have owned 7 different Canon DSLR bodies. 5 of those had AF at f/8. And there exist a few others also

Mar 13, 2011 at 06:27 AM

denoir
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p.4 #6 · Risk of focus shift issues in new 1.4/35?

Yes, center point AF - i.e crippled AF. So I would certainly not call it "very good". On a 7D and f/5.6 you have full use of the outer points as well, which you do not on a 1D with an f/8 lens. So how much the AF works is a question of definition. Certainly it does not work as well at f/8 as it does at f/5.6.

It makes me wonder though about the el-cheapo zoom lenses like the Sigma 18-200 that goes between f/3.5 and f/6.3 - they can still use full AF at f/6.3 even on the most basic models..

Mar 13, 2011 at 06:32 AM

Lars Johnsson
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p.4 #7 · Risk of focus shift issues in new 1.4/35?

I would prefer to only have the my 1Ds3 or 1D4 center point. Compared to have all the AF points on a 7D, 40D, 50D.
And at f/5,6 I will have 45 points of course. So with your logic the f/5,6 points with the other bodies are also crippled. There are an even larger difference in the number there

Mar 13, 2011 at 06:41 AM

denoir
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p.4 #8 · Risk of focus shift issues in new 1.4/35?

The point however is that after f/5.6 the AF does not work as it does for other apertures. It's crippled relative its normal operation obviously. With a 1D you don't have a normally functioning AF at f/8 as you have at f/5.6.

As for AF quality, I'd pick the 7D AF system any day over the 1Ds3 system - I've had both. And I'd pick the 1D4 AF system over the 7D system. Of the systems I've used more than just casually:

1D4> 7D > 1Ds3 > 5DII > 350D

Mar 13, 2011 at 06:47 AM

Lars Johnsson
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p.4 #9 · Risk of focus shift issues in new 1.4/35?

The AF points have different sensitivities. And some are cross-type points and others not. So you are wrong when saying "after f/5.6 the AF does not work as it does for other apertures"
it doesn't work the same for other apertures either. The 1D4 has 45 points, 39 are cross-points. But they only behave as cross-points if the lens is f/2,8 or faster. So it doesn't work the same for f/2,8 and f/5,6 either. And not for f/4 either.

Mar 13, 2011 at 06:55 AM

denoir
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p.4 #10 · Risk of focus shift issues in new 1.4/35?

Lars Johnsson wrote:
The AF points have different sensitivities. And some are cross-type points and others not. So you are wrong when saying "after f/5.6 the AF does not work as it does for other apertures"
it doesn't work the same for other apertures either. The 1D4 has 45 points, 39 are cross-points. But they only behave as cross-points if the lens is f/2,8 or faster. So it doesn't work the same for f/2,8 and f/5,6 either. And not for f/4 either.

Um, read that through again.

You are confirming that I'm definitely wrong when I'm saying that "after f/5.6 the AF does not work as it does for other apertures" and you've added further limitations - that the cross points don't work after f/2.8.

Mar 13, 2011 at 07:00 AM

Lars Johnsson
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p.4 #11 · Risk of focus shift issues in new 1.4/35?

Yes you did imply that the AF worked in one way up to f/5,6 and then it changed.

Mar 13, 2011 at 07:05 AM

Lars Johnsson
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p.4 #12 · Risk of focus shift issues in new 1.4/35?

This is your comment about Canon AF. And it's plain wrong

" Canon cameras can't for instance use AF on f/8 lenses and and the AF is barely usable for f/5.6 lenses..

Mar 13, 2011 at 07:07 AM

denoir
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p.4 #13 · Risk of focus shift issues in new 1.4/35?

Yes, that was my first comment, which I amended by the second one where I dug up the information from Canon that the top end cameras actually do have limited AF use at f/8.

Lars Johnsson wrote:
Yes you did imply that the AF worked in one way up to f/5,6 and then it changed.

Yes. 45 points at f/5.6 and 1 point at f/8. I would call that a very significant change.

Anyway, the main point originally was that AF sensors use the largest aperture to focus and that f/5.6 is not the widest but the smallest aperture at which the sensor will fully work. This holds for Canon cameras, yes including the 1D series where they have managed to circumvent the f/5.6 limitation by restricting the operation of the AF sensor to the center - so that the center point does work for f/8 lenses.

Mar 13, 2011 at 07:10 AM

Makten
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p.4 #14 · Risk of focus shift issues in new 1.4/35?

denoir wrote:
Sorry, then your 50 Planar needs adjustments. It should not give correct focus at f/2.5 - that means that it is off.

Errr, why would it be "off" just because it gives spherical aberration?? It's very well known that the Z* 50/1.4 shifts focus, and focus shift is caused by spherical aberration.
If the focusing screen cannot show shallower DOF than f/2.5, it should give correct focus at f/2.5, not at f/1.4.

sebboh wrote:
it strikes me that it could be possible that you see a shifted image through the viewfinder if you can only see a f/2.8 dof. let me know if there is an error in my thinking here: think about spherical aberration from the perspective of stopping a lens down from f/2 to f/1.4. at f/2 the rays converge to focus at specific point. when you open the lens up to f/1.4 those rays don't change their point of convergence instead a group of additional light rays are also allowed through that converge on a different point. spherical aberration means that the rays in the f/1.4 ring are in focus at a different point than the rays in the f/2 ring (and so on). what causes the classic glow is that not all the light going through the lens is focused at the same distance. if none of the additional light that comes through lens from f/2 to f/1.4 is actually visible in the viewfinder due to the reflective properties of the focus screen doesn't that mean we are seeing only the same light rays we see at f/2 and thus we can't see the correct focus at f/1.4?

Yes, exactly. Try the DOF preview button and you'll see at what aperture DOF in the viewfinder starts to change. For a stock screen it's like f/4-5.6 while a more matte screen shows something like f/2-2.8.

it seems to me the differences luka and makten are seeing may come down to differences in focus screens. makten - what kind of screen do you have in your d700?

It's from a Nikon F2.

-----------------------------------

I don't understand this AF talk at all. AF lenses can be electronically calibrated, so that an EF 50/1.4 gives correct focus at f/1.4 has nothing to do with what the AF "sees", which certainly isn't f/1.4.
An AF module seeing light at angles corresponding to f/1.4 would be totally unusable, since it would miss all of the time because of spherical aberration.

Mar 13, 2011 at 09:25 AM

denoir
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p.4 #15 · Risk of focus shift issues in new 1.4/35?

Makten wrote:
Errr, why would it be "off" just because it gives spherical aberration?? It's very well known that the Z* 50/1.4 shifts focus, and focus shift is caused by spherical aberration.

Off because the focus point doesn't match what you see in the viewfinder. However, if sebboh's theory is correct then it's probably not off but simply that you don't have a matte enough focusing screen and that the lens is not at fault.

If I'm getting f/1.4 dead on (using the viewfinder) with lenses that have significant focus shift and you are getting dead on at f/2.8 in both cases then the answer is (accepting sebboh's theory) that you are not using a focusing screen that is good enough for fast lenses.

As for the AF bit, I'm not going to speculate as I've now read credible and contradictory information from sources that seem reliable. So either some of them are wrong or there are very different AF designs on the market.

I will say this: If an AF lens simply has a lookup table, why is focus shift such a big problem with AF lenses? If there was a lookup table for different apertures there would be no backfocusing when the lens is stopped down.

Mar 13, 2011 at 10:39 AM

Makten
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p.4 #16 · Risk of focus shift issues in new 1.4/35?

denoir wrote:
Off because the focus point doesn't match what you see in the viewfinder. However, if sebboh's theory is correct then it's probably not off but simply that you don't have a matte enough focusing screen and that the lens is not at fault.

I know the lens is fine and sebbohs theory is correct.

The focus point does match what I see in the viewfinder, at the largest aperture that is visible through the screen.

Your screen is probably better, but it could also be calibrated so that it works very well with fast lenses that has a fair amount of SA. Which could be problematic with lenses with lower SA, such as the 100/2.

Edit:

I will say this: If an AF lens simply has a lookup table, why is focus shift such a big problem with AF lenses? If there was a lookup table for different apertures there would be no backfocusing when the lens is stopped down.

Probably because the AF doesn't care for what aperture is going to be used. Pretty stupid, yes.

Mar 13, 2011 at 11:05 AM

Steve Spencer
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p.4 #17 · Risk of focus shift issues in new 1.4/35?

Makten wrote:
denoir wrote:

denoir wrote:

I will say this: If an AF lens simply has a lookup table, why is focus shift such a big problem with AF lenses? If there was a lookup table for different apertures there would be no backfocusing when the lens is stopped down.

Makten wrote:

Probably because the AF doesn't care for what aperture is going to be used. Pretty stupid, yes.

The reason that autofocus can't solve this problem is because focus shift varies with shooting distance. If the lookup table changed with the aperture it would work for some distances and not others. For AF to try to overcome focus shift it would need to adjust focus for both aperture and shooting distance. This would be a much more complicated system and would require that the system could precisely measure shooting distance, which I don't think it can.

Let me say with all this focus shift talk, if one really cares about it (and you might if you often shoot at close distances where it is greatest), then there is a simple work around for manual lens shooters. Simply get a version of the lens with a manual aperture ring and set the ring to the aperture at which you want to shoot before focussing. This works very well if you have a good focus screen that allows precise focus at wide apertures and for apertures between f/1.4 and f/4 and for narrower apertures as well as long as there is plenty of light.

Mar 13, 2011 at 12:40 PM

Steve Spencer
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p.4 #18 · Risk of focus shift issues in new 1.4/35?

Lars Johnsson wrote:

Floating elements is the cure for focus shift. But it often make the bokeh less smooth/nice

I have heard the same thing about aspherical elements, but I have my doubts that either floating elements or aspherical elements necessarily lead to worse bokeh. Take for example the Canon 85L (any of them the nFD, or either of the AF Marks). It has a floating element and an aspherical element, but in my view has nice bokeh. Or consider the Pentax 31 limited. It has a floating element and an aspherical element and it has quite nice bokeh in my view as well. I also suspect that the new Zeiss 35mm f/1.4 that has a floating element and an aspherical element will have very nice bokeh (the few samples I have seen on Lloyd Chambers site seem to suggest this). And there are lots of great bokeh lenses with floating elements and no aspherical elements (the Zeiss anniversary lenses, the Pentax 77 limited, the OM 90 f/2, the Leica 100mm f/2.8 APO, the Zeiss 50mm MP, the Minolta MD 28mm f/2, etc.). Bokeh seems to be hard to predict and, IMO, there is very little one can say generally about it except that unless ones uses a lens in a lot of situations or looks at a lot of pictures from a lot of situations it is hard to get a sense of how good the bokeh is for a lens.

Mar 13, 2011 at 01:10 PM

wickerprints
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p.4 #19 · Risk of focus shift issues in new 1.4/35?

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Edited on Mar 14, 2011 at 07:18 AM · View previous versions

Mar 13, 2011 at 01:45 PM

Makten
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p.4 #20 · Risk of focus shift issues in new 1.4/35?

wickerprints wrote:
This whole issue is grossly overblown...

Interesting that your opinion is the right one. Focus shift is an issue to me and to many other photographers, regardless of what you think about it. It's an issue even for people knowing exactly how it works too, because you can't use live view in all situations. My own camera is useless with LV because it doesn't tell what aperture is used during focusing.

And please, if you suggest that someone is wrong, tell us who you are talking about and what is wrong.

Mar 13, 2011 at 02:04 PM

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