Focus Question

[nscali]

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I know focussing has been discussed ad nauseum on this forum but I just need clarification to detemine if this is nomal behaviour or my rangefinder is out of whack.

 

Using a Summicron 50m, f2:

When I focus on an object at around 1 meter, the object is in focus (file) at f2 and f2.8.

Once I get to f4 it starts to the file starts to get soft and at f5.8 it is clearly out of focus.

 

Each time I am recomposing and refocusing in the viewfinder. Shutter speed is always above 125.

 

Note: At f4 and f5.8 if I retake the shot, shifting it out of focus slightly, the image on the file is sharp. Therefore it is not a shutter speed issue at f4 or f5.8

 

Any help would be appreciated. Is this a normal characteristic?

 

Thanks

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[tashley]

I know focussing has been discussed ad nauseum on this forum but I just need clarification to detemine if this is nomal behaviour or my rangefinder is out of whack.

 

Using a Summicron 50m, f2:

When I focus on an object at around 1 meter, the object is in focus (file) at f2 and f2.8.

Once I get to f4 it starts to the file starts to get soft and at f5.8 it is clearly out of focus.

 

Each time I am recomposing and refocusing in the viewfinder. Shutter speed is always above 125.

 

Note: At f4 and f5.8 if I retake the shot, shifting it out of focus slightly, the image on the file is sharp. Therefore it is not a shutter speed issue at f4 or f5.8

 

Any help would be appreciated. Is this a normal characteristic?

 

Thanks

 

There is great debate as to whether this is 'normal' but it certainly happens and is most often reported on the 35 lux and cron, then on the summilux. It seems rare on the 50 cron.

 

Read the relevant threads to see whether your RF is correctly adjusted before investigating the lens further - just search on 'backfocus' and you will find plenty of great advice!

 

Tim

[jaapv]

This is clearly not backfocus, Tim, this is focus shift, well documented in literature over the last fifty years.

[wattsy]

This is clearly not backfocus, Tim, this is focus shift,

 

It may well be a combination of the two. Back focus (or, to a lesser extent, front focus) will exaggerate any tendency for a lens to show focus shift.

[nscali]

I just re-did the test at home using a tripod.

I also did it with a 50 lux (1.4).

Interestingly the results are the same.

The Focus starts to shift at f4 on both lenses (50 cron & 50 lux).

Both are sharp up to f2.8.

At f4, the out of focus shots show up sharper. The difference is not huge, but it is there.

 

Nicky

[jaapv]

A lot of lenses shift focus when the aperture changes. It is a fact of life in optical design. The Noctilux is notorious for this, the Summiluxes are known to do it. I think it is the first time I heard of anybody noticing on the Summcron 50.

[HaraldL]

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Ok, so the M8 makes us aware of focussing problems not noted on the previous M's. So it must be a razor-sharp and very accurate but optical demanding camera. The former is my pleasant experience so far. It creates more sharp images then I was able to obtain on my M6 / 7. When I had my 90mm apo 6-bit coded Solms also wanted to adjust something on it's focussing. However focussing accuracy was never an issue on my M7! And so far it works fine om my M8.

Now the question, has this back focussing ever caused problems on previous M's? So are we talking about a big issue, or is it all on the edge and is this discussion, with respect to the previous thread and it's authors, focussing on improvement in accuracy of the M8 compared to the M7? Maybe with some handling and learning curves? Maybe from Solms (with some unexpected adjustments like mine) as well?

 

Harald

[jaapv]

The thing is, Harald, that film has a certain thickness, which covers a multitude of sins. A sensor, however is as close to a perfect projection-plane as one can get, mercilessly showing up any imperfection.Plus, 100% crops are insane enlargements which virtually nobody did with film -and certainly never looked at from 25 cm's distance.

[HaraldL]

The thing is, Harald, that film has a certain thickness, which covers a multitude of sins. A sensor, however is as close to a perfect projection-plane as one can get, mercilessly showing up any imperfection.Plus, 100% crops are insane enlargements which virtually nobody did with film -and certainly never looked at from 25 cm's distance.

So the 'problem' is how we have to handle improvement, and for sure there is.(...?)

[carstenw]

Nicky, what about other distances? Does it ever get sharp again, as you stop down?

[jaapv]

So the 'problem' is how we have to handle improvement, and for sure there is.(...?)

 

 

Yes- that is the thing, well put

[tashley]

This is clearly not backfocus, Tim, this is focus shift, well documented in literature over the last fifty years.

 

 

Show me where I said it was...

 

 

Tim

[jaapv]

- just search on 'backfocus' and you will find plenty of great advice!

 

[tashley]

Nice try - however I was directing the poor chap to the threads in which all the relevant information is to be found - and they are mostly those with the word backfocus in their subject lines. At no point did I suggest that he was suffering from backfocus though as it happens, backwards focus shift with stopdown is often spoken of in that way. Incorrectly of course.

 

 

Tim

[jaapv]

 

Anyway, interesting post in another thread. All wide lenses have this tendency, we just have to learn to live with it.

 

I just purchased a 50 Zeiss Sonnar to use as a softer portrait lens. [...........]

From Zeiss:

"When you stop down to f/2.8 or smaller, there will be visible a slight focus shift, but sharpness will be covered within the depth-of-field. In practical use, after the adjustment the focus shift will not play an important role anymore."

 

[gman]

I, too, just ran some tests and found my 50/1.4 ASPH (LHSA model) backfocusing much more than any other lenses I own.

 

What is it about this particular lens that seems to be giving so many people this problem? Can the lens be fixed to focus correctly on the M8? As other people here have stated, if you adjust the M8 to the 50 ASPH, many other lenses will then be out of whack.

 

Do people suggest having the lens sent off for repair, and would this type of repair be covered by the Leica warranty?

 

jeff

[ho_co]

LFI has said it will be covering the issue of this focus shift in its next issue.

 

Erwin Puts has covered it here: http://www.imx.nl/photosite/leica/M8_8/m8_8.html

 

 

--HC

[tashley]

I, too, just ran some tests and found my 50/1.4 ASPH (LHSA model) backfocusing much more than any other lenses I own.

 

What is it about this particular lens that seems to be giving so many people this problem? Can the lens be fixed to focus correctly on the M8? As other people here have stated, if you adjust the M8 to the 50 ASPH, many other lenses will then be out of whack.

 

Do people suggest having the lens sent off for repair, and would this type of repair be covered by the Leica warranty?

 

jeff

This is do disappointing for you and I am sorry to hear it. I have had three 35mm lenses with this problem (probably aspherical focus shift rather than backfocus, see above, but could be either) but my 50 lux has been a gentleman throughout - as have all my other lenses. You should try to ascertain, using other lenses and bodies if possible, where the problem lies. It is MUCH more common to find this issue with 50 nocti and 35 lux, then 35 cron. 50 Lux has been reported a bit and 50 cron just once that I have seen. As HC says, it will be covered in the next LFI. The Puts article does not contain enough detail to answer the issue IMHO.

 

Best

 

Tim

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[nscali]

Tim,

To me the issue is apparent on all lenses.

When I originally tried it with the 50mm Noctilux some time ago, I just returned the lens.

It even happens on the 28mm lens BUT you really wouldn't notice the focus shift unless you were looking for it. It just seems the longer the lens, the more apparent the issue becomes.

Carsten suggested trying stopped down even further. The problem is still there at f8 and f11 but it doesn't get any worse.

My summary is as follows:

f1.4 - f2.8 Focus is accurate

f4.0 - the shift is there - slightly out of focus

f5.6 - f11 - obvious shift in focus. More apparent than at f4 but it does not worsen as you stop down further.

 

Nicky

[ho_co]

I shan't speak the unspeakable, but if I were to do so, here's what I would say:

 

1) We've always known that Leica lenses were amazingly good wide open. Other manufacturers' lenses don't come close.

 

2) We've always known that as you stopped down, the others' lenses improved and by about f/8 were comparable to the Leica lenses.

 

There's no question about it; those are simply facts.

 

3) Now with the M8 we discover a focus shift caused by spherical aberration that we hadn't noticed before.

 

Put the two together and you have one and the same thing. Zeiss says the same about their 50/1.5 C Sonnar's adjustment (http://www.leica-camera-user.com/digital-forum/21452-adjusting-zeiss-sonnar.html). Again:

 

Leica lenses are perfectly in focus wide open (our experience in point 1 and explained by Puts as cited above).

 

As we stop them down, focus shifts. It does the same with other manufacturers' lenses as well (see Puts again) BUT THE OTHERS HAVEN'T OPTIMIZED THE LENSES AS LEICA HAS. That is, the focus shift takes the Leica lenses slightly out of focus because they were perfect before. But with others' lenses, since they weren't adjusted for accuracy wide open, the focus moves INTO the rangefinder's set distance (see Puts again).

 

All lenses shift focus as you stop them down. But non-Leica lenses were adjusted with smaller apertures in mind, so we just see them improving as we stop them down. We aren't aware of the focus shift because the lens is getting better.

 

We didn't notice the focus shift with film because film isn't flat and because we couldn't blow film up to 400% at the click of a mouse. Ever wonder why it's so hard to get good scans from 35mm transparencies? It's because the film wasn't flat and we sat ten or fifteen feet from the screen, but now we're trying to extract everything and can't find it. Same thing we're seeing in digital.

 

Okay, that's my theory. It's made up out of my sheer brilliance and imagination and a little bit of whole cloth. In summary: Leica lenses work so well because they're in focus wide open, and as the focus shifts, it stays "within depth of field." But depth of field doesn't exist except in the brain, and we aren't looking at depth of field when we enlarge an image on a computer screen. The proper test is to make a print and evaluate it as an artist, not a technician.

 

(In the Zeiss thread I mentioned, the thread starter said he had asked Zeiss to adjust the focus of the 1950's style lens to work more like the Leica lenses, and they gave him exactly this explanation of what would happen with the focus adjustment. Puts says that Zeiss claims to have less focus shift but that they actually reach this claim by redefining the 'in-focus' plane to be thicker. [i suppose that has to do with how they locate the focus plane between the point of highest contrast and the point of highest resolution.])

 

Puts says the solution to this non-problem is to use the Leica lenses at or near full aperture. The "Form Follows Format" article in LFI a few months ago also said if we're not using the lenses wide open, we're missing a great artistic tool. Phrased completely differently, but a clear hint in the same direction.

 

I think we've got the world's best lenses and ought to go out and shoot them and forget about focusing on test targets. (Heaven forfend! No tests? What am I doing on the Leica Forum? )

 

Tim--I think that's all in Puts, but he puts it more genteelly. I'm also interested in LFI's approach; like Puts, they tend to tell it straight but a little hidden behind a veil.

 

--HC