M8 Backfocusing with respect to aperture

[jonoslack]

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HI Everyone

I think I might need some advice here!

I've been thinking hard about lenses - what I really want, and what I really need, and as I function of this I thought I'd check the focusing on the 5 leica lenses I have at present:

 

21mm pre-asph elmarit

new tri-elmar (coded)

2 X 50 mm 'crons (thereby hangs a tale)

90mm elmarit

 

I bunged the camera on a tripod, put a sheet of printed paper on the floor 4 ft away, and took shots at widest aperture - then f5.6, f8 and f11 for each lens.

 

All of them performed as expected wide open (I had Solms fix the focusing when it went back). However, as the aperture got smaller, whereas I'd expected the area in focus simply to get larger (which, of course it did) - I also found that the camera began to backfocus ever so slightly. This seemed to be true with all the lenses.

 

It's not an issue, but it's confusing the hell out of me - any explanations from you clever people please

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[tashley]

HI Everyone

I think I might need some advice here!

I've been thinking hard about lenses - what I really want, and what I really need, and as I function of this I thought I'd check the focusing on the 5 leica lenses I have at present:

 

21mm pre-asph elmarit

new tri-elmar (coded)

2 X 50 mm 'crons (thereby hangs a tale)

90mm elmarit

 

I bunged the camera on a tripod, put a sheet of printed paper on the floor 4 ft away, and took shots at widest aperture - then f5.6, f8 and f11 for each lens.

 

All of them performed as expected wide open (I had Solms fix the focusing when it went back). However, as the aperture got smaller, whereas I'd expected the area in focus simply to get larger (which, of course it did) - I also found that the camera began to backfocus ever so slightly. This seemed to be true with all the lenses.

 

It's not an issue, but it's confusing the hell out of me - any explanations from you clever people please

 

Hi Jono,

 

I just ran the same test to finally ascertain my feeling that my new 35mm 1.4 was dodgy. I used a ruler at every f stop with the film plane about 1 metre from the centre point of focus.

 

My 50mm 1.4 performed as you would expect, perfectly, with DOF increasing more back than forward but the point of focus remaining sharp throughout. But with the 35mm 1.4 if focussed perfectly wide open and then the point of focus moved backwards as I stopped down, to a worst case of 7 to 8cm back at F8 - which explains why so many shots I took with it at carnival were OOF though I did catch on quickly enough to do the very unscientific focus-then-shift-guess routine. It got posted back today!

 

As for the clever science bit, I'm sure someone here will have the answer!

 

Best

 

Tim

 

Tim

[nzav]

Jono, depth of field always extends further behind the subject than in front, it is not symmetrical and as you increase the DOF by stopping down, this becomes more evident.

[Guest guy_mancuso]

Jono I have been playing around with this on both bodies and made some adjustments that seem to work overall . I don't know the technical aspects of it but the results and logic that I went for. I adjusted the camera on my lenses for for wide open to be dead on. Reason is it is the most critical there and than if it shifted as i stopped down i still had excellent sharpness because the aperture covers that slight shift so overall it works fine and that is what I decided would work best for me. i need the 50 , 75 lux and 90 apo to work dead on wide open for certain work like stage and portraits and such maybe just like you so that is what i went for. I carry in my bag a 2mm allen wrench and last week was making adjustments even on a job. Now most folks would think this is bad , but the reality is it is great you can make the adjustments yourself. I can't adjust my DMR and would have to send it in but I carefull learned how to do it with the M8 and now if something feels out of whack it really is easy to do. Just turn the camera over and adjust the cam a very very slight hair one way or the other. Now everything seems dead on including all the lenses. A very easy way to check this even without a tripod is get asection of a sidewalk that has 4 groves in it going 4 different directions and focus on the center from about a 45 degree angle and you can even seein the LCD zoomed if you are nailing that cross section and if not you will see what direction it goes. Sounds somewhat crude but works great even on location

[hankg]

Guy where do you adjust the RF cam?

[tashley]

Jono I have been playing around with this on both bodies and made some adjustments that seem to work overall . I don't know the technical aspects of it but the results and logic that I went for. I adjusted the camera on my lenses for for wide open to be dead on. Reason is it is the most critical there and than if it shifted as i stopped down i still had excellent sharpness because the aperture covers that slight shift so overall it works fine and that is what I decided would work best for me. i need the 50 , 75 lux and 90 apo to work dead on wide open for certain work like stage and portraits and such maybe just like you so that is what i went for. I carry in my bag a 2mm allen wrench and last week was making adjustments even on a job. Now most folks would think this is bad , but the reality is it is great you can make the adjustments yourself. I can't adjust my DMR and would have to send it in but I carefull learned how to do it with the M8 and now if something feels out of whack it really is easy to do. Just turn the camera over and adjust the cam a very very slight hair one way or the other. Now everything seems dead on including all the lenses. A very easy way to check this even without a tripod is get asection of a sidewalk that has 4 groves in it going 4 different directions and focus on the center from about a 45 degree angle and you can even seein the LCD zoomed if you are nailing that cross section and if not you will see what direction it goes. Sounds somewhat crude but works great even on location

 

 

Guy, where on the underside of the camera is the slot for the allen wrench? I can't see it...

 

Thanks in advance

 

Tim

[stunsworth]

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Nick beat me too it :-)

 

Look at the DOF markings on a lens with the lens set to say 3 metres and look at the DOF in front and behind the point of focus.

[Guest guy_mancuso]

Tim it's a little strange becuase it does not look like a allen wrench can fit there but take the lens off and you see a round rolling bearing looking cam right in front on top and as you focus it moves the cam back and forth . Now turn the camera over and in the middle is were the allen wrench goes if your backfocusing turn slightly to the left than check it. Wish i had some pics of this , I think someone has posted it before butonce you get the hang of this it becomes pretty easy to do

[lars_bergquist]

Jono, I agree that what you have encountered is the asymmetric nature of the depth of field. Just one example from my 28 mm Summicron-M: If I focus it at 3 meters, then (if we may believe the d.o.f. scale, but see below) at f:5.6 I have everything sharp out to 10 meters, = 7 meters of leeway. In the other direction however depth of field extends only to about 1.8 meters = 3 -- 1.8 = 1.2 meters.

 

So it would seem that when stopping down to this aperture, the center of the zone of focus would have moved 4.5 meters forward! However, if you bring out your Sherlockian magnifier, you will find that the distance of BEST focus is still 3 meters.

 

Now of course that d.o.f. scale is perfectly antediluvian. It presupposes a circle of confusion in the negative (or on the sensor) of 1/30th of a millimeter. This 'industrial standard' was introduced in the 1920's, when the idea was that a Leica enlargement had to measure up to the sharpness of a contact print from a 6 X 9 cm roll film negative ... Today, a circle of confusion of 1/50th to 1/60th of a mm is a more viable proposition. To work at a 1/60th standard, just double the aperture values. So that if you want to know the depth of field or the hyperfocal distance at f:8, read the scale at f:4.

 

This is bad news, but it's realistic. Still, such a standard is based on the assumption that you do not need more than a visually sharp 18 X 24 cm or 8 X 10" print. And that is a realistic assumption!

 

The old man from the Age of Roll Film

[barjohn]

Here is the thread with pictures on how to adjust.

 

http://www.leica-camera-user.com/digital-forum/14684-back-focus-adjustment-question.html?highlight=back+focusing+adjustment

 

By the way, what is the circle of confusion (CoC) for the M8?

[hankg]

does this screw up infinity focus?

[Shootist]

I have a question to go with this thread. I did a focus test on all my lenses last night and my older 50mm f/2 has severe backfocus but all other lenses, 35mm f/2 40mm summicrom-C F/2 and both my 90mm Tele Elmarit's f/2.8 focus correctly. I did a similar test on all 3 of my M3's and they came out OK, that is I don't notice any severe backfocus with the 50mm on any of the B&W negatives or when I scan them into the computer.

Is it possible that the older 50mm just doesn't like my M8? I find it hard to believe that all lenses work good on the M3's and I have a BF problem with just the 50mm on the M8.

[mjh]

However, as the aperture got smaller, whereas I'd expected the area in focus simply to get larger (which, of course it did) - I also found that the camera began to backfocus ever so slightly. This seemed to be true with all the lenses.

While I suppose that Nick and Steve are correct in assuming that this is caused by an asymmetrical increase in depth of field, there is also a real, if very subtle effect with some lenses. If you stop down, the focus may actually shift, as explained here: Spherical aberration.

[pemayeux]

I performed the same test on my M8. I used the focus test chart that I printed out from one of the previous threads, mounted the camera on a tripod and used a release cable. I found my 35lux was razor sharp wide open. The 50 lux backfocused and the 75 lux backfocused. The 50 was the worse of the two. I adjusted the M8 to get the 50 correct but then the 35 was off and the 75 was close. I ended up adjusting the M8 back for the 35 to be correct. One thing I noticed while doing the test, and something I had not noticed prior when shooting the 50 with my MP was that when focused at infinity and looking at a distant flag pole, the image would never coincide. The same was true for the 75 though not as much. I checked this on the MP body and it was the same as the M8. Therefore I think the only solution on mine is to send the 50 and the 75 in for adjustment. Or compensate when shooting. If I focus the 50 1 stop more toward infinity it is razor sharp and the 75 gets razor sharp focused 1/2 stop closer to infinity. The new 28/2,8 asph is razor sharp like the 35.

[stunsworth]

While I suppose that Nick and Steve are correct in assuming that this is caused by an asymmetrical increase in depth of field, there is also a real, if very subtle effect with some lenses. If you stop down, the focus may actually shift, as explained here: Spherical aberration.

 

Indeed there is, but given all the talk about 'focus shift' and 'backfocussing' it makes me wonder how people have ever managed to get anything in focus before the M8 came along :-)

 

Paul, try focusing on simething a long, long way away such as the moon, it could be that the flagpole wasn't far away enough.

[jonoslack]

While I suppose that Nick and Steve are correct in assuming that this is caused by an asymmetrical increase in depth of field, there is also a real, if very subtle effect with some lenses. If you stop down, the focus may actually shift, as explained here: Spherical aberration.

Thank you so much.

I wasn't sure how to anser Nick and Steve - I am, of course, aware that dof extends forwards further than backwards, but that effect wouldn't account for the fact that the point of 'best focus' when wide open was different (close to the camera) than when the lens was stopped down.

 

What I observed with all the lenses was exactly what is described here, and I imagine that the situation was exacerbated by the fact that my target was quite close to the camera.

 

Excellent link - I'm now a happier bunny!

[jonoslack]

What a wonderful place this is.

 

Today I have learned:

 

1. that I should search past threads

2. that I wasn't going mad, and that there is a backwards focus shift when you use a smaller aperture (despite the increased depth of field).

3. that I can alter the focus distance myself without having to resort to Solms.

 

All I need now is a 2mm allen key!

 

Many thanks to everyone who has chipped in. Specially to Guy for the explanation of how to change the focusing, to John who linked to the old thread, and to Michael who explained about focus shift.

 

And yesterday it was how to use Aperture with the M8! (thanks to Eoin for that one)

[flavio]

What a wonderful place this is.

 

Today I have learned:

 

1. that I should search past threads

2. that I wasn't going mad, and that there is a backwards focus shift when you use a smaller aperture (despite the increased depth of field).

3. that I can alter the focus distance myself without having to resort to Solms.

 

All I need now is a 2mm allen key!

 

Many thanks to everyone who has chipped in. Specially to Guy for the explanation of how to change the focusing, to John who linked to the old thread, and to Michael who explained about focus shift.

 

And yesterday it was how to use Aperture with the M8! (thanks to Eoin for that one)

Jono,

and I thank you to share the Eoin's use of M8/Aperture (i did not see before)!

Great site indeed.

 

Flavio

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Hallo,

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[sean_reid]

There are two issues at play. One is DOF which runs, very roughly, 2/3 behind and 1/3 ahead. The second is that many lenses will shift focus slightly with changes in aperture (as posted above, I now see).

 

If the lenses focus correctly wide open, all is as it should be and one should probably stop testing and go take more pictures. I would not recommend messing around with the RF adjustment on a camera that focuses correctly wide open. <G>

 

Cheers,

 

Sean

[jonoslack]

There are two issues at play. One is DOF which runs, very roughly, 2/3 behind and 1/3 ahead. The second is that many lenses will shift focus slightly with changes in aperture (as posted above, I now see).

 

If the lenses focus correctly wide open, all is as it should be and one should probably stop testing and go take more pictures. I would not recommend messing around with the RF adjustment on a camera that focuses correctly wide open. <G>

 

Cheers,

 

Sean

Hi Sean

No - I think I'll leave mine as it is (focusing correctly wide open) but it's good to know that as one stops down it's worth adjusting the focus slightly (and understanding why).