# Zeiss Distagon very different AFMA with different temperatur?



## Viggo (Dec 21, 2016)

Hi guys!

Having a frustrating issue with my Zeiss 100mm f2.0 makro-planar.

Indoors my absolute sharpest setting (identical to LiveView) is afma +20, it's very dramatic adjustment, but it's 100% consistent at getting optimal sharpness at any focusing distance.

However, going outside in colder weather +2 and up to +11 Celsius "0" is the perfect value giving me very consistent optimal sharpness.

Is it really that sensitive to temperatures??

I tried to take some "studio-shots" of the kids indoors at "0" because that worked outside, but even if pressed the shutter exactly at the beep, I couldn't nail proper focus. It's nothing to do with dslr's viewfinders being poor for mf or
Something like that, because the beep and focus lock lamp is what I rely on and it's bang on every time with the correct afma value.


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## Mt Spokane Photography (Dec 21, 2016)

I have found the beep and focus light to be only roughly accurate, and it depends on which way the lens is moving, from infinity to focus point, or from close up out to focus point. That's the reason why the dot tune method of AFMA is often inaccurate, at least, it does not give the results obtainable with manual focus.

Expansion and contraction of the various mechanical parts in a lens is likely to require a slightly different position for best focus. Its not something a manual focus lens maker would worry about. It might affect a AF confirm chip accuracy, but I don't understand why.


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## Viggo (Dec 22, 2016)

Thanks for your reply. 

I'm 100% sure it doesn't have anything to with inaccurate beep and viewfinder. I test all manual lenses to see which way I should turn to find focus, and for example both the 50 f2 mp and the 21 works best if going from mfd to infinity for focus indoors and from infinity outdoors. The 100 f2 is extremely sharp both ways indoor and outdoor. It's just the afma that changes in a big way.


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## NancyP (Dec 22, 2016)

Zeiss barrels are metal, therefore have some degree of expansion / contraction in response to temperature change. Just listen to your car cool down. 
Apparently one of the positive features of engineering plastics as used by various manufacturers (Canon, new Sigma lenses) is less thermal expansion, at least that is what I gathered from a taped interview with the Sigma CEO.
https://en.wikipedia.org/wiki/Thermal_expansion


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## Zeidora (Dec 22, 2016)

AF confirm is a camera function and is optically determined by the camera body. If you want to blame a piece of equipment, then it is the body, not the lens. 

The metal expansion/contraction would come into play, if you have an object at distance x, and set focus at the x mark and you get differences in sharpness in different temperature settings. 

If you need critical focus, get a matt focusing screen, or use live view and magnify the area of interest. If you are concerned about focus breathing (I have not noticed any issues with the 100 MP), then do use live view with depth of field preview pressed.


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## Zeidora (Dec 22, 2016)

PS. The 100 MP is not a Distagon (wide angle) design. The longest distagon design, afaik, is the Otus 55.


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## Viggo (Dec 22, 2016)

Zeidora said:


> PS. The 100 MP is not a Distagon (wide angle) design. The longest distagon design, afaik, is the Otus 55.



You're of course right! It's a makro-planar, I have a 21 Distagon, it haven't seen this issue with that one, maybe because of the greater dof.


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## Viggo (Dec 22, 2016)

NancyP said:


> Zeiss barrels are metal, therefore have some degree of expansion / contraction in response to temperature change. Just listen to your car cool down.
> Apparently one of the positive features of engineering plastics as used by various manufacturers (Canon, new Sigma lenses) is less thermal expansion, at least that is what I gathered from a taped interview with the Sigma CEO.
> https://en.wikipedia.org/wiki/Thermal_expansion



Absolutely, but from 0 to +20 afma within 10 degrees Celsius difference? My car gets about a 95-100 degrees Celsius so that's a bit different


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## Viggo (Dec 22, 2016)

Zeidora said:


> AF confirm is a camera function and is optically determined by the camera body. If you want to blame a piece of equipment, then it is the body, not the lens.
> 
> The metal expansion/contraction would come into play, if you have an object at distance x, and set focus at the x mark and you get differences in sharpness in different temperature settings.
> 
> If you need critical focus, get a matt focusing screen, or use live view and magnify the area of interest. If you are concerned about focus breathing (I have not noticed any issues with the 100 MP), then do use live view with depth of field preview pressed.



Not to be rude, but if you read the first and second post I wrote this has nothing to do with the viewfinder or mattscreens and lack of presicion in the dslr. Critical focus is actually very easy, it just changes afma value with temperatur, and is that common for others within such a small temp change requiring that huge adjustment?


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