What Sean Reid Knew When?

[robsteve]

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Steve:

 

Guy's shots were not done with a Leica lens and the focus shift may also be a lens induced problem.

 

I bet if he reshot with a Leica lens it would look better. I have read the Leica APO lenses focus the IR on the same plane as the visible. This was mentioned with the R lenses, I don't know about the M lenses.

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[sean_reid]

Hi Steve,

 

I'll see what happens when I do the actual tests. No ideas but in things, you know...No need to salt any food before it's tasted.

 

Cheers,

 

Sean

[ho_co]

I have read the Leica APO lenses focus the IR on the same plane as the visible.

Robert--

I don't think that's the case. Some of E Puts's curves say otherwise. (And no, I don't mean MTF charts.) For several lenses some of his reviews show which wavelengths are brought to focus, and the apochromaticity applies only in the visible spectrum. (I think these are called 'through-focus diagrams,' but I'm not sure.)

 

Zeiss made some 'superapochromats' for Hasselblad. That's a marketing term, not an optical one. In the superapochromats, Zeiss brought three visible colors to the same focus. And they thought that was a big enough accomplishment to give it a new name.

 

If I'm not mistaken, an apochromat brings two colors to the same focus, and it wouldn't be logical for Leica to make one of those wavelengths one that is outside the visible spectrum.

 

(Or maybe Zeiss focused six wavelengths to the same plane, and 'normal' apochromats only three. Or maybe I'm all wet.)

 

--HC

[dannirr]

There is another dimension to the IR issue and using filters. I use a BetterLight scanning back and we always use a cut filter to filter the IR - but focussing is done with the filter in place, as there would otherwise be a focus shift.

 

With the filter in place over the Leica lens on an M8, this shift is not accounted for as it is a rangefinder system.

 

Other than depth of field, how do you propose overcoming this? After all, low light and critical sharpness are the advantages of any Leica.

 

Danni

[Guest stevenrk]

There is another dimension to the IR issue and using filters. I use a BetterLight scanning back and we always use a cut filter to filter the IR - but focussing is done with the filter in place, as there would otherwise be a focus shift.

 

With the filter in place over the Leica lens on an M8, this shift is not accounted for as it is a rangefinder system.

 

Other than depth of field, how do you propose overcoming this? After all, low light and critical sharpness are the advantages of any Leica.

 

Danni

 

Danni, that pretty much captures the depth of the problem, and why this has no apparent simple fix, with or without red tint mirrors.

[brucek]

With the filter in place over the Leica lens on an M8, this shift is not accounted for as it is a rangefinder system.

 

 

Maybe I'm missing something but I would think that the focus would be more accurate with the filter on the lens as you are now focusing just the visible light and not the IR as it's been blocked.

 

Bruce

[mfunnell]

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Maybe I'm missing something here, but I would have thought that with a rangefinder, of all things, the presence or absence of a filter over the lens has pretty much nothing to do with focusing.

 

...Mike

[brucek]

Maybe I'm missing something here, but I would have thought that with a rangefinder, of all things, the presence or absence of a filter over the lens has pretty much nothing to do with focusing.

 

...Mike

 

Mike:

 

I'm certainly not an expert on this, but I think you're correct for visible light. However, with the M8's increased sensitivity to IR, there is concern that that increased sensitivity will shift the focus of a lens becuase IR doesn't focus at the same point that visible light does.

 

Bruce

[Joseph S. Wisniewski]

Software doesn't receive any IR-specific data. All it gets is the intensities at each photosite, and knows what sort of filter (RGB) was at each site. It may conceivably be provided with the IR characteristics of each of these filters. Presumably there might be quite a bit of IR coming through the red filters, probably less or even none coming through the green and blue filters. In the demozaicing, it *might* be possible to isolate the IR components to some extent that way, but it would probably only be a partial fix. I think the IR issue is going to need hardware modifications - probably to the sensor cover glass.

Plenty of IR comes through the blue filter: that is why IR leakage causes a magenta cast, not a red cast. That's for the most problematic IR, around 700-750nm. From 750nm on up, plenty of IR also gets through the green filter, so the cast is more brownish.

[Joseph S. Wisniewski]

Steve:

 

Guy's shots were not done with a Leica lens and the focus shift may also be a lens induced problem.

 

I bet if he reshot with a Leica lens it would look better. I have read the Leica APO lenses focus the IR on the same plane as the visible. This was mentioned with the R lenses, I don't know about the M lenses.

Common myth. APO lenses are typically designed for no focus shift at three colors: red, green, and blue. Shift between red and green and between green and blue is minimal, because the corrections are working against each other. Shift beyond red is dramatic, because the second order corrections are working in the same direction.

 

Leica even issued a statement about this once: their reason for not putting IR "correction" marks is that there is no one correct place to put a mark. The amount of correction you need is dependent on the lighting conditions and the IR filter you're using: i.e. a mark that's right for Kodak infrared with a Wratten 25 filter outdoors is nowhere enough correction for a Wratten 89b filter under hot light...

 

But still the myth that some Leica lenses don't need correction marks persists.