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M8, sensor filter thickness, and astigmatism

Brian Caldwell , Aug 02, 2007; 06:18 a.m.

If you place a filter between the lens and the image plane you will introduce aberrations, particularly astigmatism. The problem is worse for short exit pupil distances common for Leica M lenses. I've seen the issue alluded to in Leica's press announcement regarding the IR problem, but I've never seen any meaningful discussion in any forum.

So, I analyzed what would happen to a perfect f/2 lens if you put various thickness filters near the image plane. The chart below shows MTF at 40 cycles/mm for filter thicknesses of 0, 0.5mm, and 2.0mm. I think the damage is alot bigger than most people realize.

Leica state that the filter they use over their sensor is 0.5mm thick. In addition, all sensors have a cover glass that ranges from 0.5mm to 1.0mm in thickness, so the actual total thickness is getting into the uncomfortable range.

Of course, if you stop down the problem gets reduced substantially, but still, if you buy the latest 50/1.4 ASPH do you really want its performance degraded to such an extent?

There has been much talk from various manufacturers in recent years about "designed for digital" lenses. A lens truly designed for digital must be corrected for a nominal filter glass near the image plane, but, strangely, nobody ever mentions this.


Monochromatic MTF @ 40lp/mm

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Vivek . , Aug 02, 2007; 06:51 a.m.

Thank you for your brilliant post of your analysis, Dr. Caldwell!

I think the Leica M8 has one of the thinnest sensor cover plates as you note.

I wondered about it when I use a Canon 50/0.95 TV lens on my Epson R-D1s (almost always wide open) if there are anything else induced other than strictly what the lens projects.

Also, from the Nikon's bodies and lenses, oddly, the old 18-70 kit zoom (issued with D70, cover glass 1mm plus another ~0.1 to 0.2 mm thick BG/AA filter) appears to perform poorly on a D40x (cover glass is > twice as thicker and has more efficient coatings to eliminate IR and UV)while its own kit lens (18-55 AF-D g, etc version II) does much better on the D40x. Both are 'DX' lenses 'designed for digital.

Can't a software fix be applied to correct astigmatism?

Vivek . , Aug 02, 2007; 07:50 a.m.

About the performance degradation (of a 50/1.4 Asph) due to aberrations from the sensor cover plate:
would anyone notice it?

Prairie dogs, Epson R-D1s, Canon 50/0.95 @f/0.95

Brian Caldwell , Aug 02, 2007; 09:31 a.m.

Vivek: I think that cover plate aberrations were a design-driver for Leica, and they were forced to do everything they could to minimize the effect, short of re-designing all the lenses to have a longer exit pupil distance. It certainly explains the unusually thin 0.5mm IR filter, to an extent it explains the absence of an AA filter, and it also suggests one reason why they may have chosen 1.33x instead of 1.0x. After all, they already used the trick of laterally shifting the microlenses to minimize corner shading and color shifting, so why not go ahead with a full size sensor except for the added astigmatism.

In theory you could correct for a known amount of astigmatism by image processing, but this is way beyond the realm of photoshop, DXO, or my favorite panotools.

If you did a careful test, either by measuring MTF or photographing test charts, you would probably notice the degradation with a really good f/1.4 lens like the 50/1.4 used wide open. However, the effect will be a subtle one due to all the design choices made by Leica.

Brian

Kelly Flanigan , Aug 02, 2007; 09:50 a.m.

The curve reminds me of the old microscope objective curves with and without coverglass; for non and coverglass corrected objectives.

Tom Murray , Aug 02, 2007; 10:12 a.m.

"A lens truly designed for digital must be corrected for a nominal filter glass near the image plane, but, strangely, nobody ever mentions this."

Maybe that's because most photographers aren't science nerds and pay attention just to what we and the clients can see with our nekkid eyes. In most (not all) cases if there are side effects from digital with top grade lenses (not el cheapo kit zooms) they cause less visible image degridation than all the compounded errors we used to get along the chain from shutter push to final print back in the days when we used those same lenses on film. I've used some old Hasselblad C lenses from like 1960 adapted on a 39MP body, and a few the same age Nikors on a 1DS-II and got nothing I didn't see on film with the same lenses, other than a bit of C/A here and there that was not to hard to fix in CS3.

Vivek . , Aug 02, 2007; 11:13 a.m.

Brian, Thanks again for the explanations and shedding some light on the cover glass construction in M8.

The "designed for digital" lenses would again be quite specific for a given camera from the same brand/system (Leica does not have that problem at the moment), given that different cameras from one system use cover glasses of different thicknesses. The low priced Nikkor digital zooms that I checked appear to indicate that would be the case.

Vivek . , Aug 02, 2007; 11:49 a.m.

Yeah, why bother using any camera for imaging when it is so easy to lift one from the web for any occasion.

John Shriver , Aug 02, 2007; 01:18 p.m.

Two things Brian could add to the computerized analysis.

1. Show how film non-flatness affects MTF. Add in emulsion thickness for good measure, if you like.

2. Do the same charts assuming a non-perfect lens. Like use some published MTF curves. Of course, I suppose the problem is that the effects aren't simply additive or linear.

Brian Caldwell , Aug 02, 2007; 03:33 p.m.

Kelly: Microscope objectives are typically even more sensitive to cover glass thickness. However, the culprit here is mainly spherical aberration rather than astigmatism because the objectives are very fast and are more or less telecentric.

Vivek: I'm surprised to hear about the Nikon zoom example because these are slower lenses. However, its an interesting observation. I recently talked to an engineer at one of the companies that replaces the filter pack with an IR filter to make dedicated IR cameras, and apparently there is a wide range of filter thicknesses; from the 0.5mm of the M8 to about 3mm in the case of some Canon cameras. And this doesn't include the coverslip that protects the actual sensor surface.

Tom: I admit to being an optics nerd, and that the total number of MTF curves I've calculated in my career is probably greater than the total number of pictures that I've taken. I also agree that digital photography works pretty darn well, even using lenses that aren't ideal for the purpose. However, I think I'm able to shed light on the reasons behind design tradeoffs in the M8. I also think we are a long way from reaching a resolution plateau in digital photography. By the time you reach 40-50 megapixels the astigmatism problem due to filter thickness looms much larger.

John: I've no idea what the exact shape of a piece of film is. Its straigtforward to show what happens to MTF when you defocus, but I'm not sure if this would lead to any real understanding of what happens on film. I used a perfect lens to isolate the filter effect. The only way to show what happens with a real lens is to either measure the effect or calculate it using the optical prescription. You can't do an accurate calculation using a published MTF curve because aberrations in the lens will interact with aberrations from the filter in a manner that can't be predicted from MTF curves alone.


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