[carrrlangas]

I just read this article LensRentals.com - The Glass in the Path: Sensor Stacks and Adapted Lenses and started wondering how does this affects Pentax DSLRs and K mount lenses designed for digital vs film.

In summary, the thickness of the stack in front of the sensor (UV, IR, AA filter, etc) is an important design consideration. As light leaves the lens rear element, it expects to find a specific stack thikness. This will help a lot with aberrations.
In the case of Pentax, this means all the film era lenses are designed for the thinest possible sensor stack; film. What about DFA lenses? Do they take stack thickness into consideration? DA lenses surely do.. right?
also, knowing Pentax is using a Sony sensor, do we assume stack thickness is standard for this sensor or is it specified by Pentax?

[stevebrot]

Interesting question. The article mentioned Leitz lenses which are used on both film and digital cameras in the Leica line. Leica seems to do fine using its film-designed lenses on its digital bodies. Pentax film era lenses also do well in lens tests on modern digital sensors. Canon uses a "thin" stack. I would be curious as to where the Pentax line falls and whether the stack thickness has changed in the models since the *ist D.

As for the thickness of the film filter stack, the light sensitive emulsion is on the surface of the film. The total thickness of the emulsion is less than 1/1000" (25.4 micrometers). Stack thickness is essentially zero for both B&W film and color films.

This article from "The Online Photographer" was linked from the LensRentals article and addresses the specific issue of adapted rangefinder lenses.* I found the explanation to be a bit easier to follow.

http://theonlinephotographer.typepad.com/the_online_photographer/2013/10/two-reasons.html


Steve

* Leica M wide-angle lenses mounted to M4/3 and larger digital formats have shows a disturbing tendency to have incredibly poor edge performance.

[seventhdr]

This is a very interesting article from Roger at LensRentals. It does call into question the performance of film lenses on digital cameras where there is extra glass between the rear of the lens and the sensor/film location.

Since Pentax has sourced sensors from multiple manufacturers (Kodak for the K-1, Sony and Samsung for the rest) who specifies the thickness of glass in front of the sensor. How much glass is supplied by the sensor manufacturer and how much does Pentax use? And has this varied since the *ist D was introduced in 2003.

Both Canon and Nikon will be in the same situation as a substantial number of their lenses are carried over from their film days. Others like Olympus and Panasonic have designed new lens lines from scratch.

We also have the reverse situation for Pentax users who might choose to use DA lenses on film cameras.

Does the presence of such a thick glass (4mm) in micro 43 cameras mean they can be less stringent in their lens designs?

Regards

Chris

[OregonJim]

Interesting, but I think he's chasing the wrong ghost. My gut feeling says the differences have more to do with the refractive index of the glass than thickness. If I had any lenses faster than f/1.2, I might pay closer attention, though.

[stevebrot]

So the question is, to which "stack" has Pentax, or Canon, or Nikon designed their various digital lenses. As noted above, the sensor technologies for all three brands has evolved. Has there been a matching evolution of lens tuning? For example, are the original DA lenses inappropriate or obsolete when used on a current generation body? On thing is for certain, the composition of the "stack" has changed between the K-3 and K-5. Surely the refractive properties of the "stack" has changed as well.


Steve

P.S. Before we get too worked up about all of this, it would be good to consider the nature of the issue. The angle of incidence drives the potential for aberrations. We have a couple of optical engineers active on this site. With any luck one of them will weigh in.

[glasbak]

Looking at the cut through picture of the K5 made by Falc Lumo, the K5 sensor glass and IR/antialias stack looks about 3mm thick !

[carrrlangas]

Looking at the cut through picture of the K5 made by Falc Lumo, the K5 sensor glass and IR/antialias stack looks about 3mm thick !

Probably right.

Here´s a follow up on the topic: LensRentals.com - Sensor Stack Thickness: When Does It Matter?

Using Lenses on an Adapter
The practical importance comes when we want to use a lens designed for one camera on a different camera. (I’m assuming the adapter contains no optics itself.) Several factors come into play here.
1) The difference in sensor-stack thickness between the camera the lens was designed for and the camera actually being used.
2) The maximum aperture of the lens. Wide-aperture lenses are going to be more sensitive than narrow aperture lenses.
3) How telecentric the lens is. (More specifically, how far forward the exit pupil of the lens is.) A lens with the exit pupil far away from the sensor is not affected by the thickness of the sensor stack very much. A lens with the exit pupil very close to the sensor is affected a lot.
The exit pupil is an optical phenomenon – the exit pupil is not the physical location of the rear aperture or the rear element. It can be measured, but those measurements aren’t readily available. In general more telephoto lenses have very forward exit pupils and aren’t affected by sensor stack thickness very much. Wide-angle lenses may have very close exit pupil distances. Reverse-telephoto design wide-angle lenses (SLR lenses basically) have the exit pupil more forward than Rangefinder wide-angle lenses, generally.
So in theory, a 135mm f/4 SLR lens isn’t going to care much about the sensor stack thickness. A 24mm f/1.4 rangefinder lens can be hugely affected.

In the case of Pentax DSLRs, Exit Pupil distance should be at least the register distance of 44mm and later there´s a table that says the Pentax 50mm f/1.4 Super Takumar has an exit pupil distance of 70mm.. Not that bad.
Also, there are few fast Pentax lenses so, another "migitator" of the problem . And finally, legacy lenses are made for a bigger format than the one used on DLSRs so we don´t see much of the problems of the sensor stack, as they increase as you get further from the center of the image.

The further from the image's center (where the principal ray is perpendicular, and the cover glass has thus essentially no influence), the worse the image quality will be.

From The Online Photographer article

Thinking out loud: The DA23/2.4 and DA50/1.8 where redesigned to take the sensor stack into account or just had their maximum aperture restricted to hide the sensor stack effects?

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Last edited by carrrlangas; 06-24-2014 at 08:35 AM.

[Nicolas06]

As for the thickness of the film filter stack, the light sensitive emulsion is on the surface of the film. The total thickness of the emulsion is less than 1/1000" (25.4 micrometers). Stack thickness is essentially zero for both B&W film and color films.

Because film move from photo to photo, there are mechanical margins. You also need to account build precision too. In the end the real thickness or thickness variation could be far closer than one might think to a digital sensor.

[stevebrot]

Because film move from photo to photo, there are mechanical margins. You also need to account build precision too. In the end the real thickness or thickness variation could be far closer than one might think to a digital sensor.

Huh? The issue is refraction caused by the cover slip of the digital sensor. The physical position of the film is a completely separate issue (think that is what you are saying). If you are thinking that there is a cover layer over the light-sensitive portion of the film, that is not the case. The emulsion layer is "naked to the world" and is (as noted above) a few orders of magnitude thinner than a sensor cover slip (25 micrometers = 0.025 mm). As for "build precision"...that is very high for thin-film technologies used for any photographic film made in the last couple of decades.

All this aside, as noted above, cover slip thickness is not a huge real-world issue for SLR lenses.* For non-retrofocus lenses (common to rangefinder, view, and mirrorless cameras), the angle of incidence to the sensor cover slip can be quite large (60 degrees or more). Cover slip thickness may significantly impact optical performance with those lenses.


Steve

* The location of the rear exit pupil is never much less than mount register on an SLR, which is 45.46mm for Pentax.