can the design of a lens affect a light meter?

[Ces1um]

I recently purchased lomography's new daguerreotype art lens in K mount. This is a beautifully crafted lens but I've hit quite a snag with it. Let's say I have a different lens on my k1000 and work out my exposure settings. For arguments sake let's say these settings are F4 1/125. If I then put my daguerreotype lens on and the F4 aperture plate in, the light meter now says the shot is underexposed. Radically underexposed. Now for this to work my settings are F4 1 second exposure. An independent light meter confirms the F4 1/125 to be correct.

Any idea what could be going on here? To confirm, I am using the correct F4 aperture plate. This issue persists with all apertures. The lens itself seems to be affecting the meter. These aperture plates are located in front of the element which is different from all other lenses I own. My question is- do I trust my light meter with the lens on, or do I trust my external light meter? Any help would be appreciated.

 

[etn]

Not sure, but just to bring a hypothesis on the table...
The lens could possibly change the polarization of the light? some meters are sensitive to that.

Have you tried to take 2 identical shots using the same exposure settings, preferably given by an independent/external meter, using both lenses?

 

[Nodda Duma]

That is a 3 stop difference, right? So to answer which is correct, it should be easy enough to take a photo with the camera meter settings and take another with the external meter settings and see which gives proper exposure.

To answer why is a little trickier.

I don't think polarization is an issue, but easy enough to check with polarized sunglasses (look through the lens and sunglasses, rotate the sunglasses and see if the view through the lens blacks out)

My first question is: does the K1000 meter on-axis, or by picking off from the edges of the light cone going through the viewfinder prism? The daguerrotype lens may be vignetting in a way the meter doesn't expect.

My second question: how do you know the aperture plates are correct? Depending on the prescription of the lens, the physical diameter of the opening isn't necessarily f.l. divided by f/# (think magnification of the entrance pupil).

If I knew the prescription or at the least the design type, I could tell you what the hole sizes should be.



-Jason

 

[DWThomas]

(Confess I'm not familiar with the K1000 but ...) most SLRs meter with the lens wide open and only close down to the set aperture during exposure. You probably need to do "stopped-down metering" with the art lens if your camera has a way to do that.

 

[GRHazelton]

IIRC the K-1000 is full aperture, full screen metering with no stop-down metering possibility. I'd try a test roll with the art lens; BW is cheap and should certainly indicate a 4 stop problem.

 

[grussmir]

Pentax has one lever for stop down, coupled to the iris diafragm and another signaling the selected f-Stop to the camera (by angle).... so i guess theese are both missing and in the end the camera estimates the working aperture (f4 in your example) as its open aperture and f22 or something as working aperture.
Strange that Lomo missed this point in the lens/mount design, but it makes sense, since all digital SLRs with K-mount do stop-down metering (they don't have a blesator... thats what the guys here in the german digital pentax forums call it... shorthand for blendensimulator, meaning the lenscoupling lever). So actually there is no workaround but metering manually.

 

[Chan Tran]

No meter coupling is one thing. However, it should only indicate 2 stops under as the lens I believe is rated at f/2.9. Whatever, it's the lens that is wrong and not the camera.

 

[markbarendt]

(Confess I'm not familiar with the K1000 but ...) most SLRs meter with the lens wide open and only close down to the set aperture during exposure. You probably need to do "stopped-down metering" with the art lens if your camera has a way to do that.

Yep.

I'd say the OP would do fine finding the setting with the regular K mount lens then ignoring what the camera suggests with the Lomography lens. Same idea as using a spot meter.

 

[AgX]

No meter coupling is one thing. However, it should only indicate 2 stops under as the lens I believe is rated at f/2.9. Whatever, it's the lens that is wrong and not the camera.

If this was an issue of an autoexposure-open-aperture-TTL camera then not the speed of that Lomo lens would be the point of reference but the speed such camera expects when a lens is not coupled.
(To overrun that reference point the stopped-down mode has to be chosen.)

 

[Ces1um]

Not sure, but just to bring a hypothesis on the table...
The lens could possibly change the polarization of the light? some meters are sensitive to that.

Have you tried to take 2 identical shots using the same exposure settings, preferably given by an independent/external meter, using both lenses?

I only just got this lens on the 25th and where I'm shooting him I haven't yet had a chance to see what the actual final results will accomplish. I plan to take a few photos of a sheet of paper labelled "pentax control lens" "daguerreotype using pentax settings" "daguerreotype using inbuilt light meter settings" and she which ones actually work.

 

[Ces1um]

My first question is: does the K1000 meter on-axis, or by picking off from the edges of the light cone going through the viewfinder prism? The daguerrotype lens may be vignetting in a way the meter doesn't expect.

The pentax k1000 uses a full scene averaging light meter.

My second question: how do you know the aperture plates are correct? Depending on the prescription of the lens, the physical diameter of the opening isn't necessarily f.l. divided by f/# (think magnification of the entrance pupil).

The aperture plates are labelled at the factory with their F stop number




-Jason

 

[Ces1um]

(Confess I'm not familiar with the K1000 but ...) most SLRs meter with the lens wide open and only close down to the set aperture during exposure. You probably need to do "stopped-down metering" with the art lens if your camera has a way to do that.

This lens is unique where it uses individual aperture plates that you physically insert into the lens. It's a fairly old design. I have a keyring of labelled aperture plates that I need to carry around. The k1000's light meter is "always on" and only shuts off when the lens cap is placed over the lens.

 

[Ces1um]

IIRC the K-1000 is full aperture, full screen metering with no stop-down metering possibility. I'd try a test roll with the art lens; BW is cheap and should certainly indicate a 4 stop problem.

I wish I read your reply before answering everyone's replies! Thanks for your help! I'm likely going to assume that I will need to always use an external light meter and that the issue is likely how the lens interacts with the light meter. I have a roll of colour film in there right now, but once I'm done with it I'll shoot some black and white and process that at home. I should have the results back before the lab can process my film.

 

[Ces1um]

Pentax has one lever for stop down, coupled to the iris diafragm and another signaling the selected f-Stop to the camera (by angle).... so i guess theese are both missing and in the end the camera estimates the working aperture (f4 in your example) as its open aperture and f22 or something as working aperture.
Strange that Lomo missed this point in the lens/mount design, but it makes sense, since all digital SLRs with K-mount do stop-down metering (they don't have a blesator... thats what the guys here in the german digital pentax forums call it... shorthand for blendensimulator, meaning the lenscoupling lever). So actually there is no workaround but metering manually.

My pentax is a fully manual, fully mechanical film camera. it' the pentax k1000. The lens does not communicate with the camera whatsoever either, being an old school fully "mechanical" lens. There are no electronic contacts between lens and camera. I'm guessing though that if it was connected to a modern pentax dslr then this issue may or may not exist. I'm not aware how new dslr light meters operate.

 

[MattKing]

Although the lens has a "native" aperture of f/2.9, the xxxxlens mount may be communicating false information to the metering lever in the camera.

You may not be able to use the meter in your camera with this lens.

 

[MattKing]

The manual for the K-1000 does refer to the use of stop-down metering when using accessories or lenses that don't couple to the open-aperture metering system. As per my previous post, I'm guessing that the xxxxxlens mount you are using is actually communicating false information - rather than no information - to the camera.

If lomography offers a way to ask questions, yours would be an excellent one.

 

[Ces1um]

Although the lens has a "native" aperture of f/2.9, the adapter may be communicating false information to the metering lever in the camera.

You may not be able to use the meter in your camera with this lens.

Unfortunately I strongly suspect you are correct that I will not be able to use the meter. That will certainly slow things down for me.

 

[Ces1um]

The manual for the K-1000 does refer to the use of stop-down metering when using accessories or lenses that don't couple to the open-aperture metering system. As per my previous post, I'm guessing that the adapter you are using is actually communicating false information - rather than no information - to the camera.

If lomography offers a way to ask questions, yours would be an excellent one.

I've sent an email to their "tech support". Unfortunately they're closed until the new year so I'm concerned I may be wasting film in the meantime. I've decided to use an external meter until then and trust it's results rather than using my k1000's meter.
As for the "adapter" I'm not entirely sure what you mean here. The lens is native k mount. I'm not sure if providing a link to a product is ok within the forum so if this is "taboo" may the ops please forgive my transgressions and delete my post. Maybe seeing the lens in question would be helpful?
https://microsites.lomography.com/daguerreotype-achromat-art-lens/

 

[MattKing]

The lens is native k mount.

Sorry, I missed the fact that it was available in K mount, and assumed you were using it with a lens mount adapter.
I've amended my posts accordingly.
I'm not familiar with how far the compatibility of current, digital K mount cameras to legacy K mount lenses extends (does it include open aperture metering?) but it may be that your lens was designed for a later metering system.

 

[Ces1um]

I'm not familiar with how far the compatibility of current, digital K mount cameras to legacy K mount lenses extends (does it include open aperture metering?) but it may be that your lens was designed for a later metering system.

I suspect you may have hit the nail on the head there. Even though lomography is all about film cameras, I think their art lenses are geared more towards use on current generation dslr's. It may be that there are compatibility issues within the k mount system over the years. I'll have to leave that up to people who know much more about this than I do. To be honest, I thought all metering was totally done in body with my camera as the meter functions without a lens on the camera at all. I didn't realize that these old cameras/lenses "spoke" with each other in any way. I had no idea that the lenses couple with the metering system. I always thought that the camera itself took a reading off the front of its own film curtain.

 

[BMbikerider]

Is it not the angle of acceptance of the light which will make the difference? I have a 20/35 Nikon AFD with a constant apperture of F2.8 and a reading taken at 20mm and then at 35mm of the same scene, the exposures are different. Somewhere between 1 and 2 full stops is quite normal, which is why I always take meter reading at 28mm then alter the focal length. Likewise with my Nikon 28/105AFD lens, the readings are different at 28mm and 105mm but not so marked. (Due allowance made for the 1/2 stop difference in appertures at the shortest and longest focal length)

I think if you use a hand held meter and compare it to the reading for that lens, you will find that the readings are significantly different or indeed the other lens you are using.

 

[Theo Sulphate]

Below is a Pentax K mount lens. Lever (i) is used by the camera body to stop the lens down during exposure - obviously your art lens won't have that since you manually insert the aperture. However, lever (ii) is a coupling that the body needs to determine the maximum aperture of the lens and also read the selected aperture on the aperture ring. With that information, the meter can give an accurate wide-open reading. If your art lens doesn't have lever (ii), then your meter won't read correctly with that lens. In that case, the reading from your original K lens should be used.

http://www.pentaxforums.com/content/uploads/files/77/p1463/K.jpg

 

[Ces1um]

Theo- you're a genius. My lens has NEITHER of those levers. No wonder it won't work properly. Problem solved! Light meter it is then. Thanks to everyone for your help on this!

Now should I stick with the light meter app on my phone or should I buy a handheld meter? Suggestions?

 

[Ces1um]

Also- MattKing- Thanks for all your efforts. I wasn't getting what you were saying until I saw the photo.

 

[Theo Sulphate]

Actually, grussmir mentioned it first in post #6, but having a photo of the mechanism helped.