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06-02-2010, 05:36 PM   #1
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MTF program line overexposes?

When doing some test comparison shots between the DA 15mm and A 15mm lenses, I noticed that the camera always exposed two stops higher for the A 15mm. For example, when the DA was shooting at 1/750s F4, the A would be at 1/350s (and also F4, as I was manually setting the aperture trough hyper program).

Can the MTF program line I was using be blamed for this? Obviously, darker images typically have less CA or aberrations than lighter images, so I wouldn't be surprised that the camera would attempt to do this with the DA lens. Has anyone experienced something like this before? I was using my K-7.


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06-02-2010, 06:27 PM   #2
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I'm not sure about your K7, but my K20D manual says (under Program Line, p.82): "MTF priority... is effective when combined with a DA, D FA, FA, or FA J lens". So an A lens won't get the benefit of MTF, at least not on the K20D. Check the small print in your K7 manual.
06-02-2010, 07:18 PM   #3
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According to this thread: What MTF in K10D does exactly? - Photo.net Pentax Forum

FA, FA-J, D-FA, and DA lenses have a chip that tells the camera the best exposure for sharpness/resolution based on the MTF that is encoded.

So, if the A lens doesn't have the chip with the MTF information, then it may just revert to the regular Program line. Have you tested the A lens with the normal Program line as a comparison?
06-02-2010, 08:10 PM   #4
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I did some tests and it seems that the A 15mm ALWAYS overexposes on Pentax DSLRs. I will now do a comparison with the MZ-S to see if it's the lens's or the firmware's fault. (I'm thinking firmware right now)

You sure were right about the MTF!


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06-02-2010, 09:02 PM   #5
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On the Super Program, the lens exposes 100% correctly in both P and Tv modes. Lighting is also fine on our digital bodies when using the aperture ring + stop-down metering. However, when set to 'A', both the K10D and K-7 overexpose by about 1.5 stops! Same deal with the FA-series film bodies: the PZ-1 and MZ-S metered just like the digital bodies when the lens was set to 'A'. I guess that the conclusion we can draw from this is that this lens's aperture data isn't accurately stored within the digital bodies, and that Pentax should fix this via firmware.

Can anyone else with this lens confirm such behavior?

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06-02-2010, 11:47 PM   #6
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Wouldn't your results suggest that the more advanced bodies are mis-reading the maximum aperture of the lens from the A contacts?
06-04-2010, 02:42 PM   #7
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I don't have the lens, but I have theories. There are three things I can think of that might be wrong: The camera is wrong about the aperture range, the camera sees less light when metering than when shooting, or the camera fails to set the selected aperture.

Aperture range is trivial to check, just spin the dial and see where the numbers stop. If it's right, this is not an electrical issue. (And even if it is, it's not a firmware issue. A lenses have aperture range and no other data. Unless all bodies are broken for all lenses with this particular aperture range.)

I really don't see how it could see less light when metering. Even if that seems most consistent with your results, except it was right when using stop down metering...

Left then is failure to set the aperture. This should mean it exposes correctly wide open. You can also use DOF preview and look at the aperture blades. Does an aperture set on the ring match the same aperture set on the camera? If it doesn't, I assume it would match on the super program (if it had DOF preview).

I actually have a theory on how that might come about too: The stop down lever might be worn where a modern body presses it (making it move less), but not where an older body presses it.
06-04-2010, 09:29 PM   #8
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QuoteOriginally posted by Adam Quote
When doing some test comparison shots between the DA 15mm and A 15mm lenses, I noticed that the camera always exposed two stops higher for the A 15mm. For example, when the DA was shooting at 1/750s F4, the A would be at 1/350s (and also F4, as I was manually setting the aperture trough hyper program).

Can the MTF program line I was using be blamed for this? Obviously, darker images typically have less CA or aberrations than lighter images, so I wouldn't be surprised that the camera would attempt to do this with the DA lens. Has anyone experienced something like this before? I was using my K-7.
Adam, my A 70-210/4 always exposed correctly no matter which program line I set on the K10d. I have this sharpness thing in my head that makes me use MTF program line quite often. I doubt my daughter who is now using the lens would even know how to change the program line, so I cannot check with her and her K200d.

06-09-2010, 07:08 AM   #9
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QuoteOriginally posted by Steinback Quote
Wouldn't your results suggest that the more advanced bodies are mis-reading the maximum aperture of the lens from the A contacts?
For the A 15mm, yes. It really seems like this is the case.

QuoteOriginally posted by drougge Quote
I don't have the lens, but I have theories. There are three things I can think of that might be wrong: The camera is wrong about the aperture range, the camera sees less light when metering than when shooting, or the camera fails to set the selected aperture.

Aperture range is trivial to check, just spin the dial and see where the numbers stop. If it's right, this is not an electrical issue. (And even if it is, it's not a firmware issue. A lenses have aperture range and no other data. Unless all bodies are broken for all lenses with this particular aperture range.)

I really don't see how it could see less light when metering. Even if that seems most consistent with your results, except it was right when using stop down metering...

Left then is failure to set the aperture. This should mean it exposes correctly wide open. You can also use DOF preview and look at the aperture blades. Does an aperture set on the ring match the same aperture set on the camera? If it doesn't, I assume it would match on the super program (if it had DOF preview).

I actually have a theory on how that might come about too: The stop down lever might be worn where a modern body presses it (making it move less), but not where an older body presses it.
Wide-open, exposure is always correct, so that eliminates the first theory.

The super P has DOF preview, so I can check your theory about the aperture not closing properly in A mode. Either the lens is at fault, the firmware has a bug, or the camera is simply misinterpreting the data that the lens provides.

However, I'm very reluctant to believe the first of the above. In P mode, the super P tells you what aperture will be used in the viewfinder. The resulting shutter speed matched what was displayed when using the aperture ring (non-crippled, computational metering).

The second theory is likely due to the consistency among all the electronic cameras.

The third theory is also feasible- what if one of the contacts on the lens was used unlike those of other A lenses?

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06-09-2010, 07:52 AM   #10
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Check aperture range on your DSLR.
Maybe there is a weak contact and you cam thinks aperture range is different then it really is.
06-09-2010, 08:26 AM   #11
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Adam

It sounds like time for my brick wall test,

Using AV mode run the lens through all the aperture range and plot the histogram grey scale value vs F stop.

Also, as someone else suggested check the range of aperture adjustments for the lens.

You say it meters correctly wide open, so is it possible that the contacts in the aperture coding for minimum aperture are dirty and you have lost one contact, making the camera body think you have a wider range than is actually there? This would cause the aperture activation lever to not move as far for each F stop selected, causing over exposure at all but wide open, this would, I believe be a linear trend with increasing over exposure as a function of smaller and smaller apertures.

the other thing to consider is that the A series lens may not have ideal aperture variation as a function of area. note that a 15mm lens has a pretty small aperture to begin with, making adjustments much more difficult. Again, doing an exposure test against a uniform lit wall is a great way to map out the lens performance
06-09-2010, 09:17 AM   #12
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I haven't done it in front of a lens wall, but across the board from F4 to F16, my K10 and K-7 overexposed consistently by 1.5 stops (compared to the DA15).

I'll try cleaning the contacts, but really doubt that's a problem, as the lens is pretty much brand new!

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06-09-2010, 12:49 PM   #13
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adam

just a curious point, if you look at the mark roberts page on the coding of the contacts, the A15 should have a pattern 001*11 for F3.5-22. If you lost contact with the first pin beyond the * (i.e. the A M switch) it would think the lens was F4-45.

You might not notice a 1/2 stop error wide open unless you had a "metering wall" but you sure would notice the error from not stopping down enough after that.
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