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09-29-2010, 11:11 PM   #16
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JeffJS - I tried what you said, spot metering and it does the same thing.

Canada_Rockies - I'm adjusting my aperture in camera by turning the dial on the back by my screen, I change my ISO 2 stops higher once I go from F1.4 to F2.8 to get my exposure the same. This is really frustrating because I drove last night to pick up the lens 160 miles round trip....ugh!

09-29-2010, 11:14 PM   #17
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QuoteOriginally posted by Canada_Rockies Quote
Are you adjusting your settings with the camera in M mode with your ISO set to a single value, not automatic? If you are not, the camera might be screwing with your settings. Other than that, I'm just as stumped as everyone else.
According to the Exifs, the lens is changed by 2 stops from f1.4 to f2.8. The ISO changes accordingly from 1600 to 6400. The shutter speed is held constant at 1/125. Effectively should be the same Exposure with the only thing being different, the DOF (and noise at the higher exif). This does not take into account, any post processing that may have been done however.

QuoteQuote:
JeffJS - I tried what you said, spot metering and it does the same thing.
Well, I'm stuck then. I'm back to blaming the lens/camera combo and it not knowing what to do with the f1.4 aperture. I'll pull out a couple of my A50mm lenses sometime tomorrow and see if I can trip up the K7 in the same way.

QuoteQuote:
I drove last night to pick up the lens 160 miles round trip....ugh!
One would assume that you made sure that the Aperture blades are not sticking in any way? On A, the lens should be stopped down to f22 (or as small as it will go). You should be able to pull the aperture lever on the back and it should immediately snap back when released. Is that happening? or does it drag slightly? New lens? Used lens? Clean Contacts?


3000

Last edited by JeffJS; 09-29-2010 at 11:22 PM.
09-29-2010, 11:37 PM   #18
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No post processing was done. The blades are smooth and I tested them to see if it goes back to f22 which it does so immediately. It's a used lens and the seller spoke to me today and said he never had that issue. I cleaned the contacts now and still does the same thing...Maybe my lens should be in Ripley's Believe It or Not haha
09-29-2010, 11:38 PM - 1 Like   #19
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Since the iso, shutter & aperture were set manually, the metering is a non-issue here. There are 5 possible explanations that I can think of:

1) The actual wide open t-stop of this lens is way smaller than 1.4 (not rare for fast lenses).

2) Mechanial aperture is not linear (and often they aren't).

3) The aperture blade position was not probably calibrated. You can tell by turning the aperture from f2 to f1.4 slowly. The aperture blades should just disappear completely at 1.4, but not too soon.

4) The rear aperture arm was not properly calibrated. You can tell by flipping the rear aperture arm slowly and the aperture blades should disappear just when the arm hits the end, but not too soon.

5) The stop down mechanism of the camera was not properly calibrated. This will affect other lenses too.

PS: Pentax lenses with aperture rings & A setting often disagree with each other slightly. For instance, you can take an identical shot at certain aperture, say f4, one set from the lens, one through the camera. The actual exposure could be different (by up to 1/2 stop based on experience) due to the reasons above. This is not neceassily faulty, just the limitation of mechanical aperture and loose QC for mass production. Fully electronic apertures of EOS lenses can be made more precise for the same reason.

09-29-2010, 11:50 PM   #20
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QuoteOriginally posted by crossover37 Quote

Click on Pentax K-x on the top right, it will bring up the EXIF info
QuoteOriginally posted by Canada_Rockies Quote
You need to open the "original" size to see EXIF on Flickr. Click all sizes, then original.
Thanks! I actually found it through ACTIONS->View EXIF info before I made that post. So many ways to view it and I was missing them. I swear you used to be able to right click the image then I would use my in-browser app to view the info. Oh well, I don't visit flickr much anymore and this is the result. Thanks again for offering a helping hand.
09-30-2010, 03:10 AM   #21
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I second the theory about the sticky aperture/not being able to stop down properly. My kit lens was exposed to a mild sand storm, and afterwards had a similar problem. When I played a little with the aperture lever the problem was solved
09-30-2010, 03:15 AM   #22
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I have the same experience with my FA43 using the AV mode from f1.9-f2 the exposure is a little dark compared when i use from f2.2 and up..
09-30-2010, 03:37 AM   #23
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QuoteOriginally posted by wlachan Quote
There are 5 possible explanations that I can think of:

1) The actual wide open t-stop of this lens is way smaller than 1.4 (not rare for fast lenses).

2) Mechanial aperture is not linear (and often they aren't).

3) The aperture blade position was not probably calibrated. You can tell by turning the aperture from f2 to f1.4 slowly. The aperture blades should just disappear completely at 1.4, but not too soon.

4) The rear aperture arm was not properly calibrated. You can tell by flipping the rear aperture arm slowly and the aperture blades should disappear just when the arm hits the end, but not too soon.

5) The stop down mechanism of the camera was not properly calibrated. This will affect other lenses too.
I agree with Alan.
Regarding point 5 it would be better for the Pentax cameras to have a stop-down coupler (as the better Nikon cameras have!), which would be more accurate.

We are pleading for years already, but Pentax does not see the need.
Of course with their latest lenses not even have an aperture ring anymore, there really is no need.

09-30-2010, 06:23 AM   #24
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QuoteOriginally posted by wlachan Quote
Since the iso, shutter & aperture were set manually, the metering is a non-issue here. There are 5 possible explanations that I can think of:

1) The actual wide open t-stop of this lens is way smaller than 1.4 (not rare for fast lenses).

2) Mechanial aperture is not linear (and often they aren't).

3) The aperture blade position was not probably calibrated. You can tell by turning the aperture from f2 to f1.4 slowly. The aperture blades should just disappear completely at 1.4, but not too soon.

4) The rear aperture arm was not properly calibrated. You can tell by flipping the rear aperture arm slowly and the aperture blades should disappear just when the arm hits the end, but not too soon.

5) The stop down mechanism of the camera was not properly calibrated. This will affect other lenses too.

PS: Pentax lenses with aperture rings & A setting often disagree with each other slightly. For instance, you can take an identical shot at certain aperture, say f4, one set from the lens, one through the camera. The actual exposure could be different (by up to 1/2 stop based on experience) due to the reasons above. This is not neceassily faulty, just the limitation of mechanical aperture and loose QC for mass production. Fully electronic apertures of EOS lenses can be made more precise for the same reason.
I would have to agree here also with respect to possible causes, BUT.....

I looked at the images and put them side by side,, then did a screen capture to evaluate the overall image in PSP X3

what I can tell you is the following.

There is appearance of more depth of field in the F2.8 shot, than in the F1.4 shot, although to tell if the aperture really stopped down It would have been nice to have one distant Out of Focus light, that would take the shape of the aperture blades, when stopped down.

If we look at the greyscale value of the carpet next to the bottle, in the first shot it is 68 and in the second it is 87. This represents about a 1/2 stop difference. I don't know what you metered on to get the exposure but it does show that there is some error in the aperture.

If you look at the edges of the photos, and again the greyscale value of the carpet the wide open shot shows a value of 51 in the lower left, the F2.8 shot shows 84. This is vignetting and is typical of all lenses wide open, and at F2.8 the difference center to edge is less than 1/10 of a stop or pretty good.

I think the lens is functioning properly, within the tolorances.

What you should do, is take the lens and using spot metering, shoot only the carpet, uniformly lit, and in AV mode, run a set of shots at each aperture setting and plot the center and edge greyscale value on a spread sheet. this will give you your system's exposure accuracy and also the lens performace in terms of vignetting. I have found nearly all lenses will have some exposure errors, for example my tamron 28-75 F2.8 has a drift upwards which ius just about uniformly increasing with aperture,starting with perfect exposure wide open to +1 stop at F32. It is always food to map the performance of your kit so you know how it all works together. The worst time to learn about exposure errors on any part of your system is after you screw up a once in a lifetime shot.
09-30-2010, 07:26 AM   #25
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Thanks everyone for the tips. The supervisor of Pentax emailed me asking for the images and he will try and help me out. I was on chat with them yesterday and they had to get back to me because they coudlnt answer the question. The exposure should be fine because the depth of field is shallower at f1.4, the difference in exposure shouldnt be that substantial...sucks.
09-30-2010, 07:32 AM   #26
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QuoteOriginally posted by crossover37 Quote
Thanks everyone for the tips. The supervisor of Pentax emailed me asking for the images and he will try and help me out. I was on chat with them yesterday and they had to get back to me because they coudlnt answer the question. The exposure should be fine because the depth of field is shallower at f1.4, the difference in exposure shouldnt be that substantial...sucks.
I think you need to re-read my post. I think you will find there is nothing wrong at all with your lens.

I suggested above that you do a series of tests and map out the exposure.

you can also do more shots at different apertures to see how consistent each aperture is, but I think at the end of it, if the camera meters correctly +/- 1/2 stop at all exposures, there is nothing you will be able to do.

Note, a long time ago now, I proposed to pentax that they allow users to input exposure curves for each lens they own, (just like adjusting focus) because no lens will be perfect, especially at shorter focal lengths where the aperture is actually pretty damm small already controlling the area of a small circle (or hexagon or octagon) is pretty hard.
09-30-2010, 06:32 PM   #27
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Thanks Lowell. I tested the lens at difference apertures and the exposures matched once I adjusted the iso or shutter speed accordingly but at F1.4 it is always significantly darker. For example, I took a shot at f1.4 and then one at f1.7 and the difference in the exposure was different so it seems it only happens at f1.4 and I don't know if I can fix that. If it's vignetting, you would think it would effect the f1.7 aperture also. Any ideas on why it only happens at f1.4 and if there's something I can do about it?
09-30-2010, 06:53 PM - 1 Like   #28
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You should post some shots and also consider measuring the center and corner. When I looked at your first 2 shots clearly the lens is perhaps a little slower than F1.4 in the middle but it vignettes also. On e stopped down it seems pretty good. It makes me think I should also test my two fast 50's. One other thing, exactly how close are you to the subject. This might also be influencing things as aperture is only really true at infinity. Try an exposure test at about 5-10 feet and see if it improves
09-30-2010, 08:05 PM   #29
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I guess the other question is to repeat the test using the aperture ring to adjust aperture. That will tell us if the lens can open fully or not
09-30-2010, 10:20 PM   #30
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Here is the response I got from the Pentax supervisor. He experiences the same issue with his FA 50mm F1.4 so I guess it's just the lens and how it reacts at f1.4

Jon,
Looking at your 2 RAWs, I can see that you are not doing anything
wrong, and the camera does not look like it is doing anything funky
either.

I tested a different model camera(K20D) and a different ISO set (560
vs 2200) with my FA 50mm f1.4 and got similar results as you. This to
me proves that it is a lens specific issue, not a camera or setting
specific issue.

I also tested some other very fast lens and noticed that at their most
wide open aperture setting there was some under-exposure as well.

The lens also vignettes more at f1.4, which is a common result when
shooting at or near wide open apertures, that usually disappears
completely by stopping down 2 stops. This of course makes the photo
look even darker.

I would say that your lens is not defective, but this is just the
result of the design of the lens, and the way light interacts at wide
open apertures. Replacing or repairing the lens would not give you a
different result.

Essentially, your best solution is just to be aware that if you shoot
wide open you may have to compensate a half stop.

I know this is probably not what you were hoping to hear, but I
believe it to be the correct answer.

That said, if you have doubts, you are always welcome to send the lens
into our shop, where they can test it for issues.
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