K lens

I've just bought a 28mm K lens with the A setting. Very nice it is too but my K100D seems to overexpose it by about 2/3 stops each time so the image is useable but over exposed.

I think I must be doing something wrong. I had the idea with the lens on A I could use Av or Tv and I tend to set the aperture and let the speed sort itself out.

Actually now I look again the ISO seems to be 1600 where the kit lens say would be 200 - but that makes no sense.

Any ideas?

Mike

It is possible that the iris is not operating perfectly. Take the lens off the camera and move the lever on the lens that stops the iris down. Does that work snappily and smoothly ?

I would also give the lens contacts a clean.

As far as the ISO.....you set that yourself ! Does it overexpose with ISO 200 ?

Well I tried that lever as the same thought occurred to me and it is indeed snappy etc.

Now contacts - which ones are they? There is one 'ball' that I can see. As always all help is welcomed!

The ISO was in fact a red herring as it looks like it's being selected at the various settings within the range I allow - 200-1600 ISO.

Could it be the style of metering I chose which as I recall isn't spot but the middle one?

Could it be a slightly grubby lens? I think there may be some graese on the lens surface?

Thanks for your help thus far.

Mike

Close the iris down to it's smallest aperture and check that there is no evidence of oil on the blades (check both sides).
If you suspect lens surface contamination, clean it with lens cleaning tissues or cloth, slightly wetted with lens cleaning fluid. If you suspect the grease is on the inside you'll need to get it serviced unless you have the tools to disassemble it. You also have to ask yourself where di the grease come from?
Your metering could be to blame, try the full pattern metering mode (usually the green one).
As far as cleaning contacts goes, clean the whole of the mating surface of the lens mount. You'll clean all of the contacts then. A very small amount of WD40 or similar lubricant is what I use, but it must be a type that dries, and of a non-solvent type.
Personally, I would never fit an unknown lens to any of my camera bodies without first cleaning at least the contact surface.

Give any lens a good clean...and not just the glass ! Dont forget there is a a build up of 20 years of gunge on some of these old lenses, you dont want to introduce that to you shiny new digital sensor.

The type of metering you use will make a difference, but as I understand it the A series lenses can use spot, centre weighted, or matrix.

Post some pictures and let us have a look.

OK - just tried some tests indoors.Using the kit lens and the 28mm lens the metering is showing 1/10 for one lens and 1/15 for the other which really is no difference. I have just shot 4 identical pictures one on each lens at f4 and then at f8 (thinking that maybe the lens wasn't stopping down OK) with foixed ISO. Each time the 28mm lens uses a longer shutter speed but 0.3 against 1/4 say which in reality is no difference although the image is brighter.

Blades look oil free and I did clean the contacts before the last test just with a tissue as I have found it's effective at leaching grease off surfaces. I think I'll avoid WD40 as it creeps all over the place, I have some carburretor cleaner which will get grease off anything in my experience!

Maybe the clean with tissue has done the trick?

Once again many thanks for advice.

Mike

WD40 is designed to get into joints etc. where other lubricants can't reach. A drop on a tissue is all that is needed to effectively clean a lens mount without any harm coming to the mount, lens, or camera.

As for carburretor cleaner, check it's active components thoroughly. If it contains solvents, it could damage the SMC coating.

Glad your mostly sorted anyway.

Well I'm not convinced that all is well but we'll see.

OK ignorance time: What does the contact actually do on the 28mm lens?

Here's a picture from this afternoon centre weighted 5.6 @ 1/90 but for some reason it chose ISO 1600.

http://www.flickr.com/photos/9890138@N08/

Overall the lens is good condition no obvious crap anywhere but it does look like the glass needs a decent clean up externally.

Mike[/img]

Does the histogram for that pic confirm your suspicion that it is overexposed.

To me, the white in the brickwork doesn't look blown out, and the inside of the broken iron pipe looks black enough.

Well no the histogram was OK actually if I'm reading it right - I'm new to all this digital stuff. For the shot the histogram is a fairly nice curve with the peaks well centered so not all up the 'bright' end but the curve goes right up to the 'top' as it were and is quite a wide peak. No clipping anywhere. However, I am more used to seeing histograms with multiple peaks not one large sugar loaf shape which is what this lens seems to deliver.

I guess that's OK and I did check when I was shooting and saw the histogram was OK.

Looking at a very similar shot (yes dull it may be) taken on my Sigma 10-20 lens the histogram is not so 'high' although the basic shape and position of the histogram is much the same.

Am I being just totally dumb and this is just different lens characteristcs?

The images are certainly lighter than the other lenses I have but then the 50-200 zoom creates a different image quality to the kit lens as well. Not much but a bit.

I think I need to be more scientific and shoot the same scene with each lens and see what turns up.

My head hurts.

Mike

Mike,

The above statements are contradictory.

The histogram represents black level on the left and white level on the right. The bit in between represents all the colours between black and white.

The height of the histogram shows each colours saturation and ranges from 0% at the bottom to 100% at the top.

If you have a curve which reaches the top (100%), it shows that the sensor cannot capture any more of that particular colour. In other words it's clipped.

If you have a flat topped curve and it is at the topmost (100%) part of the histogram, you have overexposure. If the curve is flat topped but it doesn't reach the 100% mark anywhere along its length, no problem.

Beg pardon I wasn't clear enough.

I have a curve that never clips at the top but is close to do doing so and it is pretty much the same in the middle 60% of the horizontal axis. The histogram from the 28 mm lens is more consistently higher than the one produced by the Sigma.

So, as I have nothing at the extreme left I have no significant dead black - true? Equally on the right the curve tails off so there is nothing totally (blown) white - true?

But I do have lots of all colours between those two extremes as I have a high but unclipped curve - true?

That said I think you'll agree the photograph is unsatisfactory?

My head still hurts and I do appreciate your help.

Mike

Mike, your wall picture is not over-exposed. There's nothing wrong with it.

I wonder if you need to check your monitor. Or your eyesight.

Seriously.

G

Mike,

Your reading of the histogram is correct, nothing is too black, nothing is too white, and nothing is clipped.

This confirms what I said above, and George has reiterated, there is nothing wrong with the image in that it is not overexposed.

The differences between your Sigma and the Pentax 28mm are acceptible in my opinion. The histograms from both lenses are bound to be slightly different as are the rendered images.

As George says, check out your monitor or view the test images on a different one, it all looks fine to me.

Regards,

Well I agree that both my monitor and eyesight may be at fault - seriously. I have had loads of trouble getting a cheap flat screen to render photographs properly.

I have visited sundry sites that offer subjective calibration from grey scales etc. and while I have the monitor set up as well as I can I'm still not convinced it's right.

As for own eyes they're not getting any younger!

Is this one too dark then?

http://www.flickr.com/photos/9890138@N08/

Looking at that wall picture again I suppose it's sort of OK but I'd expect it to be darker and I'm sure it lacks contrast - I have a bad feeling my ignorance is part of the problem here.

I am very grateful for all you guys taking time to help!

Mike