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05-16-2011, 06:42 PM   #1
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under-exposure problems

hi guys,
I'm am unhappy photographer. I own 2 pentax cameras a K10D and a K7. The K7 was purchased Aug 2010 in the hope of solving the apparent underexposure issue with the K10D. The lens is a Sigma DC 18 - 200, purchased new by myself.

I VERY rarely use auto mode as 80% of photos will turn out under exposed even when the exposure meter is "centred" for correct exposure.
As a result I always use manual mode and find I am usually setting exposure to ~ + 1 - 1.5 stops. to get a respectable amount of exposure.
I put it down to some fault in the K10D and after only owning it just on 2 yrs splurged out on the K7 in the hope of solving the issue. NO such luck. I have the same issue with the K7

I dont get it... what's the point of an exposure meter if its always reading significantly low ?? Or am I doing something wrong ?
Surely the meter is accurate to at least a 1/3 stop or thereabouts ?

In all other ways the cameras work well. The softer focus with the K7 pix I have yet to narrow down to the camera or the lens or a mix of both.
My local camera shop has offered for me to bring my camera in and take a set of pics with my lens and with a new lens ... go home and have a look at any differences in focussing. .... I find I have to sharpen the majority of the pix that come off the K7

thanks for your time

Dave

05-16-2011, 06:47 PM   #2
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Can you post any examples? I've found I need to adjust EV comp not just according to scene, but also according to lens. So my DA21, DA40 & DA70 get +1/3 stop, whilst my FA31, DA15 & FA77 get 0. No idea why that is, but maybe your sigma lens is a "dark" lens that has overly ambitious aperture claims.

The other possibility is that your eye just prefers over exposed photos
05-16-2011, 06:58 PM   #3
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The aperture control of lenses are calibrated differently. Some lenses need some EV compensation to be dialed in. In your case just dial in a positive EV compensation of +1 and leave it at that. Some lenses even vary over the range of F-stops so that the appropriate EV compensation value differs over the range. Thats's harder to deal with.
05-16-2011, 07:14 PM   #4
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QuoteOriginally posted by twitch Quote
Can you post any examples? I've found I need to adjust EV comp not just according to scene, but also according to lens. So my DA21, DA40 & DA70 get +1/3 stop, whilst my FA31, DA15 & FA77 get 0. No idea why that is, but maybe your sigma lens is a "dark" lens that has overly ambitious aperture claims.

The other possibility is that your eye just prefers over exposed photos
hi twitch,
thanks for responding...
yes I can... will do so when I get home from work, see if I can find some that I havent already adjusted else will take a few more pix

as to your second part ... no not my eye .... these pics are usually really dark!!

Dave

05-16-2011, 07:20 PM   #5
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QuoteOriginally posted by Ole Quote
The aperture control of lenses are calibrated differently. Some lenses need some EV compensation to be dialed in. In your case just dial in a positive EV compensation of +1 and leave it at that. Some lenses even vary over the range of F-stops so that the appropriate EV compensation value differs over the range. Thats's harder to deal with.
greetings Ole,
thanks .... now thats an interesting comment. I have learnt something!

I was sorta under, what now seems a misunderstanding, that the exposure meter would just take the light coming onto it via the lens and give the appropriate exposure reading. That is if you then adjusted aperture/shutter speed/ISO accordingly to give the centre reading of correct exposure. Then any effects the lens had would have already been taken into account ?

Dave
05-16-2011, 07:33 PM   #6
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Question: How are you determining it's underexposed, and compared to what? (It's possible the lens and camera just aren't talking to each other right, but let's start from what you're seeing.) Chances are, of course, that the same problem with the same lens and two cameras is either the lens or you, though. First thing to do is clean those contacts on that lens, just for form's sake if nothing else.

If you're in manual, what are you metering on and in what mode?
05-16-2011, 07:51 PM   #7
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Hi,
Thanks

QuoteOriginally posted by Ratmagiclady Quote
Question: How are you determining it's underexposed, and compared to what?
well I take a pic and look at the pic on the display and its really dark, so I may go through 1 or 2 more shots of the scene, decreasing shutter speed or increasing aperture (or both) till I get an acceptable image.
getting home and looking at the images on the computer only confirms the under-exposure of the initial and sometimes second shot.

QuoteQuote:
(It's possible the lens and camera just aren't talking to each other right, but let's start from what you're seeing.)
Yes that thought has crossed my mind that it may be the lens

QuoteQuote:
Chances are, of course, that the same problem with the same lens and two cameras is either the lens or you, though. First thing to do is clean those contacts on that lens, just for form's sake if nothing else.

If you're in manual, what are you metering on and in what mode?
To answer that I would have to check the settings. Off the top of my head I suspect I'm spot metering rather than multipoint and I rarely change that setting.

Any thoughts on my response to Ole's comments on how the metering works ?
I would have thought (maybe incorrectly) that if the lens is a little "dark" to use one of the other guy's comments, then that would obviously result in lower light falling on the meter sensor. Therefore if I adj shutter and or aperture to bring the meter up to central correct setting then the "dark" issue should be taken care of. ? I shouldnt have to add yet more + exposure to bring up the brightness ?

Cheers
Dave
05-16-2011, 08:28 PM   #8
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QuoteOriginally posted by davenn Quote
greetings Ole,
thanks .... now thats an interesting comment. I have learnt something!

I was sorta under, what now seems a misunderstanding, that the exposure meter would just take the light coming onto it via the lens and give the appropriate exposure reading. That is if you then adjusted aperture/shutter speed/ISO accordingly to give the centre reading of correct exposure. Then any effects the lens had would have already been taken into account ?

Dave
The exposure meter measures the scene at full aperture opening. The camera then calculates the proper exposure at the aperture you have set (e.g. f/11). If the aperture mechanism is not perfectly calibrated, you get an exposure error.

We recently tested two 85 mm lenses, a Sigma and a Vivitar. The aperture mechanism on the Sigma was perfect; the Vivitar was not and the error increased as the lens was stopped down.

The bottom line is that you need to learn your lenses and their quirks and set the EV compensation accordingly.

05-16-2011, 09:42 PM   #9
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thanks so much Ole

Of course .... the lens aperture only gets stopped down to whatever I set it at right at the time of exposure !! DOH forgetting the basics in my old age

Dave
05-16-2011, 10:17 PM   #10
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Posted examples of the types of scenes you're getting underexposures would definitely help.
My old K20D would regularly need +0.7EV with the kit lenses, but 0EV was fine with the FA limiteds. This also occurred with flashed exposures. Now, it also depends on the scene you capture. Bright scenes in general are likely to be metered by the camera to produce a slightly underexposed final result, since it tries to average out light intensity according to your metering mode (which is another source of 'exposure problems' people often report).
05-17-2011, 12:02 AM   #11
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If you are having the same problem with two different bodies then it's unlikely to be anything to do with the bodies themselves. Are you using the same lens(es) on both? Obviously it's the same finger on the shutter...

Do you check the histogram? Have you done some sort of basic calibration of your monitor at home?
05-17-2011, 02:49 AM   #12
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We're all shooting in the dark here without sample images with exif intact. In the meantime, what does the meter indicate when you follow the sunny 16 rule? Also, here's a good site that explains why you shouldn't be too reliant on a camera's light meter: Ultimate Exposure Computer
05-17-2011, 03:14 AM   #13
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Are images underexposed when the aperture is wide open?
Are they underexposed when using a different lens?
05-17-2011, 04:29 PM   #14
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QuoteOriginally posted by davenn Quote
Hi,


To answer that I would have to check the settings. Off the top of my head I suspect I'm spot metering rather than multipoint and I rarely change that setting.
This is likely your problem.
If you are spot metering, you pretty much always have to adjust exposure from what the spotmeter reading gives you.
Your spotmeter thinks everything is a gray card.
If you meter something brighter than that, the meter will knock the exposure back until it has made whatever you've metered 18% gray.
05-18-2011, 09:08 PM   #15
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Thanks for everyone's responses ...

I will do some pics this weekend, by the time I get home from work on week days the sun has pretty much set. Im not sure how I can resize pics to make them easily viewable without wiping the EXIF data

@ Timh ... if its a bright sunny day no, they will be well overexposed assuming an avg shutter speed (~ 250th sec).

@ Wheatfield ... ok ... so what do most people use for general outdoor scenery pics ....
spot or multipoint metering ?

@ Ash ... ok you are indicating similar thoughts to Ole, re not all lenses are created equal and having a permanent +EV compensation setting may well solve my probs.

I currently only have one fully auto lens, the one mentioned in my first post. It pretty much lives on the camera and only comes off when I wanna use the very old manual 400mm and x2 teleconverter for doing sun and moon images.

I will add a comment ..... This is a thing that only really hassles me when doing outdoor pics under moderate to good daylight conditions. And the camera exp meter is telling me that the exp is setting is good but I get dark pix. Its not a prob when doing indoor portrait experiments where I have respectable flash and softboxes etc that result in a well lit subject.

cheers
Dave
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