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06-07-2010, 04:20 AM   #1
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77ltd underexposing on k10d

I recently purchased a 77ltd and have found that when using the green button the images tend to be underexposed. Is this normal for this lens.

Any comments would be much appreciated

thanks

06-07-2010, 04:57 AM   #2
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I always found that the K10 couldn't give an accurate exposure no matter what lens was on it.
06-07-2010, 05:05 AM   #3
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I was going to say the same thing. The K10D (which I have one of) always underexposes everything. Learn to compensate and you'll be fine.

A lot can depend on metering modes, scene etc.
06-07-2010, 05:24 AM   #4
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Having a K10D and a bag full of manual lenses, I got intoa practice that every photographer should do.

I check the exposure accuracy of every lens I have, on every body. (Let's imagine I do this, I am a little behind since I got my K7)

My favourite target is the block wall on the south side of my house. It is uniformly lit on sunny days, and is a good big constant color surface that I can use with every lens to check the accuracy of the lens with the camera.

I shoot using green button, or Av mode, depending on lens (K or M42 or KAxx) and then measure and plot the greyscale histogram value of the central 10% of the frame, in a spreadsheet.

Those who have viewed threads on the K10D metering with K mount lenses will have seen my graphs posted before.

The reason I do this is because I want to know how every lens behaves, and I then use that information to adjust the camera settings to get as close to perfect as possible.

I have found, for example, my tamron 28-75F2.8 starts about 1/3 stop over exposure, and drifts up about an additional 1/2 stop over the range F2.8 to F32. I can then make a decision to split the difference and set exposure compensation to -1/3 or -1/2 stop, and have very close to perfect (+/- 1/3 stop) over the entire range.

WHile tests are boring, and measurement tedious, the 1/2 hour per lens spent on understanding exactly how your equipment performs is worth the time. At the end of the day, one of the first rules of photography is "Know your equipment"

PS, on the lens what does the aperture report as in the EXIF data. One possible thing is that the 77 is F1.8 and the coding of apertures for pentax through the lens contact pattern is F1.7 or F2,

Although the dfifference of F1.7 to F1.8 is minimal, this can also be part of the error.

06-07-2010, 06:17 AM   #5
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Thanks for all the comments. Now that I know it is not a fault with the camera. I will compensate as necessary.

Thanks again
06-07-2010, 07:49 AM   #6
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Lowell, Could you please elaborate on the prodecure to "measure and plot the greyscale histogram value of the central 10% of the frame". For example, what exactly is a grayscale histogram. The software I use has luminosity, R, B, and G histograms. Do you convert the image to black and white and then use one of the above histograms? Also, what is the "histogram value" that you use to determine the correctnes of the exposure.

I use ACDSee Pro 2.5 and Corel Photo Paintshop Pro X2 software.
06-07-2010, 08:32 AM - 1 Like   #7
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QuoteOriginally posted by wiseman Quote
Lowell, Could you please elaborate on the prodecure to "measure and plot the greyscale histogram value of the central 10% of the frame". For example, what exactly is a grayscale histogram. The software I use has luminosity, R, B, and G histograms. Do you convert the image to black and white and then use one of the above histograms? Also, what is the "histogram value" that you use to determine the correctnes of the exposure.

I use ACDSee Pro 2.5 and Corel Photo Paintshop Pro X2 software.
I use PSP X3 (although the features I use have not changed since PSP X)

I have the histogram displayed in it's own window the side pallet with the layers etc.

In PSP X? you have the option with a check box to have the histogram for the whole frame or the selection. I check selection, and then with a rectangular selection tool, select a section at about 1/3 height 1/4 width (assume horizontal section) in the middle of the frame, representing about 10% (+/- error) of the frame.

I would imagine in your software this is luminance, because in PSP I get options for the three colors, lightness, saturation, and hue as well as greyscale.

This gets the measurement, part done, then in an excell spreadsheet I enter aperture and greyscale reading, (x vs y chart) with the aperture on a log scale.

What I have found is that correct exposure is in the order of 110-120 greyscale, and each stop with neutral contrast and saturation (i.e. all jpeg settings at zero) is in the order of 45 grey scale.

You coud use the whole image value but then there will be slight errors perhaps due to vignetting if it occurres.

If you do the test with boosted contrast you will exagerate the errors, if any, so I stick with neutral.

If you find a lens is several stops over or under (like some M42 lenses) I run 2 tests, the first with no compensation, the second with exposure compensation where I think I may wish to set it for the lens, and then letting the remainder of the results go where they want but usually being averaged at equally split above and below the middle..

This basic test method can also be used to check for vignetting, by selecting the center 10% and then taking a selection of 5-10% of the frame at each corner. It can also be used to check the exposure lattitude of the sensor, bu keeping the aperture fixed, and changing shutter speed above and below the metered value in .5 or 1 stop increments, and then plot out greyscale vs shutter speed (on log plot) to see the dynamic range of the sensor, with the exposure settings.

I did this on my *istD and found that as stated above between about 25 and 225 greyscale there are about 4 1/2 stops where the greyscale value changes linearly with exposure value, then above 225 and below 25 the next stop changes greyscale by about 15, then the next stop (above 240 or below 10) changes by 7, the next stop by 3 and the last by 1. It is an interesting curve to test for.

Repeating this with contrast at max and min changed really only the middle section, adding 1/2 stop of range at minimum and taking away 1/2 stop at maximum contrast.

On the K7 the highlight preserve and shadow detail preserve added an additional stop above and below the mid point in the linear section respectively so that with highlight and shadow detail preserve you can get about 7 stops of linearly changing exposures in the midrange of the image.

Last edited by Lowell Goudge; 06-07-2010 at 08:39 AM.
06-08-2010, 02:34 PM   #8
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Thanks Lowell for the further explanation.

This cannot be done in my ACDSee Pro2.5 which is what I am more familiar with.

In PSP X2 I was able to follow your steps exactly. I did not realize all of the extra information that PSP had in their histograms! Does the use of the grayscale histogram cancel out any differences which might appear from "targets" of different colors or white balance errors?

Thanks again for your help. This is very interesting and I hope to try some of these plots when time permits.

06-09-2010, 04:35 AM   #9
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QuoteOriginally posted by wiseman Quote
Thanks Lowell for the further explanation.

This cannot be done in my ACDSee Pro2.5 which is what I am more familiar with.

In PSP X2 I was able to follow your steps exactly. I did not realize all of the extra information that PSP had in their histograms! Does the use of the grayscale histogram cancel out any differences which might appear from "targets" of different colors or white balance errors?

Thanks again for your help. This is very interesting and I hope to try some of these plots when time permits.
Thanks I have done the test so many times with different lenses that it is easy to describe.

With respect to greyscale, color targets and WB, I believe they are unrelated.

Greyscale is simply the average illumination, similar to the histogram displayed on the camera back (unless you look at the separation of colours)

for WB correction in PSP, what I do is select the colour balance option under adjust.

there, there is a selection window that allows you to pick a pure colourless surface, this can be any shade of grey from pure black to pure white, for which there should be (in your mind) no apparent colour. Once you select this, the program makes an adjustment to pure white or shades of grey using what ever colour shift was found in the sample and applying it across the entire frame/selection. I find it a little cold in terms of overall appearance so I generally reduce color temperature a little when I do this
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