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12-14-2008, 03:52 PM   #1
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K20D metering differently depending on lens?

I am wondering if this is normal. In another thread with some pictures posted, one member said something must be wrong with my camera.

I have my camera on a tripod looking at toy figure with a white piece of paper as a background. f/5.6, ISO 1600. When pressing the focus button 1/2 way, I get the following different shutter speeds depending on the lens:

25 = smc DA f/3.5-5.6 18-55mm
20 = smc DA* f/2.8 16-50mm
13 = smc FA f/1.4 50mm prime

Taking the actual picture, 20 seems correctly exposed. 13 is over, 25 is under. This is similar spread to all photos I have taken with these three lenses.

Thanks for any input!

Jeremy

12-14-2008, 03:58 PM   #2
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QuoteOriginally posted by jeremy_c Quote
I am wondering if this is normal. In another thread with some pictures posted, one member said something must be wrong with my camera.

I have my camera on a tripod looking at toy figure with a white piece of paper as a background. f/5.6, ISO 1600. When pressing the focus button 1/2 way, I get the following different shutter speeds depending on the lens:

25 = smc DA f/3.5-5.6 18-55mm
20 = smc DA* f/2.8 16-50mm
13 = smc FA f/1.4 50mm prime

Taking the actual picture, 20 seems correctly exposed. 13 is over, 25 is under. This is similar spread to all photos I have taken with these three lenses.

Thanks for any input!

Jeremy
I suspect that the differences are due to the changing field of view. The white background will cause things to be underexposed as the camera tries to average out the whole scene. If you want to do this, try it with spotmetering on the toy figure. If you really suspect that the lenses are not exposing identically, switch the camera to Manual exposure, and set it once, then take a shot with each of the three lenses. The exposures should be identical, if you set the lens' f/stop to the same value.
12-14-2008, 04:02 PM   #3
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I will do that. I did set each lens to 50mm, I did not mention that in my previous post. My biggest concern though is the metering of the camera, not the lens. For instance, I can snap a picture w/the 18-55 and it will almost always be underexposed. When using that lens I've just gotten in the habit of giving it +1EV. This was just a more specific test for the purpose of asking this question.

In a totally different setup, you can see the exact same outcome. I've posted 3 pictures through these lenses (on Av) in the post: https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/43739-50mm-fa-...m-da-pics.html

Thanks for any further input.

Jeremy
12-14-2008, 05:49 PM   #4
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I think you got a pretty good response from your first thread on this subject. I can tell you that I've experienced lens-to-lens exposure variations since I started shooting in '78. Some of the differences are due to FOV, but some are due to . . . . . something else.

About a year ago, I bought a DA 18-250. This lens invariably renders images about a half stop brighter than my other Pentax zooms - on both my K10D and DS2. I have no idea why such is the case; doubtless, the those more technically astute than I can explain why.

Jer

12-14-2008, 07:37 PM   #5
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OK, I see several previous threads, each discussing a slightly different issue.

1) It is absolutely true that setting a zoom lens at 50mm won't necessarily yield exactly the same field of view as using a 50mm lens. Nor will two different zooms lens set to 50mm yield the same results results as each other. Nor will the field of view you get when focused 10 feet away be the same as what you get focused at infinity. It's just a fact of life that this will vary. This was the bottom line from one of your threads.

2) It is absolutely true that if you meter a scene using two different angles of view, you will very possibly get two different meter readings. If one lens shows more of a bright background in the scene, the camera would not be doing its job if it did not suggest a higher shutter speed to compensate. This was the bottom line in another of your threads.

3) These two factors mean it is difficult to conduct a controlled test of exposure between lenses. The best ways to do that would be using a gray card and spot metering or a plain featureless and evenly-lit wall, but as far as I can tell, you haven't tried that. Based on what you've posted thus far, I don't think there are any conclusions to be made aside - too many uncontrolled variables.

4) It is senseless to talk about the "correct" exposure for any given scene. there is no such thing. There is always a range of potentially acceptable exposures. Expecting to always get the same results - and expecting it to alwys match what you happen to *want* - is quite simply unreasonable. If in your opinion 1/20 produced the best exposure, then having one half a stop higher and one half a stop lower is completely within the bounds of the sort of normal variation that one might expect, and wouldn't in itself be anything to be concerned about.

5) In one thread I you posted a picture that was supposedly made by the kit lens with +1EV exposure compensation but was nonetheless much darker than the pictures from the other lenses. Even despite my comments above, if that picture really was taken with +1EV compensation, something *is* wrong. Although it's fashionable to claim a little underexposure with Pentax in general and the kit lens in particular, the reality is that there is no actual problem - just normal variation within the bounds of what might be considered acceptable. if the picture you posted really was taken taken at +1EV, that would clearly outside the bounds of propert exposure, and would make only the second example of this I've seen after hundreds of claims of underexposure over the years that turned out to actually be without basis.

So if I were you, I'd concentrate on quantifying this more. Do the best controlled test you can - with a gray card or featureless wall to eliminate the effect of changing angle of view - and see if the kit lens really is over a full stop away from what it should be.
12-14-2008, 08:00 PM   #6
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QuoteOriginally posted by Marc Sabatella Quote
So if I were you, I'd concentrate on quantifying this more. Do the best controlled test you can - with a gray card or featureless wall to eliminate the effect of changing angle of view - and see if the kit lens really is over a full stop away from what it should be.
Thank you for the very detailed answers. About the kit lens, it was taken +1EV. In fact, when I put it on the camera, I always add +1EV and my pictures turn out properly exposed *most* of the time. Some times it's still under exposed a little.

I do not have a gray card. Will something else suffice? I would like to know if something is wrong or it is just my wrong doings. However, now that I purchased the 16-50 DA*, I don't think I'll be using my kit lens at all. It works great and exposes pictures correctly with no alterations.

Thanks,

Jeremy
12-15-2008, 06:52 AM   #7
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Hello!
Well, with the risk of starting a bad habit of mine, I'll again give 'my' standard answer: you suffer from the k10/k20 focus screen non-linearity...

What I mean is that this screen does not react in a linear way to the lens' aperture.
This is easily experienced when using m42 or PK-M lenses : going from f/2.8 to f/4 does not double the exposure time.

And this behavior thus has effect when using different PK-A lenses : a f/2.8 lens won't give the same exposure time of a f/4 lens, at the same aperture...

Only thing that bother me in your measures is the fact that the 18-55 is darker than the other two...
My measures did show that f/1.4 is indeed "brighter" than f/2.8, and slightly "darker" than f/5.6 (by "brighter" and "darker", I mean that the results are brighter or darker than what would have been obtained by adjusting both speed and aperture accordingly).

A LL60 screen lessens this behavior and give nearly identical results between my FA50 and DA18-55...
12-15-2008, 07:43 AM   #8
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This brings up a question I have been wanting to ask. I believe that f-stop is a mathmatical ratio of focal length to aperature size if i remember correctly. No how does filter size (ie front objective diameter) affect light gathering ability of a lens. I have read somewhere about another number to reference lens speed refered to as a T number. In ther reference I read the discussion was about a mirror lens listed as an f8 with the depth of field of an f8, but actually metered more like a f11. I have been wanting to try an experiment checking the exposure on different lenses of the same focal length and f-stop and check and see if there may be a difference in metering due to different filter sizes.

12-15-2008, 08:19 AM   #9
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Almost every lens i own meters differently, well the camera meters using that lens anyway. My Tamron underexposes by 2/3 stop, the DA lens by 1/3 stop and the FA lenses 1/2 stop, then a further 1/3 to 1/2 a stop underexposed if shooting wide open.
12-15-2008, 10:32 AM   #10
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QuoteOriginally posted by jeremy_c Quote
I do not have a gray card. Will something else suffice?
As I said, a featureless and evenly lit wall also makes a good exposure target. You want something that is exactly the same everywhere across the whole viewfinder, so there is no chance that a difference in framing from one shot to another is causing the difference. A dark figure against a white background is about the worst possible test target, since the exposure will depend entirely on the exact ratio of figure to background, and this will change if the angle of view is even slightly different.

Also, it is normal for a lens to underexpose wide open - most lenses are "optimistic" about their stated maximum aperture. So do the test at f/8.
12-15-2008, 05:04 PM   #11
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QuoteOriginally posted by shuie Quote
This brings up a question I have been wanting to ask. I believe that f-stop is a mathmatical ratio of focal length to aperature size if i remember correctly. No how does filter size (ie front objective diameter) affect light gathering ability of a lens. I have read somewhere about another number to reference lens speed refered to as a T number. In ther reference I read the discussion was about a mirror lens listed as an f8 with the depth of field of an f8, but actually metered more like a f11. I have been wanting to try an experiment checking the exposure on different lenses of the same focal length and f-stop and check and see if there may be a difference in metering due to different filter sizes.
I suspect lens collimation has more of an influence then filter size. Fast primes shot wide open may have light beam characteristics that can disturb a meter. Most likely any minor deviation from perfect will not be perceived in the viewfinder but seen by the meter. Not to mention contrast has an influence as well. As to "A" or better lenses the camera meters "wide open" (normally) and interpolates all exposures from that. A small timing error or non-linearity of an aperature mechanics can change exposure greatly.
Personally none of these "issues" will disappear till Pentax goes to an all electronic mount and more precise aperture blades.
Should add ....and increase the density of the metering cell...

Last edited by jeffkrol; 12-15-2008 at 05:27 PM.
12-15-2008, 07:33 PM   #12
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QuoteOriginally posted by shuie Quote
This brings up a question I have been wanting to ask. I believe that f-stop is a mathmatical ratio of focal length to aperature size if i remember correctly. No how does filter size (ie front objective diameter) affect light gathering ability of a lens. I have read somewhere about another number to reference lens speed refered to as a T number. In ther reference I read the discussion was about a mirror lens listed as an f8 with the depth of field of an f8, but actually metered more like a f11. I have been wanting to try an experiment checking the exposure on different lenses of the same focal length and f-stop and check and see if there may be a difference in metering due to different filter sizes.
Actually what you state is true, but when you add in the front filter, it only holds true for long lenses. on wider lenses the filter grows disproportionally large in order to avoid vignetting due to the very wide field of view. The lenses are designed to take a filter without serious vignetting.

Now back to the OP and his question. As Marc has suggested, teh only way to really compar exposure of lenses is to use a uniformly lit uniform surface. I don't use a grey card, but I use paved surfaces, block walls etc. I have checked every one of my 9 primes and 7 zooms, with the zooms at both max and min focal length. I have done this on both the K10D and my *istD.

What have I found?

Aside from the K10D's rather interesting behavior with manual apature lenses, and a tendancy to over expose with teleconverters, I have found my Tamron 28-75 F2.8, and my SMC-P 50mm F1.4 both seem to have a slight error in apature, leading to a gradual over exposure as you stop down, with minimum apature being +1/2 stop,

it is, in my view, a necessary function to test exposure on each of my lenses on each body, so I know what to expect. Out in the field is not the place to discover an inherent exposure error.
12-15-2008, 07:57 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
it is, in my view, a necessary function to test exposure on each of my lenses on each body, so I know what to expect. Out in the field is not the place to discover an inherent exposure error.
So, you are to expect variations in lenses and the metering of one camera? Thus something is not totally wrong with my camera? I have yet to shoot a uniform setting for comparison sake, but I hope to have time to do that tonight yet.

Jeremy
12-16-2008, 01:07 PM   #14
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QuoteOriginally posted by jeremy_c Quote
So, you are to expect variations in lenses and the metering of one camera? Thus something is not totally wrong with my camera?
Yes, any number of things can cause variation, as I and others have detailed. From what you've posted thus far, the only thing thing that seems potentially problematic was the one picture you posted from the kit lens allegedly at +1EV and still quite a bit darker than the others. But one shot does not prove a problem.
12-16-2008, 01:21 PM   #15
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QuoteOriginally posted by Marc Sabatella Quote
Yes, any number of things can cause variation, as I and others have detailed. From what you've posted thus far, the only thing thing that seems potentially problematic was the one picture you posted from the kit lens allegedly at +1EV and still quite a bit darker than the others. But one shot does not prove a problem.
The EXIF information should be intact, you can easily see it was indeed +1EV. I *have* to shoot +1EV on almost all shots w/the 18-55. It will be very dark with out it. This is not alledged, this is fact. I may not understand some other aspects before they were explained here, but I do know that *any* shot I take with the 18-55 is very dark. I always have to add +1EV to make it even close to normal. Indoors, outside, a book, a sky, anything I've ever shot with it needed +1EV. Others as well have said the 18-55 is dark, some have had to add +0.7EV. I don't think anyone said they need the +1EV that I do, but a few have the exact problem I do with the 18-55.

Jeremy
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