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07-09-2011, 04:39 AM   #16
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QuoteOriginally posted by RonHendriks1966 Quote
Yes it is, but it isn't always what is stated on the sticker of the lens:

DxO measures it with lenses. Non of the two lenses are tested, but here is one Zeiss: DxOMark - Carl Zeiss Distagon T 28mm f/2 ZE Canon Look at measurements and transmission.
I am afraid no. Aperture is a determined by two physical measures: focal length/diameter of the physical aperture. T-stops (transmission) are a measurement of the light transmitted.

07-09-2011, 05:55 AM   #17
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QuoteOriginally posted by wildman Quote
Perhaps I'm being simple-minded about this but...

...is not aperture an absolute measurement of light?

If both lens' are set at the same marked aperture but they do not deliver the same amount of light to the sensor then they are not at the same effective aperture because they are not transmitting the same amount of light.

It has nothing to do with efficiency - f4 is simply f4 a certain absolute quantity of light. If for instance the Pentax when set at f4 is, in fact, a true f4 at that setting and any other lens transmits more or less than the Pentax then that lens is operating at some aperture other than f4.

If you want to test this just set up your camera on a tripod and plug in different lens' set at the same f stop under the same light. If using AV for instance they should all give you the same shutter speed all else being equal. I just did this with my 6 modern lens' and this was true.

BTW in my test. for instance, was included the Zeiss 50mm f/1.4 ZK and a version 1 kit lens and they both gave me an identical shutter speed of 125/sec with this particular test.

By definition two different lens' can not be at the at the same f stop and give two different levels of light.
I am going to do some blatant plagiarism here (with attribution) .... so from
QuoteQuote:

f Stops - The f-number (f/#) is often notated as N and is given by
N = f/D
where f is the focal length, and D is the diameter of the entrance pupil. By convention, "f/#" is treated as a single symbol, and specific values of f/# are written by replacing the number sign with the value. ......

T Stops - Since all lenses absorb some portion of the light passing through them (particularly zoom lenses containing many elements), T-stops are sometimes used instead of f-stops for exposure purposes, especially for motion picture camera lenses. The T in T-stop stands for transmission.[5] The practice became popular in cinematographic usage before the advent of zoom lenses, where fixed focal length lenses were calibrated to T-stops: This allowed the turret-mounted lenses to be changed without affecting the overall scene brightness. Lenses were bench-tested individually for actual light transmission and assigned T stops accordingly.
Now some of that long ago forgotten optics engineering class is coming back ....

f Stops and T Stops are equal if the lens' transmission efficiency is perfect or at 100%. If there is some level of absorption by the glass, then f Stops and T Stops diverge. Also, f Stop effectively normalizes any difference in focal length across the lenses being appraised.

Now a new source to plagiarizer from ...
QuoteQuote:

T-stops - In practice, even the best lenses exhibit light absorbance, effectively “stealing” some of the light going through them. This means that if you calculate the exposure based on the f-stop of the lens, you will end up underexposing the image, because less light is reaching the film plane than is expected in theory. T-stops are the f-stop of the lens corrected for its absorbance and reflectance. The T-stop is the true speed of the lens, calculated by compensating for its light absorbance and reflectance, and will result in accurate exposure.

Lens speed - The speed rating of a lens, expressed as an f-top or T-stop, is its maximum aperture. It is known as “speed” because it affects how long it would take to achieve correct exposure on a film of a given ISO rating. Fast lenses have a large maximum aperture (e.g. f2.0) and allow a faster exposure time for a given film sensitivity (ISO rating).
I ran across this explanation also...Then I ran across this, which might explain wildman's experience with his Zeiss ZK 50/1.4 where Zeiss is leveling the T Stops across their newer lens. My Contax/Zeiss is 15 to 20 years old, and who knows what their aim at that time was....Wildman effectively observes, that essentially the light meter should measure and negate any differences between lenses with respect to light transmission efficiency for the indicated f Stop being used.

I have been thinking about this. At first, I agreed, but then after more thought, f Stop does not specify the physical dimensions of the aperture - i.e., the absolute physical diameter of the aperture's opening, but the ratio of the focal length to the physical size of the front lens element, thereby any focal length differences should be negated. So between the two lenses, the physical diameter of the aperture could be different and is probably different. Therefore, if the transmission efficiency of the glass differs between lenses, the camera body's light meter will measure the light coming in (and since in AP mode the only variable able to be set is the shutter speed, aperture and ISO being fixed), the shutter speed will vary.

So if the physical aperture diameter on a lens is larger than that of another lens for a given f Stop, and if the glass of the physically larger aperture lens is more effective in its transmission, then the body's light meter will register differently and produce a difference in shutter times - i.e., a shorter shutter time.

... so that's as far as I have gotten. I can understand some difference, however the observed difference is still much larger than what I expected.


Last edited by interested_observer; 07-09-2011 at 06:07 AM.
07-09-2011, 07:09 AM - 1 Like   #18
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Here' my start on an analysis of the situation:


I stopped at this point because I realized there's a missing piece of data. What the camera thinks is the CZ's minimum f-stop (because the camera thinks the CZ is an A type lens it must assume a minimum f-stop).

These photos were shot in Av mode. The camera thought the CZ photo was taken at f2.4 and the FA photo at f4 according to EXIF's.

The camera's sequence in Aperture priority mode is to assume the lenses are wide open & measure the light then calculate exposure time using the fnumber selected.

The camera knows the FA31 is f1.8 wide open & I'll guess it thinks the CZ28 is f1.2 wide open (that's what it thinks if all the contacts are shorted).

Now things get complicated.

It would be much easier to figure out what's going on if you did some strictly manual tests. Take a shot with the FA then set the CZ to the same f-number, set the camera on manual, and take the photo with the same exposure time. Then we'll know the camera isn't making any assumptions about the CZ.

Why does the camera think the CZ is a A type lens? it is probably making wrong calculations based on that assumption.

At minimum when the CZ is mounted you should turn the ewheel to decrease the f-number shown in the viewfinder to its minimum value - then the camera will not try to close the aperture at exposure time. It may still be off a bit but there'll be an in-viewfinder f-number that will give good results.

Last edited by newarts; 07-09-2011 at 08:50 AM.
07-09-2011, 08:22 AM   #19
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QuoteOriginally posted by wildman Quote
...It has nothing to do with efficiency - f4 is simply f4 a certain absolute quantity of light. If for instance the Pentax when set at f4 is, in fact, a true f4 at that setting and any other lens transmits more or less than the Pentax then that lens is operating at some aperture other than f4...
But when it comes to actual lenses and cameras, there is a considerable amount of vagueness allowed. The openings are circular, aperture blades are some polygon approximation of a circle. The meter doesn't measure light at f4, it measures light wide open and assumes that it can move the aperture lever a certain amount to get f4. One lens here has a converted mount; who knows what difference that makes, but Dave has a good idea in his post.

07-09-2011, 08:50 AM   #20
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I initially thought something like this but when I use my MF lens with no contacts on my K7 in AV mode it automatically recognizes that the aperture cannot be changed in camera. It is metering at whatever light is coming through the lens and stopped down aperture to the camera right?

However the FA lens is metering at 1.8(wide open) then calculating what it needs when it stops down. I could see this causing calculation differences if it actually was 1.9 rather than 1.8 and was then using that data to scale to 4.0. The issue is probably somewhere around the initial metering and scaling with the 31. That is where all the data is collected and calculations are made. Zeiss 28 meter exactly where the aperture will be when the photo is taken, no calculations.

I don't think there is a way to stop the 31 down to 4.0 manually and meter through the reduced aperture is there? I wonder if they would be similar if you did this though.

QuoteOriginally posted by newarts Quote
Here' my start on an analysis of the situation:


I stopped at this point because I realized there's a missing piece of data. What the camera thinks is the CZ's minimum f-stop and the fact the camera thinks the CZ is an A type lens.

These photos were shot in Av mode. The camera thought the CZ photo was taken at f2.4 and the FA photo at f4 according to EXIF's.

The camera's sequence in Aperture priority mode is to assume the lenses are wide open & measure the light then calculate exposure time using the fnumber selected.

The camera knows the FA31 is f1.8 wide open & I'll guess it thinks the CZ28 is f1.2 wide open (that's what it thinks if all the contacts are shorted).

Now things get complicated.

It would be much easier to figure out what's going on if you did some strictly manual tests. Take a shot with the FA then set the CZ to the same f-number, set the camera on manual, and take the photo with the same exposure time. Then we'll know the camera isn't making any assumptions about the CZ.

Why does the camera think the CZ is a A type lens? it is probably making wrong calculations based on that assumption.

At minimum when the CZ is mounted you should turn the ewheel to decrease the f-number shown in the viewfinder to its minimum value - then the camera will not try to close the aperture at exposure time. It may still be off a bit but there'll be an in-viewfinder f-number that will give good results.
07-09-2011, 09:00 AM   #21
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Another take on this:

I remember back in the day with my Rollei and my hand held Weston light meter.

I could tell the meter what film sensitivity I was using and what aperture I wanted to use - meter the scene and it would give the shutter speed.

No reference to what particular lens or camera I was using just a abstract mathematical calculation. All the meter needed was one constant - f stop value.
07-09-2011, 09:02 AM   #22
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QuoteOriginally posted by atlnq9 Quote
I initially thought something like this but when I use my MF lens with no contacts on my K7 in AV mode it automatically recognizes that the aperture cannot be changed in camera. It is metering at whatever light is coming through the lens and stopped down aperture to the camera right?

However the FA lens is metering at 1.8(wide open) then calculating what it needs when it stops down.
Now that raises an interesting question: Mount a non-A-type MFL on the camera, set to any auto mode, and the camera goes to Av. Then it meters the transmitted light. Now mount an A-type or AFL lens and set Av. Does the camera make a calculation based on the signaled aperture, or does it still actually just meter the transmitted light?

I suppose this could be tested by metering with the A-type / AFL, then waxing its contacts so the camera thinks it's non-A-type, and metering again. Something like that.
07-09-2011, 09:09 AM   #23
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QuoteOriginally posted by atlnq9 Quote
I initially thought something like this but when I use my MF lens with no contacts on my K7 in AV mode it automatically recognizes that the aperture cannot be changed in camera. It is metering at whatever light is coming through the lens and stopped down aperture to the camera right?
....
It is "metering at whatever light is coming through the lens and stopped down aperture to the camera right?" yes. That's what it is actually doing but might not be what it thinks it is doing! If it thinks the lens is an A type it will think it is wide open and will do its calculations based on that assumption.

I don't know exactly what a k7 does in Av mode with a preset manual K or m42 lens; a lot depends on whether it thinks it is a A type lens or not. I think you are probably supposed to use m mode & the green button to set exposure with a manual lens.

When you look in the viewfinder you should see F--- or something similar; if you see f2.8 or something similar the camera thinks it is a A type lens and will attempt to close the aperture to f2.8 at exposure time. Clearly this leads to exposure errors.

OP's EXIF data shows the camera thought the CZ lens was f2.4 which implies it thought it was an A type lens (at least that's what my k100d and kx would do).

07-09-2011, 10:34 AM   #24
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I hope the following helps us understand what the camera has to deal with concerning A and non-A type lenses.

The camera cannot tell the difference between an f:1.2-F:22 A type lens in non-A mode and a simple K bayonet type lens or m42 lens with a PK adapter. That's because in all cases all of the contacts are shorted.

But the exposure situation is different for each type lens; the A and K lenses have levers which do different things when moved (proportional to f stop or f stop squared) while the m42 lens has no lever at all but may be preset to an actual aperture.

Say I put an a f1.2-22 A type lens on with the A switch off - this lens looks identical to an smc m f1.2-22 lens... there is no way the camera can tell the difference. If the camera measures the exposure wide open then closes the aperture lever at exposure time the smc's exposure must be wrong.

Here's some experiments with a k100D & non-A lens (A lens set to non-A position; f-- shows in viewfinder):

-manual mode; pressing Green Button closes aperture and sets exposure; then closes aperture at exposure time - therefore it doesn't matter if lens is A type in non-A mode or an earlier PK lens - it'll push the lever to stop down whatever lens then set the exposure rather than calculating the exposure from the wide-open measurement.

-manual mode & Green Button also works properly with a preset m42 lens & PK adapter (lens in manual vs automatic mode.)

-Av mode & Green Button does *not* close aperture when it sets exposure and does *not* close the lens at exposure time. This works fine with a preset m42 lens in manual mode because it is already stopped down when the exposure is set.

Last edited by newarts; 07-09-2011 at 10:40 AM.
07-09-2011, 12:10 PM   #25
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QuoteOriginally posted by newarts Quote
Here' my start on an analysis of the situation:

I stopped at this point because I realized there's a missing piece of data. What the camera thinks is the CZ's minimum f-stop (because the camera thinks the CZ is an A type lens it must assume a minimum f-stop).

These photos were shot in Av mode. The camera thought the CZ photo was taken at f2.4 and the FA photo at f4 according to EXIF's.

The camera's sequence in Aperture priority mode is to assume the lenses are wide open & measure the light then calculate exposure time using the fnumber selected.

The camera knows the FA31 is f1.8 wide open & I'll guess it thinks the CZ28 is f1.2 wide open (that's what it thinks if all the contacts are shorted).

Now things get complicated.

It would be much easier to figure out what's going on if you did some strictly manual tests. Take a shot with the FA then set the CZ to the same f-number, set the camera on manual, and take the photo with the same exposure time. Then we'll know the camera isn't making any assumptions about the CZ.

Why does the camera think the CZ is a A type lens? it is probably making wrong calculations based on that assumption.

At minimum when the CZ is mounted you should turn the ewheel to decrease the f-number shown in the viewfinder to its minimum value - then the camera will not try to close the aperture at exposure time. It may still be off a bit but there'll be an in-viewfinder f-number that will give good results.
This does appear to have and excellent basis. The CZ is mounted and displays F-- in the top window. The FA is set to the A position on the lens, and the aperture is dialed in and set at f4. A way to equalize things to a degree is to just take the FA's aperture ring out of "A" and set it to f4 manually.

QuoteOriginally posted by atlnq9 Quote
I initially thought something like this but when I use my MF lens with no contacts on my K7 in AV mode it automatically recognizes that the aperture cannot be changed in camera. It is metering at whatever light is coming through the lens and stopped down aperture to the camera right?

However the FA lens is metering at 1.8(wide open) then calculating what it needs when it stops down. I could see this causing calculation differences if it actually was 1.9 rather than 1.8 and was then using that data to scale to 4.0. The issue is probably somewhere around the initial metering and scaling with the 31. That is where all the data is collected and calculations are made. Zeiss 28 meter exactly where the aperture will be when the photo is taken, no calculations.

I don't think there is a way to stop the 31 down to 4.0 manually and meter through the reduced aperture is there? I wonder if they would be similar if you did this though.
I don't think there is a way to stop the 31 down to 4.0 manually and meter through the reduced aperture is there? I wonder if they would be similar if you did this though. - Yes, this can be done and should level the playing field some, in terms of how the body is handling the light meter measurement.


QuoteOriginally posted by wildman Quote
Another take on this:

I remember back in the day with my Rollei and my hand held Weston light meter.

I could tell the meter what film sensitivity I was using and what aperture I wanted to use - meter the scene and it would give the shutter speed.

No reference to what particular lens or camera I was using just a abstract mathematical calculation. All the meter needed was one constant - f stop value.
That was my initial assumption going into all of this, but I had not counted on how the body handles the initial light measurement - and thus I went to transmission efficiency, which I still believe contributes some to this, but the more I think about it, I am leaning towards newarts' train of thought as the major contributor.

QuoteOriginally posted by newarts Quote
It is "metering at whatever light is coming through the lens and stopped down aperture to the camera right?" yes. That's what it is actually doing but might not be what it thinks it is doing! If it thinks the lens is an A type it will think it is wide open and will do its calculations based on that assumption.

I don't know exactly what a k7 does in Av mode with a preset manual K or m42 lens; a lot depends on whether it thinks it is a A type lens or not. I think you are probably supposed to use m mode & the green button to set exposure with a manual lens.

When you look in the viewfinder you should see F--- or something similar; if you see f2.8 or something similar the camera thinks it is a A type lens and will attempt to close the aperture to f2.8 at exposure time. Clearly this leads to exposure errors.

OP's EXIF data shows the camera thought the CZ lens was f2.4 which implies it thought it was an A type lens (at least that's what my k100d and kx would do).
When you look in the viewfinder you should see F--- or something similar [Yes, this is the case]; if you see f2.8 or something similar the camera thinks it is a A type lens and will attempt to close the aperture to f2.8 at exposure time [No, this isn't the case]. Clearly this leads to exposure errors.

QuoteOriginally posted by newarts Quote
I hope the following helps us understand what the camera has to deal with concerning A and non-A type lenses.

The camera cannot tell the difference between an f:1.2-F:22 A type lens in non-A mode and a simple K bayonet type lens or m42 lens with a PK adapter. That's because in all cases all of the contacts are shorted.

But the exposure situation is different for each type lens; the A and K lenses have levers which do different things when moved (proportional to f stop or f stop squared) while the m42 lens has no lever at all but may be preset to an actual aperture.

Say I put an a f1.2-22 A type lens on with the A switch off - this lens looks identical to an smc m f1.2-22 lens... there is no way the camera can tell the difference. If the camera measures the exposure wide open then closes the aperture lever at exposure time the smc's exposure must be wrong.

Here's some experiments with a k100D & non-A lens (A lens set to non-A position; f-- shows in viewfinder):

-manual mode; pressing Green Button closes aperture and sets exposure; then closes aperture at exposure time - therefore it doesn't matter if lens is A type in non-A mode or an earlier PK lens - it'll push the lever to stop down whatever lens then set the exposure rather than calculating the exposure from the wide-open measurement.

-manual mode & Green Button also works properly with a preset m42 lens & PK adapter (lens in manual vs automatic mode.)

-Av mode & Green Button does *not* close aperture when it sets exposure and does *not* close the lens at exposure time. This works fine with a preset m42 lens in manual mode because it is already stopped down when the exposure is set.
I have been using Av mode exclusively in order to force the body's metering and calculating/selecting the shutter speed. However, this brings up an interesting situation. Trying to remember back 3 months or so, I was more intent on quickly exchanging the lens, while not moving the CZ's aperture setting (or moving the focus), I can't swear that I pushed the Green Button each time.

Also, since I was doing a 5 frame bracket, given that I did press the Green Button to meter, would that provide the base metering value to the entire bracketed sequence - in terms of calculating the shutter times?

Going back to the initial sequence of bracketed shots that I extracted the two images from...

lens......0ev..-.5ev..+.5ev..-1ev..+1ev
CZ 28......4......3........6........2........8 sec
PK 31 ....10....10......20.......8......30 sec
I can make sense of the CZ's shutter times across the bracketed sequence. However, the shutter speeds for the FA are a bit different, I would not have expected the 10 seconds for the 0ev and 10 seconds again for the -.5ev. Same for the +/- 1ev times of 8 and 30 seconds respectfully. There appears to be a clear difference in how the shutter speeds were determined. But all of that may be a red hearing.

This evening, I think I'll trek back up to my vantage point and try some additional exposure sequences - in manual with the FA out of "A" and with the aperture ring set to f4, I'll take some tin foil and short the FA lens to make it appear as close to the CZ as possible.

07-09-2011, 03:02 PM   #26
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Can you explain how this is done? I don't see how it is done with my FA 43. Taking the lens out of a and setting the aperture does not engage the aperture blades when it is mounted on a camera. The lever on the lens is still held open by the camera. Unless I am missing something.

QuoteOriginally posted by interested_observer Quote
I don't think there is a way to stop the 31 down to 4.0 manually and meter through the reduced aperture is there? I wonder if they would be similar if you did this though. - Yes, this can be done and should level the playing field some, in terms of how the body is handling the light meter measurement.
07-09-2011, 03:39 PM   #27
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QuoteOriginally posted by atlnq9 Quote
Can you explain how this is done? I don't see how it is done with my FA 43. Taking the lens out of a and setting the aperture does not engage the aperture blades when it is mounted on a camera. The lever on the lens is still held open by the camera. Unless I am missing something.
I think one can do it by mounting the FA31 partway rather than turning it all the way until secure. Of course, you have to be careful not to jar the camera too much, lest that nice lens of yours falls off; but partially mounting the lens will cause an FA lens to stop down and allow you to meter at that point. I don't have an FA31 any longer to test this, but use the Zeiss 28mm so interested in the results.

You can also use optical DOF preview to do the same as well.

I'm with freewheeler...try shooting both lenses set at f4 and at same shutter speed, and take a look at the exposure differences b/w the lenses using software.
07-10-2011, 07:04 AM   #28
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QuoteOriginally posted by atlnq9 Quote
Can you explain how this is done? I don't see how it is done with my FA 43. Taking the lens out of a and setting the aperture does not engage the aperture blades when it is mounted on a camera. The lever on the lens is still held open by the camera. Unless I am missing something.
You are right, I had forgotten that the body has control over the FA in terms of engaging the aperture blades. I just shoot in in A all the time, and was just thinking in terms of a fully manual lens. Thanks for the correction!

QuoteOriginally posted by taurus9 Quote
I think one can do it by mounting the FA31 partway rather than turning it all the way until secure. Of course, you have to be careful not to jar the camera too much, lest that nice lens of yours falls off; but partially mounting the lens will cause an FA lens to stop down and allow you to meter at that point. I don't have an FA31 any longer to test this, but use the Zeiss 28mm so interested in the results.
I am already afraid that I am going to stumble over the tripod's legs when moving around in front to change the lens, so one more thing to be concerned about - the lens falling off is something that wont happen. However, its a good functional suggestion.

So, I went up last night to shoot some images in manual mode, and low and behold - it was some what of a mixed bag. 1 set of shots at 2x and 2 sets of shots at equal shutter times. Here are the sets. The two images provided below are in red.
lens......0ev..-.5ev..+.5ev..-1ev..+1ev ........ ISO
CZ 28.....1/4.....1/6.....1/3....1/8....1/2. sec ... 400
PK 31 ....1/2.....1/3.......1......1/4.....1 sec ..... 400

lens......0ev..-.5ev..+.5ev..-1ev..+1ev ........ ISO
CZ 28......1......1/2.....1......1/3.....1.5 sec ... 400
PK 31 .....1......1/2.....1......1/3.....1.5 sec ... 400

lens......0ev..-.5ev..+.5ev..-1ev..+1ev ........ ISO
CZ 28.....3......2........4.......1.5......6 sec ... 100
PK 31 ....3......2........4.......1.5......6 sec ... 100
The last two sets are the same. One thing that I did was to turn off/turn on the body between lens changes, so as to force a restart ensuring that when the lens is changed that it goes through the asking for the focal length process. I also ensured to hit the green button before each bracketed sequence.

QuoteOriginally posted by wlachan Quote
Without picture, I vote for user error because 1 stop difference is just too much. When testing lenses, you don't rely on Av mode, but have to try different shutter speeds for identical exposures at each f-stop. Another possibility is that the apertures of the Zeiss and the FA31 were calibrated to the opposite end of tolerance. I have seen aperture differed as much as 0.5EV, but not 1EV.
Does this beg the question that it all is chalked up to user error, due to an inappropriate selection of shooting in Av originally ? That may be a good assessment here. I don't know. What I should probably do is shoot both modes again.

Anyway, here are the images. Its interesting to note, that even with equivalent shutter speeds, given an equal set of apertures (f4) and ISO (400 - in this particular case), the FA is brighter, and the CZ still appears to be some what underexposed (even though it is a more accurate representation of the actual lighting conditions) in comparison. With the shutter speeds now equalized for the most part, that questions the original premise of Lens Efficiency.... Is it a matter of mode selection and how the body treats the mode vs metering, or user error, or a combination of both.

Bottom line - I like both lenses - regardless.

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07-22-2011, 03:03 PM   #29
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The camera meters wide open and then guesses that the stopping down process is linear/corrects for known non-linearities. As I recall, for some Pentax lenses the camera can adjust for non-integer steps between stops. This means any off brand lenses won't meter as well. The only way to compare transmission effects, without aperture affects confounding your results, will be to shoot wide open. This way you will be able to isolate the transmission properties of the glass. I'd be surprised if there was any difference at all. Someone further up the thread mentioned FOV...you have a load of sky in your scene which will change metering from 31-28mm. Dont think of the step as being 3mm, rather 20% (1.1^2) in terms of area and therefore potential differences in light being gathered.
07-22-2011, 09:06 PM   #30
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I would think that light transmission could easily be different in one brand of glass to another. The f stop is a ratio of the aperture to the length. Putting a neutral density filter on a lens doesn't change the aperture but certainly changes the exposure. When doing some basic research on windows I was considering for a passive solar sunroom I planned to build ( I never did) , there were huge differences in the efficiency of different brands of glass. Why wouldn't lenses have these differences? The lens coatings will likely affect light transmission also.
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Rare Carl Zeiss Jena STASI lens set for M42 Pentax mt. netuser Pentax SLR Lens Discussion 6 02-18-2010 06:07 PM
Zeiss lens to a pentax mount? Is there a way? Nubi Pentax SLR Lens Discussion 4 10-04-2009 01:12 PM
What Zeiss lens in Pentax mount? FLASH Pentax SLR Lens Discussion 8 11-18-2007 06:27 PM



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