Why is my FA31mm 1 stop brighter

pcarfan · 05-09-2010, 03:44 PM

The first shot is taken with the FA*28-70/2.8 at 31mm at f11 and 1/25.

The FA31 with the same exposure settings (iso100, f11, 1/25). The light was the same and yet the image is overexposed. (histogram shows a 1-stop over exposure)

Then I took the image with the FA31 at 1/50 and this time the image and histogram is identical to the shot takjen with the FA*28-70 at 1/25.

Can someone explain to me what is going on ?

KjetilH · 05-09-2010, 03:47 PM

I would suggest trying the 31 wide open. Over-exposure is typically from slow aperture blades on the lens, maybe try closing the aperture with the lever manually (with the finger) a couple of times to try out. May loosen it up a bit.

pcarfan · 05-09-2010, 04:00 PM

It is a new lens to me and the previous owner didn't have it for that long either. It is a MIV version so can't be too old. So, I doubt whether it is the stuck aperture. Anyhow, it gave me the idea to test it with setting the aperture with the aperture ring and then leaving it in 'A' and setting it in the camera. There is a slight difference of about 1/4 stop between the two ????

rustynail925 · 05-09-2010, 05:27 PM

Dont you like it that way? you got an extra 1 stop

pcarfan · 05-09-2010, 05:59 PM

Originally posted by rustynail925

Dont you like it that way? you got an extra 1 stop

I wish it works that way

I think the aperture opening is differnet and thus the aberration, so the f2.8 is not the true f2.8 aperture opening.

I tested it further, It looks like there is a ~1/4 stop difference between the FA43, FA31 and the FA* with the FA31 being the brightest, and then there are slight differneces based on whether the aperture was set in the lens or the body. So, it all added up together to give an almost 1-stop difference there.

I am surprised by such a discrepency, but not a big deal I guess...

**Digitalis** · 05-09-2010, 06:41 PM

are you sure your other lenses have their polarisers taken off? - I have seen plenty pf errors caused by those things...people wearing polarised sunglasses and then they take a look through the viewfinder with a lens that has a polariser on it...and all they see is black and they think something is wrong with their camera ( He just bought a canon 1DSMKII all professional lenses, and had not a clue what he was doing... he honestly thought he would have to send his camera back to to the manufacturer for repairs...canon photographers)

I think the hypothesis about the faulty aperture mechanism should be looked into. I have had some old LF lenses that had a similar problem...but in a new lens. I would get that looked at.

creampuff · 05-09-2010, 07:10 PM

Were you using a third party focusing screen?
Spot metering? Spot metering results can be inaccurate if you're using a third party focusing screen.

What metering and exposure mode were you shooting with? Did you inadvertently dial in exposure compensation accidently?

Did the light level change in between changing lenses? A passing cloud perhaps?

Had any filters on?

aerodave · 05-09-2010, 07:34 PM - **1 Like**

I've found similar behavior with some of my lenses, though it's always been less than a stop so I never got too worried about it.

I think the issue is that while our cameras are phenomenal precision instruments in most respects, the quick stopping down of the aperture is an inexact science. In the path from the camera's aperture lever mechanism to the linkage in the lens to the blades themselves, there's a lot of opportunity for play, backlash, and slop. And when you get down to it, the difference in actual aperture size between f/8 and f/11 on a lens like this is very tiny. It seems quite possible that the tolerances in the moving parts (especially ones actuated dynamically, and quite rapidly at that) could be on the order of an f-stop's worth of error.

I'm willing to bet that manually putting the lens at f/11 would create a more accurate exposure, because you're taking the camera's precision (or lack thereof) out of the equation. With an aperture ring-selected f-stop, the camera just releases the lever all the way, and the final position of the blades is determined by the lens only. When using the "A" position on the ring, however, the camera controls how much the lens is stopped down by only moving the actuator part of the way. It's trying to hit the right spot on the fly, and there's no hard limit imposed by guts of the lens itself.

I also suspect that lenses can have some nonlinearity inherent in the aperture linkage. This can be compensated for by an aperture ring whose indices are calibrated after fixing the design of the blades and linkage. So moving your lens's ring to f/11 is truly f/11 because the mechanism was designed that way. But when the camera does it in "A" mode, it's based on an interpolated value that may not be true for the nonlinear mechanism. In other words, the camera may think to itself "Okay, min aperture is f/2.8, and max is f/22. f/11 is 4 stops down out of 6, i'll move the lever 67% of the way". But in reality, your lens's nonlinearity means that a true f/11 is, say, 80% of the way down the range. When the camera stops it down by the calculated value, it turns out to be not enough. The aperture ring, on the other hand, may be indexed so that it moves the blades the perfect amount at all settings, by assuming that nonlinearity in its own design.

How's that for a big ol' pile of speculation?

blende8 · 05-10-2010, 12:20 AM

I agree with aerodave.
This is nothing unusual and just shows that stopping down a lens without stop-down coupler is inaccurate.
To find out if this is the cause, stop the lenses down manually (off-A) with the aperture ring and compare again.

axl · 05-10-2010, 12:30 AM

It would be worth trying what aerodave suggests IMO

wlachan · 05-10-2010, 01:03 AM

From what you have described, I can only think of 4 possible causes:

1) The rear aperture lever was somehow bent outward and stuck with the camera when stops down. You can check this by setting the camera to B then turn the aperture ring manually and see of the aperture blades stop down properly.

2) Greasy aperture blades, unlikely imho.

3) The aperture blades weren't calibrated properly in the factory. One way to check is to see if the aperture blades "just" fully open with the aperture ring at f1.8. If it is already fully or almost fully open at f2.5, you know the problem. The piece of retaining ring that holds the blades in place from the front if loosen (either due to loose screws or insufficient threadlock), can also cause this problem.

4) The rear aperture lever was not calibrated properly in the factory. This lever is used only when the aperture ring is at 'A'. That means if the exposure is correct with the aperture ring at any position except 'A', the lever is the problem. It can be re-adjusted easily but the problem should not be confused with #3 above.

Also, many mechanical aperture stop downs aren't completely linear (like the FA43). The difference from one end to another can be up to 1/2 stops based on the lenses that I have. Pentax usually calibrate them at f4, 8 & 16.

bdery · 05-10-2010, 05:14 AM

Remember that, in theory, aperture area is halved each time you close down the aperture by oen stop. HOWEVER, do the math yourself and you will see that the stops numbers are not Square root of 2 = 1.44 times larger or smaller than the stop before or after. They are an approximation.

What does that mean? It means that each lens can be tuned ever so slightly differently when you close down the aperture. Hence the differences (yeah, I used "hence

Also, you seem to assume that your prime is the culprit. Why not your zoom? It's much more likely to cause these kinds of problems, and it could very well vary with focal length.

Finally, take a lens with an aperture ring and play with it while looking at the blades. You will see that the difference between, say, f11 and f16 is very, very small. So at those small apertures it might be harder to tune correctly.

RolloR · 05-10-2010, 05:53 AM

it's an SMC thing perhaps.

maybe they have different coatings.

some coatings let light pass through more than others.

OR

the FA*28-70/2.8 has got 14 elements.

The FA*31/1.8 has got 9 elements.

If each of those elements are SMC'd...

**Digitalis** · 05-10-2010, 05:55 AM

If the lens coatings were causing this kind of a problem, then Pentax would have gone the way of patrika by now...

RolloR · 05-10-2010, 06:05 AM

Originally posted by Digitalis

If the lens coatings were causing this kind of a problem, then Pentax would have gone the way of patrika by now...

just a thought:

light will pass through ~100% through a pipe's hole.

put a clear glass to that hole and light maybe will pass ~98%.

then let's put some coatings to that glass, myabe only ~96% will pass.

light passes through 14 coated glass elements (assuming they're all coated?) with the FA*28-70/2.8, and only 9 through the FA*31.

Although i'm just trying to guess here hehe

05-09-2010, 03:44 PM	#1
pcarfan Veteran Member Join Date: Sep 2007 Location: Dayton, Ohio Photos: Gallery \| Albums Posts: 2,978	Why is my FA31mm 1 stop brighter The first shot is taken with the FA28-70/2.8 at 31mm at f11 and 1/25. The FA31 with the same exposure settings (iso100, f11, 1/25). The light was the same and yet the image is overexposed. (histogram shows a 1-stop over exposure) Then I took the image with the FA31 at 1/50 and this time the image and histogram is identical to the shot takjen with the FA28-70 at 1/25. Can someone explain to me what is going on ?

05-10-2010, 01:03 AM	#11
wlachan Veteran Member Join Date: Sep 2006 Location: Canada Photos: Albums Posts: 2,625	From what you have described, I can only think of 4 possible causes: 1) The rear aperture lever was somehow bent outward and stuck with the camera when stops down. You can check this by setting the camera to B then turn the aperture ring manually and see of the aperture blades stop down properly. 2) Greasy aperture blades, unlikely imho. 3) The aperture blades weren't calibrated properly in the factory. One way to check is to see if the aperture blades "just" fully open with the aperture ring at f1.8. If it is already fully or almost fully open at f2.5, you know the problem. The piece of retaining ring that holds the blades in place from the front if loosen (either due to loose screws or insufficient threadlock), can also cause this problem. 4) The rear aperture lever was not calibrated properly in the factory. This lever is used only when the aperture ring is at 'A'. That means if the exposure is correct with the aperture ring at any position except 'A', the lever is the problem. It can be re-adjusted easily but the problem should not be confused with #3 above. Also, many mechanical aperture stop downs aren't completely linear (like the FA43). The difference from one end to another can be up to 1/2 stops based on the lenses that I have. Pentax usually calibrate them at f4, 8 & 16. Last edited by wlachan; 05-10-2010 at 01:09 AM.

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05-09-2010, 03:47 PM	#2
KjetilH Veteran Member Join Date: Mar 2008 Location: Oslo Photos: Gallery Posts: 309	I would suggest trying the 31 wide open. Over-exposure is typically from slow aperture blades on the lens, maybe try closing the aperture with the lever manually (with the finger) a couple of times to try out. May loosen it up a bit.

05-09-2010, 04:00 PM	#3
pcarfan Veteran Member Join Date: Sep 2007 Location: Dayton, Ohio Photos: Gallery \| Albums Posts: 2,978 Original Poster	It is a new lens to me and the previous owner didn't have it for that long either. It is a MIV version so can't be too old. So, I doubt whether it is the stuck aperture. Anyhow, it gave me the idea to test it with setting the aperture with the aperture ring and then leaving it in 'A' and setting it in the camera. There is a slight difference of about 1/4 stop between the two ????

05-09-2010, 05:27 PM	#4
rustynail925 Veteran Member Join Date: Aug 2009 Location: Philippines Photos: Gallery Posts: 1,551	Dont you like it that way? you got an extra 1 stop

05-09-2010, 05:59 PM	#5
pcarfan Veteran Member Join Date: Sep 2007 Location: Dayton, Ohio Photos: Gallery \| Albums Posts: 2,978 Original Poster	Originally posted by rustynail925 Dont you like it that way? you got an extra 1 stop I wish it works that way I think the aperture opening is differnet and thus the aberration, so the f2.8 is not the true f2.8 aperture opening. I tested it further, It looks like there is a ~1/4 stop difference between the FA43, FA31 and the FA* with the FA31 being the brightest, and then there are slight differneces based on whether the aperture was set in the lens or the body. So, it all added up together to give an almost 1-stop difference there. I am surprised by such a discrepency, but not a big deal I guess...

05-09-2010, 06:41 PM	#6
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	are you sure your other lenses have their polarisers taken off? - I have seen plenty pf errors caused by those things...people wearing polarised sunglasses and then they take a look through the viewfinder with a lens that has a polariser on it...and all they see is black and they think something is wrong with their camera ( He just bought a canon 1DSMKII all professional lenses, and had not a clue what he was doing... he honestly thought he would have to send his camera back to to the manufacturer for repairs...canon photographers) I think the hypothesis about the faulty aperture mechanism should be looked into. I have had some old LF lenses that had a similar problem...but in a new lens. I would get that looked at.

05-09-2010, 07:10 PM	#7
creampuff Veteran Member Join Date: May 2007 Location: Singapore Photos: Gallery Posts: 3,953	Were you using a third party focusing screen? Spot metering? Spot metering results can be inaccurate if you're using a third party focusing screen. What metering and exposure mode were you shooting with? Did you inadvertently dial in exposure compensation accidently? Did the light level change in between changing lenses? A passing cloud perhaps? Had any filters on?

05-09-2010, 07:34 PM - 1 Like	#8
aerodave Veteran Member Join Date: Aug 2007 Location: Dayton, OH Posts: 365	I've found similar behavior with some of my lenses, though it's always been less than a stop so I never got too worried about it. I think the issue is that while our cameras are phenomenal precision instruments in most respects, the quick stopping down of the aperture is an inexact science. In the path from the camera's aperture lever mechanism to the linkage in the lens to the blades themselves, there's a lot of opportunity for play, backlash, and slop. And when you get down to it, the difference in actual aperture size between f/8 and f/11 on a lens like this is very tiny. It seems quite possible that the tolerances in the moving parts (especially ones actuated dynamically, and quite rapidly at that) could be on the order of an f-stop's worth of error. I'm willing to bet that manually putting the lens at f/11 would create a more accurate exposure, because you're taking the camera's precision (or lack thereof) out of the equation. With an aperture ring-selected f-stop, the camera just releases the lever all the way, and the final position of the blades is determined by the lens only. When using the "A" position on the ring, however, the camera controls how much the lens is stopped down by only moving the actuator part of the way. It's trying to hit the right spot on the fly, and there's no hard limit imposed by guts of the lens itself. I also suspect that lenses can have some nonlinearity inherent in the aperture linkage. This can be compensated for by an aperture ring whose indices are calibrated after fixing the design of the blades and linkage. So moving your lens's ring to f/11 is truly f/11 because the mechanism was designed that way. But when the camera does it in "A" mode, it's based on an interpolated value that may not be true for the nonlinear mechanism. In other words, the camera may think to itself "Okay, min aperture is f/2.8, and max is f/22. f/11 is 4 stops down out of 6, i'll move the lever 67% of the way". But in reality, your lens's nonlinearity means that a true f/11 is, say, 80% of the way down the range. When the camera stops it down by the calculated value, it turns out to be not enough. The aperture ring, on the other hand, may be indexed so that it moves the blades the perfect amount at all settings, by assuming that nonlinearity in its own design. How's that for a big ol' pile of speculation?

05-10-2010, 12:20 AM	#9
blende8 Veteran Member Join Date: Feb 2007 Location: Bremen, Germany Photos: Albums Posts: 1,521	I agree with aerodave. This is nothing unusual and just shows that stopping down a lens without stop-down coupler is inaccurate. To find out if this is the cause, stop the lenses down manually (off-A) with the aperture ring and compare again.

05-10-2010, 12:30 AM	#10
axl Veteran Member Join Date: Mar 2007 Location: Nove Zamky, Slovakia Photos: Gallery \| Albums Posts: 7,183	It would be worth trying what aerodave suggests IMO

05-10-2010, 05:14 AM	#12
bdery Pentaxian Join Date: Mar 2008 Location: Quebec city, Canada Photos: Gallery Posts: 9,357	Remember that, in theory, aperture area is halved each time you close down the aperture by oen stop. HOWEVER, do the math yourself and you will see that the stops numbers are not Square root of 2 = 1.44 times larger or smaller than the stop before or after. They are an approximation. What does that mean? It means that each lens can be tuned ever so slightly differently when you close down the aperture. Hence the differences (yeah, I used "hence Also, you seem to assume that your prime is the culprit. Why not your zoom? It's much more likely to cause these kinds of problems, and it could very well vary with focal length. Finally, take a lens with an aperture ring and play with it while looking at the blades. You will see that the difference between, say, f11 and f16 is very, very small. So at those small apertures it might be harder to tune correctly.

05-10-2010, 05:53 AM	#13
RolloR Senior Member Join Date: Feb 2010 Photos: Albums Posts: 264	it's an SMC thing perhaps. maybe they have different coatings. some coatings let light pass through more than others. OR the FA28-70/2.8 has got 14 elements. The FA31/1.8 has got 9 elements. If each of those elements are SMC'd...

05-10-2010, 05:55 AM	#14
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	If the lens coatings were causing this kind of a problem, then Pentax would have gone the way of patrika by now...

05-10-2010, 06:05 AM	#15
RolloR Senior Member Join Date: Feb 2010 Photos: Albums Posts: 264	Originally posted by Digitalis If the lens coatings were causing this kind of a problem, then Pentax would have gone the way of patrika by now... just a thought: light will pass through ~100% through a pipe's hole. put a clear glass to that hole and light maybe will pass ~98%. then let's put some coatings to that glass, myabe only ~96% will pass. light passes through 14 coated glass elements (assuming they're all coated?) with the FA28-70/2.8, and only 9 through the FA31. Although i'm just trying to guess here hehe