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04-04-2013, 06:03 PM   #1
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40mm xs is f2.8 or f3.5 really?!?!?

Firstly, I don't want to explain anything at the beginning. I will start with my 3 experiments and results then let you guys think and discuss.

My equipments in this experiment : Pentax K5 with 40mm XS lens and a bag of snack.
Environmental conditions : Indoor, WB auto, ISO1600, JPEG.

--------------------------------------------------
1ST experiment, I just took 4 shots by changing f and shutter speed manually. However, they were changed in order to get the same amount of light by step of 1/3EV.

Picture 1 was taken with f2.8 and speed shutter of 1/100 s
Picture 2 was taken with f3.2 and speed shutter of 1/80 s
Picture 3 was taken with f3.5 and speed shutter of 1/60 s
Picture 4 was taken with f4.0 and speed shutter of 1/50 s

You guys better check EXIF of them.
Here is the result.

Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 

Last edited by zany225; 04-04-2013 at 06:32 PM.
04-04-2013, 06:06 PM   #2
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2ND experiment, I took 3 shots by changing f only. Keep speed shutter at 1/100s.

Picture 1 was taken with f2.8 and speed shutter of 1/100 s
Picture 2 was taken with f3.2 and speed shutter of 1/100 s
Picture 3 was taken with f3.5 and speed shutter of 1/100 s
Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
04-04-2013, 06:11 PM   #3
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LAST experiment, I took 3 shots by changing f and shutter speed manually. The aperture was changed from f4. The changes of parameters were done with 1EV step.

Picture 1 was taken with f4 and speed shutter of 1/50 s
Picture 2 was taken with f5.6 and speed shutter of 1/25 s
Picture 3 was taken with f8 and speed shutter of 1/13 s
Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
04-04-2013, 06:23 PM   #4
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Anyway, I did these experiments because I noticed that when I changed aperture from f2.8-3.5 and pressed DOF check button, I could not see the aperture blades movement. So I looked carefully to bokeh from f2.8-3.5 and, IMO, they look identical. After that, I tried to find out what happened and the results explain my curiosity better than anythings.

However, it could be a result from either camera or my lens copy. I wish somebody could try the 40mm XS with the other cameras, like K7, K20D, K01, etc. Also, I still wonder how it will be with the regular 40mm limited.

Best Regards!
Sunny


Last edited by zany225; 04-04-2013 at 06:32 PM.
04-04-2013, 06:30 PM   #5
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QuoteOriginally posted by zany225 Quote
I think my thread is wrong. I don't know how to fix it. It should be "40mm XS is f2.8 or f3.5 really????"

lol

Anyway, I did these experiments because I noticed that when I changed aperture from f2.8-3.5 and pressed DOF check button, I could not see the aperture blades movement. So I looked carefully to bokeh from f2.8-3.5 and, IMO, they look identical. After that, I tried to find out what happened and the results explain my curiosity better than anythings.

However, it could be a result from either camera or my lens copy. I wish somebody could try the 40mm XS with the other cameras, like K7, K20D, K01, etc. Also, I still wonder how it will be with the regular 40mm limited.

Best Regards!
Sunny
Click on edit then go advanced in the first post to change your thread title.

BTW, to test this properly, you need to make sure your light source doesn't change. Try shining a lamp at your wall and taking a picture of just that. Then you can measure the average luminance of the image to see if there are any changes.

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04-04-2013, 06:32 PM   #6
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Thanks! Adam!

Your advised sound great for more accuracy.
For my tests, I have done in the apartment and weather was cloudy here. I think the light didn't change that fast. If I have time, I will do as your advised definitely.

So far, I could say that, at least my copy, this tiny lens has maximum aperture of f3.5 instead of f2.8.!

Really wanna see results from everyone as well!

Last edited by zany225; 04-04-2013 at 06:42 PM.
04-04-2013, 07:33 PM   #7
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I'd suggest using a focus chart instead.
04-04-2013, 08:02 PM   #8
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QuoteOriginally posted by drei3 Quote
I'd suggest using a focus chart instead.

I think it's NO need to use the focus chart cause, I'm talking about exposure not sharpness.

I just realized I have the regular 40mm limited in the dry box! Totally forgot about it!
I'll do these experiment again tonight compare to the bigger 40mm!

04-04-2013, 08:06 PM   #9
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You are barely going to see any movement at all of the aperture blades between f/2.8 and f/3.5, particularly with such as small diaphragm.
04-04-2013, 08:24 PM   #10
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QuoteOriginally posted by Ash Quote
You are barely going to see any movement at all of the aperture blades between f/2.8 and f/3.5, particularly with such as small diaphragm.

That's what I kept in mind before testing. However, even it's difficult to see but the brightness in those pictures on the top should be the same. In addition, when I changed aperture but kept speed shutter constant at 1/100s, pictures should be darker as I narrower the aperture.

1/3 EV step of light is noticeable.
And also, f2.8-3.5 is 2/3 EV!


Beyond f3.5, the aperture blades seem working perfectly. The blades keep narrowing. And that become my 3rd experiment as a reference testing. The brightness in the pictures are identical which is reasonable.

Last edited by zany225; 04-04-2013 at 08:29 PM.
04-05-2013, 11:44 AM   #11
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Alright, I tried again indoor. No sunlight from outside. So the light should not change.

But this time I also tried the big brother, 40mm limited, to compare with.
-------------------------------------------------------------------------------------------------------------------------------
First test is to keep exposure constant by changing speed shutter 1/3EV step once aperture has been changed from f/2.8-f/8.
The pictures follow by this order

f T
1. 2.8 100
2. 3.2 80
3. 3.5 60
4. 4.0 50
5. 4.5 40
6. 5.0 30
7. 5.6 25
8. 8.0 13
Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
04-05-2013, 11:49 AM   #12
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Second test is to keep shutter speed constant. Changing only aperture by 1/3EV step
The pictures follow by this order

f T
1. 2.8 100
2. 3.2 100
3. 3.5 100
4. 4.0 100
5. 4.5 100
6. 5.0 100
7. 5.6 100
Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
04-05-2013, 11:51 AM   #13
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Third test is just compare between f/2.8 and f/4
f T
1. 2.8 60
2. 4.0 50
3. 2.8 50
Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
04-05-2013, 12:00 PM   #14
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By the results I found, it seems that beyond f3.5 the camera and lens work properly. Brightness is almost constant.

The conclusion of my first 3 tests is that "my 40mm xs copy" has the size of aperture constantly between f/2.8-f/3.5. Even though the camera can recognize f/2.8 but that just makes metering confused and leads to under exposure by 2/3 stop.

As I emphasized, this could be only my copy and I would be happy if any of you can try your 40mm XS.

NEXT results will be from the big bro, 40mm limited.
04-05-2013, 12:05 PM   #15
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These following results are from the 40mm limited!

-------------------------------------------------------------------------------------------------------------------------------
Again, first test is to keep exposure constant by changing speed shutter 1/3EV step once aperture has been changed from f/2.8-f/8.
The pictures follow by this order

f T
1. 2.8 100
2. 3.2 80
3. 3.5 60
4. 4.0 50
5. 4.5 40
6. 5.0 30
7. 5.6 25
8. 8.0 13
Attached Images
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
View Picture EXIF
PENTAX K-5  Photo 
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