1.4 Teleconverter and F Stop

Kevriano · 05-13-2016, 02:26 PM

Originally posted by UncleVanya

Ok That makes sense. Although (get ready for irony) some have measured the field of view and find that the 1.4x teleconverter actually is closer to 1.5x - lol.

I'm guess Pentax prefers to avoid confusion and also ensure that a slight understabilization is applied vs overcompensation.

Ah well. 1.5 is fine by me. You can never have enough focal length for wildlife. . Actually, that's a lie it's been a bit too long at times today, but I didn't want to keep swapping around, I just walked backwards a bit 😊

pentasonic49 · 05-14-2016, 01:24 PM

Have a look in the "custom setting" under EVsteps. There is a choice of 1/2 or 1/3 steps. You may find that 1/3 steps are better.

Kevriano · 05-14-2016, 02:16 PM

Originally posted by pentasonic49

Have a look in the "custom setting" under EVsteps. There is a choice of 1/2 or 1/3 steps. You may find that 1/3 steps are better.

Ooo cool. I will do just that. Thanks.

**Des** · 05-14-2016, 03:50 PM

Originally posted by mohb

I have a Kenko 1.5 and K5ll and the options for manually changing the SR seem greyed out in most modes?

Originally posted by UncleVanya

I had not realized that the option to adjust the focal length was disabled when using a teleconverter with a data pin.

Yes it is with mine too. I haven't figured out a way to set the focal length for SR purposes with my Kenko TC. I tried turning the camera on without a lens and inputting the equivalent focal length manually, but that setting seems to get overridden when I attach the TC+lens. If anyone has a bright idea for how to do it, I'd be interested to hear it.

As for the EXIF, I guess there are programs that would allow you to edit the focal length entry - although none of the programs I have used seem to be able to do it.

**UncleVanya** · 05-14-2016, 08:42 PM

I would bet it you block the data pin it would but you would likely lose autofocus also.

Kevriano · 05-21-2016, 05:36 AM - **1 Like**

I thought I'd report back on this thread, that I really cant see any noticable difference in sharpness between wide open at f8, and f11 or higher. I wonder if this is because the 150 - 450 is designed for full frame and I'm using the K3, so get the generally accepted better part of the lens.

**UncleVanya** · 05-21-2016, 06:04 AM

Originally posted by Kevriano

I thought I'd report back on this thread, that I really cant see any noticable difference in sharpness between wide open at f8, and f11 or higher. I wonder if this is because the 150 - 450 is designed for full frame and I'm using the K3, so get the generally accepted better part of the lens.

That's good news. As mentioned before you are shooting at f8, and by f11 on the k-3 you may see diffraction effects starting to reduce sharpness. These effects may cancel out any gains from stopping down. You should still get increased depth of field and you may still want to use the f11 setting - diffraction is not the end of the world like it sometimes is portrayed.

**Des** · 05-21-2016, 02:11 PM

Originally posted by UncleVanya

Although (get ready for irony) some have measured the field of view and find that the 1.4x teleconverter actually is closer to 1.5x

Whereas my Kenko 1.5x TC has a (tested) field of view equivalent to 1.4x: http://www.dpreview.com/forums/post/52044051 And to confuse matters further, you will see in that thread that there is a different version of the Kenko with the same name which is actually just over 1.5x.

**UncleVanya** · 05-21-2016, 02:13 PM

Originally posted by Des

Whereas my Kenko 1.5x TC has a (tested) field of view equivalent to 1.4x.

Isn't that hilarious? I think it is pretty dang silly for them not to label them correctly. I haven't ever tested the HD to see if it is 1.5 or 1.4x but there are reports from various people that I have read that say it. Personally the difference is so slight I just couldn't be bothered to figure it out. LOL.

**Des** · 05-21-2016, 03:13 PM

It is strange, UV. But then, I suspect the stated focal length on most lenses isn't correct either.

Kevriano · 05-22-2016, 12:23 AM

Originally posted by Des

It is strange, UV. But then, I suspect the stated focal length on most lenses isn't correct either.

I think that's very true. My wife's 150 -500 Sigma (Nikon fit) seems to shoot beyond its length.

estost · 06-01-2016, 06:13 AM

The logic behind the TC factors is the loss of light... 1,4 means one f-stop, 2 means 2

1,4 = radical 2 is the factor between full f-stops, due to circular geometry. At the same time the f-stop number indicates how many times the diameter of the aperture measured in mm fits into the focal length. So by increasing the focal length by using a 1,4x TC any given aperture diameter suddenly also fits 1,4x more into the now extended focal length, thus very elegantly making the f-stop number jump to the next full value, i.e. from 5,6 to 8 in our case

Kevriano · 06-01-2016, 09:13 AM

Youtu.be

audiobomber · 06-02-2016, 05:08 AM

Originally posted by Kevriano

I think that's very true. My wife's 150 -500 Sigma (Nikon fit) seems to shoot beyond its length.

Actually, the Sigma supertele zooms and all other Internal Focus lenses can only reach their stated focal lengths when focussed to infinity. As focus distance decreases, so does the true focal length.

**UncleVanya** · 06-02-2016, 08:51 AM

Originally posted by audiobomber

Actually, the Sigma supertele zooms and all other Internal Focus lenses can only reach their stated focal lengths when focussed to infinity. As focus distance decreases, so does the true focal length.

Not ALL internal focus lenses have focus breathing issues however common it might be for most. The 50-135 Pentax has very little if any focus breathing. The 60-250 has a ton - both are internal focus (the 50-135 is also internal zoom).

05-21-2016, 02:11 PM	#23
Des Loyal Site Supporter Join Date: Jan 2014 Location: Victoria Australia Photos: Albums Posts: 6,423	Originally posted by UncleVanya Although (get ready for irony) some have measured the field of view and find that the 1.4x teleconverter actually is closer to 1.5x Whereas my Kenko 1.5x TC has a (tested) field of view equivalent to 1.4x: http://www.dpreview.com/forums/post/52044051 And to confuse matters further, you will see in that thread that there is a different version of the Kenko with the same name which is actually just over 1.5x. Last edited by Des; 05-21-2016 at 02:21 PM.

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05-13-2016, 02:26 PM	#16
Kevriano Veteran Member Join Date: May 2016 Location: West Sussex Posts: 472 Original Poster	Originally posted by UncleVanya Ok That makes sense. Although (get ready for irony) some have measured the field of view and find that the 1.4x teleconverter actually is closer to 1.5x - lol. I'm guess Pentax prefers to avoid confusion and also ensure that a slight understabilization is applied vs overcompensation. Ah well. 1.5 is fine by me. You can never have enough focal length for wildlife. . Actually, that's a lie it's been a bit too long at times today, but I didn't want to keep swapping around, I just walked backwards a bit 😊

05-14-2016, 01:24 PM	#17
pentasonic49 Veteran Member Join Date: Oct 2012 Location: weston-super-mare Posts: 395	Have a look in the "custom setting" under EVsteps. There is a choice of 1/2 or 1/3 steps. You may find that 1/3 steps are better.

05-14-2016, 02:16 PM	#18
Kevriano Veteran Member Join Date: May 2016 Location: West Sussex Posts: 472 Original Poster	Originally posted by pentasonic49 Have a look in the "custom setting" under EVsteps. There is a choice of 1/2 or 1/3 steps. You may find that 1/3 steps are better. Ooo cool. I will do just that. Thanks.

05-14-2016, 03:50 PM	#19
Des Loyal Site Supporter Join Date: Jan 2014 Location: Victoria Australia Photos: Albums Posts: 6,423	Originally posted by mohb I have a Kenko 1.5 and K5ll and the options for manually changing the SR seem greyed out in most modes? Originally posted by UncleVanya I had not realized that the option to adjust the focal length was disabled when using a teleconverter with a data pin. Yes it is with mine too. I haven't figured out a way to set the focal length for SR purposes with my Kenko TC. I tried turning the camera on without a lens and inputting the equivalent focal length manually, but that setting seems to get overridden when I attach the TC+lens. If anyone has a bright idea for how to do it, I'd be interested to hear it. As for the EXIF, I guess there are programs that would allow you to edit the focal length entry - although none of the programs I have used seem to be able to do it.

05-14-2016, 08:42 PM	#20
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	I would bet it you block the data pin it would but you would likely lose autofocus also.

05-21-2016, 05:36 AM - 1 Like	#21
Kevriano Veteran Member Join Date: May 2016 Location: West Sussex Posts: 472 Original Poster	I thought I'd report back on this thread, that I really cant see any noticable difference in sharpness between wide open at f8, and f11 or higher. I wonder if this is because the 150 - 450 is designed for full frame and I'm using the K3, so get the generally accepted better part of the lens.

05-21-2016, 06:04 AM	#22
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by Kevriano I thought I'd report back on this thread, that I really cant see any noticable difference in sharpness between wide open at f8, and f11 or higher. I wonder if this is because the 150 - 450 is designed for full frame and I'm using the K3, so get the generally accepted better part of the lens. That's good news. As mentioned before you are shooting at f8, and by f11 on the k-3 you may see diffraction effects starting to reduce sharpness. These effects may cancel out any gains from stopping down. You should still get increased depth of field and you may still want to use the f11 setting - diffraction is not the end of the world like it sometimes is portrayed.

05-21-2016, 02:13 PM	#24
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by Des Whereas my Kenko 1.5x TC has a (tested) field of view equivalent to 1.4x. Isn't that hilarious? I think it is pretty dang silly for them not to label them correctly. I haven't ever tested the HD to see if it is 1.5 or 1.4x but there are reports from various people that I have read that say it. Personally the difference is so slight I just couldn't be bothered to figure it out. LOL.

05-21-2016, 03:13 PM	#25
Des Loyal Site Supporter Join Date: Jan 2014 Location: Victoria Australia Photos: Albums Posts: 6,423	It is strange, UV. But then, I suspect the stated focal length on most lenses isn't correct either.

05-22-2016, 12:23 AM	#26
Kevriano Veteran Member Join Date: May 2016 Location: West Sussex Posts: 472 Original Poster	Originally posted by Des It is strange, UV. But then, I suspect the stated focal length on most lenses isn't correct either. I think that's very true. My wife's 150 -500 Sigma (Nikon fit) seems to shoot beyond its length.

06-01-2016, 06:13 AM	#27
estost Junior Member Join Date: May 2016 Photos: Gallery Posts: 33	The logic behind the TC factors is the loss of light... 1,4 means one f-stop, 2 means 2 1,4 = radical 2 is the factor between full f-stops, due to circular geometry. At the same time the f-stop number indicates how many times the diameter of the aperture measured in mm fits into the focal length. So by increasing the focal length by using a 1,4x TC any given aperture diameter suddenly also fits 1,4x more into the now extended focal length, thus very elegantly making the f-stop number jump to the next full value, i.e. from 5,6 to 8 in our case

06-02-2016, 05:08 AM	#29
audiobomber Veteran Member Join Date: May 2008 Location: Sudbury, Ontario Photos: Albums Posts: 6,806	Originally posted by Kevriano I think that's very true. My wife's 150 -500 Sigma (Nikon fit) seems to shoot beyond its length. Actually, the Sigma supertele zooms and all other Internal Focus lenses can only reach their stated focal lengths when focussed to infinity. As focus distance decreases, so does the true focal length.

06-02-2016, 08:51 AM	#30
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by audiobomber Actually, the Sigma supertele zooms and all other Internal Focus lenses can only reach their stated focal lengths when focussed to infinity. As focus distance decreases, so does the true focal length. Not ALL internal focus lenses have focus breathing issues however common it might be for most. The 50-135 Pentax has very little if any focus breathing. The 60-250 has a ton - both are internal focus (the 50-135 is also internal zoom).