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05-13-2016, 12:39 AM   #1
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1.4 Teleconverter and F Stop

Hi. Hopefully this is the right place to ask, forgive the forum newbie if not (I tried search first btw).

I am using the K3 and very recently purchased 150 - 450 lens with the Pentax 1.4 Teleconverter, all of which are amazing, but I have a question about loss of stops.
I am aware that the converter means you lose one F Stop, not an issue at all, but this should mean that at the far end oif the zoom I should be able to select f7.1, given that it's maximum aperture is f5.6 without the tele, but I can't select anything less than f8.

Hs anyone else come across this at all, or does anyone have an explanation? It's not a big issue, I am shooting f8 at this range normally anyway, but I noticed it yesterday when trying to gain a bit of shutter speed.

Thanks!


Last edited by BigMackCam; 05-13-2016 at 12:50 AM.
05-13-2016, 12:49 AM   #2
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QuoteOriginally posted by Kevriano Quote
Hi. Hopefully this is the right place to ask, forgive the forum newbie if not (I tried search first btw).

I am using the K3 and very recently purchased 150 - 450 lens with the Pentax 1.4 Teleconverter, all of which are amazing, but I have a question about loss of stops.
I am aware that the converter means you lose one F Stop, not an issue at all, but this should mean that at the far end oif the zoom I should be able to select f7.1, given that it's maximum aperture is f5.6 without the tele, but I can't select anything less than f8.

Hs anyone else come across this at all, or does anyone have an explanation? It's not a big issue, I am shooting f8 at this range normally anyway, but I noticed it yesterday when trying to gain a bit of shutter speed.

Thanks!
Hi, and welcome to the forums...

I don't use the 1.4x tele-converter, but... one full f-stop down from f/5.6 is f/8 (f/7.1 would be 2/3rds of an f-stop down, wouldn't it?). So if the tele-converter robs you of one stop, f/8 would be correct at the long end.

Last edited by BigMackCam; 05-13-2016 at 01:01 AM.
05-13-2016, 01:05 AM   #3
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QuoteOriginally posted by BigMackCam Quote
Hi, and welcome to the forums...

I don't use the 1.4x tele-converter, but... one full f-stop down from f/5.6 is f/8 (f/7.1 would be 2/3rds of an f-stop down, wouldn't it?). So if the tele-converter robs you of one stop, f/8 would be correct at the long end.
Ah, therein lies my confusion with stops then! Perfect answer. f8 it is then, perfect. Happens to be where the lens is sharpest anyway
05-13-2016, 01:18 AM   #4
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QuoteOriginally posted by Kevriano Quote
Ah, therein lies my confusion with stops then! Perfect answer. f8 it is then, perfect. Happens to be where the lens is sharpest anyway
Unfortunately, that is not correct. At f8 with TC, the lens will be wide open, therefore not at its sharpest aperture.

05-13-2016, 01:22 AM   #5
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QuoteOriginally posted by Kevriano Quote
Ah, therein lies my confusion with stops then! Perfect answer. f8 it is then, perfect. Happens to be where the lens is sharpest anyway
Glad to help, Kev. There's a useful wikipedia article on f-stops - part of which gives a handy f-stop scale broken into 1/3rds.

As @audiobomber has said above, although you'll be at f/8 in terms of the amount of light getting through to the sensor (due to light losses from the tele-converter), the lens itself is still wide open. That said, I don't think you'll be unhappy with the sharpness
05-13-2016, 02:22 AM   #6
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QuoteOriginally posted by audiobomber Quote
Unfortunately, that is not correct. At f8 with TC, the lens will be wide open, therefore not at its sharpest aperture.
Sof 9 or 11 then? Man this is confusing. Yesterday's f8 pics look sharp as a tack mind...
05-13-2016, 02:35 AM   #7
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At full telephoto, the aperture of your lens wide open is f/5.6. From what you're saying, it allegedly performs best at f/8, which is one full f-stop down from f/5.6. By adding the 1.4x teleconverter, you're not changing the aperture of the lens... you're simply losing an amount of light that is equivalent to one f-stop. For the purposes of exposure, then, it is equivalent to f/8 - and the camera will see the lens as being at f/8, even though it is still wide open. To get the best performance from the lens, you'd still need to stop it down one full f-stop as before. So, based on your camera settings, you'd stop down from f/8 to f/11 (which is one full stop). Does that make sense? That said, if you're getting satisfactory images with the lens wide open, there's no need to stop down further... I have several lenses that don't perform at their best unless stopped down (most lenses are that way, to a greater or lesser extent), but I regularly use them wide open because I need the extra light, or shallower depth of field, or simply because I like the look of the images produced
05-13-2016, 06:34 AM   #8
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Great discussion. What do you do when the camera doesn't factor in the TC?
300mm F4 + 1.4 TC (not HD 1.4) the camera doesn't indicate an aperture change? Does this mean it will be exposing incorrectly?

05-13-2016, 07:16 AM   #9
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QuoteOriginally posted by wissink Quote
Great discussion. What do you do when the camera doesn't factor in the TC?
300mm F4 + 1.4 TC (not HD 1.4) the camera doesn't indicate an aperture change? Does this mean it will be exposing incorrectly?
No, the light is measured directly. It just mislabeled the f stops on the EXIF. Think of it this way, wide open it may think that you are at f/5.6, but it is really f/8. The exposure is set based on light seen which is accurate since it is measured through the lens. If you were using an external meter you would adjust manually. Then as you stop down to let's say f8, really f11 due to 1.4x tc, your camera adjusts for a one stop decrease in aperture. That is accurate. The EXIF will be wrong and the shake reduction will be wrong unless you manually enter the focal length.

Also, unlike what was said by someone above, the actual f stop is changed. The focal length increased but the aperture diameter stayed constant. This can sharpen the image, but there may still be room for improvement. Diffraction effects will still happen based on the actual f stop adjusted by the converter and not the lens reported original f stop.
05-13-2016, 07:25 AM   #10
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QuoteOriginally posted by UncleVanya Quote
Also, unlike what was said by someone above, the actual f stop is changed. The focal length increased but the aperture diameter stayed constant. This can sharpen the image, but there may still be room for improvement. Diffraction effects will still happen based on the actual f stop adjusted by the converter and not the lens reported original f stop.
Oh. I stand corrected, then, and learned something too Thanks, UV
05-13-2016, 07:44 AM   #11
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QuoteOriginally posted by UncleVanya Quote
the shake reduction will be wrong unless you manually enter the focal length.
How do you do this, I have a Kenko 1.5 and K5ll and the options for manually changing the SR seem greyed out in most modes?
05-13-2016, 09:47 AM   #12
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QuoteOriginally posted by mohb Quote
How do you do this, I have a Kenko 1.5 and K5ll and the options for manually changing the SR seem greyed out in most modes?
Yep, the option to manually enter isn't there with the Pentax HD 1.4 either. The EXIF on the pictures repoerts the focal length at the far end as 690mm, which is what the crop sensor gives, it doesn't show the teleconverters effect. So far I cant fault the images though. I have been shooting at f9, 10 and 11 today, as an experiment. It's a lovely sunny day, so light and speed haven't been an issue. I will see if there's a noticab;e image difference from yesterdays f8 shots.
05-13-2016, 11:07 AM   #13
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---------- Post added 05-13-16 at 02:08 PM ----------

QuoteOriginally posted by mohb Quote
How do you do this, I have a Kenko 1.5 and K5ll and the options for manually changing the SR seem greyed out in most modes?
See answers below.

QuoteOriginally posted by Kevriano Quote
Yep, the option to manually enter isn't there with the Pentax HD 1.4 either. The EXIF on the pictures repoerts the focal length at the far end as 690mm, which is what the crop sensor gives, it doesn't show the teleconverters effect. So far I cant fault the images though. I have been shooting at f9, 10 and 11 today, as an experiment. It's a lovely sunny day, so light and speed haven't been an issue. I will see if there's a noticab;e image difference from yesterdays f8 shots.
I don't think that is correct. The only reason the HD 1.4x reports a different (greater) focal length is that it computes the amount it adds to the lens focal length - it should never take crop factor into account. So a 300 mm is reported as 420mm not 450mm for example. At least that is what I understand and have seen in the past. I have my TC on loan to my dad at the moment so I cannot verify this exact detail on focal length but I can confirm that this is what I recall and what I have read.

Which lens reports as 690mm?

Also as for manual input - I sometimes used to use a Pentax A series 1.4x or 2x K series teleconverter until I sold these. With either of them you manually input your focal length just as you would with an M42, K, M, or A series lens since it doesn't have a data pin to transmit that info from the lens to the camera. I had not realized that the option to adjust the focal length was disabled when using a teleconverter with a data pin.
05-13-2016, 01:53 PM   #14
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QuoteOriginally posted by UncleVanya Quote


---------- Post added 05-13-16 at 02:08 PM ----------



See answers below.



I don't think that is correct. The only reason the HD 1.4x reports a different (greater) focal length is that it computes the amount it adds to the lens focal length - it should never take crop factor into account. So a 300 mm is reported as 420mm not 450mm for example. At least that is what I understand and have seen in the past. I have my TC on loan to my dad at the moment so I cannot verify this exact detail on focal length but I can confirm that this is what I recall and what I have read.

Which lens reports as 690mm?

Also as for manual input - I sometimes used to use a Pentax A series 1.4x or 2x K series teleconverter until I sold these. With either of them you manually input your focal length just as you would with an M42, K, M, or A series lens since it doesn't have a data pin to transmit that info from the lens to the camera. I had not realized that the option to adjust the focal length was disabled when using a teleconverter with a data pin.
Apologies required, it is reporting as 630. It's the Pentax 150 - 450mm. Clearly it is recognising the Teleconverter and reporting the full focal length achieved using that, not the crop, so you are correct. Im glad I asked this though, interesting points being raised. Thanks all.
05-13-2016, 01:59 PM   #15
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QuoteOriginally posted by Kevriano Quote
Apologies required, it is reporting as 630. It's the Pentax 150 - 450mm. Clearly it is recognising the Teleconverter and reporting the full focal length achieved using that, not the crop, so you are correct. Im glad I asked this though, interesting points being raised. Thanks all.
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.
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