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03-29-2016, 09:23 AM   #151
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QuoteOriginally posted by clackers Quote
Well, it doesn't, SJF.

Focal length isn't a property of any sensor.

Don't listen to anyone who claims otherwise! ☺
QuoteOriginally posted by clackers Quote
Focal length isn't a property of any sensor.
I'm not referring to focal length, which I totally understand why it doesn't change.

03-29-2016, 12:26 PM   #152
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QuoteOriginally posted by EssJayEff Quote
Now that it's no longer 6:00 in the morning, let me clarify my question. I've never worked with a "crop factor" that was less than 1.0. I understand that a full frame 35mm lens produces an image circle that is larger than an APS-C sensor, hence a crop factor larger than 1.0. I saw something (OK I'll blame Ken Rockwell's chart "645 Lens 35mm Equivalents") that says the crop factor for the 645 film format to 35mm sensor is 0.62075x, with an angle of view for a 645 35mm lens being equivalent to that of a 21mm on a full frame 35mm camera. This makes no sense to me. The 645 lens is creating an image circle that is larger than a 645 digital sensor, so naturally the image circle is also larger than a full frame 35mm sensor. So why wouldn't the crop factor still be larger than 1.0 when using a 645 lens on a 35mm full frame sensor? A crop factor less than 1.0 to me implies that the sensor is larger than the image circle. Why would a 645 35mm lens act like a 21mm on a camera with full-frame 35mm sensor? Where am I getting confused?
The sensor IS larger than the image circle. That is, the 645 sensor is larger than the 35mmFF image circle. The crop factor is being given from the smaller sensor's frame of reference - the reverse of what you're probably used to. If you were converting 35mmFF Field of View to 645 equivalents, then the crop factor would be greater than 1, just as it is when you convert APS-C to 35mmFF. However, the chart is converting from a larger format to a smaller format. If you wanted to express 35mmFF Field of View in APS-C equivalents, the crop factor would be 0.667 - because it would be from the frame of reference of the smaller sensor.

For lenses of the same focal length, a 645 sensor captures a wider field of view than a 35mmFF sensor (in the same way that a 35mmFF sensor captures a wider field of view than an APS-C sensor). Therefore, you need a wider lens for the 35mmFF sensor to capture the same field of view as the 645 sensor - just as you need a wider lens for an APS-C sensor to capture the same FoV as a 35mmFF sensor.

To get the same field of view, 35mm on 645 ≈ 21mm on 35mmFF ≈ 14mm on APS-C.

Last edited by THoog; 03-29-2016 at 01:13 PM.
03-30-2016, 04:03 PM   #153
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QuoteOriginally posted by THoog Quote
The sensor IS larger than the image circle. That is, the 645 sensor is larger than the 35mmFF image circle. The crop factor is being given from the smaller sensor's frame of reference - the reverse of what you're probably used to. If you were converting 35mmFF Field of View to 645 equivalents, then the crop factor would be greater than 1, just as it is when you convert APS-C to 35mmFF. However, the chart is converting from a larger format to a smaller format. If you wanted to express 35mmFF Field of View in APS-C equivalents, the crop factor would be 0.667 - because it would be from the frame of reference of the smaller sensor.

For lenses of the same focal length, a 645 sensor captures a wider field of view than a 35mmFF sensor (in the same way that a 35mmFF sensor captures a wider field of view than an APS-C sensor). Therefore, you need a wider lens for the 35mmFF sensor to capture the same field of view as the 645 sensor - just as you need a wider lens for an APS-C sensor to capture the same FoV as a 35mmFF sensor.

To get the same field of view, 35mm on 645 ≈ 21mm on 35mmFF ≈ 14mm on APS-C.
Thanks fellow Tar Heel THoog! I think I got it . . . now if I can just remember that in the future. If not, I can always pop back here for a refresher.
03-30-2016, 05:03 PM   #154
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is theee any possibility the DA ltd 20-40/2.8-4 will work well on the K1? if so, that might be a nice solution for a lot of people.

03-30-2016, 08:07 PM - 1 Like   #155
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QuoteOriginally posted by rbelyell Quote
is theee any possibility the DA ltd 20-40/2.8-4 will work well on the K1? if so, that might be a nice solution for a lot of people.
Adam says "not even close":

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/290959-hd-20-40-ltd-ff.html
03-31-2016, 07:08 AM - 1 Like   #156
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QuoteOriginally posted by rbelyell Quote
is theee any possibility the DA ltd 20-40/2.8-4 will work well on the K1? if so, that might be a nice solution for a lot of people.
As THoog said, it's not going to work, it's an APS-C only lens.

The closest thing to it would be the FA 20-35mm f/4. That lens should work well on the K-1 - considering its age and the fact that it was built for film cameras. It probably won't compare well to the new 15-30mm, of course, but it should still be a fun lens to use for a reasonable price.
04-18-2016, 12:57 PM   #157
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Hey Pentaxians, I would appreciate a piece of advice. Just found a FA 20-35/4 and thought about might getting it and giving up my 12-24/4 since the K-1 is on its way. Would you recommend and encourage me to do this or do you think the 12-24 (from 18mm up) will have a better overall performance on the K-1? Wonder how the 20-35 will be on the whole 36mpx sensor and not just covering the sweetspot...

Hopefully this lens lab will soon open up and we have in addition to this the sample search database to help deciding such questions.
04-18-2016, 01:13 PM   #158
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QuoteOriginally posted by micromacro Quote
I did not have luck to find that lens, only DC (for cropped format), but never for FF. It's a great lens, although it's a bit back focusing on my K5iis wide open (not in LV).


There has been several Sigma DG series lenses in the Marketplace recently. All great lenses for full frame, all autofocus.

04-19-2016, 07:48 AM   #159
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Nobody has mentioned the 24-90 FA lens yet. It works very well on my K3 so the larger pixels on the K-1 won't over tax its resolution capability. Besides I already own that - so that makes it a great wide angle choice for me.

As far as an ultra wide - the lens I am looking forward to using is my original (mark 1) Sigma 12-24 f4-f5.6 DG EX. I have a shot I want to take with that lens when the K-1 gets here.
04-19-2016, 08:30 AM   #160
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QuoteOriginally posted by HoustonBob Quote
It works very well on my K3 so the larger pixels on the K-1 won't over tax its resolution capability.
this is an interesting thought. thanks. using K3/ K3 II shots to check lenses since they have smaller pixels. good one!
04-20-2016, 02:28 PM   #161
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I hope someone will try Samyang 14mm since it's an FF lens.
04-20-2016, 03:22 PM   #162
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The A series 28mm ƒ2 has been my most used lens on APS-C. It has a nice even quality like the DA 40 across the frame which is probably my second most used lens. I am looking forward to using the A-28mm ƒ2 on the K-1 as a wide angle lens.

I have an M 20mm ƒ4 that has gotten a lot of use.

One AF lens not mentioned is the F series 24-50mm ƒ4. This is a very capable lenses. It also has 8 blades. It has a nice size that should fit well with the K-1.

Another K-series and Takumar ultra wide option is the 17mm ƒ4 fish eye. This lens is a lot of fun.
04-20-2016, 03:27 PM   #163
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QuoteOriginally posted by Nitrogliserin Quote
I hope someone will try Samyang 14mm since it's an FF lens.
Works very well on FF.

04-20-2016, 05:22 PM   #164
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QuoteOriginally posted by HoustonBob Quote
Nobody has mentioned the 24-90 FA lens yet. It works very well on my K3 so the larger pixels on the K-1 won't over tax its resolution capability. Besides I already own that - so that makes it a great wide angle choice for me.

As far as an ultra wide - the lens I am looking forward to using is my original (mark 1) Sigma 12-24 f4-f5.6 DG EX. I have a shot I want to take with that lens when the K-1 gets here.
My 24~90 is best - really very, very nice - from 30 to 80. It has soft edges at extremes, but not offensively so. My only concern will be whether edge softness will be more pronounced on the FF sensor.

On film, though, it is great.
04-21-2016, 12:19 AM   #165
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Dear Pentaxians, I have two qustions regarding Samyang 14mm f/2.8 and Sigma 20mm f/1.8:

1. Does every Samyang (not Rokinon) 14mm F2.8 ED AS IF UMC has 'A' position on aperture ring like on picture here Samyang 14mm F2.8 ED AS IF UMC Lens Reviews - Samyang Lenses - Pentax Lens Review Database ?
I'm asking this because I could buy that lens in K mount only through online store but pictures there are not for K mount and I'm a little suspicious because of that.
It's a deal breaker for me if it doesn't have 'A' position.

2. What are your experiences and opinions about Sigma 20mm F1.8 EX Aspherical DG ?
There are a few cheap second hand Sigmas 20mm f/1.8 on local market. I don't have any experience with that lens in any mount and I'm a a little suspicious because of a bad ratings here Sigma 20mm F1.8 EX Aspherical DG DF RF Lens Reviews - Sigma Lenses - Pentax Lens Review Database
From reviews I see two disadvantages: it's not sharp wideopen @1.8 and it is big and heavy for what it does. I need the lens to be very sharp at f/2.8 and weight is not a problem because it is a temporary solution until I save enough money for D-FA 15-30.

Thank you in advance!
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