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12-07-2018, 09:23 AM   #1
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35mm Focal Length?

Hello everybody,


I have a general question that I'm curious about.
I just got a used lens for my K-50 that I've mentioned in another thread, and was testing it out. It is a Pentax 28-80 F/3.5 - 5.6 SMC FA AL Silver.


Well, it seems relatively 'okay', and so far I'm satisfied with it for what I paid. While looking at the image files on my computer, looking at the properties, details, I noticed under the Camera section the usual information, but also two focal lengths.


Of course, since my camera recently acquired the aperture block failure <frown>, I'm having to use nothing but manual lenses, because I can't set the aperture with the camera. So,I'm setting the aperture physically on the lens itself and using the green button for metering.


In the properties of the image, it shows "F-Stop f/0". That figures, because I'm in manual mode, how would it know? It also shows the exposure time, the ISO speed, and the Focal length of 28mm (which is what I had the lens set on). But a few lines further down, it shows "35mm focal length 42". When I adjusted the lens to 80mm, those pictures had details showing "35mm focal length 120". Do those numbers sound correct? 28mm = 42mm and 80mm = 120mm?


Is the camera able to electronically detect the focal length of the lens, even though the aperture has failed the mode is manual, and I'm not using the "A" setting? I'm thinking that may be the case. Maybe it is somehow connected enough to the lens to sense this, and automatically calculates the equivalent 35mm focal length. Is that about right? Or, if not, how does the camera know the focal length?


Maybe it depends on the lens? This isn't a terribly old lens, maybe a really old manual one with no "A" setting might give less information to the camera? Thanks for your time and responses!


Scott

12-07-2018, 09:38 AM   #2
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Yes, the data pin transmits the focal length and the lens contains some electronics that measure the zoom ring position and send that data to the camera.

The camera multiplies that focal length by 1.5 to get the "35mm equivalent" that's in the EXIF. There should also be an EXIF field with the actual focal length.
12-07-2018, 10:13 AM   #3
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QuoteOriginally posted by Astronomersmith Quote
But a few lines further down, it shows "35mm focal length 42". When I adjusted the lens to 80mm, those pictures had details showing "35mm focal length 120". Do those numbers sound correct? 28mm = 42mm and 80mm = 120mm?
It is telling you the equivalent field of view on 35mm/Full Frame. ie a 120mm lens on a FF camera would give you the same FOV as your 80mm lens on a aps-c camera. If you dont use more than one format (aps-c/FF) the information is pretty much irrelevant.
12-07-2018, 10:26 AM   #4
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QuoteOriginally posted by photoptimist Quote
Yes, the data pin transmits the focal length and the lens contains some electronics that measure the zoom ring position and send that data to the camera.

The camera multiplies that focal length by 1.5 to get the "35mm equivalent" that's in the EXIF. There should also be an EXIF field with the actual focal length.






Thanks!
I looked further down the properties and found this under the "Advanced photo" section: "EXIF version 0230" - is this what you are speaking of? If not, then what is that exactly? It is the only listed item with EXIF, and it seems too long to be a unit of measurement?


Pardon the na´ve questions (that I probably should already know!). I'm wanting to learn and understand.


Scott

12-07-2018, 10:57 AM   #5
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QuoteOriginally posted by Astronomersmith Quote
Thanks!
I looked further down the properties and found this under the "Advanced photo" section: "EXIF version 0230" - is this what you are speaking of? If not, then what is that exactly? It is the only listed item with EXIF, and it seems too long to be a unit of measurement?


Pardon the na´ve questions (that I probably should already know!). I'm wanting to learn and understand.


Scott
EXIF is short for Exchangeable Image File Format. EXIF a standard established by the Japan Electronic Industries Development Association (JEIDA) for the purpose of embedding metadata in image files. Like most standards, it gets updated from time to time, and each update gets a version number. That's all "0230" means - version 0230, (or 2.30).

Just to be clear, all that information (properties) such as focal length, shutter speed aperture, etc. are collectively called "metadata" or "EXIF" information. Most software that shows EXIF data does not show all of it. Most likely your software is showing you a limited number of items that someone thought you would be most interested in. Some other software might show more or fewer items, but I would think most would show actual focal length, and not just 35mm equivalents.

Last edited by runswithsizzers; 12-07-2018 at 06:44 PM.
12-08-2018, 10:09 PM   #6
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QuoteOriginally posted by Astronomersmith Quote
Hello everybody,

I have a general question that I'm curious about.
I just got a used lens for my K-50 that I've mentioned in another thread, and was testing it out. It is a Pentax 28-80 F/3.5 - 5.6 SMC FA AL Silver.

Well, it seems relatively 'okay', and so far I'm satisfied with it for what I paid. While looking at the image files on my computer, looking at the properties, details, I noticed under the Camera section the usual information, but also two focal lengths.

Of course, since my camera recently acquired the aperture block failure <frown>, I'm having to use nothing but manual lenses, because I can't set the aperture with the camera. So,I'm setting the aperture physically on the lens itself and using the green button for metering.

In the properties of the image, it shows "F-Stop f/0". That figures, because I'm in manual mode, how would it know? It also shows the exposure time, the ISO speed, and the Focal length of 28mm (which is what I had the lens set on). But a few lines further down, it shows "35mm focal length 42". When I adjusted the lens to 80mm, those pictures had details showing "35mm focal length 120". Do those numbers sound correct? 28mm = 42mm and 80mm = 120mm?

Is the camera able to electronically detect the focal length of the lens, even though the aperture has failed the mode is manual, and I'm not using the "A" setting? I'm thinking that may be the case. Maybe it is somehow connected enough to the lens to sense this, and automatically calculates the equivalent 35mm focal length. Is that about right? Or, if not, how does the camera know the focal length?

Maybe it depends on the lens? This isn't a terribly old lens, maybe a really old manual one with no "A" setting might give less information to the camera? Thanks for your time and responses!

Scott
Focal length of a lens is needed for IBIS and AF.

If your lens provides AF services - such as screw drive {like your Pentax-FA lens} or in-lens AF motor - it will report focal length to the body; otherwise (*) body will ask you for that value when you mount the lens.


(*) Thus, when I used a Takumar, Pentax-M, or Pentax-A {which does have an "A" setting}, body would ask for focal length.
5 Days Ago   #7
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QuoteOriginally posted by reh321 Quote
Focal length of a lens is needed for IBIS and AF.

If your lens provides AF services - such as screw drive {like your Pentax-FA lens} or in-lens AF motor - it will report focal length to the body; otherwise (*) body will ask you for that value when you mount the lens.


(*) Thus, when I used a Takumar, Pentax-M, or Pentax-A {which does have an "A" setting}, body would ask for focal length.


Good, thanks for the information. It seems like I ran into that situation you describe with one of my really older lenses. I have a Cosinon F/4 200mm MC, and while I've not used it in a long time, I was playing around with my *ist DS on a beach vacation and tried it out (with a 2x converter to boot!)...it asked me for a focal length.


And regarding my 28-80 lens we were talking about to begin with, it does do AF, so I should have figured it is 'connected' to the camera electronically too. Sorry for the silly questions and asking before thinking!


Scott

---------- Post added 12-10-18 at 12:17 PM ----------

QuoteOriginally posted by runswithsizzers Quote
EXIF is short for Exchangeable Image File Format. EXIF a standard established by the Japan Electronic Industries Development Association (JEIDA) for the purpose of embedding metadata in image files. Like most standards, it gets updated from time to time, and each update gets a version number. That's all "0230" means - version 0230, (or 2.30).

Just to be clear, all that information (properties) such as focal length, shutter speed aperture, etc. are collectively called "metadata" or "EXIF" information. Most software that shows EXIF data does not show all of it. Most likely your software is showing you a limited number of items that someone thought you would be most interested in. Some other software might show more or fewer items, but I would think most would show actual focal length, and not just 35mm equivalents.




Thanks for the useful reply! And just for the record, I wasn't even using real photo software, I was just in Windows 10, file explorer, clicked on the Properties option to look. It gave a good bit of details, but like you said, there is some it probably didn't.


Scott
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