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12-14-2013, 02:17 PM   #1
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Quick Method: What Is The Equivalent Focal Length

While shopping for lenses from different eras like I am doing lately I wonder if there is a quick and easy way to determine what the actual equivalent focal length will be on my K-5 in real terms or whatever of any particular given lens? Because I can't make heads or tails of the range of numbers on the front of the lenses, to be honest, and I've even read stuff where they say f/2.8 isn't even really f/2.8 for example on this camera or that camera, because the sensor is different or bigger or smaller or whatever. What's the deal with my new camera? Is there like a simple formula to translate the numbers or something? So like I could say to myself okay this 70-210 will actually be like f/5.6-f/11 and 100-500 on the K-5 or whatever?


Dave


Last edited by DavidSKAF3; 12-14-2013 at 02:25 PM.
12-14-2013, 02:21 PM - 2 Likes   #2
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A 50mm made in 1950 is a 50mm made in 1990 is a 50mm made in 2013.

Focal length never changes, equivalence does. You'll get (almost*) the exact same size image with a 50mm screwmount manufactured in the 70's as you would with a DA50mm made yesterday.

* - of course this is provided that the 50mm is actually a 50mm and doesn't stray a bit in either direction. All the same, whatever the focal length is marked on the lens is the same as it is no matter when the lens was made.
12-14-2013, 02:22 PM   #3
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QuoteOriginally posted by davids8560 Quote
While shopping for lenses from different eras like I am doing is lately there a quick and easy way to determine what the actual equivalent focal length will be on my K-5 in real terms or whatever of any particular given lens? Because I can't make heads or tails of the range of numbers on the front of the lenses, to be honest, and I've even read stuff where they say f/2.8 isn't even really f/2.8 for example on this camera or that camera, because the sensor is different or bigger or smaller or whatever. What's the deal with my new camera? Is there like a simple formula to translate the numbers or something? So like I could say to myself okay this 70-210 will actually be like f/5.6-f/11 and 100-500 on the K-5 or whatever?


Dave
What it says on the lens is what you get. No conversions or multiplications are necessary. Aperture is aperture, focal length is focal length, the only thing that the sensor influences is the field of view. So a 50mm lens on APS-C would have a narrower field of view then a 50mm on full-frame since the latter sensor has a larger area and therefore sees more of the frame.

Adam
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12-14-2013, 02:28 PM   #4
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F2.8 is always F2.8. F stops are a function of geometry, it is simply a ratio of the lenses focal length to the diameter of the entrance pupil.

What changes between different sensors is the apparent DoF. If you take two images, one with a FF sensor and one with an APS-C sensor, then crop the FF image the DoF will be the same in the two images.

The only other thing that changes is the field of view. Don't get hung up on Formatfaktor.

12-14-2013, 02:29 PM   #5
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QuoteOriginally posted by Adam Quote
What it says on the lens is what you get. No conversions or multiplications are necessary. Aperture is aperture, focal length is focal length, the only thing that the sensor influences is the field of view. So a 50mm lens on APS-C would have a narrower field of view then a 50mm on full-frame since the latter sensor has a larger area and therefore sees more of the frame.

Thanks. So like what are the charts on the bottom part of this web page supposed to be talking about?


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12-14-2013, 02:35 PM   #6
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That the 6-64.3mm lens gives you the same field of view with a 1/1.7" sensor as a 28-300mm lens would on a film/full-frame camera. Compact cameras always have very short focal lengths so the authors are just trying to give users something to relate to as far as the field of view goes.

For APS-C DSLRs, the basic breakdown is (roughly) as follows:

< 15mm = ultra-wide
15-25mm = wide-angle
25-40mm = normal
40-70mm = short tele
70-200mm = tele
200mm+ = extreme tele

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12-14-2013, 02:38 PM   #7
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QuoteOriginally posted by Adam Quote
That the 6-64.3mm lens gives you the same field of view with a 1/1.7" sensor as a 28-300mm lens would on a film/full-frame camera. Compact cameras always have very short focal lengths so the authors are just trying to give users something to relate to as far as the field of view goes.

Thanks Adam. To me the chart for example is like saying the f/3.5 - f/5.6 on the 18-135 lens is actually way ahead of the so-called f/2.8 on the other cameras. And that the numbers are not the real numbers if you know what it all means.

Last edited by DavidSKAF3; 12-14-2013 at 02:43 PM.
12-14-2013, 02:39 PM   #8
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Simply put, don't stress over "equivalence" as they are rather pointless these days, unless you are using two cameras with different film/sensor size and you want to capture the exact same.. Field of view, or depth of field, or some other thing that you want to keep constant.
A 50mm from 1970s will give you the same FoV as the latest DA 50mm f1.8. Just the film plane is slightly smaller than the 70s film. So if you compare a modern APS-C sensor with film, it appears that the edges are cut off and effectively you have a relatively reduced FoV. You can even get a medium format 50mm and adapt it on your camera. It will just project a large image circle that the sensor won't record, and so the photo will look like other 50mm lenses.
Unless you shot a lot of film, you shouldn't worry about equivalences. Just get the lenses you like. If you buy old lenses, I suggest you get a nice hood along with them, because lens coatings back then were not as great and a lens hood can really improve older lens' IQ. And I suggest you read up on Catch in focus - a great Pentax feature that gives you some AF-like functionality to manual lenses.

12-14-2013, 02:42 PM   #9
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Unless you routinely use both digital and film cameras, or cameras with different sized sensors like for example a FF Nikon and Olympus m4/3 then please just ignore all the equivalent talk. It means nothing to you. A 50mm lens is always a 50mm lens the focal length is a physical property of the lens and cannot be changed. What the view through that lens will look like might change if you change the sensor size, but if all you are using is the one camera then just ignore all the talk.
12-14-2013, 02:42 PM   #10
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QuoteOriginally posted by Na Horuk Quote
Simply put, don't stress over "equivalence" as they are rather pointless these days, unless you are using two cameras with different film/sensor size and you want to capture the exact same.. Field of view, or depth of field, or some other thing that you want to keep constant.
A 50mm from 1970s will give you the same FoV as the latest DA 50mm f1.8. Just the film plane is slightly smaller than the 70s film. So if you compare a modern APS-C sensor with film, it appears that the edges are cut off and effectively you have a relatively reduced FoV. You can even get a medium format 50mm and adapt it on your camera. It will just project a large image circle that the sensor won't record, and so the photo will look like other 50mm lenses.
Unless you shot a lot of film, you shouldn't worry about equivalences. Just get the lenses you like. If you buy old lenses, I suggest you get a nice hood along with them, because lens coatings back then were not as great and a lens hood can really improve older lens' IQ. And I suggest you read up on Catch in focus - a great Pentax feature that gives you some AF-like functionality to manual lenses.

Thanks. Yes a few people have suggested the hood thing to me. And they said authentic hoods that go with the old lenses can be really hard to find. I am wondering if those generic collapsible rubber hoods will suffice, or should I look for really long hoods, or whatever, because to be honest the flower petal ones look so small I don't see how they could make a difference!
12-14-2013, 02:51 PM   #11
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QuoteOriginally posted by davids8560 Quote
I am wondering if those generic collapsible rubber hoods will suffice
They might, if they were designed for the same FOV as the lens they are mounted to. A few bullet points:
  • Hood design is a balance between adequate shielding vs. vignette caused by too much hood depth
  • Petal hoods are designed to fine tune to the proportions of the image frame
  • Hoods for zooms are a compromise gesture, particularly if the zoom range is extreme
I would avoid aftermarket petal hoods. I use a collapsible rubber hood made for normal focal length on my film cameras. That same hood works OK with the same 50mm lens on APS-C digital, but I would probably do better with one a bit deeper.


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12-14-2013, 02:57 PM   #12
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QuoteOriginally posted by Na Horuk Quote
Simply put, don't stress over "equivalence" as they are rather pointless these days, unless you are using two cameras with different film/sensor size and you want to capture the exact same..
There is another time you'd need to think about it, when using a flash with a zoom head, the zoom positions equate to full frame field of view. When the head is at 35mm, it will be covering a 63 degree field of view, but 35mm on a APS-C crop sensor is 45 degrees. It is normally not an issue, since too wide is better than too narrow, but it is something to keep in mind if you're trying to get as much useful light as possible.

Of course there is also the matter of hoods, as others have mentioned.
12-14-2013, 02:58 PM   #13
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Don't get fooled again! Join the Equivalence Gang TODAY!

Seriously, equivalence only matters if one is an anal-retentive neckbeard who never actually takes photos. It has very little practical relevance.
12-15-2013, 09:06 AM   #14
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Lots of members here like the rubber hood because it has adjustable depth. You can also find chinese knock-offs of Pentax hoods that don't cost much, you just have to type into the ebays the hood code, like PH-RBA 52 or something. Those petal hoods can also be good, but just make sure its not too deep or too wide for the lens you want to use it on. Petal design is actually pretty advanced and can be more effective than a circular hood. So, yes, try to find a hood that claims to be made for a field of view similar to your lens. If the hood is too tight/deep, it might vignette (this means the edges of the photo get darker). And if its too shallow/wide, it doesn't protect much.
Sigh, lens hoods are a whole science. A knock-off or a rubber hood is a good place to start. Im sure you will figure out what exactly you prefer once you get more experience Photography is a process
12-15-2013, 01:55 PM   #15
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QuoteOriginally posted by davids8560 Quote
While shopping for lenses from different eras like I am doing lately I wonder if there is a quick and easy way to determine what the actual equivalent focal length will be on my K-5 in real terms or whatever of any particular given lens? Because I can't make heads or tails of the range of numbers on the front of the lenses, to be honest, and I've even read stuff where they say f/2.8 isn't even really f/2.8 for example on this camera or that camera, because the sensor is different or bigger or smaller or whatever. What's the deal with my new camera? Is there like a simple formula to translate the numbers or something? So like I could say to myself okay this 70-210 will actually be like f/5.6-f/11 and 100-500 on the K-5 or whatever?


Dave

Equivalence makes it easy to compare two camera systems and determine which is going to give you the capability you need.

For instance, you'd have approximately the same image noise level with a Nikon D7100 + 17-55 f/2.8 (or whatever that Nikon lens is) and a Nikon D600 + 24-85 f/3.5-4.5.

It's not a perfect comparison of course as the larger format will generally give you better colors and in practice will always give you sharper images.

All Pentax DSLR's have the same sensor size so it's not really a big concern in the Pentax world.

What exactly are you trying to take pictures of... where is your current camera 'failing' right now?
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