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03-18-2017, 04:12 PM   #1
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K1:200mm, 2.8 True Focal Length?

OK, I've searched wide for this, it appears that my 200mm prime 2.8 lens might be 300mm on my K5 but what is it on my new K1?

K1: 200mm, 2.8; whats the real focal length on this full frame body? 200 or 300?

Thanks

03-18-2017, 04:18 PM - 1 Like   #2
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200mm
03-18-2017, 04:21 PM   #3
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QuoteOriginally posted by BullsOnParade81 Quote
200mm
cheers
03-18-2017, 04:42 PM   #4
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QuoteOriginally posted by Mark (MC) Quote
OK, I've searched wide for this, it appears that my 200mm prime 2.8 lens might be 300mm on my K5 but what is it on my new K1?

K1: 200mm, 2.8; whats the real focal length on this full frame body? 200 or 300?

Thanks
That's not quite correct, a 200mm lens is a 200mm lens, so that's what it is on both the K-1 and the K5. However, on APSC the 200mm lens will give you the equivalent field of view to that of a 300mm lens. You can read all about it here: The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

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03-18-2017, 05:02 PM   #5
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Let's make that "a 200mm lens on APS-C will give you about the same field of view as a 300mm lens on FF".

When using the same lens but changing formats (even by switching the K-1 to and from APS-C crop mode, or by cropping in camera) the focal length doesn't change; field of view does.
03-18-2017, 05:05 PM   #6
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QuoteOriginally posted by Mark (MC) Quote
OK, I've searched wide for this, it appears that my 200mm prime 2.8 lens might be 300mm on my K5 but what is it on my new K1?

K1: 200mm, 2.8; whats the real focal length on this full frame body? 200 or 300?

Thanks
200mm on both cameras.

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03-18-2017, 07:52 PM   #7
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Now it can get complicated when you focus at different distances.. focus breathing.. due to lens design it can actually be slightly less or more (usually less I think) then the rated focal length. This is usually true at closer distances.
03-18-2017, 09:25 PM   #8
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I own the 100, 200, and 300. The 200 is about 100 narrower than the 100, and about 100 wider than the 300.
I've tested this on the k-3 and k-1, true in both cases.
The difference is between the k-1 and k-3, and not the lenses. The k-3 has a crop factor of 1.5, which means they are approximately a 150, 300, and 450 lens when mounted on the k-3 (I.e those are the focal lengths I'd need to buy for my k-1 to match those lenses when mounted on the k-1)

It doesn't matter though. A good lens mounted on the k-1, is still a good lens on the k-3. If the lens is good, enjoy what it gives you. If the lens is bad, get rid of it and buy an alternative.

03-19-2017, 02:47 AM   #9
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A 200 mm lens has 200 mm focal length. The focal length has nothing to do with the sensor.
The concept cannot be any simpler.

DOF is calculated based on focal distance, focal length and pixel pitch and is not depending on total sensor dimensions.
Field of view can be wider or narrower depending on what size sensor you put behind the lens.
03-19-2017, 05:53 AM   #10
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thanks everyone
03-19-2017, 09:11 PM   #11
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It is also a matter of convenience, and cost effectiveness. The above information is indeed correct. In practical terms, what it boils down to is what you get in your picture out of the camera with your 200mm lens on your K-5 APS-C body is like the image you would get from the same position and subject with a 300mm lens on your full-frame K-1 body. But putting a 300mm f/2.8 lens on your K-1 would present you with a VERY large, heavy ensemble to carry, and the cost of such a lens is very high. Of course, you could take the same shot from the same position using your K-1 and 200mm f/2.8 lens, in which your subject would be smaller with more surroundings in the photo, then do post-process work in a program like photoshop to crop the photo to match what it would be by using your K-5. That however, entails extra work, and with the K-1 unnecessary. Fortunately, the K-1 can be switched to the APS-C format with the same advantage, and about the same resolution as a K-5! For the same reasons, it is more difficult to design a compact wide-angle lens for APS-C use. Especially one having a faster larger aperture. If you could find a zoom lens having f/2.8 aperture, but built for APS-C use and giving the same view as the Pentax 15-30mm f/2.8 on your K-1, the thing would be huge, cost twice the price, and have greater distortion at the wide end.

Your K-5 can still serve you well when you don't need the special advantages of the K-1, will be doing quite a bit of tele work where the APS-C added image size will be helpful, and the K-5's smaller size and weight will be advantageous to your needs.
03-21-2017, 10:42 AM - 1 Like   #12
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QuoteOriginally posted by mikesbike Quote
But putting a 300mm f/2.8 lens on your K-1
Actually, that lens would give a different image than a 200/2.8 on a K-5. The difference is DOF, depth-of-field. A 300/2.8 has a shallower DOF than a 200/2.8.

Let's compare the numbers, assisted by the online DOF calculator* on http://www.dofmaster.com/dofjs.html

300/2.8 on K-1, 10m subject distance gives a DOF from 9.91m to 10.1m, resulting in roughly 18cm.
200/2.8 on K-5, 10m subject distance gives a DOF from 9.83m to 10.2m, resulting in about 35cm

Thus the DOF is nearly twice as deep.

Again, with a larger subject distance:

300/2.8 on K-1, 100m subject distance gives a DOF from 91.4m to 110.4m, in total 19m
200/2.8 on K-5, 100m subject distance gives a DOF from 85m to 121.4m, in total 36.4m

Here the DOF is also twice as deep.

Let's fiddle with the aperture, step down on the K-1 and look at the resulting DOF:
(for sake of brevity I'll only list the total values)

300/2.8 - 19m
300/3.2 - 21.3m
300/3.6 - 24m
300/4.0 - 27.1m
300/4.5 - 30.5m
300/5.0 - 34.5m
300/5.6 - 39m

To obtain an image similar to one taken with a 200/2.8 on the K-5, you'll need a 300mm lens stopped down (or just not being faster than) something between F5.0 and F5.6.

This is between 1⅔ and 2 f-stops slower than the lens on the K-5.

You could spare both money and weight ...

were it not for the higher sensor sensitivity you'd need shooting with a lens 2 f-stops slower.

*) Just for clarification purposes, I've chosen "35mm film" and "aps film" for the comparison
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