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01-13-2015, 08:53 AM   #16
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QuoteOriginally posted by BrianR Quote
I may be mistaken, but I think you've misunderstood Adam. Your example changes the sensor size and keeps the lens the same (field of view changes here). His quote above is referring to changing between two 55mm lenses but keeping the sensor the same. Like comparing the field of view of a FA-645 55mm on a K-3 and the field of view of a SMC Pentax-DA* 55mm on a K-3, in this case the field of view is the same.
I think we're on the same page- I just went on to explain the whole crop factor thing, as it sounded like mee was still confused after reading the article.

QuoteOriginally posted by mee:
So how can someone say the field of view (that is, the framing) doesn't change -- it does change!
If you're referring to this:

QuoteOriginally posted by Adam Quote
Two lenses with the same focal length used on any given camera will produce the same field of view. The crop factor just refers to the difference between the field of view of say a 55mm lens on 645 film vs its field of view on APS-C.
I was talking about using a 55mm 645 lens on APS-C vs using a DA* 55mm on APS-C, for instance. Same FoV there. Much different if you compare the DA* on APS-C and the 55mm 645 lens on a 645Z.


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01-13-2015, 09:12 AM   #17
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QuoteOriginally posted by owl Quote
FLx1.5, simple as that.
No, that's not right. 1.5 is FF vs APS-c. He asked about MF to APS-C. As Adam said above, it's 2.4.
01-13-2015, 09:52 AM   #18
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QuoteOriginally posted by Adam Quote
I think we're on the same page- I just went on to explain the whole crop factor thing, as it sounded like mee was still confused after reading the article.
Uh oh, now I may be confused about who was confused. You (Adam) and me (BrianR) are on the same page, and were from the start. The bit of yours (Adam) that I quoted is part of what mee also quoted and where I thought mee was tripped up on, and I think if he/she was reading the page we (Adam/BrianR) are on it was upside down so I tried to right it. Possibly without success and not as thorough as other explanations


More seriously, I think it's beneficial for anyone confused to get their hands on a k-mount film slr with a 50mm lens. Semi functional k1000's can be had cheap or by digging though film-era photographers closets. Try the 50mm on the film camera and on a pentax dslr and you can play with what happens first hand.
01-13-2015, 10:09 AM   #19
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QuoteOriginally posted by BrianR Quote
Uh oh, now I may be confused about who was confused. You (Adam) and me (BrianR) are on the same page, and were from the start. The bit of yours (Adam) that I quoted is part of what mee also quoted and where I thought mee was tripped up on, and I think if he/she was reading the page we (Adam/BrianR) are on it was upside down so I tried to right it. Possibly without success and not as thorough as other explanations
Oh boy- I think this just proves how confusing of a concept the crop factor thing is to begin with

QuoteOriginally posted by Parallax Quote
No, that's not right. 1.5 is FF vs APS-c. He asked about MF to APS-C. As Adam said above, it's 2.4.
Yup, and that's a rough figure for film 645 cameras (the 645z/d would be closer to 2.0).


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01-13-2015, 12:16 PM - 2 Likes   #20
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You know one thing that might help is to look at the "normal" focal length on all the different formats, and use that as a basis for comparison. First however, it might be helpful to understand how the "normal" focal length is determined, and why it is significant.

The "normal" focal length for any format is equal to the measurement of the sensor or film diagonal. It's easy to calculate this if you know the dimensions of the image by using the Pythagorean Theorem, a2 + b2 = c2 where a and b are the sides. So for instance on 35mm film the dimensions are 24mm by 36mm. Plug that into the equation and the hypotenuse is 43mm. So 43mm is the "normal" focal length for 35mm film (which is why Pentax chose to create the FA 43mm limited, but that's another discussion), not 50mm, though plenty of people will be willing to tell you otherwise. So what's so important about that number?

Well it comes down to field of view and perspective. Lenses have the ability to render the world in a different way than what we observe with our eyes. A wide angle lens, which has a shorter focal length, takes a wider-than-normal image of the world, and squeezes the sides in until it fits in the image circle. This causes spatial relationships (i.e. perspective) to appear to elongate from front to back. A telephoto lens on the other hand, has a longer focal length, and compresses the scene from front to back. So for example, wide angle lenses are generally not used for portrait photography, because they tend to elongate the subject's nose to an unflattering degree. At the "normal" focal length though, the lens neither expands nor compresses the scene, it just leaves it as-is. With a "normal" lens on 35mm film 43mm from the lens gets mapped across 43mm of film plane. In a 24mm wide angle, 24mm from the lens gets stretched to 43mm of film plane, and in a 300mm telephoto, 300mm of focal length gets compressed to 43mm of film plane. Keep in mind though that 43 is only the number for 35mm film.

So, now with that concept in mind let's look at "normal" focal lengths across different formats.
  • 8"x10" Large format (203mm X 254mm) = 325mm
  • 4"X5" Large Format (101mm X 127mm) = 162mm
  • 6 X 7cm Medium Format = 92mm
  • 6 X 6cm Medium Format = 85mm
  • 6 X 4.5cm Medium Format = 75mm
  • 33x44mm 645D/Z "Cropped MF" = 55mm
  • 24x36mm "35mm" aka "Full Frame" = 43mm
  • 16X24mm APS-C aka "Cropped Sensor" = 29mm
  • 13x17mm Micro Four Thirds = 21mm
  • 4.5X6.1mm Pentax Q (original, not Q7) = 7.6mm
325mm would be considered a super telephoto lens on an APS-C camera like the K-3, but we can see from the chart above that it is only a "normal" lens on an 8x10 film camera. Likewise, a 7.5mm lens would be a normal lens for the Pentax Q, but if you could somehow mount it to a K-3 it would be an ultra-wide angle lens, though it would almost certainly not fill the image circle since it was designed for a smaller format.

In the original example given by Adam in this thread, we had a 55mm 645 lens. If you look at the chart above, 55mm is shorter than the 75mm "normal" length of the format for which that lens was designed, hence we call it a wide angle lens. If we mount that lens on the new 645Z though, it now IS a true normal lens with no perspective compression or expansion. Looking again at the chart, we can see that it becomes a short telephoto lens (i.e. longer than 29mm) if we mount it on the K-3. And if we mount it on the Pentax Q, it becomes a super telephoto.

Will you see the same thing in the viewfinder of all of these cameras? No. Assuming you do not move the camera, you will see a smaller and smaller section of the same frame. Only the field of view will change, the perspective in the parts of the frame that you can see will remain the same (because you have not moved the camera). Now, here's the tricky part, in practice you would most likely want to move the camera to maintain the same field of view, and in moving the camera, you would change the perspective. For instance, if you put the 55mm 645 lens on the Pentax Q, you would have to move the camera WAY back in order to get the same scene in the frame that you had in your 645 viewfinder. But because you've moved the camera back, you've changed the perspective, and the spatial relationships between objects in the scene become compressed front-to-back.

Like I said before, there are different ways to understand the same phenomenon. There is likely one that will make more sense to you than the others, so latch on to that and don't let go until you are completely secure in the knowledge that you can always fall back on your preferred method of understanding. Don't be concerned that others may explain it differently, and seemingly contradictory to what you understand. They are just coming at the problem from a different angle.
01-13-2015, 03:12 PM   #21
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QuoteOriginally posted by Parallax Quote
No, that's not right. 1.5 is FF vs APS-c. He asked about MF to APS-C. As Adam said above, it's 2.4.
I stand my the statement, at least as I understood the question. The whole point of crop factor is to have a common reference point to 35mm film format, so why does a lens made for MF get any crop factor adjustment? It is whatever focal length it is, just because it has a larger image circle to start with is irrelevant.
01-13-2015, 03:56 PM - 1 Like   #22
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QuoteOriginally posted by owl Quote
I stand my the statement, at least as I understood the question.
You're certainly welcome to stand by your statement. His original question was:
QuoteQuote:
Please help me figure out the Crop factor between Pentax 645 lens and K3 APS-C
I think everyone else interpreted that as being a 645 lens used on 645 format vs APS-c. I can see your interpretation as well.

Last edited by Parallax; 01-13-2015 at 04:09 PM.
01-13-2015, 09:54 PM   #23
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It's quite interesting seeing how different people explain crop factor - and as it has been said all of them are correct.
With APS-C format, when I pick a lens I have to calculate what FOV that would relate to in 35mm film format (or full frame if you wish), before I can say whether it will right for the shot.
I think the reason for this is for decades I had only 3 lenses for my full frame, a 35mm for landscapes, 55mm for streetwork and gatherings and a 135mm for portraits.

01-15-2015, 06:31 PM   #24
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QuoteOriginally posted by Adam Quote
I was talking about using a 55mm 645 lens on APS-C vs using a DA* 55mm on APS-C, for instance. Same FoV there. Much different if you compare the DA* on APS-C and the 55mm 645 lens on a 645Z.
Huh? You seem to have said both are the same FoV there but then, the very next line, say they are much different?!?!?!

The chart on the webpage here explaining this shows different FoV's for those lenses.




Look at a 19mm lens in 645d format -- 110.5 deg vs 19 mm lens in 19 mm lens in APS-C format -- 75.4 degrees.

I thought the FoV would be the same but, in APS-C, the smaller sensor size would mean some form of cropping of that normal FoV (vs the larger format compared). It seems a big confusing mess.

Obviously, I don't NEEEED to understand this no more than a race car driver needs to understand many of the technicalities behind how their car functions.. but it would be nice to know.
01-15-2015, 09:04 PM   #25
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QuoteOriginally posted by mee Quote
Huh? You seem to have said both are the same FoV there but then, the very next line, say they are much different?!?!?!
This is what I had tried to clarify. He's giving two examples, one where the FoV is the same, the other where it is different:

1) Take two different 55mm lenses, one the DA* 55mm made for aps-c, the other the FA-645 55mm made for medium format. Mount both lenses on 2 different k-3's. Field of view will be the same (29.4 degrees by your chart). The point here that a 55mm lens is a 55mm lens, it doesn't matter what format it's made for, it's still 55mm.

2) Use the two lenses from above. Put the DA* 55mm on a k-3. Put the FA-645 55mm on a 645z. Field of view will be different (29.4 vs 53.4 degrees by your chart). Here's where 'crop factor' comes in due to different sensor sizes.
01-16-2015, 06:12 PM   #26
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Haha I will be your hardest case. Not because I'm attempting to be difficult, but because I really don't seem to get it.

If a 55mm lens is a 55mm lens then why do they show different FoV on the chart per sensor size? That is the part I don't get. You and others keep saying a 55mm lens is a 55mm lens but the chart shows a 55 DA* lens would not have the same Fov as one would on a 55mm 645 lens!?

You even mention it in your two examples.. It seems example 1 contradicts example 2. That is the part that is blowing my mind. I'm looking at a chart that shows 50 in one format at one FoV and 50 in another format at a different FoV!

Perhaps, the issue is the 55mm 645d lens on a K-3 would really behave like a 29mm lens.. I'm guessing due to the crop factor? But, if that is the case, the 55mm lens (while it says 55mm on the tin) is no longer a true 55mm lens. It would behave similar to a 29mm lens in FoV. right? If so, the chart itself is confusing. If not, then I'm simply confusing. haha
01-16-2015, 06:38 PM   #27
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Yes, it is a true 55mm lens. It behaves like a 55mm lens no matter what camera it's used on. Pick a picture in your collection that was printed from a 35mm negative. Take scissors and cut 25% off of the left, right, top, and bottom of the photo and throw the scraps away. Now look at the center chunk that's left. Did cutting the the center out of the photo change the physical properties of the lens that was used on the camera?
Xmm is Xmm. Xmm acts as Xmm.
Here's another way to look at it. Position a chair several feet from a window, sit down and take note of what you see. Now get some masking tape and tape off a few inches of the window on all 4 sides. Sit back down and notice what you see now. You see less on the sides, top and bottom, right? Did that change your eyes any? Are they now telephoto eyes?

Last edited by Parallax; 01-16-2015 at 06:49 PM.
01-16-2015, 08:44 PM - 1 Like   #28
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QuoteOriginally posted by mee Quote
If a 55mm lens is a 55mm lens then why do they show different FoV on the chart per sensor size? That is the part I don't get. You and others keep saying a 55mm lens is a 55mm lens but the chart shows a 55 DA* lens would not have the same Fov as one would on a 55mm 645 lens!?
The chart gives FoV for a lens and camera together. The 55mm entry in the orange column is not saying what happens to a 645 55mm lens, it's giving the FoV of any 55mm lens on a 645d/z (*see below). The body is important.

Likewise the 55mm entry in the yellow column gives the FoV for any 55mm lens on a k-3, whether it's the DA* or one made for 645.

Focal length and sensor size are the keys. Example 1) had two different lenses of the same focal length mounted on the same sized sensor (both setups use the yellow column), same FoV. Example 2) had two different lenses of the same focal length on different sized sensors (one setup uses the yellow column, the other the orange), so different FoV. I suggested earlier you get a k-mount film camera and dslr and swap lenses between them for a practical example, I'd do this if you can.


*I say" any lens", but there are practical problems mounting any lens on any camera, let's just ignore that for now
01-16-2015, 10:10 PM   #29
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QuoteOriginally posted by mee Quote
If a 55mm lens is a 55mm lens then why do they show different FoV on the chart per sensor size?
Because 55mm means something different on each format. On APS-C it means a short telephoto, and on 645 film it means a "polite" wide angle.

If you wanted to extend the comparison, on 8x10 sheet film 55mm is an extreme ultrawide, the equivalent of a 5mm lens on APS-C (that is, you would need to use a 5mm lens for your K-3 to get the same shot you took with a 55mm on your Sinar 8x10). But if you took a 55mm lens from a large format camera and mounted it to a K-3, it would give you the same image that any 55mm would on that camera. (A large format lens would project a huge image circle, only a small portion of which would be captured)

That's why they say a 55mm lens is a 55mm lens. The lens doesn't know to which body it is mounted, it just behaves like a 55mm lens. It is the sensor/film size that determines what 55mm means.
01-16-2015, 11:10 PM - 1 Like   #30
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QuoteOriginally posted by mee Quote
If a 55mm lens is a 55mm lens then why do they show different FoV on the chart per sensor size? That is the part I don't get. You and others keep saying a 55mm lens is a 55mm lens but the chart shows a 55 DA* lens would not have the same Fov as one would on a 55mm 645 lens!?
As BrianR explained earlier, FOV is a combination of camera and focal length. Do you understand the drawing that I posted earlier in this thread?

And a little more theory (in case you don't know what focal length is )

Focal length is that distance from a lens where parallel incoming beams of light come together in a single point. In the drawing below, the lens is drawn in black, the light comes in from the right (two parallel beams drawn in red) and comes together at the left of the lens. The distance from the lens to the point where the red lines cross each other is the focal length.

To get real parallel incoming beams of light, you need to have a subject at infinity distance from the lens.



For a given focal length, this point is always the same. This point is also the point where the film / sensor must be placed to get an 'in focus' picture of a subject at infinity.

Maybe you played with a magnifying glass in earlier days to put a piece of paper on fire? You placed the magnifying glass between the piece of paper and the sun and adjusted the distance of the magnifying glass to the paper so the sun was just the smallest possible dot on the paper; that distance between magnifying glass and paper was (is) the focal length of the magnifying glass.
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