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10-04-2019, 12:44 PM - 2 Likes   #16
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So, are you more or less confused at this point?

10-04-2019, 01:25 PM   #17
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This table from PF Staff is very useful:
The Crop Factor Unmasked - Articles and Tips | PentaxForums.com



And remember: FoV in this table is diagonal!

Last edited by angerdan; 10-06-2019 at 04:25 AM.
10-04-2019, 02:21 PM - 5 Likes   #18
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QuoteOriginally posted by clickclick Quote
So, are you more or less confused at this point?
If not, you should be!
10-04-2019, 03:20 PM   #19
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QuoteOriginally posted by photoptimist Quote
All 150 mm lenses have the same FoV on APS-C as long as they have a large enough image circle. The intended camera format of the lens does NOT affect the FoV of that lens when it is mounted on a smaller format camera.

A 150 mm lens mounted on an APS-C camera would have the same FoV as a 225 mm lens on FF camera and the same as about* a 420 mm lens on a 6x7 camera.

A 100 mm lens mounted on an APS-C camera would have the same FoV as a 150 mm lens on FF camera and the same as about a 280 mm lens on a 6x7 camera.

A 60 mm lens mounted on an APS-C camera would have the same FoV as a 90 mm lens on FF camera and the same as about a 150 mm lens on a 6x7 camera.

*Note: the 6x7 numbers are approximate because the differences in aspect ratio for 6x7 (versus FF & APS-C) might force slightly different equivalent focal lengths for landscape versus portrait shots.
It is essentially as simple as this ^^. What you see in the field of view is what you will get in your image when you take the shot. If you have a 35mm film body, especially an older one with a 100% VF, plus say a 100mm FF lens, you can alternatively put that lens on your APS-C body and note the difference in what you will see in the VF from the same location between the two cameras.

10-04-2019, 03:45 PM - 2 Likes   #20
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Here's how I like to describe it. Lets say you have a painting, and for the sake of argument, let's say it is 64 inches wide. If I place a framing mat on it, but the square hole on the mat is only 50 inches wide, then I can only see 78% of the painting, but it's the same painting. If instead, I placed a framing mat on it with only a 33 inch hole in it, I can see only 52% of the painting, but again the painting is the same. This is how crop factor works. A 150mm lens gives you a certain "painting" of the scene. How big your sensor is determines how much of that painting you get to see. If it's a Medium Format sensor, you get to see the whole painting. If it's "full frame", then you get to see 78% of it, and it it's APS-C, you get 52%. Note that 78/52 = 1.5, so the ratio between APS-C and Full Frame we see is the commonly-known "1.5".

Where we get confused is we have a digital representation, and we can print that out at any size, so we if we print the MF image at the same size as the APS-C image, then the MF image appears to have been almost twice as wide with the same lens. But on the sensor, the images were exactly the same size, we just have more of the image with the MF sensor. Most of the time we are after a specific image, so if a 100mm lens gives us a certain image on MF, you would get the same image represented on APS-C by a 52mm lens. You could look at it like this: the 100mm MF combination lets you see "all" of the 100mm painting, whereas with the 52mm on APSC you really are seeing 52% of a 52mm image, and 52mm/52% = 100mm, and this is why a 100mm MF image has the same content as a 52mm APS-C image.

On top of that is the Image Circle of a given lens, this tells us the biggest "painting" we can make with that lens. If I put my DA15 on a medium format camera, it's still a 15mm, and I get a 15mm photo, but half the sensor will get no data (because the image circle is too small), and I'll get only a small circle of image. If I get a 15mm MF lens, I get the EXACT SAME image on the sensor, but I get light all the way to the edge of the sensor. If I take both photos, and print them at the same size, I will see the same image, only the APS-C lens will only expose a circle in the middle of the photo, while the MF lens will have exposed all the way to the edge.

Does this analogy help?

Last edited by Kozlok; 10-04-2019 at 03:51 PM.
10-04-2019, 03:49 PM   #21
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QuoteOriginally posted by onlineflyer Quote
Okay, I understand the focal length of a lens doesn't change if you mount the same lens on a difference size sensor body but I'm trying to understand how the field of view (I believe I'm using the correct term) changes from medium format, to full frame, to APS-C. I asked this question in another thread and don't want to get into a major discussion there, so as not to thread on that thread.So, let me be specific.
If you don't use multible cameras with different sensor sizes, my advise is forget everything about equivalent focal lenght, you won't need it.

QuoteOriginally posted by onlineflyer Quote
It is my understand if you mount a full frame 150mm lens on an APS-C body you get a crop factor of 1.5, so the focal length equivalent is 225mm.
Yes, when you take a picture from the same position with a 150mm lens on an APS-C camera and a 225mm lens on a "fullframe" camera you will get two pictures with the same field of view.
But if you take two pictures with the APS-C camera, one with a 150mm lens designed for APS-C and one with a 150mm lens designed for FF, both pictures will have the same field of view, that is why equivalent calculations are useless if you don't have/use various sensor formats interchangeably.
10-04-2019, 03:53 PM   #22
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QuoteOriginally posted by Kozlok Quote
Here's how I like to describe it. Lets say you have a painting, and for the sake of argument, let's say it is 64 inches wide. If I place a framing mat on it, but the square hole on the mat is only 50 inches wide, then I can only see 78% of the painting, but it's the same painting. If instead, I placed a framing mat on it with only a 33 inch hole in it, I can see only 52% of the painting, but again the painting is the same. This is how crop factor works. A 150mm lens gives you a certain "painting" of the scene. How big your sensor is determines how much of that painting you get to see. If it's a Medium Format sensor, you get to see the whole painting. If it's "full frame", then you get to see 78% of it, and it it's APS-C, you get 52%. Note that 78/52 = 1.5, so the ratio between APS-C and Full Frame we see is the commonly-known "1.5".Where we get confused is we have a digital representation, and we can print that out at any size, so we if we print the MF image at the same size as the APS-C image, then the MF image appears to have been almost twice as wide with the same lens. Most of the time we are after a specific image, so if a 100mm lens gives us a certain image on MF, you would get the same image represented on APS-C by a 52mm lens.On top of that is the Image Circle of a given lens, this tells us the biggest "painting" we can make with that lens. If I put my DA15 on a medium format camera, it's still a 15mm, and I get a 15mm photo, but half the sensor will get no data (because the image circle is too small), and I'll get only a small circle of image. If I get a 15mm MF lens, I get the EXACT SAME image on the sensor, but I get light all the way to the edge of the sensor.Does this analogy help?
Simple but effective, nicely done
10-04-2019, 04:04 PM - 1 Like   #23
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QuoteOriginally posted by onlineflyer Quote
Where an I wrong?
Where you are wrong is by overthinking it.
Draw a small square and a larger square (to represent sensor size) on a piece of white paper on a table under a light.
Select one of your lenses and hold it above the paper and focus the light on the paper and observe.
You do not have to change the lens to paper distance to focus on one square rather than the other do you(obvious) ? So the focal length is the focal length independent of format.
Observe that the light intensity is the same over both the squares (obvious). So aperture is independent of format.
Observe that the image of the light takes up a bigger proportion of the small square compared to the larger square and that reflects the increased telephoto effect of putting a lens onto a smaller format.
Think of this is a slightly different way and observe the the smaller square restricts the views out on the edges of the larger square - your field of view has changed.
Apply these observations to the real world of cameras and it all gets simple.

10-04-2019, 04:16 PM   #24
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QuoteOriginally posted by GUB Quote
Observe that the light intensity is the same over both the squares (obvious). So aperture is independent of format.
Aperture relating to exposure is independent of format, but when it comes to depth of field I am not so sure about that.
10-04-2019, 04:19 PM - 1 Like   #25
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I got some new tyres for my Land Rover recently.

I saw some real big tyres there and the fitter told me they were for an earth moving vehicle. I said I wanted a set of tyres on my Land Rover that would be the equivalent of those real big tyres on an earthmover.

He gave me an odd look and said "have you ever driven an earthmover"
10-04-2019, 05:12 PM - 1 Like   #26
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Part of the confusion in all this is that the term "equivalent" is usually shorthand for "equivalent field of view on a 24x36 Full Frame 35mm sensor". Keep in mind that the field of view is a function of focal length AND the sensor. When you bring other sensors into the conversation, you really need to specify which sensor you mean.

To edit your question a little:
QuoteOriginally posted by onlineflyer Quote
So, let me be specific. It is my understand if you mount a full frame ANY 150mm lens on an APS-C body you get a crop factor of 1.5 when compared to a 24x36 Full Frame 35mm body, so the focal length equivalent field of view is that of 225mm on a 24x36 Full Frame 35mm body.

Okay, now let's use the example of a 150mm 67 format lens. If you mount this lens on a APS-C camera, what is the focal length equivalent on a 24x36 Full Frame 35mm body?
As others have said, the type of lens just changes the size of the projected image circle. It does not change the focal length of the lens. So the equivalent field of view would be 225mm on a 24x36 Full Frame 35mm sensor.

QuoteOriginally posted by onlineflyer Quote
From the following chart, it would appear it is in the area of 450mm. Where am I wrong?
You aren't wrong, you just didn't keep in mind that the sensor is different in each column. 450mm is the equivalent field of view on a 6x7 sensor, but only on that sensor. On the 24x36 35mm "Full Frame" sensor, it is 225mm.
10-04-2019, 05:57 PM   #27
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This post has some good graphics with image circles and the areas of the different formats:

APS-C in 70- 200/ 2.8 range - Page 3 - PentaxForums.com

and earlier in the same thread:

APS-C in 70- 200/ 2.8 range - Page 2 - PentaxForums.com
10-04-2019, 08:51 PM   #28
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QuoteOriginally posted by othar Quote
Aperture relating to exposure is independent of format, but when it comes to depth of field I am not so sure about that.
Yes that is why I said light intensity. The depth of field situation is a bit of a can of worms.
10-05-2019, 12:36 AM   #29
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As photographer, all you care about is field of view and depth of field, the rest is poetry.
10-05-2019, 01:54 AM - 1 Like   #30
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QuoteOriginally posted by GUB Quote
Yes that is why I said light intensity. The depth of field situation is a bit of a can of worms.
Not really, DOF is just a factor of subject distance, f-stop and focal length.

Let us assume that two photographers are taking a picture of a model standing at a given distance, but one of them shoots a K-1 with a 135 mm lens and the other one shoots a KP. Assume f-stop is equal (say 3.5 for the sake of it).

Option a: KP uses 135 mm lens as well. DOF is the same but the framing is 1.5 times tighter because the sensor records less of the *identical* image circle.

Option b: KP uses an 85 mm lens to get the *same framing*, but the DOF increases because it is now a shorter focal length.

Optiom c: KP moves backwards with the 135 mm, but that changes subject distance and thus increases DOF as well.
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