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10-04-2019, 09:47 AM   #1
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Focal length = focal length, I know . . . . . . . . . . BUT

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. 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. 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? From the following chart, it would appear it is in the area of 450mm. Where an I wrong?

Field of View Tables, APS-C, 24x36, 645D, 645, 6x7 - PentaxForums.com

10-04-2019, 09:53 AM   #2
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AFAIK....

Medium format would have a crop of 0.79 (I think). So if you multiple full frame focal length by 1.5x to get APS-C, for medium format you'd multiple 0.79x...

Reading your question...
I think a 150mm medium format lens on an aps-c sensor would look like a 343.5mm

1.5+0.79 = 2.29x
150 x 2.29 = 343.5

I am very possibly wrong on this.
10-04-2019, 10:03 AM - 2 Likes   #3
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Focal length is focal length. The FOV of a (let's say) 50mm lens on APS-C is the same whether the lens is made for APS-C, FF, 645 or 8x10 view cameras. The term "crop factor" only makes sense because it's always relative to the FOV of a given focal length on a 24x36 mm sensor (so FF).
10-04-2019, 10:04 AM   #4
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The 6x7 150mm would be equivalent to a Full Frame 75mm lens in Field of View terms, so on a aps-c camera the crop factor/FoV, it wiuld be more of a 112mm lens

10-04-2019, 10:05 AM - 4 Likes   #5
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QuoteOriginally posted by FozzFoster Quote
AFAIK....

Medium format would have a crop of 0.79 (I think). So if you multiple full frame focal length by 1.5x to get APS-C, for medium format you'd multiple 0.79x...

Reading your question...
I think a 150mm medium format lens on an aps-c sensor would look like a 343.5mm

1.5+0.79 = 2.29x
150 x 2.29 = 343.5

I am very possibly wrong on this.
No. A 150mm medium format lens would have the same FoV on aps-c as one designed for 135FF.

The only difference is the size of the image circle projected by the lens.
10-04-2019, 10:06 AM - 1 Like   #6
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Any 150mm lens, whether originally for APS-C or FF or 67 format, will all give the same magnification and same field of view on an APS-C camera.

Crop factor is relevant when comparing the same focal length on different size sensors. Ignore crop factor when sensor size remains constant.
10-04-2019, 10:06 AM   #7
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Accordingly to the chart 150mm on 67 gives 26.3 degrees of horizontal field of view, and to get the same horizontal FoV, you need 77mm on FF, and roughly 50 mm on APS-C. The same chart shows that 450mm lens on 67 (not on FF!) has got roughly the same field of view as 150mm on APS-C.

Last edited by pentageek; 10-04-2019 at 10:18 AM. Reason: clarification
10-04-2019, 10:09 AM   #8
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QuoteOriginally posted by FozzFoster Quote
AFAIK....

Medium format would have a crop of 0.79 (I think). So if you multiple full frame focal length by 1.5x to get APS-C, for medium format you'd multiple 0.79x...

Reading your question...
I think a 150mm medium format lens on an aps-c sensor would look like a 343.5mm

1.5+0.79 = 2.29x
150 x 2.29 = 343.5

I am very possibly wrong on this.
It's 0.79 for *digital* 645, yes.
A 150mm lens (assuming it covers the image circle of the 645Z) would "look like" a 120 mm lens when mounted on the 645Z, like a 150mm mounted on the K-1 and it would look like a 225 mm lens when mounted on the KP.

But this is all because we have grown used to relate everything to the 35mm (24x36, "Full Frame") format... If the standard were Digital-645, we would say that the K-1 has a crop factor of 1.3, and the KP has a crop factor of 1.9...

10-04-2019, 10:23 AM   #9
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QuoteOriginally posted by onlineflyer Quote
From the following chart, it would appear it is in the area of 450mm. Where an I wrong?

Field of View Tables, APS-C, 24x36, 645D, 645, 6x7 - PentaxForums.com
You are comparing the crop vs fullframe or the crop compared to 6x7. Look at that chart with apsc field of view and 150 on apsc is 9.1.
9.1 in the other columns are all at different places. Where that 9.1 falls are equivalent. So 9.1 on apsc is equivalent to where 9.1 is on 24x36. They are both equivalent to where 9.1 falls on 645D, which in turn are all equivalent to where 9.1 falls on 645. Ditto with 6x7 or any numbers like 1.8 million x 3.2 million.
10-04-2019, 10:34 AM   #10
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QuoteOriginally posted by onlineflyer Quote
...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.
The crop factor is simply the ratio of the frame diagonal lengths relative to a 24x36mm frame. It is useful when the aspect ratio is the same (e.g. 3:2 for FF and APS-C), but not so much when it is not (12:9, 1:1, 7:6, 5:4, 3:1, 2:1, etc.).

The calculator at the linked page below is a bit obtuse, but covers most use cases.

https://www.scantips.com/lights/fieldofview.html


Steve

Last edited by stevebrot; 10-04-2019 at 10:41 AM.
10-04-2019, 10:43 AM - 1 Like   #11
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I found this

[ follow the link to find an easy chart to read ]

QuoteQuote:
Using FF and 35mm FL as the reference, here are the crop factors & "equivalent FF FLs" (for the same FOV):

MF (film) Pentax 67N ("6x7"format 6x7cm - actually 55x70mm 0.49x 17mm
MF (film) Pentax 645N ("645" format 6x4.5cm - actually 56x41.5mm) 0.62x 21.5mm
MF (digital) Pentax 645D (44x33mm) 0.79x 27.5mm
FF (actually 36x24mm) 1x 35mm
APS-H (Canon) 1.26x 44mm
APS-C (Sony, Pentax, Nikon) 1.52x 53mm
APS-C (Canon) 1.62x 56.5mm
Four-Thirds (Olympus, Panasonic) 2x 70mm
P&S (various sizes around 1/1.8"-1/2.5") 4.8x-6x 170mm-210mm

Note 1: Since some of these formats do not have a 3:2 AR, the comparison has been made using the FF diagonal length of 43.3mm.


Note 2: MF has other sizes besides the 3 shown.
Read more at: Crop Factor, Focal Length and Field of View - PentaxForums.com


does it help out ?
10-04-2019, 10:57 AM - 2 Likes   #12
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QuoteOriginally posted by onlineflyer Quote
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?
Equivalent to what ? A 150mm lens is a 150mm lens. The magnification it provides on the sensor can not change.

You ask what is the equivalent focal length ? To what? To a 35mm camera, to a 67 format camera, to a 4/3 camera, to a Large Format View camera ? To a shoebox with a hole in the end. The list is endless.

If you are not familiar with the other formats and only use a aps-c camera then knowing its equivalence is meaningless.
10-04-2019, 11:15 AM - 2 Likes   #13
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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.
10-04-2019, 11:17 AM   #14
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Well there are a lot of numbers being thrown around here so maybe this will clear things up, or it just might make things worse:
A 150mm lens on a 6x7 format camera that lens would give you the normal field of view for a 150mm lens so will will use this as our starting point. That field of view will be be equivalent to about a 75mm lens on full frame (6x7 to FF crop factor is about 2), or about the same as a 50mm lens on APS-C (6x7 to APS-C crop factor is about 3).

So this means that if you want a field of view similar to a 150mm lens on a 6x7 camera but with a full frame camera you should use something like a 75mm lens or if you want that same field of view but on an APS-C camera you should use a 50mm lens

Any 150mm lens that you can stick to the front of a camera that will provide a large enough image circle to cover the sensor of film will provide basically the same field of view. It doesn't matter if the lens is a real 150mm lens for a u4/3 camera or if it is a 150mm lens designed for a 8"x10" glass plate camera the field of view provided by that lens is the same on the same sensor/film.

To confuse matters lots of cheaper cameras like to quote the equivalent 135 format (full frame) focal length which is why you see some cameras that claim an equivalent focal length up to like 1500mm or other silly number but they are using a tiny sensor (think cellphone sized sensor) that have a crop factor of like 8 (compared to full frame) or something so a that real 150mm focal length ends up giving a field of view similar to a 1200mm lens on full frame or an 800mm lens on APS-C.

The whole crop factor discussion is rather meaningless unless you are trying to figure out how to get a similar image with a different format camera. I know that andromeda frames up nicely with my 400mm on APS-C but if I were to switch to full frame I would need a 600mm lens to get a similar framing. If instead I jumped to u4/3 I could get similar framing with a 300mm lens. If I were to jump up to a 645Z because apparently I won the lottery I would need like an 800mm lens to get similar framing. Finally if I was dumb and wanted to try astrophotography with a Pentax 67II I would need something like a 1200mm lens to get similar framing.
10-04-2019, 12:34 PM   #15
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QuoteOriginally posted by onlineflyer Quote
...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.

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?
The 6x7 format 150mm equivalent focal length would also be a 225mm on an APSC camera (1.5 x 150) as compared to FF; hence, the saying focal length is focal length. In other words, if you mount a 150mm K-mount film lens on your APSC camera you will see the same field of view as a 6x7 150mm lens. However, the image circle is much larger on the 6x7 lens than the K-mount lens.

Last edited by tuco; 10-04-2019 at 01:13 PM.
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