[swanlefitte]

Can someone please explain to me how this is even possible? This goes against my understanding of everything I have read on this forum as well as every website .

I am also sorry to hear this. Many of us hate this flippant use of terminology because it confuses so many. It happens time and time again. It was an expensive lesson for you and that sucks.

[SEVASTIANOS]

Thank you to everyone for being so kind to explain. I had been researching focal length and field of view for a month and every time I thought I had grasped the concept I would read a different piece of information that would make me confused all over again. I found this article easy to understand. https://photographylife.com/equivalent-focal-length-and-field-of-view
I have to say I am still a bit confused by what I saw when mounting this 45mm 67 lens onto my 645 body. Even though the 645z sensor is significantly smaller than the 6x7 film I saw zero difference between the FOV of the 45mm 6x7 lens and the 45mm Pentax 645 lens. Even more confusing was the fact that when I mounted the 45mm 6x7 lens onto my Full Frame I did see a difference in the FOV (looked more zoomed in although I know this is due to the size of the sensor not being the full projected image from the lens). Maybe the missing piece to the puzzle is that I never compared the 45mm 67 lens mounted on the P67 versus the same lens on my 645z, but I assumed that by mounting my 45m 645 lens on my MF and comparing that to the 45mm 67 lens mounted on my same MF would produce different fields of view. Shouldn't the 45mm 67 lens look more like a 55mm 645 lens on the MF since the 645 sensor would only be picking up the center of the lens? Then I see charts saying the 45mm 67 lens is equivalent to a 23mm lens on a full frame which is not at all what I saw when comparing a photo of the 45mm (67 and 645) on my MF to a photo of my 24mm FF lens on my FF camera.
I was under the assumption that when they say the 45mm 6x7 lens is equivalent to a 23mm on full frame and a 35 on medium format, that meant that what I would see when looking through a 6x7 camera with that 45mm would resemble what I would see when mounting a 23mm to my full frame and a 35mm on my medium format. As I said, what I saw on my medium format (don't own 6x7) with the 45mm was nowhere near to what I saw when I took a photo with my full frame with a 24mm mounted (closest I had to 23mm).
Although I let my frustration get to me earlier, I am really liking this 45mm 6x7 prime lens. One day I would like to test it out versus my 45-85mm 645 lens, directly. If it is an improvement then I would be happy to keep this lens. Also since purchasing the 67 adapter I am starting to look at other 67 lenses. If anyone has any suggestions would appreciate it. Need a nice close up portrait lens (already have the 90mm f2.8 for wider portraits).
When I have more time I will read through again and maybe fully understand these equivalences. Im a little confused as to why the 45mm for the 6x7 looks exactly the same on my MF camera as the 45mm for my MF, but when I put it on my FF it looks more zoomed. On the one hand people are saying the 45 will always be a 45 so I am totally confused.

1st image is Pentax K1 with 24mm full frame lens
2nd image is Pentax K1 with 45mm 67 lens
3rd picture is Pentax 645z with 45mm 645 lens
4th picture is Pentax 645z with 45mm 67 lens

[GUB]

Thank you to everyone for being so kind to explain. I had been researching focal length and field of view for a month and every time I thought I had grasped the concept I would read a different piece of information that would make me confused all over again. I found this article easy to understand. https://photographylife.com/equivalent-focal-length-and-field-of-view
I have to say I am still a bit confused by what I saw when mounting this 45mm 67 lens onto my 645 body. Even though the 645z sensor is significantly smaller than the 6x7 film I saw zero difference between the FOV of the 45mm 6x7 lens and the 45mm Pentax 645 lens. Even more confusing was the fact that when I mounted the 45mm 6x7 lens onto my Full Frame I did see a difference in the FOV (looked more zoomed in although I know this is due to the size of the sensor not being the full projected image from the lens). Maybe the missing piece to the puzzle is that I never compared the 45mm 67 lens mounted on the P67 versus the same lens on my 645z, but I assumed that by mounting my 45m 645 lens on my MF and comparing that to the 45mm 67 lens mounted on my same MF would produce different fields of view. Shouldn't the 45mm 67 lens look more like a 55mm 645 lens on the MF since the 645 sensor would only be picking up the center of the lens? Then I see charts saying the 45mm 67 lens is equivalent to a 23mm lens on a full frame which is not at all what I saw when comparing a photo of the 45mm (67 and 645) on my MF to a photo of my 24mm FF lens on my FF camera.
I was under the assumption that when they say the 45mm 6x7 lens is equivalent to a 23mm on full frame and a 35 on medium format, that meant that what I would see when looking through a 6x7 camera with that 45mm would resemble what I would see when mounting a 23mm to my full frame and a 35mm on my medium format. As I said, what I saw on my medium format (don't own 6x7) with the 45mm was nowhere near to what I saw when I took a photo with my full frame with a 24mm mounted (closest I had to 23mm).
Although I let my frustration get to me earlier, I am really liking this 45mm 6x7 prime lens. One day I would like to test it out versus my 45-85mm 645 lens, directly. If it is an improvement then I would be happy to keep this lens. Also since purchasing the 67 adapter I am starting to look at other 67 lenses. If anyone has any suggestions would appreciate it. Need a nice close up portrait lens (already have the 90mm f2.8 for wider portraits).
When I have more time I will read through again and maybe fully understand these equivalences. Im a little confused as to why the 45mm for the 6x7 looks exactly the same on my MF camera as the 45mm for my MF, but when I put it on my FF it looks more zoomed. On the one hand people are saying the 45 will always be a 45 so I am totally confused.

Focal length is a function of a lens only.
Aperture is a function of a lens only.
But the Field of View (FoV) is a function of a lens' Focal length and it's relationship with the sensor size.
This means for a given camera format all lenses of a given focal length will have the same FoV .
And if you change camera format with a given lens the FoV will change. (just like if you change lens focal length with a given format the FoV will change)

[SEVASTIANOS]

Focal length is a function of a lens only.
Aperture is a function of a lens only.
But the Field of View (FoV) is a function of a lens' Focal length and it's relationship with the sensor size.
This means for a given camera format all lenses of a given focal length will have the same FoV .
And if you change camera format with a given lens the FoV will change. (just like if you change lens focal length with a given format the FoV will change)

Thank you for the reply. Why does the FOV for the 45mm 67 lens look exactly the same as the 45mm for the 645? See photos 3 and 4 in my comment above.

[GUB]

Thank you for the reply. Why does the FOV for the 45mm 67 lens look exactly the same as the 45mm for the 645? See photos 3 and 4 in my comment above.

Because they are both taken with the same format sensor and they are both 45mm.
45mm is a function purely of the lens. So all lenses defined as 45mm will behave the same on a given format.
Question - did you do the good old optics diagrams at school? They are a great help here.

[pschlute]

Maybe the missing piece to the puzzle is that I never compared the 45mm 67 lens mounted on the P67 versus the same lens on my 645z, but I assumed that by mounting my 45m 645 lens on my MF and comparing that to the 45mm 67 lens mounted on my same MF would produce different fields of view. Shouldn't the 45mm 67 lens look more like a 55mm 645 lens on the MF since the 645 sensor would only be picking up the center of the lens?

Two important aspects here...magnification and FOV.

Magnification of a lens is dictated by it's focal length. All 45mm lenses made for any format will magnify to the same extent, you can see this in your pictures above 3 and 4. In practice there may be small differences as one 45mm lens may have an actual FL of 46.2 and a different 45mm lens may be 44.1

Note that magnification may appear to have increased when you use the same lens on a smaller format as in your picture 2. That is only because when you look at a smaller sensor (K-1) image, you are effectively enlarging the image.



The FOV will change when mounting the same lens on different formats. This is because the smaller sensor takes a smaller "bite" out of the lens's image circle. This is shown in your images 2 and 4.

Im a little confused as to why the 45mm for the 6x7 looks exactly the same on my MF camera as the 45mm for my MF, but when I put it on my FF it looks more zoomed

As I said above, all 45mm lenses will look the same on the same format The zoomed affect is due to enlargement of the smaller K-1 image.


I understand that the 645Z sensor is a 0.79 crop factor, so an equivalent view to using 645Z + 45mm lens is to use K-1 + 35mm lens.

You also need to ensure that any adapter you use does not increase/decrease the FL

[Serkevan]

Thank you for the reply. Why does the FOV for the 45mm 67 lens look exactly the same as the 45mm for the 645? See photos 3 and 4 in my comment above.

45mm is 45mm - the P67 lens just covers the entire 6x7 film. If you were to mount the 645's 45mm on the P67 (I know it cannot happen, but let's assume so) you'd have the same FOV but would see black corners as the lens won't cover the entire area of the film.


When people talk about "equivalent", they don't mean that the lens changes focal length when used on a different sensor size - they mean that to get the same FoV on a 645Z+45mm you'd need a K-1+35mm lens. Conversely - if you were to mount your 45mm on the K-1 you'd be getting the same FoV as you do when mounting a 57mm lens on your 645Z.

[Lowell Goudge]

I will start by admitting that I have read only the first post and the last post in this thread, and will begin by saying that I despise the concept of equivalent focal length, however, I am going to throw my hat in the ring here and offer a comment.

Yes focal length is focal length. And regardless of what format you put the lens on, the image magnification and field of view on that format is unchanged.

The issue of equivalence is, equivalent to what?

When changing formats, normally the equivalent focal length ratio is expressed as a factor that is based upon the ratio of the diagonal of the format you are coming from, to the diagonal of the format you are going to.

For full frame 35mm film to an APSC sensor, that factor is 1.5. , someone asked about going from a 6x7 to APSC this ratio is 3.2 x compared to the equivalent lens on 6x7

But regardless for example any 150mm lens on APSC will behave the same on APSC

[SEVASTIANOS]

Thank you again for all the replies. I understand that the focal length doesn't actually change when mounting the same lens to another body. I understand that it is the size of the sensor that gives the illusion of a different focal length.Yet this still doesn't explain what I am seeing in my tests.
For example, the 645z sensor is a lot smaller than the 67 negative yet when I place the 45mm 67 on my 645 and then place my 45mm 645 lens on my 645 they look exactly the same. Some people here explain that away as (focal length doesnt change) but as you saw in my post above, placing the 45mm 67 lens onto my full frame camera gives the illusion that it is zoomed in more (compared to 645 camera with same lens).

Also look at these photos below.
I placed the 45mm 67 lens on my K1 and then placed my 45mm 645 lens on my K1 and the result shows that the 45mm 67 lens gives the illusion of being wider than the 45mm 645 lens.
So how is this even possible that my 645z shows no difference with those lenses mounted but my K1 does? Feel like I am losing my mind here.
Just to be sure I didnt mess up something I re did the test (645 camera with 45mm 67 lens mounted and then with 45mm 645 lens mounted) and again they looked exactly the same compared to each other.
There are no glass elements in my adapters and I am using the p67-645 adapter for my 645 tests and then for my k1 tests I am attaching the p67-645 adapter to my p645-pk adapter.

[othar]

For example, the 645z sensor is a lot smaller than the 67 negative yet when I place the 45mm 67 on my 645 and then place my 45mm 645 lens on my 645 they look exactly the same.

They have the same focal lenght (both 45mm) so they are supposed to give you the same field of view when used on the same camera. The only difference of those lenses is, you can't use the lens designed for the smaller sensor (645) on a 67 camera, because it doesn't cover the whole image circle and the flange focal distance is shorter (the lens of the smaller format is usually closer to the film plane because the mirror box doesn't have to be as big as with greater sensor/film formats)

Some people here explain that away as (focal length doesnt change) but as you saw in my post above, placing the 45mm 67 lens onto my full frame camera gives the illusion that it is zoomed in more (compared to 645 camera with same lens).

Focal length doesn't change, but field of view does when you use different sensor sizes (that's the whole reason why equivalent charts exist)

So how is this even possible that my 645z shows no difference with those lenses mounted but my K1 does? Feel like I am losing my mind here.

As mentioned above can focal lenghts slightly differ from the stated number, but I think the discrepancies you see in your images are there because focal lenght is stated for infinity focus, but depending on how the different lens designs focus on closer objects can there be something called "focus breathing" -> that means that the actual focal lenght of the lens will change with focus (usually it gets shorter the closer you focus)
Take a test shot with each lens of something close to infinity focus and the field of view of both lenses will get the same again.

[GUB]

Interesting one. As Othar said the different focus can account for some of the difference - one of your shots is focused a lot closer than the other. I opened your images in my editor and the distance from the black spot above the bananas to the white vertical line is 626 pixels in one and 518 in the other. That is probably too much for focus alone. Was the viewing point exactly the same - ie on a tripod?.

[Lowell Goudge]

I can only suggest that given the focal length of the lens, 45 mm is quite wide for a 6x7 lens and may be highly corrected for such a wide FOV. As a result, by looking only at the central 24 x 16 mm out of the full FOV of tap 70 x 60 mm frame, perhaps globally the distortion of the lens, as you move to the frame edges might result in a different calculated focal length. This would be true as well for my Sam Yang 14mm/2.8 if I looked at a crop out of the middle using my q.

[GUB]

I can only suggest that given the focal length of the lens, 45 mm is quite wide for a 6x7 lens and may be highly corrected for such a wide FOV. As a result, by looking only at the central 24 x 16 mm out of the full FOV of tap 70 x 60 mm frame, perhaps globally the distortion of the lens, as you move to the frame edges might result in a different calculated focal length. This would be true as well for my Sam Yang 14mm/2.8 if I looked at a crop out of the middle using my q.

I was thimking down the same lines ie projection. But thought the centre of the image would be the most stable. And that is why I took the measurement above near the centre of the image.
EDIT I don't think that is the solution -I downloaded the last two shots at the top (2 45mms on 645) and the same dimension as I measured above comes out identical. In that case 400 pixels.

[Sandy Hancock]

Are you absolutely certain that bottom image in post #144 was from the K-1? It looks more like it should if it was from the 645Z. I can't see any EXIF to check it myself. You have clearly taken a lot of test images, and getting them confused is not beyond the limits of possibility

The images in post #137 are exactly as one would expect from using a 45mm lens on different format cameras, and two different format 45mm lenses on the same format camera.

[GUB]

Are you absolutely certain that bottom image in post #144 was from the K-1? It looks more like it should if it was from the 645Z. I can't see any EXIF to check it myself. You have clearly taken a lot of test images, and getting them confused is not beyond the limits of possibility

The images in post #137 are exactly as one would expect from using a 45mm lens on different format cameras, and two different format 45mm lenses on the same format camera.

Yes I too would like to see the exif.