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10-20-2021, 08:19 AM   #1
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Understanding "Equivalents" for Medium Format Lenses

More specifically, the 6x7 lenses.

It's pretty common to find people talk about the 6x7 lenses in what they correspond to. However, I recently found a listing that also mentioned the aperture? Like they referred to the 75mm f4.5 as corresponding to a 35mm f2.2, and the 135mm f4 lens as corresponding to a 68mm f2. That confused me a little, since I understand somewhat that the "zoom equivalent" is due to the difference in size of the film, but I'm not sure I understand how the aperture changes?

Like, should I be metering for their aperture equivalent, or were they merely referring to a more specific quality of aperture like depth of field? Or is aperture equivalent not a thing and I just found a weird listing?

Perhaps my understanding of this subject is inadequate, if that is the case can someone point me to some resources that explain these concepts in greater detail?

10-20-2021, 08:39 AM - 2 Likes   #2
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QuoteOriginally posted by Fobbo Quote
Like, should I be metering for their aperture equivalent
Nope. f4 is f4 whatever format you use
10-20-2021, 09:09 AM - 2 Likes   #3
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They are talking about


1) Depth of field. Depth of field of an f/4.0 lens on a 6x7 camera is roughly the same as the depth of field of an f/2.0 lens on a 35mm (or "full frame") camera. Whether the lens itself is a 6x7 lens or a full frame lens makes no difference.


2) Field of view. The field of view of a 100mm lens on a 6x7 camera is roughly the same as the field of view of a 50mm lens on a 35mm camera. Again, whether the lens itself is a 6x7 lens or any other lens, doesn't matter. If you put a 50mm 6x7 lens on a 35mm camera, it will have the same field of view as a 100mm 6x7 lens on a 6x7 camera.


Since depth of field and field of view are two very important properties, and for some shooters, like portrait shooters, wedding shooters, etc, they are basically what matters, you'd hear things like "a 100mm f/4 lens on 6x7 is the same as 50mm f/2 lens on a 35mm camera". While that statement is incomplete, it allows you to quickly compare properties of cameras and lenses from different systems.
10-20-2021, 10:03 AM - 3 Likes   #4
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QuoteOriginally posted by Fobbo Quote
Like, should I be metering for their aperture equivalent, or were they merely referring to a more specific quality of aperture like depth of field? Or is aperture equivalent not a thing and I just found a weird listing?Perhaps my understanding of this subject is inadequate, if that is the case can someone point me to some resources that explain these concepts in greater detail?
It sounds like your understanding is absolutely fine. Your 75mm/4.5 is a 75mm/4.5 and you only ever need to meter it as a 75mm/4.5.

Sadly there is a particular subspecies of gear collector that believes that the goal of all photography should be to exactly match the photo that a 24x36mm format camera would take, and when you hear people claiming that your medium format 75mm/4.5 is "equivalent" to a small format 35mm/2.2 that's what you've come across. Just ignore them.

10-20-2021, 10:09 AM - 1 Like   #5
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QuoteOriginally posted by Dartmoor Dave Quote
Your 75mm/4.5 is a 75mm/4.5 and you only ever need to meter it as a 75mm/4.5.
QuoteOriginally posted by pschlute Quote
Nope. f4 is f4 whatever format you use
Nothing changes. You get the same photo whatever format is used
10-20-2021, 10:15 AM - 1 Like   #6
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QuoteOriginally posted by biz-engineer Quote
Nothing changes. You get the same photo whatever format is used
I think you're being naughty and trying to turn this into yet another argument. It's probably best if the mods close this thread now, because the question in the original post has already been answered.
10-20-2021, 10:15 AM   #7
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QuoteOriginally posted by Fobbo Quote
Like they referred to the 75mm f4.5 as corresponding to a 35mm f2.2, and the 135mm f4 lens as corresponding to a 68mm f2. That confused me a little, since I understand somewhat that the "zoom equivalent" is due to the difference in size of the film, but I'm not sure I understand how the aperture changes?
Equivalent focal length and aperture is answering the question: what would be the equivalent lens that would allow to take the same photo using a full frame format.

10-20-2021, 10:21 AM - 1 Like   #8
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I'm going to ask again for the mods to stop this before things get out of hand. Meanwhile, I'm out as a contributor but I'll be reading with popcorn in hand.
10-20-2021, 10:26 AM - 2 Likes   #9
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QuoteOriginally posted by pschlute Quote
Nope. f4 is f4 whatever format you use
Yeah, f4 is a key you may find on a PC keyboard.
10-20-2021, 10:50 AM - 3 Likes   #10
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Watching this thread now. I see no need to close it yet. The forums allow for dreary topics that aren't political, just not mean ones. As long as no name calling starts and the thread remains respectful, it can go on. Probably if it becomes dull it will wither away on its own.
10-20-2021, 12:01 PM - 1 Like   #11
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First, as others have said "equivalence" is too strong a word when people start misconstruing it as a lens in one format becoming a different lens. A 75mm lens on 6x7 isn't really "equivalent" to a 35mm on full frame. Instead, it's more accurate to say that a 75mm lens on 6x7 makes images with a similar field of view as a 35mm on full frame.

Differences in formats also create "similarity" for some the effects of lens aperture but not others.

1) the aperture for similar exposures of most subjects (exceptions: see #4 & 5): no difference between formats

2) the aperture for similar depth-of-field or hyperfocal distance: use the crop factor

3) the aperture for similar diffraction effects: use the crop factor

4) the aperture for similar brightnesses of stars and other tiny points of light (not moons, planets, or nebulae): use the crop factor

5) the aperture for similar brightnesses of meteors, satellites, and thin lines of light (not moons, planets, or nebulae): use the square root of the crop factor


As long as people talk about how images made with different formats may look similar (or dissimilar) depending on choices of focal lengths, apertures, or ISO settings, then things are OK. Equivalence is too strong a term and it's deeply wrong to say that one lens becomes a different lens due to so-called equivalence.
10-20-2021, 12:17 PM   #12
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QuoteOriginally posted by Fogel70 Quote
Yeah, f4 is a key you may find on a PC keyboard.
But that's a CAPITAL F4. Big difference.
10-20-2021, 12:18 PM   #13
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QuoteOriginally posted by photoptimist Quote
aperture for similar depth-of-field or hyperfocal distance: use the crop factor
I have such a terrible time wrapping my head around any of this. I've previously only used aps-c sensors so I didn't care what 'equivalence' even meant, since I just used the one format.
but... I've been shooting 35mm and some 120 film and the topic is a bit more interesting now...
It seems to me that the quality of light leaving the rear element towards the sensor would be the exact same no matter what format it's mounted on.
All that changes is the amount of 2D surface area captured from a lenses potential image circle.
For instance, the quality of brightness does not change between formats.
So why is it that an image quality such as depth-of-field change between formats?
Wouldn't the depth of field have been 'decided' at the time the light left the lens?
Yet I've seen images where people display the bokeh and field differences and equivalences.
I just don't get it haha
10-20-2021, 12:41 PM - 5 Likes   #14
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QuoteOriginally posted by Fobbo Quote
Perhaps my understanding of this subject is inadequate, if that is the case can someone point me to some resources that explain these concepts in greater detail?
There is one thing you can do to alleviate the confusion, headaches, and heartburn associated with equivalency, crop factor, etc.......
Do yourself a favor and forget you ever heard of the concept or term. Learn how the lens you're using renders on the format you're using it on. If you're using a 6x7 lens adapted to an APSC body then the results it would give on a 6x7 or 24x36 format are meaningless. If you're using an FA 31 Ltd on a K-3 the FOV and DOF it would produce on a K-1 is useless information.
10-20-2021, 12:52 PM - 2 Likes   #15
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If folk are not willing to learn (by taking pictures) how their particular format camera and lens perform with regard to image size/field of view/depth of field....... using a crop-formula is not going to help them.
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