Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions
Chikoo
Chikoo Senior Member • Posts: 1,630
Focal length 35mm equivalent, but not F-stop?

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

Example:

Pentax MX-1

A 4X optical zoom (28-112mm in 35mm format) at f1.8-2.5 for outstanding depth of field control and beautiful bokeh.

Sony DT 18-135

Focal Length (35mm equivalent) : APS: 27-202.5 mm (35mm Equivalent: 18-135mm)

  • Aperture (Max.) : f/3.5 - 5.6
  • Aperture (Min.) : f/22 - 36
Pentax MX-1
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Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?
10

Because it is not necessary.  The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.  This is why we refer to lenses with wide apertures as fast, not shallow.  That ability does not magically change with format.  In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions.  The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens.  That is how most fast lenses have been marketed and the primary reason those lenses are purchased.  In many cases the shallower depth of field of the faster lens was looked upon as a negative.

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EinsteinsGhost
EinsteinsGhost Forum Pro • Posts: 11,977
Re: Focal length 35mm equivalent, but not F-stop?
1

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field. This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format. In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions. The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens. That is how most fast lenses have been marketed and the primary reason those lenses are purchased. In many cases the shallower depth of field of the faster lens was looked upon as a negative.

Amen!

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AlbertTheLazy
AlbertTheLazy Veteran Member • Posts: 8,134
Re: Focal length 35mm equivalent, but not F-stop?
1

EinsteinsGhost wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field. This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format. In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions. The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens. That is how most fast lenses have been marketed and the primary reason those lenses are purchased. In many cases the shallower depth of field of the faster lens was looked upon as a negative.

Amen!

Hallelujah!

Extra comment: the small number of people who are interested in 'equivalence' in the DoF/noise way are usually perfectly capable of working out their arcane calculations for themselves. Angle of view equivalence ('APS-C 35mm = FF 50mm' type stuff) is meaningful for the great unwashed wanting to buy a travel zoom. All the rest would just confuse them.

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Albert
Every photograph is an abstraction from reality.
Most people are more interested in the picture than the image.

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Tom_N Forum Pro • Posts: 16,819
Re: Focal length 35mm equivalent, but not F-stop?
3

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

f-stops are already normalized.  The letter 'f' stands for the focal length of the lens, which will vary with the sensor size for the same field of view.

Thus, for instance,

f/2 on 50mm on an APS-C body => 25mm absolute aperture

f/2 on 75mm on a full-frame body => 37.5mm absolute aperture

The (APS-C, 50mm) and (full-frame, 75mm) combinations will produce the same field of view, and the different absolute apertures will keep the light intensity per unit sensor area the same – for the same subject and lighting conditions.

Without the 'f' notation, photographers would need to keep the different absolute sizes in mind as they went from one sensor size to another.  With it, they can just assume that f/8 means f/8 means f/8 regardless of the size of the sensor or the focal length of the lens.

Allan Olesen Veteran Member • Posts: 3,391
Re: Focal length 35mm equivalent, but not F-stop?
5

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

In fact, a 50 mm f/8 on full frame will have the same effective lens area as a 12.5 mm f/2 on 4x crop.

This is why we refer to lenses with wide apertures as fast, not shallow.

"Fast" is very vague in comparisons between different sensor sizes. You can always get a fast shutter speed by increasing the ISO. You don't want to do this because of noise. So the only relevant comparison is at ISOs which produce the same amount of noise. Usually, a modern FF camera will have the same amount of noise at a 16x higher ISO than a modern 4x crop camera.

See how the numbers compare?

The difference between ISOs with the same noise (4 stops) is the same as the difference between equivalent f-stops (4 stops).

So at the same shutter speed, you can expect the FF camera at f/8 and ISO 1600 to have the same noise as the 4x crop camera at f/2 and ISO 100.

So equivalent f-stops tells us much, much more about a camera's low light abilities.

And it tells us much, much more about how fast the lens really is: Which shutter speed can we get with a given amount of noise?

Comparing real f-stops across different sensor sizes is a waste of time as it tells us nothing about the low light abilities.

(If you do a comparison between real cameras, you may find that the numbers are 1/2 stops off because small sensors of the same generation are usually slightly more efficient than large sensors. But this 1/2 stop error is nothing in comparison to the 3.5 stop error you would make if you just looked at the real f-stops.)

Sadly, this simple truth is rejected by large groups of forum members which appear to either...:

  • ...feel threatened on their photographic knowledge.
  • ...or feel threatened on their small sensor cameras.

So they make these kneejerk reactions in the same way as a creationist being told about Darwin.

ultimitsu
ultimitsu Veteran Member • Posts: 6,650
Re: Focal length 35mm equivalent, but not F-stop?
1

Chikoo wrote:

Why?

Because it would make lenses sound a lot less impressive.

LX3 was the first consumer grade camera to sport an F2.0 lens at the time, a lot of people, including myself, went "wow!" I was so "wow-ed" I bought not one, but two.

3 years later Panasonic released an even more amazing F1.4 LX7, but by then I already understood that it actually isnt all that amazing. it didnt really offer much over XZ1, and later RX100 eclipsed it significantly with an F1.8.

The whole point of understanding equivalence is to help one with camera selection. know what you are really getting.

Pixel Pooper Veteran Member • Posts: 3,235
Re: Focal length 35mm equivalent, but not F-stop?
2

F numbers are great because they let us use the same exposure settings regardless of format, but that's only useful when you are taking a picture. When comparing specs to make a buying decision they tell you nothing about the camera's performance and lead to people thinking that a compact with an f/1.8 lens will be better in low light than a full frame with an f/4 lens.

MoreorLess Veteran Member • Posts: 4,670
Re: Focal length 35mm equivalent, but not F-stop?
1

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

Example:

Pentax MX-1

A 4X optical zoom (28-112mm in 35mm format) at f1.8-2.5 for outstanding depth of field control and beautiful bokeh.

Sony DT 18-135

Focal Length (35mm equivalent) : APS: 27-202.5 mm (35mm Equivalent: 18-135mm)

  • Aperture (Max.) : f/3.5 - 5.6
  • Aperture (Min.) : f/22 - 36

There are arguments about equivalents on both sides but lets be honest we all know the real reason, they want to manipulate the specs they give to make their product look as good as possible.

Lets be realistic as well this is a serious issue, perhaps not for many people who post here but consumers as a whole are being conned as to performance in these kinds of areas(just listing ISO numbers is another).

AlbertTheLazy
AlbertTheLazy Veteran Member • Posts: 8,134
Re: Focal length 35mm equivalent, but not F-stop?
2

Pixel Pooper wrote:

F numbers are great because they let us use the same exposure settings regardless of format, but that's only useful when you are taking a picture.

And that's something the equivalence-heads don't do much of?

When comparing specs to make a buying decision they tell you nothing about the camera's performance and lead to people thinking that a compact with an f/1.8 lens will be better in low light than a full frame with an f/4 lens.

I strongly suspect that most people base their buying decisions on things like ergonomics or pose factor rather than seriously sitting down to work out that a 1/2.3" compact is equivalent to a FF with a pinhole.

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Albert
Every photograph is an abstraction from reality.
Most people are more interested in the picture than the image.

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Donald B
Donald B Forum Pro • Posts: 14,449
Re: Focal length 35mm equivalent, but not F-stop?

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

In fact, a 50 mm f/8 on full frame will have the same effective lens area as a 12.5 mm f/2 on 4x crop.

This is why we refer to lenses with wide apertures as fast, not shallow.

"Fast" is very vague in comparisons between different sensor sizes. You can always get a fast shutter speed by increasing the ISO. You don't want to do this because of noise. So the only relevant comparison is at ISOs which produce the same amount of noise. Usually, a modern FF camera will have the same amount of noise at a 16x higher ISO than a modern 4x crop camera.

See how the numbers compare?

The difference between ISOs with the same noise (4 stops) is the same as the difference between equivalent f-stops (4 stops).

So at the same shutter speed, you can expect the FF camera at f/8 and ISO 1600 to have the same noise as the 4x crop camera at f/2 and ISO 100.

So equivalent f-stops tells us much, much more about a camera's low light abilities.

And it tells us much, much more about how fast the lens really is: Which shutter speed can we get with a given amount of noise?

Comparing real f-stops across different sensor sizes is a waste of time as it tells us nothing about the low light abilities.

(If you do a comparison between real cameras, you may find that the numbers are 1/2 stops off because small sensors of the same generation are usually slightly more efficient than large sensors. But this 1/2 stop error is nothing in comparison to the 3.5 stop error you would make if you just looked at the real f-stops.)

Sadly, this simple truth is rejected by large groups of forum members which appear to either...:

  • ...feel threatened on their photographic knowledge.
  • ...or feel threatened on their small sensor cameras.

So they make these kneejerk reactions in the same way as a creationist being told about Darwin.

if what you say is true show us all some pics to back up your knowledge ? make sure you leave data in tact. and some of YOUR shots not from other testing sites.

cheers don

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Pentax k7,fz150,xz1 my toys.

Allan Olesen Veteran Member • Posts: 3,391
Re: Focal length 35mm equivalent, but not F-stop?
4

Donald B wrote:

if what you say is true show us all some pics to back up your knowledge ? make sure you leave data in tact. and some of YOUR shots not from other testing sites.

I am not taking your bait.

This is very simple technical stuff. It should be possible for you to understand it without seeing photos.

Pixel Pooper Veteran Member • Posts: 3,235
Re: Focal length 35mm equivalent, but not F-stop?

AlbertInFrance wrote:

Pixel Pooper wrote:

F numbers are great because they let us use the same exposure settings regardless of format, but that's only useful when you are taking a picture.

And that's something the equivalence-heads don't do much of?

I take quite a lot of pictures, but not while comparing specs to make a buying decision. I take my pictures with cameras that I've already purchased.

When comparing specs to make a buying decision they tell you nothing about the camera's performance and lead to people thinking that a compact with an f/1.8 lens will be better in low light than a full frame with an f/4 lens.

I strongly suspect that most people base their buying decisions on things like ergonomics or pose factor rather than seriously sitting down to work out that a 1/2.3" compact is equivalent to a FF with a pinhole.

A lot of people compare specs directly and think that a 20MP compact with f/1.8 has better resolution and is faster than a 16MP full frame with f/4. The misleading specs help a lot with the "pose factor".

Pixel Pooper Veteran Member • Posts: 3,235
Re: Focal length 35mm equivalent, but not F-stop?
1

MoreorLess wrote:

There are arguments about equivalents on both sides but lets be honest we all know the real reason, they want to manipulate the specs they give to make their product look as good as possible.

It would take a brave manufacturer to start calling their small cameras f/16 equivalent while everyone else was still calling theirs f/2.8.

Pixel Pooper Veteran Member • Posts: 3,235
Re: Focal length 35mm equivalent, but not F-stop?
3

Donald B wrote:

if what you say is true show us all some pics to back up your knowledge ? make sure you leave data in tact. and some of YOUR shots not from other testing sites.

cheers don

I don't think that's really relevant to this discussion. This is pretty basic stuff that even a beginner with no photographic skills should be able to understand. It would be more useful to ask to see his purchase history so we could evaluate how well he has applied his knowledge to making informed buying decisions.

Allan Olesen Veteran Member • Posts: 3,391
Re: Focal length 35mm equivalent, but not F-stop?
1

Pixel Pooper wrote:

MoreorLess wrote:

There are arguments about equivalents on both sides but lets be honest we all know the real reason, they want to manipulate the specs they give to make their product look as good as possible.

It would take a brave manufacturer to start calling their small cameras f/16 equivalent while everyone else was still calling theirs f/2.8.

Correct. But at least dpreview provides this service for us. Look in any recent test of a compact camera, and you will find a diagram like this, comparing equivalent f-stops of the tested camera to equivalent f-stops of other cameras:

http://www.dpreview.com/reviews/panasonic-lumix-dmc-fz1000?utm_campaign=internal-link&utm_source=reviews-latest-widget&utm_medium=image&ref=reviews-latest-widget

Allan Olesen Veteran Member • Posts: 3,391
Re: Focal length 35mm equivalent, but not F-stop?
1

Pixel Pooper wrote:

 It would be more useful to ask to see his purchase history so we could evaluate how well he has applied his knowledge to making informed buying decisions.

I haven't applied it. I went from compact to APS-C DSLR because of the typical shooting delay and very limited background blur of compact cameras.

At that time I only knew that the larger sensor would give more background blur at the same f-stop, but I had not yet figured out the exact relationship.

I dream of FF, but I also did that before. So no influence there neither.

But if I were to buy a compact camera again for some purpose, the knowledge of equivalent f-stops would heavily influence my decision.

Donald B
Donald B Forum Pro • Posts: 14,449
Re: Focal length 35mm equivalent, but not F-stop?

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

In fact, a 50 mm f/8 on full frame will have the same effective lens area as a 12.5 mm f/2 on 4x crop.

This is why we refer to lenses with wide apertures as fast, not shallow.

"Fast" is very vague in comparisons between different sensor sizes. You can always get a fast shutter speed by increasing the ISO. You don't want to do this because of noise. So the only relevant comparison is at ISOs which produce the same amount of noise. Usually, a modern FF camera will have the same amount of noise at a 16x higher ISO than a modern 4x crop camera.

explain this then 16 x ? I forgot to add these are 300% crops aprox

See how the numbers compare?

The difference between ISOs with the same noise (4 stops) is the same as the difference between equivalent f-stops (4 stops).

So at the same shutter speed, you can expect the FF camera at f/8 and ISO 1600 to have the same noise as the 4x crop camera at f/2 and ISO 100.

So equivalent f-stops tells us much, much more about a camera's low light abilities.

And it tells us much, much more about how fast the lens really is: Which shutter speed can we get with a given amount of noise?

Comparing real f-stops across different sensor sizes is a waste of time as it tells us nothing about the low light abilities.

(If you do a comparison between real cameras, you may find that the numbers are 1/2 stops off because small sensors of the same generation are usually slightly more efficient than large sensors. But this 1/2 stop error is nothing in comparison to the 3.5 stop error you would make if you just looked at the real f-stops.)

Sadly, this simple truth is rejected by large groups of forum members which appear to either...:

  • ...feel threatened on their photographic knowledge.
  • ...or feel threatened on their small sensor cameras.

So they make these kneejerk reactions in the same way as a creationist being told about Darwin.

-- hide signature --

Pentax k7,fz150,xz1 my toys.

Allan Olesen Veteran Member • Posts: 3,391
Re: Focal length 35mm equivalent, but not F-stop?
1

Donald B wrote:

explain this then 16 x ? I forgot to add these are 300% crops aprox

Both of these photos are incredibly blurred. I have no way of seeing if this blur was caused by noise removal or bad lenses.

Any camera can give low noise with low detail.

So I see nothing here which needs explaining.

Edit:

If you say that each pixel on the sensor was magnified to 3x3 pixels in the photo you showed us, some heavy processing has certainly been performed. There is no way the photos could look like that without it.

Leo "Zoom" Senior Member • Posts: 1,620
Re: Focal length 35mm equivalent, but not F-stop?

That would cause even more confusion. Equivalent f is needed only to compare dof. But it doesn't change for exposure calculations. If equivalent f would be widely used - I'm sure, that lots of people would think, that lens is faster that it actually is.

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