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

Started Jul 24, 2014 | Discussions
Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: What ordinary people want

mostlyboringphotog wrote:

Chikoo wrote:

Chikoo wrote:

Gerry Winterbourne wrote:

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?

Ordinary people just want to take photos. Most of them know absolutely nothing about photography and nothing relating to 35mm cameras means anything to them. Quite a few, though, have some former experience with 35mm, or access to people with such experience.

I'll start with APS-C because that was the first widely used sensor size. In use an APS-C DSLR was, and is, very similar to an SLR , with one major difference - the crop factor. I was caught out by this (my fault for insufficient research but caught I was). Focal length has various effects but the one that most obviously affects users who know about 35mm photography is the FOV so stating FLs in 35mm equivalent as well as the actual FL is a simple courtesy to buyers.

The same thing applies to users of other sensor sizes and camera types: either FL means nothing to them or they have some knowledge about 35mm FL/FOV. So that information is useful. But anything beyond is superfluous: you only have to read the Beginners forum here to know how few newcomers understand DOF as a concept, so what earthly use to them is data on equivalency?

On the other hand, anyone who knows - and cares - about such things is able to work them out so the information isn't needed.

Ditto the noise equivalency mentioned elsewhere in the thread: get into that and you should also be asking why camera makers don't publish data on the noise performance of all cameras. Even at the same sensor size, why don't we see noise data to allow us to compare a D700 with a 6D with ...? Because without that it's pointless to look at a P&S noise performance v a specific other camera.

So what makers give is a compromise between completeness and confusion.

A person who has grown up using 35mm camera, and loves the way his f2.8 lens provide a nice bokeh, and sees the new camera in the store which is smaller and lighter, and boasts the same focal length and zoom range + F-stop, will be more than happy to purchase that camera. Only to find out that it is not the same. He is not getting the same quality of picture as his 35mm f2.8 lens gave him for the last 30 years.

That is deceptive to say the least.

This is like stating the hp of a car with a 4 cylinder engine as a ratio of power to weight and sell it saying it is the same or better than the large v8 he currently drives.

This was true even when "I, Roman" ruled the world - that's why they warned "Caveat Emptor".

So if I have a 35mm camera and a m43, will the same exact focal length, fstop give me the same picture given the ISO/DR/SNR is the same on both?

Ordinary people want photos in which their spouse look good and their kids don't look blurry because they were running around. If you have 35mm camera and m43, you don't need to ask the question and if you do (no condescension intended as we all need to learn at least once and someone like me, many time but I digress...) there are far better ways to learn than looking for the "equivalent aperture" on a spec sheet. That is my opinion.

maybe they should not print any specs. Just try and buy if you like your wife in the picture eh?

Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: What ordinary people want

Gerry Winterbourne wrote:

Chikoo wrote:

Gerry Winterbourne wrote:

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?

Ordinary people just want to take photos. Most of them know absolutely nothing about photography and nothing relating to 35mm cameras means anything to them. Quite a few, though, have some former experience with 35mm, or access to people with such experience.

I'll start with APS-C because that was the first widely used sensor size. In use an APS-C DSLR was, and is, very similar to an SLR , with one major difference - the crop factor. I was caught out by this (my fault for insufficient research but caught I was). Focal length has various effects but the one that most obviously affects users who know about 35mm photography is the FOV so stating FLs in 35mm equivalent as well as the actual FL is a simple courtesy to buyers.

The same thing applies to users of other sensor sizes and camera types: either FL means nothing to them or they have some knowledge about 35mm FL/FOV. So that information is useful. But anything beyond is superfluous: you only have to read the Beginners forum here to know how few newcomers understand DOF as a concept, so what earthly use to them is data on equivalency?

On the other hand, anyone who knows - and cares - about such things is able to work them out so the information isn't needed.

Ditto the noise equivalency mentioned elsewhere in the thread: get into that and you should also be asking why camera makers don't publish data on the noise performance of all cameras. Even at the same sensor size, why don't we see noise data to allow us to compare a D700 with a 6D with ...? Because without that it's pointless to look at a P&S noise performance v a specific other camera.

So what makers give is a compromise between completeness and confusion.

A person who has grown up using 35mm camera, and loves the way his f2.8 lens provide a nice bokeh, and sees the new camera in the store which is smaller and lighter, and boasts the same focal length and zoom range + F-stop, will be more than happy to purchase that camera. Only to find out that it is not the same.

See my comment below* about this.

He is not getting the same quality of picture as his 35mm f2.8 lens gave him for the last 30 years. That is deceptive to say the least.

A small camera can give perfectly good bokeh. It might even be smoother than the 35mm lens. What you've just said is deceptive, because bokeh is the character of the out-of-focus blur, not the amount of it. Here's the problem (or part of it) - move away from the briefest description and you need a book to tell the whole story, and even then most readers won't understand it.

I'd also dispute the general assumption that the quality of 35mm film is better than small-sensor digital. Different in several ways, yes, but not overall better or worse.

This is like stating the hp of a car with a 4 cylinder engine as a ratio of power to weight and sell it saying it is the same or better than the large v8 he currently drives.

(* from above). You've introduced a useful analogy. People know there are big cars and small cars and that they have different characteristics. Most people also have a broad view of which characteristics matter to them. Anyone buying a small car knows it's not the same as a big car; anyone buying a small camera knows it's different from a big one. The difference is that the general population knows more about what cars do so there's no need for pointing out similarities; which means there is no meaningful motoring analogy.

For cars things that tend to matter are number of seats, acceleration, fuel consumption. I have no idea of the power-to-weight ratio of any car including all those I've owned.

Gary, why can't the cameras + lenses be defined accordingly? which is by result, and not by the mathematical numbers that make it?

Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

Chikoo wrote:

I had my lens set at 35mm on my 35mm camera.

On my APS-C, Should I set it at 35mm? and get the same frame?

You are changing the subject.  Your initial post was a complaint about f stop equivalence not being published.  My posts have addressed that specific subject and shown why, for the vast majority of camera users, that information is not needed and would only add confusion.

Information is readily available from camera and lens makers about angle of view equivalence.  The vast majority of photographers find that information useful, which is why it is readily available.

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Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

Chikoo wrote:

I had my lens set at 35mm on my 35mm camera.

On my APS-C, Should I set it at 35mm? and get the same frame?

You are changing the subject. Your initial post was a complaint about f stop equivalence not being published. My posts have addressed that specific subject and shown why, for the vast majority of camera users, that information is not needed and would only add confusion.

Information is readily available from camera and lens makers about angle of view equivalence. The vast majority of photographers find that information useful, which is why it is readily available.

If my angle of view changes, so does my exposure.

bobn2
bobn2 Forum Pro • Posts: 64,435
Re: Focal length 35mm equivalent, but not F-stop?

Chikoo wrote:

Beachcomber Joe wrote:

Chikoo wrote:

I had my lens set at 35mm on my 35mm camera.

On my APS-C, Should I set it at 35mm? and get the same frame?

You are changing the subject. Your initial post was a complaint about f stop equivalence not being published. My posts have addressed that specific subject and shown why, for the vast majority of camera users, that information is not needed and would only add confusion.

Information is readily available from camera and lens makers about angle of view equivalence. The vast majority of photographers find that information useful, which is why it is readily available.

If my angle of view changes, so does my exposure.

No, it doesn't.

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Bob

Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: Focal length 35mm equivalent, but not F-stop?

bobn2 wrote:

Chikoo wrote:

Beachcomber Joe wrote:

Chikoo wrote:

I had my lens set at 35mm on my 35mm camera.

On my APS-C, Should I set it at 35mm? and get the same frame?

You are changing the subject. Your initial post was a complaint about f stop equivalence not being published. My posts have addressed that specific subject and shown why, for the vast majority of camera users, that information is not needed and would only add confusion.

Information is readily available from camera and lens makers about angle of view equivalence. The vast majority of photographers find that information useful, which is why it is readily available.

If my angle of view changes, so does my exposure.

No, it doesn't.

It does.

tko Forum Pro • Posts: 13,066
better be careful
1

If you check typical posts, it seems like people who understand equivalence take a lot more photos and they do better.

I guess understanding the technical aspects of any sport, art, or field does help one. As opposed to sticking your head in the sand . . .

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

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Just another Canon shooter
Just another Canon shooter Veteran Member • Posts: 4,691
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

Simpler?

I photograph a subject with my FF digital camera. The brightness I want in the photograph requires an exposure of 1/125 sec, F/11 at ISO 100. If I grab my old Crown Graphic, load it with ISO 100 film and shoot at 1/125 sec, f/11 the resulting photograph will have the same brightness level as the FF image. Taking a MFT camera, exposing at 1/125 sec., f/11 at ISO 100 again the resulting photograph will have the same brightness as the FF digital and medium format film cameras.

And of course, they all look the same?

Now, grab your compact pocket camera, shoot at f/11 and post the results here next to your FF.

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Erik Magnuson Forum Pro • Posts: 12,247
Re: Focal length 35mm equivalent, but not F-stop?

Just another Canon shooter wrote:

Now, grab your compact pocket camera, shoot at f/11 and post the results here next to your FF.

f/11 @ 1/125 is not far off the default for an old Pocket Instamatic shooting 110 film.  Same brightness there too.

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Erik

Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

Chikoo wrote:

If my angle of view changes, so does my exposure.

Your camera's meter may suggest a different exposure because it sees more light or dark areas.  The actual exposure required does not change.  You need to take a very basic class on exposure.  Clearly reading the technobabble spewed forth on these forums has left you confused.

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Tim Tucker Senior Member • Posts: 1,337
Re: Whoops!

Great Bustard wrote:

Wheatfield7 wrote:

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.

This is what is known as wrong.

Um, it is absolutely correct.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.

Incorrect.

Peoiple really need to learn what they are talking about before they start to spread the sort of misinformation that is quoted above..

Yes, they do. The fact of the matter is that 16x as much light will fall on the sensor for a given perspective, framing, and shutter speed with 50mm f/2 on FF as will fall on the sensor at 12.5mm f/2 on 4x.

- SNIP -

WHOA!!! Hang on there. I don't quite understand the point here, is this different with digital than it was with film?

Let's say I have one lens and I put it on my FF camera and take a picture at a shutter and aperture combination X1.

If I take the same lens and put it on a camera with a smaller sensor then it still transmits the same amount of light and the correct exposure would still be X1 or an equivelant of X1. However the smaller sensor would collect less of this light. If I used film it would be a simple crop to obtain the equivalent photo. The exposure would be the same. Yes?

What I'm getting at is that if you put film behind each of the lenses you discuss then with the exposure of X1 you will get correct exposure for all the film, just the coverage of the different lenses would change, yes? (the total light hitting the film would be the exposure - X1 times the total area exposed).

Are we just talking about noise generation in digital sensors here?

(Not forgetting that the aperture marking system is not limited to digital camera lenses, and it measures transmittance and not DoF. DoF is not even an absolute science as certain assumptions are made about enlargement and viewing distance.)

Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

Just another Canon shooter wrote:

Beachcomber Joe wrote:

Simpler?

I photograph a subject with my FF digital camera. The brightness I want in the photograph requires an exposure of 1/125 sec, F/11 at ISO 100. If I grab my old Crown Graphic, load it with ISO 100 film and shoot at 1/125 sec, f/11 the resulting photograph will have the same brightness level as the FF image. Taking a MFT camera, exposing at 1/125 sec., f/11 at ISO 100 again the resulting photograph will have the same brightness as the FF digital and medium format film cameras.

And of course, they all look the same?

As i said, they will all have the same brightness level.  Even within a format and sensor type different brands will look differently because of resolution, color rendition and noise among other things.  Between formats depth of field and other characteristics will also change.  Trying to massage numbers to render all the variables the same is a fool's errand.

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Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

Chikoo wrote:

If my angle of view changes, so does my exposure.

Your camera's meter may suggest a different exposure because it sees more light or dark areas. The actual exposure required does not change. You need to take a very basic class on exposure. Clearly reading the technobabble spewed forth on these forums has left you confused.

Probably so.

However consider that I was photographing 100 light bulbs of 10w with 160 lumens output each when I was using a 35mm lens on a FF camera.

Now if I switch to a APS-C (1.5x crop) camera with the 35mm lens at the same distance, same focal length, the number of light bulbs in my frame would decrease, which means the light coming in will also decrease.

Yes, no?

Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

Chikoo wrote:

Beachcomber Joe wrote:

Chikoo wrote:

If my angle of view changes, so does my exposure.

Your camera's meter may suggest a different exposure because it sees more light or dark areas. The actual exposure required does not change. You need to take a very basic class on exposure. Clearly reading the technobabble spewed forth on these forums has left you confused.

Probably so.

However consider that I was photographing 100 light bulbs of 10w with 160 lumens output each when I was using a 35mm lens on a FF camera.

Now if I switch to a APS-C (1.5x crop) camera with the 35mm lens at the same distance, same focal length, the number of light bulbs in my frame would decrease, which means the light coming in will also decrease.

Yes, no?

The total light will decrease but not the light per square mm of area.  Since the amount of light falling on a specific area of the sensor remains the same the exposure remains the same.  A lot of the technobabble you read hear is about total light.  Total light can be ignored.  What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.

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Erik Magnuson Forum Pro • Posts: 12,247
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

Trying to massage numbers to render all the variables the same is a fool's errand.

It's quite easy with digital: apply the crop factor in stops to both f-stop and ISO.

If DOF matters, you might use a DOF calculator -- which will give you get the exact same answer as using the crop factor  (assuming same distance, AOV, output size) . If noise vs. shutterspeed matters, you (or the camera) will end up adjusting ISO to get the same result.

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Erik

Lee Jay Forum Pro • Posts: 55,402
Re: Whoops!

Tim Tucker wrote:

Are we just talking about noise generation in digital sensors here?

No, we're talking about the noise that's in the light itself (due to the quantum nature of light).  And the signal to noise ratio that such noise generates on its own is equal to the square root of the total number of photons.  More SNR = good, therefore more light = good.  More light comes from either brighter light (faster f-stop) or more capture area (bigger sensor).  Given the same f-stop, a sensor with 16 times more sensor area will capture 16 times as much light and therefore have four times the signal to noise ratio, which is the same as if the small sensor had been using an f-stop four stops faster with the same shutter speed.

IOW, FF 1/60th f/8 = 4x-crop 1/60th f/2, as far as signal to noise ratio goes.

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Lee Jay

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Just another Canon shooter
Just another Canon shooter Veteran Member • Posts: 4,691
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

Just another Canon shooter wrote:

Beachcomber Joe wrote:

Simpler?

I photograph a subject with my FF digital camera. The brightness I want in the photograph requires an exposure of 1/125 sec, F/11 at ISO 100. If I grab my old Crown Graphic, load it with ISO 100 film and shoot at 1/125 sec, f/11 the resulting photograph will have the same brightness level as the FF image. Taking a MFT camera, exposing at 1/125 sec., f/11 at ISO 100 again the resulting photograph will have the same brightness as the FF digital and medium format film cameras.

And of course, they all look the same?

As i said, they will all have the same brightness level. Even within a format and sensor type different brands will look differently because of resolution, color rendition and noise among other things. Between formats depth of field and other characteristics will also change. Trying to massage numbers to render all the variables the same is a fool's errand.

But you can get the same brightness with a variety of different settings, and films/ISOs. Equivalence tells you how to compare the results, with regards to a few factors.

You present a perfect example why you should know about equivalence. Shooting at f/11 with m43, aside for some very specific applications like macro, degrades the IQ. For general landscape use, f5.6 would be enough, and provide better IQ. I remember a thread where Bob questioned even the need for f/11 eq. (f/5.6 on m43) for DOF purposes.

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Erik Magnuson Forum Pro • Posts: 12,247
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

The total light will decrease but not the light per square mm of area. Since the amount of light falling on a specific area of the sensor remains the same the exposure remains the same.

Why is it so important the numbers for exposure have to match?  Don't you care more about the results than the exact numbers used to get them (cross-format).

A lot of the technobabble you read hear is about total light. Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.

You really would shoot an FF and MFT camera identically?  If you were shooting 35mm and 4x5, would you'd choose the same film and f-stop/shutterspeed?

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Erik

bobn2
bobn2 Forum Pro • Posts: 64,435
Re: Focal length 35mm equivalent, but not F-stop?
1

Beachcomber Joe wrote:

Chikoo wrote:

Beachcomber Joe wrote:

Chikoo wrote:

If my angle of view changes, so does my exposure.

Your camera's meter may suggest a different exposure because it sees more light or dark areas. The actual exposure required does not change. You need to take a very basic class on exposure. Clearly reading the technobabble spewed forth on these forums has left you confused.

Probably so.

However consider that I was photographing 100 light bulbs of 10w with 160 lumens output each when I was using a 35mm lens on a FF camera.

Now if I switch to a APS-C (1.5x crop) camera with the 35mm lens at the same distance, same focal length, the number of light bulbs in my frame would decrease, which means the light coming in will also decrease.

Yes, no?

The total light will decrease but not the light per square mm of area. Since the amount of light falling on a specific area of the sensor remains the same the exposure remains the same. A lot of the technobabble you read hear is about total light. Total light can be ignored. What matters from a photographic standpoint is the amount of light per square mm hitting the sensor.

Completely wrong. What matters is the total number of photons captured. Differences in exposure can always be compensated by simply changing the relationship between exposure and output grey scale. The total light locks in a noise pattern in the image, which can't be altered apart from noise reduction, which will usually lose you image detail. Those wishing to maximise image quality deal in total light. Of course, if you stick to one sensor size, there is no difference between total light and exposure.

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Bob

Erik Magnuson Forum Pro • Posts: 12,247
Re: A Camera Club Perspective

JBurnett wrote:

  • I would estimate that 35 of them purchased their APS-C cameras with the kit lens, and had no concept of equivalencies at the time.

YMMV. In my club 35mm equivalent focal lengths are mentioned often. If your members had compact cameras previously, I'll bet almost none of them could have told you the actual focal length of the lenses. If anyone said "mm" instead of "3x", they would have used equivalence.

  • Maybe 10 understand that there is a difference in depth of field between their camera and cameras with larger or smaller sensors. Only 3 would know where to go to calculate that equivalence (or would ever care to do so).

Again, I'll bet most actually do know this intuitively -- it's obvious when they use their cell phone camera. Or when they compare the results to their previous compact camera.

  • Most members understand that if a light meter says f/4 at 1/60 at ISO 200 on one camera, they could use the same settings on a different camera

Since you say they mostly have similar cameras, that's reasonable. But would you tell the compact camera user to use ISO 3200 @ f/4 like the APS-C kit users?

  • Only one person in the club really cares about equivalencies, because he shoots both full-frame and APS-C bodies at the same time.

Or if he has to talk to the APS-C users.

  • The majority of time I've been drawn into a discussion about equivalencies, it has been a newer member who wants to move from a P&S to a DSLR or mirrorless. Equivalent focal lengths to what they see with their existing camera makes sense to them. 

The differences in noise or DOF never comes up?

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Erik

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