35mm equivalent question

[MNE]

So,

A 20 1.7 is the equivalent of a 40 3.4, but the 3.4 only has reference to DOF right? The lens still allows the f 1.7 amount of light through, right? or is the amount of light coming through the lens halved as well?

TIA

 

[Adventsam]

Will be really interesting to put the new fuji up against the 20mm f1.7 imo, especially the GH2 with 20mm f1.7 will perform almost identically in terms of fov and dof.

So,

A 20 1.7 is the equivalent of a 40 3.4, but the 3.4 only has reference to DOF right? The lens still allows the f 1.7 amount of light through, right? or is the amount of light coming through the lens halved as well?

TIA

 

[Diane B]

You're right. Light is still f/1.7
--
Diane B
http://www.pbase.com/picnic
G1 gallery http://www.pbase.com/picnic/temp_g1

 

[ANAYV]

The light coming through the lens is still 3.4.The light is not halved.
Just the focal length (with 4/3 and m 4/3 system) is x 2

It's because of the size of the sensor......Full Frame sensored D-SLR's don't have this effect.

Most APS-C sensors have either 1.5 or 1.6x crop factor (Canon 1D is 1.3)

ANAYV

So,

A 20 1.7 is the equivalent of a 40 3.4, but the 3.4 only has reference to DOF right? The lens still allows the f 1.7 amount of light through, right? or is the amount of light coming through the lens halved as well?

TIA

 

[Jay A]

The light coming through the lens is still 3.4.The light is not halved.
Just the focal length (with 4/3 and m 4/3 system) is x 2

It's because of the size of the sensor......Full Frame sensored D-SLR's don't have this effect.

Most APS-C sensors have either 1.5 or 1.6x crop factor (Canon 1D is 1.3)

ANAYV

Not sure you typed this the way you wanted. First off, you're contradicting yourself. You say it's still 3.4. Did you mean to say it's still 1.7 (which it is)? Second, you say the light is not halved. Well, if it WAS 3.4 yes it would be halved...but again, it's not 3.4 it's still 1.7.

Again, I think you just made some mistakes in typing, but I wanted to clear it up.

 

[BigBarney]

So,

A 20 1.7 is the equivalent of a 40 3.4, but the 3.4 only has reference to DOF right? The lens still allows the f 1.7 amount of light through, right? or is the amount of light coming through the lens halved as well?

This article tries to explain how f numbers for lenses are derived:

http://en.wikipedia.org/wiki/F-number

In other words the f number for any lens is a constant regardless of which camera it is attached to. The same goes for the focal length.

What differs for m43 and FF (Full frame 35mm camera image size) is that a 20mm focal length lens on m43 will capture the same angle of view as a 40mm lens on a FF camera. Those of us brought up on 35mm photography in the film era simply have to double the focal length of a m43 lens to translate it into angle of view equivalence of those former 35mm film lenses we used.

 

[Brent Gair]

I don't know why so many people manage to bungle this simple question.

Diane B posted the best response"

Yes you are right. Light is not halved. For purposes of exposure f1.7 is stil f1.7.

 

[misolo]

So,

A 20 1.7 is the equivalent of a 40 3.4, but the 3.4 only has reference to DOF right? The lens still allows the f 1.7 amount of light through, right? or is the amount of light coming through the lens halved as well?

TIA

The light density reaching the sensor is the same for the same f-number. But, for the same f-number, the total amount of light reaching the m4/3 sensor is one fourth of the total light reaching the FF sensor (same density but one-fourth the sensor area).

Both depth-of-field and and total image noise are closely proportional to the total amount of light reaching the sensor, so thinking of the lens as the same as a 40 f/3.4 in a FF camera is a good analogy for all practical purposes in terms of the final images you can get.

 

[deniz erdem]

So,

A 20 1.7 is the equivalent of a 40 3.4, but the 3.4 only has reference to DOF right? The lens still allows the f 1.7 amount of light through, right? or is the amount of light coming through the lens halved as well?

TIA

The light density reaching the sensor is the same for the same f-number. But, for the same f-number, the total amount of light reaching the m4/3 sensor is one fourth of the total light reaching the FF sensor (same density but one-fourth the sensor area).

Both depth-of-field and and total image noise are closely proportional to the total amount of light reaching the sensor, so thinking of the lens as the same as a 40 f/3.4 in a FF camera is a good analogy for all practical purposes in terms of the final images you can get.

what i would consider a practical purpose would be the exposure? how is this a good analogy i dont understand.

 

[MNE]

Thanks Misolo, That was the answer I was looking for.

Without doing the math, we are saying that for all practical purposes, the shutter speed will be the same, but the m43 sensor will require a tiny bit slower shutter speed to produce the same exposure.

How is this for an analogy:

Someone takes a flashlight and shines it into a tube with the same diameter as the rim of the flashlight. If the tube was 1/2 the diameter, the light would have to be a tiny bit brighter in order to get the same luminance at the other end. This is because more light is absorbed by the sides of the tube proportionally.

Should I stop drinking now?

 

[sacundim]

Both depth-of-field and and total image noise are closely proportional to the total amount of light reaching the sensor, so thinking of the lens as the same as a 40 f/3.4 in a FF camera is a good analogy for all practical purposes in terms of the final images you can get.

No. Depth of Field and background blur are much more complicated topics than that.

The image circles produced by a 20mm f/1.7 lens for points outside the focus plane are the same size as the corresponding circles produced by a 40mm f/3.4 lens; this is the basis for the claim that a 20/1.7 is "equivalent" to a 40/3.4. However, a photo taken with a 4/3" camera will be enlarged twice as much as a photo from a full frame camera . This means that for a proper DoF calculation, you also have to adjust the acceptable circle of confusion criterion to make it more stringent for the 4/3" output—twice as stringent. This means that on the final print, the 20mm f/1.7 on a 4/3" camera will generally have less DoF than a 40mm f/3.4 on a full frame camera, and will in many situations exhibit just as little DoF as a 40mm f/1.7.

The idea that a 20mm f/1.7 lens on 4/3" has a lot more depth of field than a 40mm f/1.7 on full frame comes from two mistakes:

1. Confusing depth of field with background blur. In cases where the two lenses show the same DoF, the out of focus areas from the 40/1.7 will be a lot blurrier. People assume blurrier = less DoF, but DoF isn't how blurry the unfocused parts of scene are; DoF is about how much of the scene is acceptably sharp.
2. There will be many circumstances where the 20/1.7 on 4/3" will indeed show more DoF than the 40/1.7 (but less than the 40/3.4), especially at longer focus distances and with smaller prints. This is because when the degree of blur falls under the CoC criterion, suddenly things "snap" into acceptable sharpness, and this happens more easily with the 20/1.7 and its smaller blur.

Check out this page, especially figures 3-5 and the sections of the text they appear in:

http://toothwalker.org/optics/dof.html

 

[misolo]

Thanks Misolo, That was the answer I was looking for.

Without doing the math, we are saying that for all practical purposes, the shutter speed will be the same, but the m43 sensor will require a tiny bit slower shutter speed to produce the same exposure.

Sorry if I added to the confusion... For the same f-number and the same ISO setting the exposure time will be the same independently of sensor size.

What I meant is that you can get very similar images on, say, a Panasonic G2 and a Canon 5DII as follows:

• G2: focal length=20mm, aperture=f/1.7, shutter speed=1/100, ISO=400.
• 5D: focal length=40mm, aperture=f/3.4, shutter speed=1/100, ISO=1600.

The two images, if printed at the same size, will be very similar -- in terms of depth of field, in terms of motion blur, and in terms of noise.

But! If you have f/1.7 on both cameras, and the same ISO setting on both, the shutter speed should be exactly the same (though in practice not exactly the same, since manufacturers differ in their interpretation of the ISO standard, lenses may have better or worse glass, etc.) You'll just have more shallow depth-of-field and less noise with the full frame camera.

 

[misolo]

what i would consider a practical purpose would be the exposure? how is this a good analogy i dont understand.

Rather than rewrite much of what I said, please see my reply to MNE. Thanks.

 

[misolo]

Both depth-of-field and and total image noise are closely proportional to the total amount of light reaching the sensor, so thinking of the lens as the same as a 40 f/3.4 in a FF camera is a good analogy for all practical purposes in terms of the final images you can get.

No. Depth of Field and background blur are much more complicated topics than that.

The image circles produced by a 20mm f/1.7 lens for points outside the focus plane are the same size as the corresponding circles produced by a 40mm f/3.4 lens; this is the basis for the claim that a 20/1.7 is "equivalent" to a 40/3.4. However, a photo taken with a 4/3" camera will be enlarged twice as much as a photo from a full frame camera . This means that for a proper DoF calculation, you also have to adjust the acceptable circle of confusion criterion to make it more stringent for the 4/3" output—twice as stringent. This means that on the final print, the 20mm f/1.7 on a 4/3" camera will generally have less DoF than a 40mm f/3.4 on a full frame camera, and will in many situations exhibit just as little DoF as a 40mm f/1.7.

The idea that a 20mm f/1.7 lens on 4/3" has a lot more depth of field than a 40mm f/1.7 on full frame comes from two mistakes:

1. Confusing depth of field with background blur. In cases where the two lenses show the same DoF, the out of focus areas from the 40/1.7 will be a lot blurrier. People assume blurrier = less DoF, but DoF isn't how blurry the unfocused parts of scene are; DoF is about how much of the scene is acceptably sharp.
2. There will be many circumstances where the 20/1.7 on 4/3" will indeed show more DoF than the 40/1.7 (but less than the 40/3.4), especially at longer focus distances and with smaller prints. This is because when the degree of blur falls under the CoC criterion, suddenly things "snap" into acceptable sharpness, and this happens more easily with the 20/1.7 and its smaller blur.

Check out this page, especially figures 3-5 and the sections of the text they appear in:

http://toothwalker.org/optics/dof.html

Sorry, but I think you are rather confused. Even the source you cite clearly says otherwise:

Generally, when two formats are compared with the purpose of taking the same picture, the larger format requires a focal length that is R times as large as the lens focal length for the smaller format, where R is the ratio of the format dimensions. The above observation may then be generalized into a rule of thumb: The smaller format employed at an F-number N yields the same DOF as the larger format at an F-number of R × N.

Nothing like the empirical method: get your hands on both a m4/3 and a FF camera and try it to convince yourself (I have a GF1 and a 5D, and did plenty of experiments to convince myself that the equivalence really was as precise as claimed).

 

[Guy Parsons]

Hells bells, this is getting into silly land again.

F/1.7 is always f/1.7 that's all you need to know in terms of exposure.

Shutter speeds are exactly the same on M4/3 as on 35mm frame as on 8"x10" film plates or pocket cameras. The f stop is the f stop and that's why it was invented, so that the quoted say "1/125 at f/8 for ISO 100" exposure means exactly the same exposure on any camera whether the sensor be the size of an ant or as big as a house with its appropriate focal length lens that happens to cover the f/8 range.

The smaller sensor and shorter focal lengths combine to give an apparently different depth of field.

It is totally misleading to talk about "the total amount of light is less so the exposure must be different" - that is rubbish talk.

If that were so then any time you cropped an image to a smaller size then the effective exposure of the crop would change? I don't think so.

Drive that "total amount of light" argument right out of your mind. It's only the actual stated aperture on the lens or what it is stopped down to by the camera that matters.

Anyway, there's a few absolute basics in photography....

An f/stop is an f/stop.

Perspective is purely dependent on the place you stand to take the photo, it has nothing to do with lens focal length.

Depth of field when keeping the subject the same size in the frame purely depends on apertures used and has nothing to do with focal length changes. This one is definitely not intuitive but a good article explaining it is over at Luminous Landscape http://www.luminous-landscape.com/tutorials/dof2.shtml

Regards........... Guy

 

[gandalfII]

but a smaller sensor is noisier. That's why you use a fast lens and low ISO with a small format, and a higher f stop and higher ISO with larger format. This is the equivalency principal, which causes heated debates, but shouldn't really- it's well established and has been around since film days.

 

[MNE]

I am sorry for triggering another DOF equivalency debate.

 

[Tenhauser]

For the same perspective, framing, f-ratio, and display size, larger sensor systems will yield a more shallow DOF than smaller sensors in proportion to the ratio of the sensor sizes. For example, 20mm f/1.7 on m43 (aperture diameter is 20/1.7=11.76mm) will yield LESS background blur (more DOF) than 40mm f/1.7 (aperture diameter is 20/1.7=23.52mm!) on FF (less DOF)

For the same perspective, framing, aperture diameter, and display size, all systems have the same DOF, i.e. 20mm f/1.7 on m43 (aperture diameter is 20/1.7=11.76mm) will yield SAME background blur as 40mm f/3.4 (aperture diameter is 40/3.4=11.76mm)

 

[novice7]

Both depth-of-field and and total image noise are closely proportional to the total amount of light reaching the sensor , so thinking of the lens as the same as a 40 f/3.4 in a FF camera is a good analogy for all practical purposes in terms of the final images you can get.

I don't understand how " DoF can be proportional to the total amount of light reaching the sensor ". Photons captured buy one well can't hardly have any effects on the other wells, let alone the image projected by the lens! DoF depends on the image projected by the lens and the print/view size (how much you magnified that image for viewing/printing).

 

[cameron2]

A 20 1.7 is the equivalent of a 40 3.4, but the 3.4 only has reference to DOF right? The lens still allows the f 1.7 amount of light through, right? or is the amount of light coming through the lens halved as well?

The m43 20mm f1:1.7 has a "field-of-view equivalent" of a 40mm lens on a 35mm camera.

The m43 20mm f1:1.7 has the DOF of a 20mm f1:1.7 on a 35mm camera.

The m43 20mm f1:1.7 has a "lets-light-in equivalent" of an f1:1.7 lens on a 35mm camera.