f-stop equivalent for digital

[chuck]

I can't find this on the web:

Does anyone know whether the f-stop on a digital camera is equivalent to the f-stop on a 35mm film camera? I'm used to using the old formula: 1/(ISO value) = f16 for pictures on bright sunlit days, then adjusting the f-stop and shutter speed for actual conditions. Could do that in my head and rarely had to use a meter in the old days. Does this same formula hold for digital? In other words, will f8 at 1/100 sec. on my FZ20 give me the same exposure as f8 at 1/00 on my old Nikon 6006 for the same ISO setting?

I'm a little surprised that the aperature only goes down to f8 on the FZ20, although I have read that the shorter focal length results in a greater depth of field at that setting. I'm not sure of the reason for this, either.

Chuck

[24Peter]

I'm no expert on this one but I think because of the small sensor (CCD) size on the FZ20, F8 = F22 on a normal 35mm camera.

[chuck]

Peter, I'm not too sure about that - the CCD is smaller, but the diaphram could be closer to the focal plane. Since f-stop is focal distance/opening diameter, these could still be the same ratios. Not that I know this, but it's a possibility.

Chuck

[dwig]

f/stops are f/stops. The system has been used for so long because f/8 on any lens is the same, in terms of light intensity, as f/8 on any other lens, discounting any small factor for light absorbtion and reflection. f/stops are the same on digital cameras as they are on film cameras.

There may be slight differences/inaccuracies in a digital camera's ISO rating. The test reviews I've read have occasionally reported ~1/3 stop variances in some models. Additionally, digital P&S cameras don't have traditionally f/stop rings and merely report the f/stop on a digital dispplay. The reported values will be rounded off to some increment that the camera is configured to display. Some only display the nearest standard full stop leaving the display somewhat inaccurate (+/- 1/2 stop at times). Also note that most modern wide range zooms vary in f/stop as you zoom so if you set an f/stop and then zoom the value may change. I say "may" as some models will shift the iris to correct for the drift as you zoom.

An f/stops relationship to depth-of-field is also not directly affected by changing from film to digital. It is affected by the "film" format (negative or images sensor size) in addition to the focusing distance and focal length relative to the image sensor's diagonal measure. Its only digital camera's tendancy to have smaller sensors than traditional film camera's that makes a digital camera's lens to have more depth of field at matching f/stops and proportional focal lengths.

[pixelator]

I can't find this on the web:

Does anyone know whether the f-stop on a digital camera is equivalent to the f-stop on a 35mm film camera? I'm used to using the old formula: 1/(ISO value) = f16 for pictures on bright sunlit days, then adjusting the f-stop and shutter speed for actual conditions. Could do that in my head and rarely had to use a meter in the old days. Does this same formula hold for digital? In other words, will f8 at 1/100 sec. on my FZ20 give me the same exposure as f8 at 1/00 on my old Nikon 6006 for the same ISO setting?

I'm a little surprised that the aperature only goes down to f8 on the FZ20, although I have read that the shorter focal length results in a greater depth of field at that setting. I'm not sure of the reason for this, either.

Chuck
Another problem ( I do not know if also in the Fz20) is that the definition in the digitals are WORSE in F8 than in F5,6 and F4. A clear example is the Sony V3. Read the review in this page.

[chuck]

OK, Peter and dwig, I have two opposing opinions on whether the digital f-stop represents the same exposure number as in 35mm.

Any tie-breakers? I would think that if the definition of f-stop is (distance from iris to focal plane) divided by (iris diameter), then the two would not necessarily be equivalent, except by chance. But then, I'm no expert. Would like to get more opinions on this.

[IanJ]

Dwig is right. f-stops are f-stops. For any camera lens the same f-number will allow the same exposure with the same ISO and shutter speed settings. The f-number is the focal length of the lens divided by the aperture diameter. So for a 35mm camera with a 50mm lens f8.0 represents an aperture diameter of around 6mm, whereas for a 8mm digital camera lens the aperture diameter at the same f-stop would be only around 1 mm. That is the reason that most digital cameras don't stop down below f8.0 because the aperture is so tiny that diffraction effects come into play. The other effect is on depth of field, a larger aperture opening will give a smaller depth of field. That's why digital cameras generally have depth of field to burn compared to 35mm stuff.

[24Peter]

Well as I said, I'm definitely no expert on this subject. But I guess my question is, if in Ian's example f8 on a 35mm camera has a 6mm aperture diameter and the digital camera only has a 1mm diameter opening, then if for the sake of argument if the 35mm camera has a 1mm opening at f22, doesnt' that make f8 on the digital camera and f22 on the film camera roughly equal? :confused:

[IanJ]

Yes, but only as far as depth of field is concerned. f22 is still f22 no matter what the format. You might like to read this article (http://dpfwiw.com/exposure.htm) which gives a good grounding on all this stuff and might help you get your head around it all.

Cheers, Ian

[chuck]

Great answers - thanks!

It makes sense that the f-stop definition would remain the same, regardless of the camera. What changes, I think, is the ISO rating. Regardless of how much light a particular f-stop lets into your camera, the digital ISO is DEFINED so that the image results are equivalent to the corresponding 35mm. ISO is the only variable that can change, because the other variables (f-stop, shutter speed, etc.) are all set by physical measurements. Film ISO is set by the granularity of the photosensitive crystals on the film media; the digital camera media has no such chemical layer, so the digital "ISO" value can be manipulated for 35mm equivalency.

It makes sense to define all digital camera variables in terms of the 35mm format. Example: for the same magnification value, the focal lengths of digital camera lenses vary depending on the size of the CCD, which is can be different between camera models and makes, unlike 35mm film which is always the same. So there has to be an equivalent standard for comparison; 35mm is the logical choice.

Or, you can just set the camera to "auto everything" and never have to worry about this stuff.

[genece]

Thanks IanJ
Good information.

[chuck]

Yes, but only as far as depth of field is concerned. f22 is still f22 no matter what the format. You might like to read this article (http://dpfwiw.com/exposure.htm) which gives a good grounding on all this stuff and might help you get your head around it all.

Cheers, Ian
That's some article, Ian! Just what I needed. I'll have to print it out and keep it as a reference manual. Thanks for pointing it out.

Chuck

[theimage1]

f-stops are f-stops ... but not necessarily with many digital cameras. Most often the inexpensve digital cameras have no IRIS meachanism in the lens path. They change their effective f-stop only when the camera is zoomed. (Changing the focal length with the same lens diameter.)

Many lower end digital cameras effectively adjust the sensitivity of the ccd cell when a "f-stop" is selected in aperature mode. This is done to force the "shutter mechanism" (Also not always present in digital cameras) to change its capture time. Hence a high f-stop number (say f-8) will require more exposure than a lower selected f-number (say f-2). The f-2 selection will give a shorter exposure time. This allows for intentionally blurring images (like a water fall) or forcing them to be sharp.

What then of depth of field (or depth of focus)? Unfortunately these cameras only change their depth of field when zoomed to a new setting. Most have such tiny effective aperatures that getting anything "out of focus" can be difficult (unless the whole image is out of focus ... on purpose?)

You would need to check with the camera manufacturer to see if any given model has an iris mechanism or if it is handled through the electronics. The higher end models sometimes do have true IRIS mechanisms and shutters.

PS- some of the camera "shutter mechanisms" are only designed to stop light from continually hitting the ccd cell, and do not actually set the shutter times ... only the effective sensitivity of the ccd controls the entire exposure.

[dwig]

f-stops are f-stops ... but not necessarily with many digital cameras. Most often the inexpensve digital cameras have no IRIS meachanism in the lens path. They change their effective f-stop only when the camera is zoomed. (Changing the focal length with the same lens diameter.)

Many lower end digital cameras effectively adjust the sensitivity of the ccd cell when a "f-stop" is selected in aperature mode. This is done to force the "shutter mechanism" (Also not always present in digital cameras) to change its capture time. Hence a high f-stop number (say f-8) will require more exposure than a lower selected f-number (say f-2). The f-2 selection will give a shorter exposure time. This allows for intentionally blurring images (like a water fall) or forcing them to be sharp.

What then of depth of field (or depth of focus)? Unfortunately these cameras only change their depth of field when zoomed to a new setting. Most have such tiny effective aperatures that getting anything "out of focus" can be difficult (unless the whole image is out of focus ... on purpose?)

You would need to check with the camera manufacturer to see if any given model has an iris mechanism or if it is handled through the electronics. The higher end models sometimes do have true IRIS mechanisms and shutters.

PS- some of the camera "shutter mechanisms" are only designed to stop light from continually hitting the ccd cell, and do not actually set the shutter times ... only the effective sensitivity of the ccd controls the entire exposure.

To clarify this:

1. Lack of an "iris mechinism" in _ABSOLUTELY NO WAY_ means that the camera doesn't control the lens aperture. For a good half century, cameras have used methods of stopping the shutter blades shy of being fully open to affect an aperture adjustment.

2. If a camera makes exposure adjustments by any method other than actually adjusting the f/stop but calls it an f/stop adjustment, that camera simply lies to the user. This lie, in absolutely no way, changes the fact that f/stops are, in truth, f/stops on _ANY_ lens used for _ANY_ purpose. F/stops are defined as the ratio of the aperture's diameter to the focal length of the lens. The focal length in the formula is the real focal length and not some logical abstraction, as the "equivalent focal length" so misused in the digital camera world.

3. Some low end cameras will have their lens stopped to a fixed aperture, or changing range associated with zoom setting, and never adjust exposure using f/stop changes. They work by adjusting the "shutter speed", either mechanically or electronically, and/or simply adjusting the signal amplification (electronic "ISO"). These simple cameras will not offer any manual control.

4. (in reference to an earlier post) "Lens" have focal lenghts and apertures and therefore have f/stops. "Cameras" do not have such. To bring the type of camera into the discussion of what f/stops are is a red herring. Only once you understand that f/stops are f/stops and that an f/stop's effect on exposure is the same _REGARDLESS_ of the focal length of the lens or the format and type of "image receptor" (film, digital, ...) can you begin a discussion of an f/stop's affect on depth-of-field where focal length, the ratio of focal lenght to format diagonal are both factors. Even here, the image receptor type (film, digital, ...) has no direct impact, though the intrinsic sharpness (resolution, contrast, tonal range, ...) and some other subtle physical properties of the image receptor, along with "print" size, have some minor impact.

[Grog]

F8 aperture size is .78 mm or something like that I found it and posted in on another thread on this site. F22 on one lense will not be the same on another, Zoom or focal lieght will cause the size to change from one lense to another. But it is around 1.0?? again don't remember the size I posted before. So F8 on the FZ20 is about the same as F22 on a 35 MM SLR.
The FZ20 has a senser that is listed as 1/2.5 (is a imperial fraction) I could not find what that means but I did find one for 1/2.7 it is 5.27 MM X 3.96 MM. 1/1.8 is 7.18 X 5.32 MM while 2/3 is 8.80 X 6.60 MM. Some Cannon us a senser that is 22.7 X 15.1. Most other except for some Pro SLR digital use a senser that is 23.5 X 15.7 MM. The 1/2.5 sensor gives a 6 X 72 MM = 432 MM. The Cannon has 1.6 X lense MM or 100 MM 35 MM lense is 160 MM and the 23.5 sensor has a 1.5 X Lense MM or a 100 MM 35 MM lense would be 150 MM.

[theimage1]

To clarify this:

1. Lack of an "iris mechinism" in _ABSOLUTELY NO WAY_ means that the camera doesn't control the lens aperture. For a good half century, cameras have used methods of stopping the shutter blades shy of being fully open to affect an aperture adjustment.

>>Agree, but in the case of Shutter Blades (also known as IRIS Sutters) the shutter is in hte same juxtiposition as the IRIS would normall be. Found in virtually all good view camera lenses. It can serve with two functions.

2. If a camera makes exposure adjustments by any method other than actually adjusting the f/stop but calls it an f/stop adjustment, that camera simply lies to the user. This lie, in absolutely no way, changes the fact that f/stops are, in truth, f/stops on _ANY_ lens used for _ANY_ purpose. F/stops are defined as the ratio of the aperture's diameter to the focal length of the lens. The focal length in the formula is the real focal length and not some logical abstraction, as the "equivalent focal length" so misused in the digital camera world.

>Yes, they simply lie to the user! Imagine that. They want ot make most users confortable with the concepts they already understand. Film cameras simply can't change the ISO on the fly. Yes, the focal length is the "real focal length" of a lens. Thus all lenses (not telephotos) have a native focal length based on their design, and they have a set f/stop based on the formula. Telephotos change their focal length and hence change in the f/stop formula, thus have different f/stops depending on their zoom position.

3. Some low end cameras will have their lens stopped to a fixed aperture, or changing range associated with zoom setting, and never adjust exposure using f/stop changes. They work by adjusting the "shutter speed", either mechanically or electronically, and/or simply adjusting the signal amplification (electronic "ISO"). These simple cameras will not offer any manual control.

> Yes, the whole point of my post. The question came up at our Photo SIG recently why certain digital cameras don't change depth of field when a different f-stop is seleced in aperture mode. It's becasue the f-stop selection (on that camera) does not "change the aperture". it's simply a ccd ISO shift in these cameras. It provides the means to give a different exposure time so at least one facet of norml photography can be maintained. I agree they offer no manual control fordepth of field, but do offer shutter speed (or at least exposure time) control.

4. (in reference to an earlier post) "Lens" have focal lenghts and apertures and therefore have f/stops. "Cameras" do not have such. To bring the type of camera into the discussion of what f/stops are is a red herring. Only once you understand that f/stops are f/stops and that an f/stop's effect on exposure is the same _REGARDLESS_ of the focal length of the lens or the format and type of "image receptor" (film, digital, ...) can you begin a discussion of an f/stop's affect on depth-of-field where focal length, the ratio of focal lenght to format diagonal are both factors. Even here, the image receptor type (film, digital, ...) has no direct impact, though the intrinsic sharpness (resolution, contrast, tonal range, ...) and some other subtle physical properties of the image receptor, along with "print" size, have some minor impact.

>Only a "red hearing" if someone can't understand why they can't control their depth of field using a "changed" aperture setting on teir digital camera. Which is why I posted the answer. I agree if the camera "has" a way to control the "true aperture" then it affects the depth of field. But not all low end digital cameras have it although they certainly "allow for it" in their controls, they simply don't change it. That was my warning. f-stops are f-stops on all cameras which actually have them, reguardless of the media they record on, or the focal length of the lenses.

[dwig]

F8 aperture size is .78 mm or something like that I found it and posted in on another thread on this site.

False statement

f/8 has a diameter of exactly 1/8th the focal length; it _NEVER_ has a finite diameter (such as 0.78 mm, or whatever) until after you state the focal length (real focal length and not some "equivalent focal length" misconception) being considered.

An aperture of 0.78mm would be f/8 only when the real focal lenght is 6.24mm. It matters not what format camera the lens is used on.