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Silly question about f: stops

Discussion in 'Help and Feedback' started by dumigron106, Oct 11, 2012.

  1. dumigron106

    dumigron106 Mu-43 Regular

    39
    Sep 26, 2012
    Asheville NC
    Emmanuel
    So i read somewhere that on the micro 4/3 system adapted lenses "ACT" in different ways ie: a 17mm c mount lens is closer to a 35mm lens in it's real world application " and i have seen the math and the formulas. Were i am somewhat confused is the f: stops ...some say it doubles too ! but i have also read that it actually goes the otherway ei: " an f:2 lens "ACTS" more like an f:1.7 lens " , the reason i ask is that of all lenses i have i also have a few f:2 and f:2.4 lenses and in low light shooting they certainly do not act like an f:4 or f:5 lenses but more like an f:1.7 and f:2 ...so what is the truth if there is one ? thanks

    It is probably a question that has been asked so please accept my apologies but i could not find the answer
     
  2. jloden

    jloden Mu-43 Hall of Famer

    May 15, 2012
    Hunterdon County, NJ
    Jay
    Light gathering capability is the same for a given f-stop on m4/3 as on any other system, that doesn't change.

    What does change is the depth of field. Due to the smaller sensor size, m4/3 cameras will have more DoF (meaning more will be in focus) than a full frame or APS-C sensor camera for the same f-stop.
     
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  3. quatchi

    quatchi Mu-43 Veteran

    326
    May 17, 2012
    Munich, Germany
    Here is the short (non-scientific) answer:

    In terms of DOF it doubles.
    In terms of light falling on the sensor it stays the same.
    (comparing u4/3 vs. 35mm)


    What does it mean:
    When exposing a frame, you would choose the same exposure time, f-stop value and ISO on a u4/3 cam as with a full frame (35mm) one.

    The depth of field, however, sort of "behaves" differently. For example, we take an imaginary lens with say 50mm and f1.7. Taking a picture with that lens on a full frame camera will give you a certain view with a shallow depth of field. If you use the same lens on a u4/3 sensor, it gives you the field of view of a 100mm lens on full frame. That's because the u4/3 sensor is only half the size of the full frame one. It's like cutting out the middle part of a picture taken with the full frame cam. Obviously the depth of field stays the same when cutting out the middle part of the image.

    Here comes the but, though. As u4/3 image's field of view equals the one of a full frame's 100mm one, you would although expect the depth of field to be the same as of an full frame 100mm f1.7 image. The depth of field of the u4/3 image, however, only has the depth of field of an 50mm f1.7 lens (remember, that the f value is related to the focal length). With an 50mm f1.7 lens, one, therefore, gets an image wich equals a 100mm full frame field of view but only with the depth of field of an 50mm f1.7 full frame image.

    I hope, this makes some sense. :)
     
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  4. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    There's a lot of misinformation floating around the internet...

    Let's address focal length first, even though you didn't specifically ask about that. A 17mm lens is a 17mm lens, no matter what camera it's mounted to. What does vary is how different sensors "crop" the image circle made by that lens. For example: a 50mm lens designed for 35mm has a specific field of view. Use that same lens on m43, and because the m43 sensor is smaller, it "crops" a part of the image out of the center of what the lens captures. Because the m43 sensor is about half the size of a FF sensor, it captures the same field of view as a 100mm lens would capture on a FF camera.

    So some people say the 50mm lens is "equivalent" to a 100mm lens on a FF camera. Or a 35mm lens on m43 is "equivalent" to a 50mm lens on FF. In the case of your 17mm c-mount lens, you could say it's field of view is the same as a 35mm lens on a FF camera.

    But this has nothing to do with it being an adapted lens. A native m43 lens of 17mm will produce exactly this same field of view. As would a 17mm lens designed for APS-C or FF or medium format. The only reason the "equivalent" field of view is narrower is because the sensor is smaller.

    To confuse matters further, that c-mount lens won't work on a FF camera at all. It was designed for a video camera which probably has an even smaller sensor than m43 cameras. So compared to it's intended use, it probably provides a wider angle of view than it was designed for.

    And since most of today's photographers have never owned a "FF" camera, comparing to FF equivalent focal lengths may not be all that meaningful anyway. If you compare to APS-C cameras, the multiplier is much smaller. Unless you have lots of experience with 35mm or FF cameras, my suggestion is to ignore this "equivalence" argument and just learn how each lens works on your current camera (e.g., a 17mm lens is a moderate wide-angle, a 14mm lens is a wide-angle, a 7mm lens is an ultra-wide angle).

    Others have addressed the f-stop question already, but a little more detail (post #8):
     
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  5. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    I find this easiest to think of this in two parts, the lens and the camera it is attached to.

    Let's take the lens. The focal length defines the light cone that the lens sees (longer lenses see less of the field of view as you are - simplistically - looking down a longer tube) and aperture defines how much light that lens can gather as it defines the hole through which the light falls. Aperture also defines how steep the cone of light can be - for a given focal length, a wider aperture will give a wider "base" to the cone. Given a fixed focal length, a wider aperture allows things to go out of focus quicker as the cone "height" (the focal length) is fixed. So a lens has a set of physical properties that is independent of the camera it is attached to.

    What changes when you attach different cameras is the sensor size. When you attach a full frame camera to the back of the lens, it takes a bigger cut out of the focal plane than if you put a :43: sensor on the back. What it changes is the field of view. When someone says that a 25mm on :43: is the same as a 50mm on full frame, what they mean is it has the same field of view as you would get on a full frame sensor, so to take the same picture (same framing) you need different focal lenths on different sized sensors.

    As for aperture equivalent, that is to do with depth of field, not shutter speed calculations. You can go and look the maths up if you want (in fact, it is here: https://www.mu-43.com/f38/m-zuiko-75mm-1-8-not-really-expensive-32924/index5.html#post329483) but what happens is that if you are taking a picture with a longer lens, to get the same out of focus effect, you need the apreture to be physically the same.

    That is, if you have a 25mm @ f/5 (5mm) aperture on a :43: camera, to get the same field of view on a full frame, you need a 50mm lens. To get the same out of focus feel, you need the aperture to be physically the same size (5mm) so for an equivalent picture, on full frame, you need 50mm with 5mm aperture or 50mm @ f/10

    This is where the "equivalence" or "acts like" comes from.
     
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  6. RobWatson

    RobWatson Mu-43 Hall of Famer

    Forget all that junk and just take some pictures.
     
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  7. dumigron106

    dumigron106 Mu-43 Regular

    39
    Sep 26, 2012
    Asheville NC
    Emmanuel
    Thanks for the feedback, i have till recently only ever shot with FF 35mm Film cameras and FF digital , what i have enjoyed about the M 4/3 World is the ability to adapt many different lenses that it be C mount, native , m39/m42 , pen f or even 35mm Cine lenses etc... since all of these lenses have their own propriety qualities and some are better than others , i have been able to get more fast lenses with incredible glass for 1/3 of the price of FF proprietary fast lens. yes at the end of the day it is just to experiment and enjoy the renewed joy of photography

    thanks for the feedback !

    Kern Paillard 25mm @ f:1.4 1/125 ISO 100
    267251_4002555535776_40754767_n.
     
  8. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    As stated above, in terms of exposure there's no difference. If the proper exposure is 1/250th at f 2.8 for a given ISO, that's the correct exposure whether you're shooting on a tiny P&S camera, m43, APS-C, FF, or an 8x10 film camera.

    In terms of depth of field, let's make sure we have the basics down. An f-stop is simply the ratio between the diameter of the aperture and the focal length of the lens (this is slightly simplified, but close enough for normal use). So a 50mm f/2 lens has an aperture that's 25mm in diameter. A 25mm f/2 lens has an aperture of 12.5mm. Why does this matter?

    Depth of field is directly related to the size of the aperture, rather than the f-stop, sensor size or focal length. Two lenses with the same physical aperture size, shot of the same subject from the same position, will have the same DOF. A 50mm lens shot at f/4 on m43 will have exactly the same DOF as that same lens shot at f/4 on a FF DSLR. (Only the framing will be different, because the smaller sensor crops the image.) Sensor size, in spite of what you'll read all over the web, has absolutely nothing to do with DOF. (Well, that's not quite true either, but the effect is the opposite of what people think.)

    So why do we say that m43 camera have more (deeper) DOF than cameras with larger sensors? Because if we want a picture that looks the same as the one taken on the FF camera, we will want to use a 25mm lens. And a 25mm lens has an aperture that is 1/2 the diameter of the one in the 50mm lens, when the same f-stop is used. To get the same sized aperture on the 25mm lens, you need to open the aperture up by 2 f-stops. So the 25mm lens needs to be opened to f/2 to have the same DOF as the 50mm lens at f/4 on a full frame (35mm) camera.

    That bolded part, above, is important. People love to talk about how m43 cameras have 2 stops deeper DOF than FF DSLRs, but remember that few people use FF DSLRs. Most people shoot APS-C DSLRs, which have a similar "issue" when compared to FF cameras.

    Compared to an APS-C camera, m43 cameras have only about 2/3 stop more DOF. Not a real big difference. And compared to P&S cameras, m43 cameras have much shallower DOF.
     
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  9. RobWatson

    RobWatson Mu-43 Hall of Famer

    From wikipedia this is the most informative blurb.

    When the “same picture” is taken in two different format sizes from the same distance at the same f-number with lenses that give the same angle of view, and the final images (e.g., in prints, or on a projection screen or electronic display) are the same size, the smaller format has greater DOF.​


    "Same picture" means different lens and camera format so not really the same at all!

    A Nikon 50mm F1.2 will present the same DOF no matter what camera it is used.

    Take a full frame camera and slap on a Nikon 50mm F1.2 and take the picture. Slap that same lens on a MFT and take the picture (same subject, distance, ap. Crop the FF image to match the MFT and, voila, "same picture", same FOV same DOF. This is the least confusing way for me to thing of "crop factor" and "equivalence" - except I don't think of such things.
     
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  10. With_Eyes_Unclouded

    With_Eyes_Unclouded Mu-43 All-Pro

    Apr 17, 2012
    Vassilios
    Nice analysis meyerweb! A note on the point below:


    "People" talking about the shallower DOF attainable by FF cameras, and sometimes this talk is accompanied by meaningless references to "artistic bokeh" and such, fail to mention that there are many applications where a deeper DOF at a given f-stop is actually preferable.

    Landscape and macro photography readily come to mind. Let's not forget that, with today's high Mp counts in digital cameras, diffraction starts to become a problem in some occassions, from a lower f-stop number. "Gaining" two f-stops may be crucial for those applications (given the same diffraction limit). Not to mention, of course, that a particular :43: lens has more chances of being optically sharper at f/5.6 than a "FF equivalent" at f/11.
     
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  11. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Thank you!

    I couldn't agree more. I find my favorite photos tend to be ones that show real depth, by having adequate detail and sharpness in both foreground and background, like this one. If this had razor thin DOF, so only the person talking on the phone was sharp, it would be a very different photo, and not a very interesting one, I don't think. MHO.

    Ferry_Terminal_11-10-09_0123.
     
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  12. With_Eyes_Unclouded

    With_Eyes_Unclouded Mu-43 All-Pro

    Apr 17, 2012
    Vassilios
    Exactly that. Apart from the fact that I'm sick of portraits where only one eye is in focus (and nothing else... for "artistic purposes" I suppose :rolleyes:) even techniques like Brenizer are overdone IMHO.

    Photography is about saying something with the picture. This means your way of telling it, and DOF is just a tool for this.

    I'm wondering if, in fact, a method of freely varying the sensor's effective surface could be a solution to more flexibility in that direction, for FF or larger sensor cameras.
     
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  13. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Nikon already does this to a limited extent. Put a DX (APS-C format) lens on a FF camera, and it will crop the usable sensor area to match. With a corresponding loss of resolution, of course.

    But this only affects DOF if you change the lenses to match the effective size. Remember, a 50mm f/2 will provide the same DOF no matter what size sensor it's in front of.
     
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  14. arad85

    arad85 Mu-43 Veteran

    477
    Aug 16, 2012
    You might think this, but you need to take into account print size too and what photo you are taking. As Wikipedia says (quite rightly)

    Depth of field - Wikipedia, the free encyclopedia

     
  15. RobWatson

    RobWatson Mu-43 Hall of Famer

    What happens between the lens and the sensor does not change DOF. How one makes a print does matter. The same lens on different cameras will give the same DOF in the image. Going from digital image to print will make a difference (unless the viewing disance is also changed). Step back from the print by a "crop factor" and the DOF becomes the same!
     
  16. meyerweb

    meyerweb Mu-43 Hall of Famer

    Sep 5, 2011
    Yes, I did ignore print size, for the sake of simplicity (and because I've found 99% of people on online forums can't get their head around the fact that the smaller sensor can actually cause shallower DOF). But I did hint at it with this statement in an earlier post:

    But the second paragraph of what you quoted really confirms my statement. If you make the same final image, with the same lens, aperture and shooting position, DOF will be identical, regardless of sensor or film size.

    To really discuss DOF fully, we need to get into Circle of Confusion (CoC) and how it's affected by a number of factors, including physical aperture (not f-stop) and degree of enlargement, which is dependent both on final image size as viewed and sensor or film size. Also viewing distance, and display medium (the resolution of your monitor and printer can affect apparent DOF, too). Camera to subject distance affects DOF, too, not only in total amount but in how it's distributed in front of and behind the subject.

    And finally, the entire concept is subjective. There's really no such things as a range of in-focus items in the photo. Only one plane is in focus, and as you move away from that plane everything is less sharp. DOF is based on how out of focus something is before the viewer perceives it as unsharp, and that will vary from person to person, based both on visual acuity, individual tolerance, and experience viewing photos critically. Joe or Jane Facebook user might look at my image above, and consider the entire bench to be in focus. Someone who selects photos for a gallery display might say "ooh, not enough DOF. The front of the bench is too fuzzy."

    We could discuss DOF for hours. In fact, I have, both as a student and teacher of photography. :)
     
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  17. flash

    flash Mu-43 Hall of Famer

    Apr 29, 2010
    1 hour from Sydney Australia.
    Gordon
    Rob. There's no such thing as depth of field at the sensor. DOF is directly related to the relationship between the sensor size, aperture and the size of the image when viewed. See the post below yours.

    Gordon
     
  18. RobWatson

    RobWatson Mu-43 Hall of Famer

    Some of the history of DOF found here is even more entertaining than the actual subject!
     
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  19. mguffin

    mguffin Mu-43 Regular

    33
    Aug 10, 2011
    Westwood, NJ
    Stick the lens on your camera and take some pictures. That's the truth.

    :smile:
     
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  20. Amin Sabet

    Amin Sabet Administrator

    Apr 10, 2009
    Boston, MA (USA)
    Coming from that background, all you need to know is that you double the focal length and the f-number to get the 35mm format equivalent lens.

    If you take a photo with a 35mm format camera, 50mm lens at f/2.8 and another photo of the same subject from the same position with a Micro 4/3 camera, 25mm lens at f/1.4, they will be the same in terms of framing, DOF, and perspective. This is true regardless of whether the Micro 4/3 camera is using a native Micro 4/3 lens or an adapted lens.
     
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