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Thread: Equivalent focal lengths - never thought of this before...

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    rpcrowe's Avatar
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    Equivalent focal lengths - never thought of this before...

    This is another of those it really doesn't make a darn bit of difference posts!

    I was reading a query on another forum about which lens will provide a wider field of view:

    10-22mm on a 1.6x camera or 17-40mm on a full frame camera.

    The immediate response would be to multiply the 10mm focal length by 1.6x crop factor which gives you 16mm. This is 1mm shorter than the 17mm of the 17-40L so will give you a wider view!

    However, I am not totally sure that that is an across the board accurate response. The manufacturer's designated focal lengths are valid ONLY WHEN THE LENS IS FOCUSED AT INFINITY. Focusing at distances other than infnity can change the focal length.

    I have never seen a table of focal lengths of lenses at varying distances other than infinity.

    Since you would probably be shooting at different distances using a full frame or a crop camera; could not that impact the exact focal length of the lenses that you are using?

    Of course, the difference between the 10mm on a crop camera and 17mm on a full frame camera could be so close that it may be a moot question, and at infinity, the 10-22mm should provide a wider view...
    Last edited by rpcrowe; 9th June 2013 at 03:26 PM.

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    Re: Equivalent focal lengths - never thought of this before...

    That makes my head hurt, Richard. But interesting question for those that may be using both types of cameras.

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    Re: Equivalent focal lengths - never thought of this before...

    You also have to factor in the characteristics of the lens. At varying focal lengths there is a big difference in the magnification and field of view between the Nikon 70-200 VR and the 70-200 VRII. Try working that out with the math. It's one of those things simply not worth worrying about; just take pictures.

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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by Andrew1 View Post
    You also have to factor in the characteristics of the lens. At varying focal lengths there is a big difference in the magnification and field of view between the Nikon 70-200 VR and the 70-200 VRII. Try working that out with the math. It's one of those things simply not worth worrying about; just take pictures.
    I totally agree Andrew - one of the biggest criticisms of the /f2.8 70-200mm VRII was that at the closest focusing distance at the 200mm setting it was really 135mm. The camera lens sites with full of whining about this shortcoming. In my view, so what. I bought the lens anyway. The lens designers have to make numerous compromises, even in a high end lens. In spite of this "flaw", it continues to be one of the lenses I shoot the most.

    The lens manufacturers don't publish this information anyways, so going through the gyrations of doing test shots to figure out what is happening doses not interest me in the least. I'd rather be out shooting....

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    Re: Equivalent focal lengths - never thought of this before...

    Why would you be shooting at a different distance?

    If you are using a 10mm on a crop and a 17mm on a FF then the field of view will be (about) the same so you will be stood in the same spot to take the equivalent photograph so both lenses will be focused at the same point.

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    Like I said... This is a post that doesn't make any ditteferce...

    This is a "Catch 22" type of a problem that really doesn't make any difference in day to day real-life shooting but which can be theoretically interesting.

    In effect, it can throw out a lot of information that is considered holy by some folks. Like, "Does 200mm on one lens equal 200mm on a different lens". Since any two lenses may not be of equal focal lengths, at different focusing distances, despite what is engraved on their lens barrels, DOF charts are not "carved in granite"

    As an example, if the Nikon lens referred to above is actually a 135mm focal length rather than 200mm when focused at some distances, the use of a chart to determine DOF would not be accurate across all focal lengths, with all focusing distances and for all lenses.

    Like I said, this probably makes as little difference in real like shooting as the differences in image quality between the 70-200mm f/2.8L IS II and the 70-200mm f/4L IS lenses. Both lenses provide excellent image quality, except perhaps for pixel peepers.

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    William W's Avatar
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    Re: Like I said... This is a post that doesn't make any ditteferce...

    On another thread here at CiC, JCUKNZ (post #8)brought up this point up of the Stated FL being only correct at Infinity; and I commented on it (Post #9), mentioning, (for that thread) it was of little practical purpose.



    BUT, another point to consider is, that the “Stated FL” on the lens might NOT be so.

    So for the exampe that Richard is considering - the 17 to 40 might be a 17.4mm to 39.1mm lens and the 10 to 22 might be a 10.5 to 21.5mm lens . . .

    And this fact does raise its head, more severely, in practical scenarios, sometimes.

    (Film days) I had a perplexed Photographer once ask me why his two lenses were “so different” - I can’t remember the lenses but one was a lesser quality zoom, (cheaper third party brand) max zoom was 135mm and the other lens (he just bought, because it was a much faster aperture) was a 135mm Prime.

    He wanted to know why the 135mm Prime “got much closer”

    The answer was: the zoom lens’s maximum zoom was in reality about 100~105mm.

    I expect the same applies with lenses today: perhaps even within Nikon and Canon lens cache ? I haven’t A/B tested any of my own lenses but I think I will just for fun, when I have some spare time.

    WW

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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by black pearl View Post
    Why would you be shooting at a different distance?

    If you are using a 10mm on a crop and a 17mm on a FF then the field of view will be (about) the same so you will be stood in the same spot to take the equivalent photograph so both lenses will be focused at the same point.
    The question I would like answered is: what happens when you put that 17mm FF lens on a crop factor camera (1.5x crop)? What would the focal length be – 25.5mm? And what then if you put a 25.5 crop lens on the crop factor camera, what is the actual focal length then – 38.25? To confuse things even further: what happens if you put a 25.5 FF lens on that crop factor camera?

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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by AB26 View Post
    The question I would like answered is: what happens when you put that 17mm FF lens on a crop factor camera (1.5x crop)? What would the focal length be – 25.5mm? And what then if you put a 25.5 crop lens on the crop factor camera, what is the actual focal length then – 38.25?
    The focal length doesn't change. That is a fixed property of the lens combination. What changes is the area covered by the sensor. A smaller sensor will cover less of the focal plane than a larger one. That's it. You get less for your money, so to speak, with a small sensor from a particular focal length. The manufacturer will make the lenses designed for large sensors cover a greater area on the focal plane so it will cover the sensor. That's why you can use a FX on a DX except the sensor, being smaller will only give you a bit of the picture (about 2/3). It's also why, if you use a DX on an FX you get a circle for a picture.

    Maybe it's time we just thought in terms of what the lens does for the camera we own and not bother trying to convert. A 35mm lens has a different effect on different size sensors. Know how it works for the camera you use and it becomes simple.

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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by tomdinning View Post
    The focal length doesn't change. That is a fixed property of the lens combination. What changes is the area covered by the sensor. A smaller sensor will cover less of the focal plane than a larger one. That's it. You get less for your money, so to speak, with a small sensor from a particular focal length. The manufacturer will make the lenses designed for large sensors cover a greater area on the focal plane so it will cover the sensor. That's why you can use a FX on a DX except the sensor, being smaller will only give you a bit of the picture (about 2/3). It's also why, if you use a DX on an FX you get a circle for a picture.

    Maybe it's time we just thought in terms of what the lens does for the camera we own and not bother trying to convert. A 35mm lens has a different effect on different size sensors. Know how it works for the camera you use and it becomes simple.
    I think that the converson factor is a valid thought when:

    1. The photographer is used to shooting full frame format (most often with film) and is used to selecting lenses based on their full frame coverage...
    2. When the owner or prospective owner of a crop camera is reading about lenses and focal lengths on a website or in a book directed toward full frame format equipment..
    3. When focal lengths are being compared across different formats..

    Other than that, the conversion really has little impact except that a photographer might take the factor in consideration when using the 1/Focal Length shuter speed recommendation for hand holding a camera (resulting in 1/Focal Length x Crop Factor = minimum shutter speed). However, since that is is only a rough recommendation, you may not need to introduce the crop factor into the formula at all. As an example, using longer, non-Image Stabilized lenses I need about a 1/Focal Length x 2 shutter speed to ensure sharp imagery as I have grown elderly. This takes the crop factor completely out of consideration for me.

    Additonally, with Image Stabilization, the entire formula for hand holding is thrown out the window...

    Focal length, is focal length, is focal length - no matter the size of your format. The only difference is the area covered by the lens and sensor as illustrated below...

    Equivalent focal lengths - never thought of this before...

    Consideration of a crop facor (or 35mm equivalent) is also valid when comparing the coverage of lenses used on different format cameras. As an example, saying that a P&S or Bridge camera has a 30x zoom lens really tells us nothing about the coverage it is capable of providing. However, saying that the lens has a 24-720mm equivalent coverage is comparing apples with apples...

    The conversion factor is also a valid consideration if a photographer has been quite content with a certain focal length lens on a crop camera and desires to achieve approximately the same coverage with a lens for a full frame camera (the opposite is also true).

    However... even though I have cut my teeth on full frame (and many other formats) I tend to think about focal lengths in relation to my crop cameras. I know by experience that, when mounted on my 7D, my 70-200mm lens will give me flattering portrait coverage across its entire focal range. I don't need to consider that this lens has a 112-320mm equivalent on my 7D.
    Last edited by rpcrowe; 10th June 2013 at 02:48 PM.

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    Re: Equivalent focal lengths - never thought of this before...

    I have heard about focus breathing especially at the longest focal lengths of a lens like the Nikon 70-200 example frequently cited. Some are discussing the same issue with the new Nikon 80-400. I have heard some say the widest fl of a lens may also be somewhat wider on such lenses. Shooting close. I was under the impression from this that the focal length changes more up close than as a gradual variation from infinity to close. Also, I have only heard this discussed with telephotos--not the ultrawides you mentioned. I guess because most people who shoot ultrawides are mainly concerned about the widest end of the lens and not so much losing zoom. I do remember one review mentioning the slight differences in actual field of view of different ultrawides at their widest. While that may be important, getting the Sigma 8-16 on dx or the Sigma 12-24 on fx would avoid that problem entirely.

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    Re: Equivalent focal lengths - never thought of this before...

    The original question is of course not of much importance when you take pictures. I seldom worry about focal length, and when I use a zoom, I frame without even thinking on what focal length I set the lens to. It will show up in EXIF data, but at the moment I take the picture, I don't bother a tiny bit about it. Neither would I worry about the actual angle of view when focusing a lens compared to when it's set at infinity, although the angle of view of a macro lens becomes narrower when focusing close, unless it is entirely inner focusing. In the latter case, its focal length becomes shorter when focusing close.

    The same happens when using a lens that has front element focusing. They are rare nowadays, but fifty years ago, many cameras used the front group for focusing. Most triplet lenses were set up that way in pocket foldables. Effectively, their focal length varied with focusing, and the field of view remained largely the same at all focus distances.

    So an old type macro lens with 90 mm focal length would have the field of view of 180 mm at 1:1, its focal length remaining the same, as it is extended 90 mm farther from the film plane than when focused at infinity and finding focus at 360 mm distance. A newer 90 mm macro lens might remain the same length while focusing and be closer to 45 mm focal length when focused for 1:1 reproduction scale at about 180 mm distance from the sensor plane. Either way, one would learn what to expect from the lens. The one with a larger working distance may be preferred by some, while others prefer a lens that won't extend.

    And of course no zoom lens has the same focal length when focused at close distance as when focused at infinity, as all zooms change focal length when focusing.

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    Re: Equivalent focal lengths - never thought of this before...

    Remember, the factor only applies to the crop factor NOT to a change in the magnification. What ever the images produced by the two different cameras, with the same focal length lens, the magnification will be the same, just less area.

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    Re: Equivalent focal lengths - never thought of this before...

    Taking a risk to confuse every one even further:
    There is no such thing as an 17mm objective for a DSLR...
    What we have is an objective that has an angle of view equivalent to a lens with focal distance of 17mm,
    (which would have to be somewhere within the camera body ).
    The same for telezooms, which tend to be shorter than the focal length would indicate.

    So the important parameter is the angle of view, which can be translated to an apparent focal length. And of course, that apparent focal length must change if you focus on
    something close to the camera with a lens with internal focusing (which stays the same length): you can focus on a close-by object by changing either the distance between lens
    and sensor, or by changing the apparent focal length of the optical system (as there is a simple relation between object distance, image distance and focal length:
    1/o +1/i - 1/f = 0)
    Last edited by revi; 2nd July 2013 at 06:19 AM.

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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by Wizard View Post
    What ever the images produced by the two different cameras, with the same focal length lens, the magnification will be the same, just less area.
    Hi Again,

    You know, you guys are brilliant. With all of these “doesn’t make a darn bit difference posts” you made me realize that all of this about “crop factor” is a lot of baloney.

    No, really guys, there is no such thing as a “crop factor”. Think of it. What is the “crop factor” of a FF camera compared to a medium format camera? Is a 600mm lens fitted to a Leica S2 in actual fact only a 300mm lens?

    A 600mm lens fitted to a FF camera is a 600mm lens. Fitted to a “crop” sensor camera it is still a 600mm lens. It is baloney to say you suddenly have the reach of a 900mm lens because it is fitted to a 1.5X “crop” camera. The darn image on the “crop” sensor is just a lot smaller than on the FF sensor, and it is going to be a darn bit more pixilated than the FF image when enlarged.

    The light turned on last night, perhaps my lens cap came off, and I think I now do understand “crop factor” and focal length.
    If I were to say the specs of the sensor is much more important than the focal length of the lens, would that make sense? Will you understand what I am aiming at?

    If my theory is wrong, photographers are probably the most ignorant idiots on earth. I am serious. Why would you spend $5000.00 on a FF camera whilst a “crop sensor” camera of $2000.00 will give you more “reach”, with the same lens, rendering the same quality of image? Does that make sense?

    Thanks “Wizard” for bringing this up again.

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    William W's Avatar
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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by AB26 View Post
    Why would you spend $5000.00 on a FF [DSLR] camera whilst a “crop sensor” camera of $2000.00 will give you more “reach”, with the same lens, rendering the same quality of image?
    Because . . . just off the top of my head a few reasons:

    About 1⅔ stops shallower DoF capacity
    Faster, wider Prime Lenses
    Faster, wider Zoom Lenses
    Often better Viewfinders
    Full capacity to exploit TS-E Lenses
    Full capacity to exploit Fisheye lenses

    And …

    I can charge more . . .

    WW

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    Re: Equivalent focal lengths - never thought of this before...

    I like the sound my full frames make. A decent thud like the old days with the F series. A real mans sound.
    Just saying, if a bloke has $5000 to spend on a camera do you think he'd spend only half of it? Not this bloke. Is bigger better? Only in the minds of those who's device is smaller. And for those who have a bigger one, its everything.
    Do the manufacturers and advertisers play on this? Bloody oath they do. Do we fall into their trap? Bloody oath we do.
    Is it important to have a ff camera? Only if you think so. Now that I am retired I find myself using the V2 more and more for general stuff and that's about 50% of the time and rising. Sometimes us amateurs forget why we take photos.

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    Re: Equivalent focal lengths - never thought of this before...

    I like the sound my full frames make. A decent thud like the old days with the F series. A real mans sound.
    Just saying, if a bloke has $5000 to spend on a camera do you think he'd spend only half of it? Not this bloke. Is bigger better? Only in the minds of those who's device is smaller. And for those who have a bigger one, its everything.
    Do the manufacturers and advertisers play on this? Bloody oath they do. Do we fall into their trap? Bloody oath we do.
    Is it important to have a ff camera? Only if you think so. Now that I am retired I find myself using the V2 more and more for general stuff and that's about 50% of the time and rising. Sometimes us amateurs forget why we take photos.

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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by AB26 View Post
    Hi Again,

    You know, you guys are brilliant. With all of these “doesn’t make a darn bit difference posts” you made me realize that all of this about “crop factor” is a lot of baloney.

    No, really guys, there is no such thing as a “crop factor”. Think of it. What is the “crop factor” of a FF camera compared to a medium format camera? Is a 600mm lens fitted to a Leica S2 in actual fact only a 300mm lens?

    A 600mm lens fitted to a FF camera is a 600mm lens. Fitted to a “crop” sensor camera it is still a 600mm lens. It is baloney to say you suddenly have the reach of a 900mm lens because it is fitted to a 1.5X “crop” camera.
    True and false

    Don't forget that a lot of the initial users of digital cameras were used to cameras using 35mm film (images the size of the current Full Format sensors). As they always thought in terms of focal length (and not angle of view), it made sense to express the angle of view as an apparent focal length for 35mm film.

    So in that sense, wrt image size as percentage of sensor size, a 600mm lens on a 1.5 crop sensor 'behaves like' a 900 mm lens on a full frame sensor (leaving aside DoF issues)

    The darn image on the “crop” sensor is just a lot smaller than on the FF sensor, and it is going to be a darn bit more pixilated than the FF image when enlarged.
    The image on the crop sensor will be smaller, but so is the sensor. So the image will take up as many pixels on a crop sensor as on a full frame sensor with the same number of pixels.
    Otoh, if you use a cropped sensor with more pixels than the full frame you compare with, it's the FF that will show the most pixelation (if you ever get to a print size where that gets important)

    The light turned on last night, perhaps my lens cap came off, and I think I now do understand “crop factor” and focal length.
    If I were to say the specs of the sensor is much more important than the focal length of the lens, would that make sense? Will you understand what I am aiming at?

    If my theory is wrong, photographers are probably the most ignorant idiots on earth. I am serious. Why would you spend $5000.00 on a FF camera whilst a “crop sensor” camera of $2000.00 will give you more “reach”, with the same lens, rendering the same quality of image? Does that make sense?

    Thanks “Wizard” for bringing this up again.
    One other difference with the full frame sensor is photo site size: for a given pixel count, the full frame sensor has more space, so photo sites can be larger. That means that they can catch more light, which tends to give a better signal to noise ratio (aka 'less noise') in the final image.

    Are those differences (noise, DoF, extra lenses available) worth the price (and weight) differences? Not for me as an amateur, but that's in large part a question of budget.

    @William: do you mean that TS-E lenses don't work on crop sensor bodies? I'd expect them to work as well or better (larger shift relative to sensor size, for one).

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    Re: Equivalent focal lengths - never thought of this before...

    Quote Originally Posted by revi View Post
    @William: do you mean that TS-E lenses don't work on crop sensor bodies? I'd expect them to work as well or better (larger shift relative to sensor size, for one).
    I meant by what I wrote: that you are not going to get the FoV of 17mm FL, on an APS-C.

    WW
    Last edited by William W; 3rd July 2013 at 01:05 AM.

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