What is Equivalence and Why Should I Care? DPreview Weighs In

by Hugh Brownstone14 Comments

We received a veritable firestorm of commentary following our post on Tony Northrup's video. In that video, he challenged the industry — and conventional wisdom — about the true nature of differences in performance among full frame and crop sensor cameras and lenses.  Now, dpreview.com has weighed in along essentially the same lines as Mr. Northrup: not all f/stops are created equal.

Their style is different — dpreview.com's tone is perhaps more palatable to some — but in any event it is fascinating that they have taken the step of reporting not just focal length equivalents in their reviews, but now depth of field equivalents as well.


The subject — essentially, when is 1.8 not 1.8 and everything that goes with it, from focal length to light gathering — is garnering the attention it deserves.  This is a good thing!

Makes you think Cosina is clearly on to something with its set of f/0.95 Nokton lenses (it isn't about bragging rights); ditto with Leica for its Nocticron and for Metabones with its SpeedBooster. Can the rest of the MFT field be far behind in deepening their optics offerings?

And what does this bode for the full frame giants?

Stay tuned!

What is equivalence?

Equivalence, at its most simple, is a way of comparing different formats (sensor sizes) on a common basis. This is already the way most lenses are talked about: it's quite common to say that a compact camera includes a '28-120mm lens' but the key and (often unspoken) word in that description is ‘equivalent.' It's a simple way of describing the range of fields-of-view that the lens offers, cancelling out the effect of sensor size by using a common reference point.

A 100mm equivalent lens on a small-sensor camera will give the same framing and perspective as an actual 100mm lens does on a full-frame camera, regardless of sensor size, because they are equivalent.

equivalence image 1

It's this logic that the idea of ‘crop factors' is based on. The ‘Four Thirds' sensor format has a diagonal very close to half that of a ‘full frame' sized sensor. And, sure enough, if you calculate the angle-of-view of a 50mm lens on a system with a crop factor of 2, it's the same as for a full frame camera with a 100mm lens.

Equivalence image 2

Continue reading about equivalence in DPReview's article “What is equivalence and why should I care?”

Note: it is our policy to give credit as well as deserved traffic to our news sources – so we don't repost the entire article – sorry, I know you want the juicy bits, but I feel it is only fair that their site get the traffic and besides, you just might make a new friend and find an advertiser that has something you've never seen before

(cover photo credit: snap from DPReview)


  1. “A http://www.bhphotovideo.com/c/search?Ntt=100mm&N=0&InitialSearch=yes&BI=2975&KBID=3910/DFF/d30-v1-t9/DFFInline=regex equivalent lens on a small-sensor camera will give the same framing and perspective as an actual http://www.bhphotovideo.com/c/search?Ntt=100mm+lens&N=0&InitialSearch=yes&BI=2975&KBID=3910/DFF/d30-v1-t9/DFFInline=regex does on a http://www.bhphotovideo.com/c/buy/full-frame/Ntt/full-frame/N/0/kw/search/BI/2975/KBID/3910/DFF/d30-v1-t9/DFFInline=regex camera, regardless of sensor size, because they are equivalent.”

    This doesn’t seem right. Equivalent lens on small sensor will give you same framing (i.e. same field/angle of view), however distortion and background compression should be linked to actual focal length of the lens.

  2. This is why I detest crop sensor cameras other than S35. What a major pain in the ass.

  3. “As you can see, although the lenses are quite different, the 50mm f/2 lens is giving the same framing and the same depth-of-field as a 100mm f/4 lens is on Full Frame. As such, you can say that a 50mm f/2 for Micro Four Thirds is equivalent to a 100mm f/4 Full Frame lens in terms of both field-of-view and depth-of-field.”
    This is absolutely correct. But it isn’t what Tony Northrup is saying. It’s his lack of understanding of optics that leads to his misstating things, and the resulting tsimmes.
    Here’s the rule… Equal lens diameters * produce equal DOFs for equal final magnification. This means that shorter lenses — regardless of format — produce greater depths of field if the picture is taken at the same f/stop.
    This is not new! I knew it and understood it 40 years ago, and it was old hat then.
    The problem is that most current photographers were trained in the era when the 24mmx36mm format predominated. They’re accustomed to thinking of field of view, depth of field, etc, in terms of the focal lengths available for that format. When using cameras having smaller-than-24×36 sensors, want some frame of reference.
    Camera manufacturers provide this reference by stating a lens’s focal length in terms of its 24mmx36mm format equivalent. If a small sensor has 5/8 the dimensions of a 24×36 sensor, then a 50mm lens for that camera will have exactly the same field of view as an 80mm lens for a 35mm camera.
    It will not have the same depth of field, except when the entrance pupils are the same diameter. Since equivalent exposures are determined by f/stop, not entrance pupil, this means that the smaller-format camera will show greater depth of field.
    f/stops are f/stops. All f/2 lenses produce the same illumination at the image plane, ** regardless of camera type. The idea that f/stops for smaller format cameras are somehow “different” than those for larger-format cameras is just… wrong.
    As for depth of field… The smaller the format, the greater the depth of field. If you’re worried about equivalence, simply divide the large format lens’s f-number by the crop factor to get an idea of the advantage of the smaller-format lens.
    IMPORTANT POINT  Please note that image perspective is controlled by one thing, and one thing only — the distance of the camera from the subject. It is not affected in any way by focal length or format size.
    * That is, equal entrance pupils — not f/stops.
    ** I’m ignoring slight differences in transmission. Which was the reason for T stops.

  4. William Sommerwerck

    “IMPORTANT POINT  Please note that image perspective is controlled by one thing, and one thing only — the distance of the camera from the subject. It is not affected in any way by focal length or format size.”

    What about distortion and compression of a lens? Doesn’t it play a role in perspective portrayed in the image?

    “I’m ignoring slight differences in transmission. Which was the reason for T stops.”

    Could you please elaborate on that or point me to proper documentation on that topic. I was always ignorant enough to assume that this was a difference of photography and cinematography nomenclature. Not some kind of technical difference.

    Thanks in advance for you answer.

  5. Filip Wesolowski William Sommerwerck  “Perspective” is defined as “the relative size of objects”. A moment’s thought will show that this is determined only by the distance to the objects — not the focal length of the lens.
    In a book on photography by Andreas Feininger, he showed photos taken with a 400mm lens and a 100mm lens, on a view camera, of the New York skyline from the harbor.
    He enlarged the 100mm image to the same size as the 400mm, and printed them side by side. Other than grain and sharpness, they were indistinguishable. The perspective was identical.
    “Distortion” really refers to geometric distortion, such as barrel or pincushion distortion. The “odd” effects seen in wide-angle shots are actually the result of a geometrically “correct” projection of the image on a flat plane.

  6. If you don’t like it switch lenses.  This crap of full frame is so out of control and most don’t even realize it’s a still photo term not motion film.

    35mm film is roughly the same size as APS C sensor and same as S35.  So 1.6 crop factor is from a full frame still film camera and there is no crop factor when comparing motion film and S35 sensor.

    Anyone coming from film won’t see the difference on a S35 sensor.

    I have a 5D Mark 3 and a GH4 and I use which ever is best for that shot.  If I don’t like the look I change the lens or camera, period.

    Let’s move on. nothing to see.

  7. almo100 Yes, exactly, but a standard would be nice in interchangeable lens cameras that shoot footage. We have what now? At least five different sensor sizes floating around in popular cameras now.

  8. William Sommerwerck Chris Santucci almo100 That’s pretty cool. Do they sell those at B&H? :-p

  9. Chris Santucci William Sommerwerck almo100  Not that I’m aware of. Some third party will have to produce one, because no manufacturer will make something that implicitly promotes a competitor’s product.

  10. Filip Wesolowski William Sommerwerck

    F stop is the ratio of the lens aperture to focal length.  T stop is the amount of light transmitted through a lens.  They are not the same thing.  See here for more:  http://youtu.be/jI8uAzX0bBw

  11. William Sommerwerck Chris Santucci almo100 There is an app for that. https://itunes.apple.com/us/app/cadrage-directors-viewfinder/id793232740?mt=8

  12. almo100 William Sommerwerck Chris Santucci It doesn’t show equivalences among different lenses for different formats, which is what we discussing..

  13. James 9 Filip Wesolowski William Sommerwerck  Please read what I wrote, Mr Wesoloski: ” I’m ignoring slight differences in transmission. Which was the reason for T stops.”

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