Implied Focal Length of EF-S Lenses

<p>Canon (as well as many other reviewers of Canon lenses) states that the 55-250mm EF-S lens is "equivalent to a focal length of 88-400mm in the 35mm format (when used on Canon EOS cameras compatible with EF-S lenses)." Since the lens was designed specifically for APS-C sensors, why wouldn't they then say that it is an 88-400mm lens? If I were able to mount this lens on my full-frame 6D would it then have the angle of view of a 55-250mm lens? In other words, would it cover the entire field of the full-frame sensor? The obverse makes sense: if I mount my 70-300mm lens on my APS-C T3i it would be the equivalent of a 112-480mm lens because the sensor is only reading the smaller rectangle in the center of the field.</p>

<p>Can anyone shed any light on this? Does the EF-S 55-250mm actually have the field of view that would cover a full-frame sensor or 35mm frame? This, of course, would no doubt also apply to all the EF-S lenses, not just the 55-250mm.</p>

<p>Focal length is a property of the lens and not the sensor that it is being mounted to, therefore the focal length of the lens is the same regardless of what camera it is mounted on. They say equivalent because it would have a similar angle of view as an 88-400mm lens on a 35mm camera, but physically it is still a 55-250. As for would it cover full frame, no it would only project a circle partially covering the frame. Also Canon makes their EF-S lenses so that they won't mount to full frame so trying to mount it to a 6D could cause damage and is not recommended.</p>

<p>It's all here <a href="http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html">http://www.bobatkins.com/photography/tutorials/crop_sensor_cameras_and_lenses.html</a></p>

<p>Focal length is focal length. Period.</p>

<p>"Equivalent focal length" is an expression relating to the field of view to that of the focal length of a lens on a different sized sensor camera giving the same field of view.</p>

<p>If we used field of view to describe the characteristic of a lens on a given camera rather than focal length, we wouldn't have all this confusion</p>

<p> </p>

<blockquote>

<p>If I were able to mount this lens on my full-frame 6D would it then have the angle of view of a 55-250mm lens?</p>

</blockquote>

<p>Yes and no. Yes, in theory but in fact the corners (and possibly edges) of the frame would be black (or at least dark) and/or not very sharp. No in that the vignetting of the lens would limit it's field of view, not the size of the sensor. The image scale would indeed be that of a 55-250mm lens because <strong>IT IS</strong> a 55-250mm lens</p>

<p>This "equivalence" business was a really bad idea, even when most people adopting digital were old 35mm film shooters.</p>

<p>Today it only causes confusion.</p>

<p>In fact, it would make just as much sense to label a 50mm 24x36mm-coverage lens an 80mm equivalent, since on a 6cm (2.25") film camera the 'normal lens' was 80mm. By a coincidence, a 35mm coverage lens has a 1.6X "crop factor" in that comparison.</p>

<p>People need just to learn what is normal, etc. on each format they use and stop trying to make that relate to the <em>invariable fact</em> of what the focal length of a lens is.</p>

<p>Thank you Matt and Bob! Your input clears it up quite well and answers my question. Thanks again.</p>

<p>Bob, I agree 100%. "Field of view" is the clearest term, not "equivalent focal length" or "reach" which to me both imply magnification versus full frame. However, from a marketing standpoint, talking about a longer "equivalent focal length" (which suggests paying less and getting more as long lenses can be very expensive), is more appealing that saying that a lens on a crop sensor camera gives you a "narrower field of view". "Long" sounds good, "narrow" sounds not so good.</p>

<p>It gets even more confusing when you start throwing in other sensor sizes besides full frame and APS-C. My Lumix LX-5 is usually described as having a "24-90mm" lens. In reality, its 5.1-19.2mm. The whole "equivalent" focal length thing should have been dumped by now. We don't deal with such nonsense when it comes to medium and large format, why so with smaller formats?</p>

<p>This excerpt from the Bob Atkins page mentioned earlier may help:</p>

<p>First, what is a crop sensor camera? Well, it's simple. A full frame 35mm camera (whether it uses film or a digital sensor) records an image that is approximately 36mm x 24mm in size. In the early days of digital sensors it was not possible to make digital sensors that big in any sort of quantity, and the ones you could make were so expensive that hardly anyone would have been able to buy a camera which used one. So camera makers decided to use a smaller sensor, around 15mm x 22.5mm. This just happens to be close to the image size which was used with the short-lived APS film format, specifically the APS-C image size of 25.1 × 16.7 mm (there was also APS-H and APS-Panoramic format).</p>

 

<center><img src="http://www.bobatkins.com/photography/tutorials/cropped_sensor_view/crop_sensor.jpg" alt="" width="403" height="284" border="0" /></center>

<p>The "crop" name comes from the fact that if you take a full frame image (24x36mm) and crop the center 15x22.5mm out of it, you get an image the size of "crop" sensor cameras.<br>

So why does the format size matter and what effect does it have on focal length? Well the answer to the second part of the question is "none". The focal length of a lens is the focal length of the lens. Whether you mount that lens on a 35mm camera, a medium format camera of a large format camera doesn't change its focal length. All 35mm lenses and lenses designed for use on APS-C DSLRs are marked with their true, actual, focal length.<br>

The problem is that most of us have been trained to think in terms of focal length rather than field of view when comparing lenses. We've been trained to think that a 50mm lens is "normal", a 35mm lens is "wide normal", a 28mm lens is "wide", a 24mm lens is "very wide", a 20mm lens is "super wide", a 16mm lens is "ultrawide" and so on. In fact this is true ONLY if that lens is making a 36mm x 24mm image. The field of view (which is what "wide" is all about) is actually determined just as much by format size as by focal length. The diagram below shows why.</p>

<p>We don't deal with such nonsense when it comes to medium and large format, why so with smaller formats?<br /> <br /> Because anyone using medium and large format equipment probably understands what they are doing and the math relating focal length to sensor size and field of view. The average user of a compact digicam is pretty clueless. Mention the word "math" and their eyes glaze over.<br /> <br /> Even I would have little idea what the FOV of a digicam with a 5.1-19.2mm lens was. I'd have an idea it was probably wide to short-telephoto, but I'd have no real feeling for how wide and how long. First I'd have to figure out how big the sensor was and very few cameras have that in the spec. If they do it's in the equally (and possibly more) confusing 1/1.7" terminology, which you then have to look up in a table somewhere to find it's approximately 9.5x7.6mm. Then you have to figure out what the "crop factor" is....</p>

<p>BTW we generally don't post text from other websites on the forums here because of a number of reasons, not the least of which is copyright. In this case since it's from my site and I don't mind, I'll leave it, but normally we don't post quotes (or images) from anywhere else.</p>

<p>[[My Lumix LX-5 is usually described as having a "24-90mm" lens. In reality, its 5.1-19.2mm.]]</p>

<p>The LX-7, however, has a 4.7-17.7mm lens, despite the fact that the field of view remains 24-90mm. </p>

<p>The equivalent field of view is the better way to compare compact cameras because sensor sizes change with some regularity. <br /> </p>

<p>Bob and Rob, point taken. It's just very confusing is all.</p>

<p><em>Re. Bob and why do we put up with this nonsense in 35mm?</em></p>

<p>Its because Canon and Nikon and all the others don't want everyone buying just full frame lenses. Canons way of preventing this is by stopping EF-S lenses mounting properly on Canon FF bodies. Nikon is not so brazen and their DX lenses will mount on FX bodies, but you suffer the black edges and corners as the circle of coverage on DX lenses is smaller, suiting the small censor dimensions.<br>

But everyone knows this don't they?</p>

<p> </p>

<p>Since it's likely most people buying an EF-S lens don't have any idea how the FOV looks on a 35mm/FF camera, even offering that description would be pretty unhelpful to the consumer. Presumably, by the time someone adds a FF body to their arsenal of gear, they would understand the FOV is different for a given focal length, so it is again unnecessary. I think making the comparison is typically unhelpful and generally serves to confuse more than it makes clear, except when you have a long time film user switching to digital and is going with an APS-C format body.</p>

<p>Using more descriptive terms would be far more helpful...ultrawide, wide, normal, telephoto, supertelephoto...then people might have some idea of how that lens might fit their needs.</p>

<p>Canon and Nikon don't make any money by being helpful... They make money by selling new things... Lenses and Cameras specifically. I certainly agree that a 'FOV' nomeclature would be much more helpful in general use than the traditional FL, but it's easier to sell a "50mm" lens than a 40deg horizontal by 27deg vertical by 46deg diagonal or "40d H x 27d V x 46d D (on 24mmx36mm)"... ;-)</p>

<p>Bob, that snipe at math retards was hardly meaningful. The simple fact is that the larger formats predate 35mm (miniature format).</p>

<p>I find it useful to think in FOV equivalents. Actual mileage obviously varies.</p>

<p>The comment earlier about medium and large formats I think is spot on - when I use my RB67, I know that the 50mm is a wide angle, the 90mm is normal and the 180mm is telephoto. I couldn't care less how that relates to 35mm</p>

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<p>The comment earlier about medium and large formats I think is spot on - when I use my RB67, I know that the 50mm is a wide angle, the 90mm is normal and the 180mm is telephoto. I couldn't care less how that relates to 35mm</p>

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<p>Maybe that works fine for you, but there are plenty of people who use a variety of different formats at FF and below (like me, with a FF/1.6-factor kit), and find it useful to have a conventional reference point – namely behaviour on FF – with which to make a comparison. So, I find it useful to remember that when I put my 10~22 in the bag with the 7D I am going to get marginally more at the wide end than with the 17~40 and 5DIII, because the 10~22 has the same FOV at the wide end as a 16mm lens on FF. But certainly we should be talking about FOV rather than implying that the focal length changes in some mysterious way.</p>

<p>Concerning EF-S lenses, Canon have, rightly or wrongly, simply chosen a different technical route from Nikon. By making EF-S lenses mountable only on 1.6-factor bodies, they have given themselves the freedom to use designs that extend further back into the camera than is possible with a FF body. No need for paranoia just because you would prefer that they had done it differebtly.</p>

<blockquote>

<p>find it useful to have a conventional reference point</p>

</blockquote>

<p>Is it useful, or merely confusing to refer to an unrelated format?</p>

<p>I use and have used everything from 4"x5" to 'half-frame' and use a kit with both APS-C and 35mm-sensor digital.</p>

<p>Somehow it doesn't confuse me at all to distinguish between my Flektogon 50mm wide angle on 6cm and my Nikkor 55mm f/1.2 normal on my 35mm formats.<br /> So far, I've had really good luck in distinguishing between my Sigma 10-20mm lens and my Sigma 15-30mm lens, too. I just think in terms of ultrawide, or whatever, and don't worry about specific focal lengths much.</p>

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<p>Bob, that snipe at math retards was hardly meaningful.</p>

 

</blockquote>

<p>To be fair, Bob didn't use <em>anything like</em> as pejorative a phrase to describe these folk; and besides, he's absolutely right - many people simply don't care/can't be arsed to expend the (small) effort that's needed, into understanding these principles. </p>