SLR Lens vs Mirrorless Lens Focal Length

<p>After using film SLRs with standard focal length lenses, i.e. 50MM, 52MM etc., I'll be switching to mirrorless

cameras. How does one determine how mirrorless focal lengths relate to SLR focal lengths? For example, a

mirrorless 14 to 42MM lens which comes as the standard lens - is this comparable to the 50 to 55MM lens that

may have been the standard lens for a SLR? If not, is there a rule of thumb for converting?</p>

<p>It varies with the system. If you're referring to the 14-42 lens in the micro-4/3 system, its field of view equates to a 28-84mm in a 35mm film camera (2x).</p>

<p>What the 'conversion factor' deals with is equivalent angle of view. The factor for micro fourthirds is 2X. Thus you multiply by 2 when you want to know what the equivalent lens focal length would be on a 24X36mm full frame sensor. This has special meaning when adapting legacy lenses to cameras using smaller than full frame sensors.</p>

<p>Such as; I have an Olympus E-410 with an adapter for my OM mount lenses. When I clip on a 135mm lens it has an angle of view the same as a 270mm lens used on a full frame camera.<br>

Somr sensors are a bit larger than fourthirds and have conversion factors like 1.3X or 1.5X or 1.6X <br>

There are a forrest of sensor sizes out there.</p>

<p>Clear as mud?</p>

<p><a href="http://en.wikipedia.org/wiki/Image_sensor_format"><strong>This Wikipedia article</strong></a> includes an illustration with multiplier factors for "crop sensor" cameras, all based on the 35mm film or 24x36mm paradigm as the baseline standard. (An arguable assumption for folks who used medium and large format films, but it's too late to stuff that cat back into the bag.)</p>

<p>Lenses for these systems are usually labeled and marketed in the actual focal lengths. It's up to the user to estimate the equivalents, again presuming the 24x36 paradigm:</p>

<ul>

<li>The various APS sensors have multipliers ranging from 1.29x to 1.62x. Nikon's APS-C DX sensor multiplier is 1.5x, which makes it easier to estimate the comparable 35mm film/full frame digital equivalent.</li>

<li>The Four Thirds, 4:3 or Micro 4:3 multiplier is 2x.</li>

<li>Nikon's CX and Sony's one-inch sensor multiplier is 2.7x.</li>

</ul>

<p>Tiny sensor digital cameras, usually P&S types with non-interchangeable lenses, typically are labeled with the actual focal lengths on the lenses, but are occasionally marketed with their multipliers for convenience since many consumers are familiar with the 24x36/35mm film paradigm. A P&S digicam with a 1/1.7" sensor has a multiplier of 4.55x, so a camera with a fixed wide angle listed as "28mm equivalent" will have an actual focal length of approximately 6mm.</p>

<p> </p>

<p>Being an old film dog I try, (not always successfully) to equate sensor size to old film formats.<br /> For instance; fourthirds is very close to 110 cartrige film's 13X17mm<br /> APS size are sometimes close to my 35mm half frame camera's 18X24mm<br /> And Nikon CX is a little smaller than my Minolta 16 size of 10X14mm <br /> I think there is also a sensor close to the Tessina's 14X21mm format.</p>

<p>Others already said the fundamental things, and in particular the Wikipedia link is quite complete. Just remember that it's not only a matter of equivalent angle-of-view, but also of equivalent depth-of-field (as the Wikipedia link also explains). For instance, a photo shot at f/2.8 on a MFT system with a 100mm lens has a depth of field roughly equivalent to f/5.6 (two stops more) on a full frame with a 200mm lens. If you are not concerned with shallow DoF (e.g. you're a landscape shooter) this won't be a problem, as you will stop down the lens; if you are concerned with photos in which subject is well separated from the background, it can be a problem.<br /> If you're also concerned with sharpness, there's another issue to take into account, and here unfortunately there's no rule of thumb, nor formula: while most lenses (full frame or APS-C) tend to give their best sharpness around f/8 - f/11, and diffraction starts to be a severe limitation beyond f/16 (typically at f/22), the best aperture in terms of sharpness might be different with other formats and the smaller the sensor the earlier diffraction starts to be a problem. For instance, for MFT many lenses give their best sharpness at full aperture (e.g. see [1]), and diffraction is a problem starting from f/16, sometimes f/11. Details are depending on the lens model and you have to look at reviews.<br>

[1] http://www.photozone.de/olympus--four-thirds-lens-tests/530-pana_714_4?start=1</p>

<p>The so called focal length multiplier or crop factor or whatever you want to call it has nothing to do with whether a camera is an SLR or mirrorless. It has to do with the size of the sensor.</p>

<p>As BeBu said for instance the 80mm on the Rollei is wider than the 50 on an SLR, and a 135 is wide for a 5x4 camera. The diagonal of MFT is 21mm so that is roughly equal to a 43mm on SLR.<br>

That is using the diagonal of the gate or usable area of the sensor as the focal length of the 'Standard ' lens. MFT is 17.3 x 13mm so the x2 is a rounded figure near enough for most purposes. As was 17x13 when I did my calculations above :-)</p>

<p>Another excellent resource:<br>

<a href="http://www.josephjamesphotography.com/equivalence/index.htm">http://www.josephjamesphotography.com/equivalence/index.htm</a></p>