Hi all, Does anyone know how to calculate the focus distance to create the equivalently framed image with two different focal lengths? For example, how much closer do I have to get with my O45mm to create the same framed image as the O75? Or same question between the 12, 15, and 17mm lenses. I'm trying to do some DOF calculations. Thanks! Sent from my iPhone using Mu-43

Doesn't work. One focal distance is infinity, what proportion of that for the other lens? (for example)

Hmm, maybe I misphrased my question? I mean, say you have a watermelon in the frame with a 45 and a 75. How far do you stand to make the watermelon fit 50% of the frame? I know it can be done (and yes, I know he exact framing won't be the same), just need the math to do it. Sent from my iPhone using Mu-43

That still depends on many factors, particularly whether you are talking about horizontal, vertical, or diagonal measures. The best way to calculate this is to use the field of view for each lens. I am not sure why you wish to calculate this. Explaining might help people give answers. In practice we just step forward or back to change our perspective.

LENS ….. < of view …… 1/2 < of view (angles are on the diagonal of the sensor) O75 …….. < = 18 …….. 1/2< = 9 O60 …….. < = 20 …….. 1/2< = 10 O45 …….. < = 27 …….. 1/2< = 13.5 O25 …….. < = 47…….. 1/2< = 23.5 O17 …….. < = 65 …….. 1/2< = 32.5 O12 …….. < = 84 …….. 1/2< = 42 Figure the longest focal length first. The formula gives 1/2 the distance along the sensor diagonal. a = 1/2 the frame length along the diagonal. ( this is calculated based on the manufacturer’s given 1/2< ) b = distance from sensor plane to subject. ( this variable is input by you ) formula ……. a = b x Tan 1/2< Figure the shorter focal length. You are looking for the new “b” dimension from the calculated “a” dimension in the above formula. formula ……. b = a / Tan 1/2< Again, the formula gives 1/2 the distance along the sensor diagonal. Yes, you can also use the law of sines and cosines. Good luck trying to figure DOF.

Just remember that while framing may be similar, perspective will be completely different due to moving close to/further away from the subject. This is also affects background blur, and in turn perceptive DOF. Longer lenses will appear to have blurrier backgrounds and thus narrower DOF, due to enlarging the background/out of focus elements further. Luckily, there is a website that calculates all of this for you: http://howmuchblur.com/#compare-1x-50mm-f1.4-and-1x-85mm-f1.8-on-a-0.9m-wide-subject This sort of calculation is great for determining subject isolation. If you want to calculate only DOF, eg, how much is actually in focus irrespective of background blur, use this calculator: http://www.dofmaster.com/dofjs.html The easiest way to do this with the above calculator is to set the focal distance relative to the focal length. For instance: Calculate 75mm at 7.5 feet, 45mm at 4.5 feet, etc. Someone can correct me if this is a dumb thing to do, but its how I've always done it and it seems to make sense. This should even it all out (and you'll find that F1.8 has the same DOF no matter the focal length when calculating like this). This sort of calculation is great for ensuring you have enough in focus, for landscape photography for instance.

Thank you!! This is exactly what I was looking for - calculating relative subject isolation. For example, I have the 14mm f/2.5. Would I get more subject isolation using the 12mm f/2 with the extra 2/3 stop, but needing to move closer to keep objects the same size? The answer is yes! Similarly, the 75mm seems to have way more subject isolation properties than the 45, especially at long distances. So, thanks! Sent from my iPhone using Mu-43

Yes it does, the 75mm actually is on par with the 42.5/1.2. Though having owned both I prefer the working distance of the Nocticron.

Thanks, good info. I'm basically considering swapping my 45 and my 45-175 for the 75, since with my PL25, I seem to never use the 45, as i still fill the frame with a face using the 25. And when I want reach, I find the 45 not enough. On the other end, the 12 and 14 are pretty close on subject isolation, but going to the 15 f/1.7 gives me more of what I want on the wide side with that extra half to full stop. Sent from my iPhone using Mu-43

There is no "equivalently framed image" between various focal length lenses (if by "equivalent" you mean the same perspective between foreground and background objects - i.e. the same relative sizes between fore- and background objects). If you adjust the camera-to-subject distance so the foreground objects are the same size with both the O45 and O75 lenses, then the background objects will be smaller in the O45 image than the O75 image. This is just a law of perspective optics, perspective being determined by where the camera is place and field of view being determined by the lens' angle of view. As for how far you'd have to adjust the camera-to-subject distance to have a foreground object be the same size on-sensor with both lenses, the distance is the ratio of the two lens's focal lengths, i.e. 45/70 = ~0.643. So an object 10 feet from the camera using the 70mm lens will be the same size on-sensor at 6.43 feet using the 45mm lens. ~Joe

Thanks Joe, that makes sense. I meant "same sized object", understanding that the perspective can't be the same. The last formula helps too. Sent from my iPhone using Mu-43

This calculator works in the reverse to the way you phrased the question, but it will still give you the answer with a bit of trial and error. http://www.cambridgeincolour.com/tutorials/camera-lenses.htm For example a 1 ft object will fill the screen at 2.75 ft with a 45mm lens, but with the 75mm you need to be 4.58 ft away for it to just fill the screen. (Yes, it's just a straight ratio of focal length versus distance)

Theres a few youtube videos which are on the same lines - affect of focal length on perspective (while trying to keep the framing the same) - https://www.youtube.com/watch?v=g77_rNL7lsE Not quite what you were after, but its useful nonetheless

Is this what you wanted? Each table corresponds to a framing so that the specified size (0.5m, 1.0m, 2.5m and 5m) fills the vertical frame. In the table is the focus distance (how far away you need to be). For each f-stop there are then two distances corresponding to the distance in front and behind the focus point that should be sharp (assuming a circle of confusion equivalent to 0.029mm in 35mm format). Sorry for the formatting - I have no idea why the forum is inserting so much space before each table. The output is from a short program that I wrote to detail lens/focal-length/fstop settings to help choose a lens for zone-focus shooting. I have a longer blog-post on this that was written a couple of days ago, but which will not be live until this evening... (thanks to the wonder of Wordpress scheduled posts). - Mark <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Head (0.5m) in Landscape Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>0.5</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td></tr> <tr><td>15</td><td>0.6</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td></tr> <tr><td>17</td><td>0.7</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td></tr> <tr><td>25</td><td>1.0</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>45</td><td>1.7</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>60</td><td>2.3</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>75</td><td>2.9</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> </table> <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Upper-Body (1.0m) in Landscape Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>0.9</td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.6m </td></td><td> 0.3m / 1.1m </td></td></tr> <tr><td>15</td><td>1.2</td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.5m </td></td><td> 0.3m / 0.9m </td></td></tr> <tr><td>17</td><td>1.3</td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.5m </td></td><td> 0.4m / 0.8m </td></td></tr> <tr><td>25</td><td>1.9</td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.4m </td></td><td> 0.4m / 0.7m </td></td></tr> <tr><td>45</td><td>3.5</td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.4m </td></td><td> 0.4m / 0.6m </td></td></tr> <tr><td>60</td><td>4.6</td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.4m </td></td><td> 0.5m / 0.6m </td></td></tr> <tr><td>75</td><td>5.8</td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.4m </td></td><td> 0.5m / 0.5m </td></td></tr> </table> <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Full-Body (2.5m) in Landscape Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>2.3</td><td> 0.3m / 0.5m </td></td><td> 0.4m / 0.7m </td></td><td> 0.6m / 1.2m </td></td><td> 0.7m / 2.1m </td></td><td> 0.9m / 4.9m </td></td><td> 1.1m / 59.6m </td></td><td> 1.3m / inf </td></td></tr> <tr><td>15</td><td>2.9</td><td> 0.3m / 0.5m </td></td><td> 0.5m / 0.7m </td></td><td> 0.6m / 1.1m </td></td><td> 0.8m / 1.8m </td></td><td> 1.0m / 3.4m </td></td><td> 1.3m / 9.7m </td></td><td> 1.5m / inf </td></td></tr> <tr><td>17</td><td>3.3</td><td> 0.4m / 0.4m </td></td><td> 0.5m / 0.7m </td></td><td> 0.6m / 1.0m </td></td><td> 0.8m / 1.7m </td></td><td> 1.1m / 3.0m </td></td><td> 1.3m / 6.9m </td></td><td> 1.6m / 80.7m </td></td></tr> <tr><td>25</td><td>4.8</td><td> 0.4m / 0.4m </td></td><td> 0.5m / 0.6m </td></td><td> 0.7m / 0.9m </td></td><td> 0.9m / 1.4m </td></td><td> 1.2m / 2.3m </td></td><td> 1.5m / 4.1m </td></td><td> 1.9m / 9.1m </td></td></tr> <tr><td>45</td><td>8.7</td><td> 0.4m / 0.4m </td></td><td> 0.5m / 0.6m </td></td><td> 0.7m / 0.9m </td></td><td> 1.0m / 1.3m </td></td><td> 1.3m / 1.9m </td></td><td> 1.8m / 3.0m </td></td><td> 2.3m / 4.9m </td></td></tr> <tr><td>60</td><td>11.5</td><td> 0.4m / 0.4m </td></td><td> 0.5m / 0.6m </td></td><td> 0.7m / 0.8m </td></td><td> 1.0m / 1.2m </td></td><td> 1.4m / 1.8m </td></td><td> 1.9m / 2.8m </td></td><td> 2.5m / 4.3m </td></td></tr> <tr><td>75</td><td>14.4</td><td> 0.4m / 0.4m </td></td><td> 0.5m / 0.6m </td></td><td> 0.7m / 0.8m </td></td><td> 1.0m / 1.2m </td></td><td> 1.4m / 1.8m </td></td><td> 1.9m / 2.6m </td></td><td> 2.6m / 4.0m </td></td></tr> </table> <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Group (5.0m) in Landscape Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>4.6</td><td> 1.2m / 2.4m </td></td><td> 1.5m / 4.3m </td></td><td> 1.9m / 9.9m </td></td><td> 2.3m / 128.3m </td></td><td> 2.7m / inf </td></td><td> 3.0m / inf </td></td><td> 3.4m / inf </td></td></tr> <tr><td>15</td><td>5.8</td><td> 1.2m / 2.2m </td></td><td> 1.6m / 3.6m </td></td><td> 2.0m / 6.9m </td></td><td> 2.5m / 19.6m </td></td><td> 3.0m / inf </td></td><td> 3.5m / inf </td></td><td> 4.0m / inf </td></td></tr> <tr><td>17</td><td>6.5</td><td> 1.3m / 2.1m </td></td><td> 1.7m / 3.4m </td></td><td> 2.1m / 6.1m </td></td><td> 2.6m / 14.0m </td></td><td> 3.2m / 173.1m </td></td><td> 3.8m / inf </td></td><td> 4.3m / inf </td></td></tr> <tr><td>25</td><td>9.6</td><td> 1.4m / 1.9m </td></td><td> 1.8m / 2.9m </td></td><td> 2.4m / 4.7m </td></td><td> 3.0m / 8.3m </td></td><td> 3.8m / 18.3m </td></td><td> 4.6m / 121.5m </td></td><td> 5.5m / inf </td></td></tr> <tr><td>45</td><td>17.3</td><td> 1.4m / 1.7m </td></td><td> 2.0m / 2.6m </td></td><td> 2.7m / 3.9m </td></td><td> 3.5m / 6.0m </td></td><td> 4.6m / 9.9m </td></td><td> 5.9m / 18.4m </td></td><td> 7.3m / 46.3m </td></td></tr> <tr><td>60</td><td>23.1</td><td> 1.5m / 1.7m </td></td><td> 2.0m / 2.5m </td></td><td> 2.8m / 3.6m </td></td><td> 3.7m / 5.5m </td></td><td> 4.9m / 8.7m </td></td><td> 6.4m / 14.5m </td></td><td> 8.2m / 27.8m </td></td></tr> <tr><td>75</td><td>28.8</td><td> 1.5m / 1.7m </td></td><td> 2.1m / 2.4m </td></td><td> 2.8m / 3.5m </td></td><td> 3.9m / 5.3m </td></td><td> 5.2m / 8.1m </td></td><td> 6.8m / 12.9m </td></td><td> 8.8m / 22.4m </td></td></tr> </table> <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Head (0.5m) in Portrait Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>0.3</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>15</td><td>0.4</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>17</td><td>0.5</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>25</td><td>0.7</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>45</td><td>1.3</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>60</td><td>1.7</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td></tr> <tr><td>75</td><td>2.2</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.0m </td></td><td> 0.1m / 0.1m </td></td></tr> </table> <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Upper-Body (1.0m) in Portrait Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>0.7</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.2m / 0.5m </td></td></tr> <tr><td>15</td><td>0.9</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td><td> 0.2m / 0.3m </td></td><td> 0.2m / 0.4m </td></td></tr> <tr><td>17</td><td>1.0</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.4m </td></td></tr> <tr><td>25</td><td>1.4</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.2m </td></td><td> 0.2m / 0.2m </td></td><td> 0.2m / 0.3m </td></td></tr> <tr><td>45</td><td>2.6</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.3m </td></td></tr> <tr><td>60</td><td>3.5</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.3m </td></td></tr> <tr><td>75</td><td>4.3</td><td> 0.0m / 0.0m </td></td><td> 0.0m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.1m / 0.1m </td></td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.3m </td></td></tr> </table> <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Full-Body (2.5m) in Portrait Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>1.7</td><td> 0.2m / 0.3m </td></td><td> 0.3m / 0.4m </td></td><td> 0.4m / 0.6m </td></td><td> 0.5m / 1.0m </td></td><td> 0.6m / 1.8m </td></td><td> 0.7m / 4.5m </td></td><td> 0.9m / inf </td></td></tr> <tr><td>15</td><td>2.2</td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.4m </td></td><td> 0.4m / 0.6m </td></td><td> 0.5m / 0.9m </td></td><td> 0.6m / 1.5m </td></td><td> 0.8m / 3.0m </td></td><td> 1.0m / 9.7m </td></td></tr> <tr><td>17</td><td>2.5</td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.4m </td></td><td> 0.4m / 0.5m </td></td><td> 0.5m / 0.8m </td></td><td> 0.7m / 1.4m </td></td><td> 0.8m / 2.6m </td></td><td> 1.0m / 6.3m </td></td></tr> <tr><td>25</td><td>3.6</td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.3m </td></td><td> 0.4m / 0.5m </td></td><td> 0.5m / 0.8m </td></td><td> 0.7m / 1.2m </td></td><td> 0.9m / 1.9m </td></td><td> 1.2m / 3.5m </td></td></tr> <tr><td>45</td><td>6.5</td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.3m </td></td><td> 0.4m / 0.5m </td></td><td> 0.6m / 0.7m </td></td><td> 0.8m / 1.0m </td></td><td> 1.1m / 1.6m </td></td><td> 1.4m / 2.4m </td></td></tr> <tr><td>60</td><td>8.7</td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.3m </td></td><td> 0.4m / 0.5m </td></td><td> 0.6m / 0.7m </td></td><td> 0.8m / 1.0m </td></td><td> 1.1m / 1.5m </td></td><td> 1.5m / 2.2m </td></td></tr> <tr><td>75</td><td>10.8</td><td> 0.2m / 0.2m </td></td><td> 0.3m / 0.3m </td></td><td> 0.4m / 0.5m </td></td><td> 0.6m / 0.7m </td></td><td> 0.8m / 1.0m </td></td><td> 1.1m / 1.4m </td></td><td> 1.5m / 2.1m </td></td></tr> </table> <table style="width:100%;text-align:center;"> <tr><td colspan="9" style="text-align:left;background-color:#eee;"><b> Group (5.0m) in Portrait Orientation</b></td></tr><tr><th>Focal Length (mm)</th><th>Focus Distance (m)</th><th>f1.4</th><th>f2.0</th><th>f2.8</th><th>f4.0</th><th>f5.6</th><th>f8.0</th><th>f11</th></tr> <tr><td>12</td><td>3.5</td><td> 0.7m / 1.2m </td></td><td> 0.9m / 2.0m </td></td><td> 1.2m / 3.6m </td></td><td> 1.5m / 9.1m </td></td><td> 1.8m / inf </td></td><td> 2.1m / inf </td></td><td> 2.3m / inf </td></td></tr> <tr><td>15</td><td>4.3</td><td> 0.7m / 1.1m </td></td><td> 1.0m / 1.8m </td></td><td> 1.3m / 3.0m </td></td><td> 1.6m / 6.0m </td></td><td> 2.0m / 19.7m </td></td><td> 2.3m / inf </td></td><td> 2.7m / inf </td></td></tr> <tr><td>17</td><td>4.9</td><td> 0.8m / 1.1m </td></td><td> 1.0m / 1.7m </td></td><td> 1.3m / 2.8m </td></td><td> 1.7m / 5.1m </td></td><td> 2.1m / 12.9m </td></td><td> 2.5m / inf </td></td><td> 2.9m / inf </td></td></tr> <tr><td>25</td><td>7.2</td><td> 0.8m / 1.0m </td></td><td> 1.1m / 1.5m </td></td><td> 1.4m / 2.4m </td></td><td> 1.9m / 3.9m </td></td><td> 2.4m / 7.0m </td></td><td> 3.0m / 16.5m </td></td><td> 3.6m / 442.0m </td></td></tr> <tr><td>45</td><td>13.0</td><td> 0.8m / 1.0m </td></td><td> 1.1m / 1.4m </td></td><td> 1.6m / 2.1m </td></td><td> 2.1m / 3.1m </td></td><td> 2.8m / 4.9m </td></td><td> 3.6m / 8.2m </td></td><td> 4.6m / 15.7m </td></td></tr> <tr><td>60</td><td>17.3</td><td> 0.8m / 0.9m </td></td><td> 1.2m / 1.4m </td></td><td> 1.6m / 2.0m </td></td><td> 2.2m / 2.9m </td></td><td> 2.9m / 4.5m </td></td><td> 3.9m / 7.1m </td></td><td> 5.0m / 12.0m </td></td></tr> <tr><td>75</td><td>21.7</td><td> 0.9m / 0.9m </td></td><td> 1.2m / 1.3m </td></td><td> 1.6m / 1.9m </td></td><td> 2.3m / 2.8m </td></td><td> 3.1m / 4.3m </td></td><td> 4.1m / 6.5m </td></td><td> 5.4m / 10.6m </td></td></tr> </table>

IMO there are two factors to consider. First as others have mentioned, perspective. http://en.wikipedia.org/wiki/Perspective_distortion_(photography) Second, depth of field. For a given aperture, depth of field is shallower as the focal length of the lens increases. Shallow DOF improves subject isolation. Lots of debate about DOF around here because for people who like shallower DOF the shorter focal lengths of the M43 lenses are a disadvantage. That's why, IMO, you are seeing f 0.95 lenses being offered. In the Bad Old Days, fast lenses were important because film speeds were slow. 400 was about it for decent color and 1600 for pushed Tri-X in B&W. Now that the cameras have much higher sensitivity the interest in fast lenses has moved towards DOF considerations. Formulas and tables really aren't IMO very helpful in understanding perspective and DOF. Just get out and start experimenting.

Of course there is no substitute to going out and shooting (you have to learn perspective, motion blur, ISO noise, composition, ...). However, if you are starting out anything to give some idea as to what to expect from a camera/lens is useful - particularly if you do not already have a given lens. Also, I have been shooting for many years, but it was only when looking at the math that I finally understood why I was finding street shooting with a 50mm so much harder than with a 24mm (because even with the same subject size, you get more DOF at the same f-stop and can use faster shutter speeds for the same ISO...).

I remember shooting ISO 50 Velvia slides, not knowing what the output would be until days or weeks later and always needing a tripod or very bright light. Bad old days indeed. Sent from my iPhone using Mu-43

Exactly - I don't have the 12 nor do I have a lens faster than f/2.5 at 14 (or a fast telephoto longer than 45), so hence trying to seek out the calculations. Sent from my iPhone using Mu-43