Sunday, February 21, 2010

Comparison Medium Format and 35mm Equivalent Focal Lenghts

Usually, equivalent lens focal length tables use the diagonal angle of view as a reference (for example comparing medium format lens focal lenghts to other formats). This does not make sense as one needs to consider both, the image's length and width if one wants to capture a similar image! It makes a large difference if you want to end up with a square image or an aspect ratio of 3:2. You cannot take a similar photo with a different film's aspect ratio unless you crop.

Even worse, the popular page at has a few conversion errors in its calculations (probably wrong metric conversion factors). Moreover, it gives its results with a misleading and irrelevant precision of one position after the decimal point and does not show the results for 645 and 6x7 film!

Therefore I provide the following table, which gives different equivalent 35mm focal lenghts for popular 6x6 lenses (first columns) and different aspect ratios you want to end up with (larger focal lenght are rounded).

6x6 Medium Format Focal Length (mm) 35mm film 1:1 35mm film 5:4 35mm film 4:3 35mm film 3:2
40 17 21 22 25
50 21 26 28 32
60 25 32 34 38
75 32 40 42 48
80 35 42 45 50
150 65 80 85 95
250 105 135 140 160
300 130 160 170 190

Let's take two examples: You want a square image and use a 80 mm lens on a 6x6 camera. You need a 35 mm lens for a 35mm camera (because you have an 24 mm x 24 mm image instead of 56 mm x 56 mm, this yields a factor of 3/7).
However, if you use the same 80mm lens on your medium format camera and do not want a square image but plan to crop it with an 3:2 aspect ratio, you need an 50 mm equivalent focal length for the 35mm film format. The frequently made comparison of medium format's 80mm with 35mm format's 50 mm lens is only valid if you want to have image with a 3:2 aspect ratio!
So essentially, you have two very popular lenses in one with your 80 mm lens on a square format! You have a wide and a normal lens. The same is true for your 150 mm lens: you have both, a long normal (65 mm equiv.) and a short tele lens (95 mm). Moreover, you will not encounter any sharpness or resolution problems when a medium format image is cropped to 3:2.
This table also shows how popular 35mm prime focal lenghts might have developed. For example, 150mm lenses are often considered as portrait lenses in medium format and the aspect ratio of 4:3 is also very popular with portraits. It requires a little cropping with both, medium and 35mm format. Now let's look at the column of the equivalent 35 mm focal length at 4:3. It's 85 mm, a very popular portrait focal length with 135 cameras! I am sure the manufacturers made the same calculations in recent times!
Similar calculations probably led to the also popular wide angle focal lengths of 28 mm and 35 mm (also in the 4:3 column).

If you have a 6x4.5 camera, you can simply use the first column for your medium format lenses and the 4:3 column for the 35mm equivalent, but as 6x4.5 is not exactly 4:3 but 112:83 (because it is 56 mm x 41.5mm), you can also use the following, more precise table which also comprises other popular focal lenghts.

6x4.5 Medium Format Focal Length (mm) 35mm film 1:1 35mm film 5:4 35mm film 112:83 35mm film 3:2
35 20 20 20 22
75 43 43 43 50
120 70 70 70 77
150 85 85 85 95

The following table relates to 6x7 cameras (70mm x 56 mm image) with their respective frequently available focal lengths.

6x7 Medium Format Focal Length (mm) 35mm film 1:1 35mm film 5:4 35mm film 4:3 35mm film 3:2
43 18 18 19 22
50 21 21 22 25
65 27 27 29 33
80 35 35 36 41
150 65 65 70 80
210 90 90 95 110

As 645 and 6x7 are not square formats, they also need to be cropped in order to become square. Therefore the equivalent 35mm focal length is the same as in their native aspect ratio.

Saturday, February 6, 2010

Testing Lens and Camera Shutter Speeds using Audio

Carl Zeiss Planar 1:2,8 f=80mmIt is very easy to test the shutter speeds of your lens or camera. You simply record it while you press the shutter and analyze the duration of the shutter sound.

I did this in order to check if my Zeiss Planar 80 mm f/2.8 lens was working correctly after its repair. Before the repair, 1 second nominal shutter speed lasted approximately 2 seconds (I did not test the exact shutter speed but it was obvious that it was too long because I could observe this with a watch!).

With old lenses, the shutter speeds in the range from 1/2 s to 1 s and 1/250 s to 1/500 s are often too slow and you have to add one or two thirds of a stop to the aperture in order to compensate for this. The intermediate speeds are usually okay.

I recorded the sound with a portable Zoom H2 MP3 recorder (it's no problem to use a Zoom in order to record fast primes!) at 48 kHz and 16 Bit. I used 48 kHz because it yields more samples and should be more accurate - at least theoretically. 44.1 kHz should also be fine. Of course any other microphone would also work.

I set the shutter speed, pressed the mirror pre-release, announced the selected shutter speed and pressed the shutter button. I release the shutter button about half a second after the shutter closed so the sound of the auxilliary shutter is separated from the sound of the diaphragm shutter. I recorded each shutter speed three times in order to test if they vary from time to time. I tested all speed from 1 s to 1/500 s.

Then I read the file into the computer and opened it with Audacity. Of course you can use any software that shows a time scale and has the ability to zoom into the track. The following image shows an example for the shutter speed of 1/2 s. The first noise is the mirror pre-release. The shutter is pressed at approximately 1:10.95 and closes at 1:11.5. Of course it is difficult to see where exacly the shutter opens and closes but I found the most reasonable values when I simply looked at the positions of the maximum peaks, in this case at 1:10.99 and 1:11.50. This means, the shutter opens for 510 milliseconds instead of the nominal 500 milliseconds. Thus it is only 2 % too slow. I think, this is pretty accurate for a mechanical shutter.

At 1:12.1 you see that the shutter button is released and the auxilliary shutter closes.

Please note that you need to compare your measured values to the acutal and not to the marked shutter speed (see table below). Use the right column in order to compare your results to the specified speed.

Marked shutter speed (1/s)   Actual shutter speed (1/s)   Duration (ms)  
1 1 1000
2 2 500
4 4 250
8 8 125
15 16 62.5
30 32 31.3
60 64 15.6
125 128 7.8
250 256 3.9
500 512 1.95
1000 1024 0.98

For fast speeds, one needs to zoom much more into the audio file and it becomes harder to determine the position of the maximum peaks. For me it worked still at 1/500 s which is the maximum speed for this lens.

I found all shutter speeds of my lens to be correct within 10 %. This means I do not need to correct for this and close the aperture anywhere in order to compensate for this.

Testing the shutter speeds this way is relatively simple, inexpensive and precise.