Thursday, April 10, 2014

The Perfect Portrait Lens for MFT?

About this lens, you have seen me writing previously, the Cosina COSINON f=55 F=1.4 M42 screw-mount lens. These lenses are presently on Ebay for really really cheap. I wonder if people actually know what they are selling. Actually I bought 2, just because... well, not really, one for photography, one for astro-photography (check out my bellow-cam, somewhere in this blog).

So, this is how the lens looks like:
Cosina COSINON 55/1.4
You're probably not so much interested in how the lens look , but rather what the importance of such a lens is in the digital motorized auto-focus zoom era.
Well, with some cameras, manual focus can actually be quicker than auto-focus. As to zoom, walking back and forth, given the environment allows for this, is of course slower. Additionally, to compress the background, a lens change has to happen.

On the positive, this lens set me back about €30,- only. For an f/1.4, that is nothing! Right, to make it work, e.g. on the Olympus E-PM2, I had to invest some more dough to get myself an adapter. However, those adapters can be found on Ebay too, where they are shipped from the Far East for free (provided one is patient enough).
The adapter M42-MFT (micro four-thirds) came by mail today. Hence, the possibility to mount the COSINON lens on the Olympus PEN mini2 for the first time.

Due to the lack of a model, I took a portrait of a brick in a wall of my house.
scaled brick portrait @ f/1.4

Nothing done to the image other than converting from RAW to PNG and scaling it down.
Have a look at the nice bokeh in both, the foreground and the background... and this very shallow island of sharpness cutting through the frame.

Speaking of, this is a crop of the center line, in original resolution.

center of the image, original resolution
Yes, there is some color fringing going on. As I said, there is no "correction" applied whatsoever.

Now to some technical details, some of my readers are potentially interested in.

The focal length of about 55mm would be referred to as a "normal" when used on a full frame camera (36mm x 24mm), since this is essentially how we see.
The MFT's (micro 4/3) sensor size has a crop factor of about 2. Meaning the field of view on such a sensor would be equivalent to a field of view of a 110mm lens on a full frame sensor.
110mm equivalent falls spot on the realm of portrait lenses, because such a focal length gives enough background compression and still allows to be relatively close to the "subject".

In terms of brightness, we loose two stops! Meaning, mounted in front of an MFT sensor, the lens is now acting as a 110/2.8, relative to full frame gear.

However, there still is a difference between a 110/2.8 used on full frame and a 55/1.4 used on MFT, and an important difference that is! The depth of field!
The depth of field is only dependent on the distance between the lens and the sensor, i.e. the focus distance. Although less of the light that went into the lens from the object side is actually falling onto the sensor (essentially a fourth only), the focal condition did not change at all! The ratio of the distances between the object and the sensor are identical, disrespectfully of how much light falls onto the sensor integrally.

It seems there is a disadvantage of loosing two stops of light.
I don't think so! In portrait photography, we have control over all our light(s). In a studio, we will just have our (three) speedlites at full power rather than quarter power. OK, some recycle time is lost here...
In bright sunlight, there is even an advantage. How often did you experience that you had to use a 3 stop neutral density filter to be able to shoot wide open (depth of field)?! You might have even missed the shot, due to a lack of a neutral density filter, when metering resulted in s/2000 @ f/11 when using ISO 50. There goes your Bokeh! OK, in this example, loosing 2 stops will get us to f/5.6 only, still a little on the deep side, in terms of depth of field. However, adding a 3 stops ND-filter here, will enable us to open to f/2.

Yep, that was a little technical... I admit!


Maybe mathematics will help. Photography has a lot to do with mathematics! I will backup the following statements by suggestions to experiment you can perform with your very own DSRL.

When the focal length of the objective is doubled, the subject's image will be twice as tall/wide on the sensing surface.
Experiment: Set the zoom of your camera to 25mm and place yourself such than an object fills the viewfinder left to right. Now change your zoom to 50mm. You will see that one half of the object now fills the viewfinder left to right. => a factor of 2 in zoom leads to a factor 1/2 in one dimension of the field of view.
That means that the same amount of light is now spread on twice the width.
That also means that the same amount of light is spread on twice the height!
Therefore, a single pixel of a sensor will only receive 1/2 x 1/2 = 1/4 the amount of light, i.e. 2 stops of light.

What the depth of field is concerned, this is a 1-demensional entity, since this is only determined by the ratios of lengths along the optical axis. This dimension, however, is perpendicular to the dimensions (width and height) of the sensor. Thereby, not being influenced by the crop size whatsoever.
You may want to perform a boring experiment with focal distances, object distances and f-numbers... I would not recommend this. It's linear, hence, the changes are less visible and the entire exercise seems a bit boring to be honest.

There you have it. A very cheap legacy lens, bough from the interwebs, can be very convenient when used with modern digital cameras.

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