What is Focal length, Sensor size in photography and why is it matters?

The intention of this article is to dig out the relationships between focal length of the lens and the camera’s sensor sizes and how does the both affects the framing independently.

I have made a simple illustration diagram below, which depicts how does the light enters from the scene to the sensor/film.

Focal Length-Photography-Sensor-Light entryImage a. – Light travelling path inside a lens and camera body from the scene. x is the height of the sensor(part of the camera), b is the focal plane(principal)- this always happens inside the lens, z is height of the frame (photograph’s vertical measurement in landscape mode), y is the focal length which is usually mentioned on the lens.

Focal length(y) is the distance between the sensor (a) and the focal plane (b) where the light rays converge to form a sharp image of an object to the digital sensor.

Now, the frame you see via the viewfinder will change, if you adjust the Focal length which is possible in zoom lenses. The same is not possible in prime lenses since the focal length has been set in the factory itself while manufacturing, in such a way that it cannot be changed. The prime lenses are made purposely for high quality at a lesser price comparatively with a zoom lens in that focal length.

Conversely, it is also possible to change the frame by changing the size of the sensor. But the sensor has been fit inside the camera while manufacturing in the factory itself and it cannot be changed by wish on the field. But you can use two different cameras having different sensor sizes as one may wish.

Now we have understood that, if we change the size of the sensor (x) or the focal length (y), in both the ways the frame will get changed.

The ranges of available lenses in the market with different focal lengths are beyond listing thus I am not getting into that.

Sensor sizesImage b – Scale of the other smaller sensors with respect to the Full frame sensor. The FF sensor size is 36mm × 24 mm. For the sake of understanding, we here discuss about the vertical  measurement(x).

The smaller the sensor sizes, the field of view gets narrower. That is, if “x” gets smaller, “z” also becomes smaller Assuming that the “y” is same in both the cases. Conversely, if “x” gets bigger the field of view (z) also gets wider.

The advantages and disadvantages of choosing which sensors (cameras) have been briefly explained here in my earlier article.

The crop ratio has been established in the industry with reference to the 35mm film sensors. 35mm width sensors are the reference one and the crop factor for those sensors is 1.00. Other sensors are classified by the crop factor with respect to 35mm sensors (1.00).

Crop factor is the ratio of the dimensions of a given sensor’s imaging area compared to the 35mm sensor’s imaging area (full frame sensors in common). If a sensor’s crop factor is more than 1(APS-C sensors), the image area will be less by that ratio. Conversely, if the crop factor of the camera is less than 1(medium format cameras), the image area will be more by that ratio.

For instance, if a camera’s crop ratio is 1.60 and you have mounted 85mm prime lens, what the camera sees is the view of 136mm (85mm × 1.60) not what is written on the lens. If the same 85mm lens been mounted on a medium format camera with 0.50 crop factor, what you will be seeing is 42.5 mm (85mm × 0.50) focal length view.

Sensor sizesImage c – Dimensions of the different sensors fixed in different cameras and the respective crop ratios.

The focal length written on the lens is true, only if it is mounted on 1.00 X crop factor sensors (full frame cameras). If you are mounting it on non-full frame cameras, you should multiply the crop factor of that camera to get the actual focal length.

I hope, I have covered all the necessary topics which are required to understand the focal length, sensor sizes and how does both affects what you see in the viewfinder. Have I missed something? or the article been helpful to you, do drop a comment and let me know.

Cheers & Happy Photographing.

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Aside

What is photography?

 

I think this is high time that the above question needs to be answered.

Answering straight to the question

  1. If you are able to tell a story /make the viewer to stare at the image for a while / frame an idea, show something really beautiful (Street,Travel)
  2. If you are documenting something (Wildlife, Poverty, Illegal acts, War)
  3. If you are making a concept  (Conceptual , Creative Ads)
  4. If you are showing off something which is not possible to see by the general naked eyes (Macro, Aerial, Underwater)
  5. If you are making something really beautiful (Landscapes, Events, Portfolios,Fashion, Products, etc.,)

1. And why street is on the top

Nothing is in your control; you and your camera only, yet you put an idea, piece of intelligence there between the four lines. Not lighting, not the subjects, nothing is in your control.

2. Here is why Wildlife, War, Poverty – Humanity

You have less control, much more risk in shooting wildlife, sometimes fatal, compromising on your comfort level staying in some remote places where there are no humans, no food, no water yet you loved your camera and went for it, recording the wildlife’s unseen actions and unseen lives.

Likewise you have risked your life for documenting Poverty, Illegal acts, War to bring attention of the common people and governments, activists.

*It is really difficult to classify these two titles in to first & second position, yet I have taken a stand and given importance to photographical point of view.  Do not be very sympathetic, ideas have made us evolved from the invention of stone wheel till cryogenic engines.

3. Conceptual Ads, Creative awareness ad Photography

Everything is under your control, but you have put your intelligence there.

4. Macro, Under water, Aerial Photography

You use so many equipments to capture that outstanding shot, much more planning, taking risk etc.,

5. Landscapes, Events, portfolios

Photographing landscapes is nothing than to take a very beautiful photograph, purely personal satisfaction. Events, portfolios, products all are commercial. Hence last.

Just because you bought an inexpensive or expensive camera, it does not mean that you have to take snapshots. Start respect the master piece of engineering, it will start giving you photographs. Believe me. Start learning, Photography is fun, it is happiness.

Here are few links where either you will learn something or you would be astonished by the masters.

Few iconic images been analyzed by experts here (Check out all the four parts)

Check this master’s images here (Outstanding photographs can be in colour as well)

The above two links are just sample, there are so many masters, start searching.

By writing this article I am not trying to establishing myself as a great photographer or master but I am trying to be one by each time I press the shutter button and most importantly I am learning by day by day. I have taken for granted to write this article assuming that, I am a photographer because one of my photographs went to 2 stories height of printing and one other image is being displayed in an international creative online gallery. Even when these two images are not selected for any display, I would be writing this because I understand what photography is. I am doing wedding photography as my profession.