What lens do I need, lens choose, old lens

How to choose the Lens ?

Choosing Lenses, miroslav potic


Another very interesting question.

On a trivial example, I will explain the essence of the issue.

Question: What’s better: cars, truck or tractor?

Answer: – If you need to get somewhere quickly will choose the car, if we need to transport large quantities of goods will select the truck, if you need to finish the field will select the tractor.

In this trivial example, I explained that the lens should choose.

To select the right lens we need to know some basic things about them.

Basic characteristics of each lens are the focal length, aperture, depth of field, angle of view.

A very important thing you need to know the size of the CMOS sensor that our device has, since the lens does not behave the same on full frame and crop sensor.

– The focal length is the distance in millimeters between the lens and the point where they intersect the rays of light that have passed through the lens. At each lens focal length writes. For example, if the lens says 50mm, this lens is a fixed focal length, and if it says 50-135mm this is a zoom lens in which we can choose the focal length in this range.

– The aperture determines the amount of light that will fall on the sensor and the depth of field. Is marked with the letter “F” which is the ratio of the focal length and aperture. In other words, if the lens has a focal length of 50mm and F4 to F16, it means that the largest aperture to be 12.5mm. Simple math 50/4 = 12.5. However, if you reduce the aperture to F16, then the aperture actually be 3,12mm which is the smallest aperture on this lens. We draw the conclusion that the greater the “F” to a smaller aperture. Look at the picture and everything will be clear.

aperture range, focal length, camera, slr, cmos sensor


Therefore, the larger the aperture more light may fall on the sensor and the other way around.

– Depth of field is the depth of focus of the image. Depends on aperture, focal length and distance from your subject. The larger the aperture, depth of field is smaller.

See the following picture where took a picture of a dog Teddy. I used a Japanese lens from 1970 Minolta Rokkor 50mm F2 on 1.5x crop sensor camera. Means that the actual focal length of about 80mm. The depth of field on this image about 20cm. As you can see the contrast is given to Teddy, while the curtain just a little blurry, fences are more blurred and the trees murkiest and completely out of focus. If I use a smaller aperture, such as the F16 then the fence and trees were in focus (sharp) in the picture, but that was not the point of this photo, but the point was to isolate and put Teddy contrast to it, especially in the his eyes.

miroslav potic, bichone dog, Teddy


A very important thing when choosing a lens that has focal length should be chosen. If we take a picture of a landscape or interior design, you should choose a lens with a smaller focal length as for example 10mm so we get a higher angle views.

Here took a picture of “Kej na Nišavi” in Serbia. I used a lens with a focal length of 18mm, equivalent to 30mm on my 1.5x crop sensor.

But if you use this lens for portraits, it will be a catastrophic mistake because it will show distortion. See the next photo.

portrait distortion, distortion portrait, miroslav potic


As you can see, in order to avoid distortion, we use the lens of at least 70mm on a full frame sensor or around 50mm on 1.5x crop sensor. In other words, the sensor looked like the human eye, the focal length must be a minimum of 70mm to ful frame sensor. Another thing we can notice is that at higher focal length background is more blurred, which means that it is easier to isolate the subject.

We came to a tele zoom lens. These lenses have a long focal length and are used for shooting distant objects. The larger the focal length, the greater the zoom. 200mm lens on 1.5x crop sensor acts as a 300mm on a full frame sensor. This is a table full frame equivalent focal length on crop sensors.

 table full frame equivalent focal length mm, crop factor, miroslav potic


I took this picture a German lens Pentacon or Meyer-Optik Görlitz Orestegor 200mm F4 which is on my 1.5x crop sensor actually like 300mm. However, I added the tele converter and got a 2x increase in focal length of a whopping 600mm.

For all of these examples show that there is no universal lens.

However, there is something that many people complain.

Question: What is the difference between a lens with a fixed focal length of 50mm and a zoom lens 50-300mm?

Answer: – Both lenses have the same focal length of 50mm, but the lens with a fixed focal length lens has a larger aperture and is much shorter and more light may fall on the sensor. It is also easier to control the depth of field. 50-300mm lens has an aperture F4.5, and 50mm lens with a fixed focal length lens can have aperture F1.2, this lens is called a fast lens, and is suitable for portraits, because it can easily isolate the subject or highlight any part of a man’s people. Of course, care must be taken to avoid distortion when shooting portraits. In practice, you best find out what it means.


One thought on “How to choose the Lens ?

  1. Thank you for a great article! I found the face distortion photos very helpful. I do have a little question though.

    I’ve read somewhere that, on a FF camera, lenses between 40mm and 65mm are considered normal; a 28mm lens will distort subjects a little (close-up shots), and a 135 mm lens will flatten a subject’s features. My question is: what determines this distorting or flattening effect? Is it angle of view or focal length?

    In other words, if I use a crop sensor camera like Canon 70D, and a 35mm EF lens, will faces in close-up shots look slightly distorted, like what we expect a 35mm lens on a FF camera would do, or will the faces look more normal, like the ones taken with a (hypothetical) 56mm EF lens on a FF camera?

    Thanks in advance!


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