About aperture

In their everyday life, many photographers often mean the same thing under the words 'Aperture', 'Aperture', 'Relative aperture'.

About aperture

About aperture

If everything is greatly simplified, then the F number (aperture number) is responsible only for the ratio of the geometric aperture of the lens to its focal length - therefore you can still find the definition that the F number is called geometric aperture. In fact, aperture Is the ability of the lens to transmit light, and this ability is influenced not only by the ratio of the focal length of the lens to its diameter (i.e., geometric indicators). A huge role in the ability to transmit light is played by the optical design of the lens, which tends to transmit not all of the incident light.

An ideal lens would let in all the light that falls on it, but due to reflection, reflection and absorption by the optical elements of a real lens, only a part of the light flux reaches the photosensitive element, which forms the final image. Therefore, different lenses with different optical schemes, but with the same relative aperture, can create different exposures in photographs, all other things being equal. This is very often encountered in a movie where you need to mount a lot of short videos, for example, shot from different angles, into one big one. At the same time, if the scene is shot from different angles with different optics with the same F value, then in the final gluing you can get different brightnesses, which will look very bad when viewing. This is the most primitive example that videographers often cite.

To make it more convenient to work with photo and video equipment, there is a so-called T number (from the English 'Transmission' - transmission, transmission). The T-number is the F-number corrected for the light transmission efficiency of the lens. The T number indicates the equivalent of a lens with a specific F number that allows 100% of the light to pass through. For example, if a 50mm, f / 1.4 lens only transmits 50% of the light, then an ideal T 2.0 lens would match. The number T can be used in the same way as the number F.

Example. If we have a 100mm T 4.0 lens, it doesn’t matter which geometrical hole it actually is and what its F number is, it will still transmit as much light as any other lens with the same T number, for example, some 50mm T 4.0 At the same time, 100mm T 4.0 and 50m T 4.0 can have completely different values ​​of the number F. If you put a neutral filter on such lenses, you can say that their values ​​of the numbers F will be preserved, and the numbers T will change to the degree of dimming by the filter. Thus T-stop (an analogue of the step of the number F) is in many ways more convenient to use.

On the network, I met information that photographers cheatedindicating on the lens barrel is not the real aperture value. In fact, no one is deceiving anyone, just between the concept of "aperture" and "relative aperture" there are certain differences that an experienced photographer knows about. On the lens, the usual value of the relative aperture is indicated (it is also called the maximum aperture, or F number), but how much light such a lens actually lets through can sometimes only be found in the instructions for the lens.

When I wrote the text for this article, I found instructions for a modern lens Nikon Nikkor AF-S 35mm 1: 1.8G DX, re-read it from cover to cover, but did not find information about the light transmission of the lens. Therefore, you can still slander the manufacturer for incomplete information about the lenses.

Due to the different light transmittance, even small paradoxes with the f-number can occur. For example, let's take two lenses - Nikon 35mm 1: 1.8G DX Nikkor (lens for cropped cameras) and Nikon 35mm 1: 2D Nikkor (full-format lens). It would seem that the first lens has a slightly higher aperture than the second. But if you try to shoot using these lenses using a cropped camera, it may turn out that the amount of light projected onto the camera’s matrix with the first lens will be less than the second. This is due to the fact that the cropped lens has stronger vignetting at F / 1.8 and with different luminous flux losses in optical circuits.

Photo for paragraph separation :)

Photo for paragraph separation :)

Many aspiring photographers tend to use high-aperture optics for the common reasons - excerpts, more flexible control of depth of field, beautiful drawing and excellent image quality. But high-aperture optics give some more very pleasant (and maybe not pleasant?) Nuances.

The first of them I want to note the brightness of the optical viewfinder. High-aperture optics give a nice bright picture in JVI. With such lenses it is much more convenient to aim manually, you do not need to peer very much at JVI and squint your right eye. The human eye adjusts very well to the light intensity, and therefore you can’t always notice the difference with different lenses, but there is one. Personally, I tried to determine my personal sense of brightness. JVI with a fast manual aperture lens - Porst Color Reflex MC Auto 1: 1.2 / 55mm. Here's what I noticed:

  • The difference between F / 1.2 and F / 1.4 is not felt at all
  • The difference between F / 1.4 and F / 2.0 is almost elusive
  • The difference between F / 2.0 and F / 2.8 can already be easily caught, but on F / 2.8 in JVI everything is clearly visible and does not cause any discomfort
  • The difference between F / 2.8 and F / 4.0 is enormous, you immediately notice it. Visually working on F / 2.8 is much nicer
  • The difference between F / 4 and F / 5.6 is not very noticeable, but at F / 5.6 after F / 2.0 there remains a feeling of extreme limitation.
  • With further closure of the diaphragm, everything becomes faded.

Based on the experience (and some others), I came to the conclusion that the most comfortable values ​​of the maximum relative aperture for sighting are F / 2.8 and lower.

You can conduct your own experiment on brightness JVI your camera. This is easiest to do if the camera supports depth of field preview via JVI. If there is no such function, then you need to use a lens with manual iris control. The electronic viewfinder is not suitable for such a test.

Helios 44 bokeh with 8 petals

Helios 44 bokeh with 8 petals. Photo separator

Aperture optics not only gives a brighter and brighter picture in JVIbut also allows in many cases where more accurately and quickly handle the autofocus system.

Roughly speaking, the stronger the light flux from the lens to the mirror, the easier it is for the phase focusing sensor to focus. The first time I felt the difference was shooting in the studio for a long time, where I had a weak modeling light from the fixtures at hand. The high-aperture lens that I used for a half-length portrait easily clung to the subject, but when I had to shoot a group of people and use the standard zoom with an average aperture, it simply refused to focus in such light.

I suppose that high-aperture optics should improve the quality of focusing also in the Live View mode.

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In addition to improvements in the focusing system, the camera, with fast lenses under certain conditions, much more accurately produces and measures exposure. I can’t say for sure how and for what reasons this or that camera improves the operation of the exposure meter, but, based on my experience, for some reason I’m sure that there are errors in exposure with aperture optics much less.

In my practice, errors in exposure most often occur when using medium-aperture optics and when shooting on covered apertures. When using high-aperture optics at the same values ​​of the number F, errors are much less. Of course, small errors in exposure not critical if you shoot in RAW, but still this is a good plus of such lenses.

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Also, I notice that fast aperture optics give less rejection due to focusing errors when used on covered apertures. I assume that if a slight error was made when focusing on a fast lens, then during shooting, when the aperture is closed, a noticeable expansion depth of field zones just make up for this mistake.

Who does not know, then modern SLR cameras always perform focusing with the aperture fully open and close it to the set value only during shutter release.

For an example we will take a fast fifty dollars with F / 1.4 and a usual regular zoom with F / 3.5-5.6. We will shoot at 50mm and F / 6.3. If the focusing error was initially made at fifty dollars, then due to the closure of the aperture to F / 6.3, the depth of field will greatly expand and will most likely capture our subject. At the same time, if there was a focus error at the zoom, then a small change in the depth of field during the transition from F / 5.6 to F / 6.3 will not be able to compensate for inaccurate focusing.

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True, high-aperture optics have obvious drawbacks. One of them I want to highlight the diffraction threshold, which sometimes starts from F / 8. Super-fast lenses with f / 1.4 and f / 1.2 and below suffer especially from diffraction at very closed apertures. Usually the minimum F number they can use is F / 16. Low-aperture optics are less prone to diffraction because they need to perform less aperture maneuver. So the stock "dark" zooms at F / 8 only come to life and show excellent photo quality. This can be critical only for certain types of shooting, and the threshold is different for different lenses. The features and subtleties I have described cannot always be clearly shown, but over time they begin to be felt in practice and affect the work :)

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Material prepared Arkady Shapoval. Please, if this material was useful to you, help my project. And don't forget that everyone can write your review for one or another photographic equipment.

Add a comment: Sergei



Comments: 146, on the topic: About the aperture

  • Land

    Guys, tell me where you need to move in order to have good aperture and low flu? For short exposures in the evening

    • Arkady Shapoval

      Toward mid-focus lenses with F / 1.0, F / 1.2, F / 1.4 and cameras with high ISO

  • Volodymyr

    Good afternoon. Where is the site where you can see the measurements of the number “T” for manual lenses, for example Radyansky ones?

    • Arkady Shapoval

      There is no such site

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