Lens focal length depending on sensor type
The far focal objective is an optical characteristic of the lens itself , regardless of where the camera is mounted or the size of the sensor.
A 50mm lens has that focal length on a full-frame sensor camera, on an APS-C sensor camera, and in any other circumstance and use.
What changes is the end result.
For example, if a lens intended for full frame is used in an APS-C camera, the result will be that the sensor only captures a part of what a full frame sensor would. Cropping occurs in the photograph, which for angle-of-view purposes is equivalent to a multiplication factor of the focal length .
It does NOT change the focal length, only the angle of view.
The small sensor gives us a more closed vision, which is roughly equivalent to what a lens with a longer focal length would give us.
For practical purposes, the APS-C sensor introduces a cropping factor (focal multiplier) of 1.6 for Canon APS-C sensors and 1.5 for the rest of APS-C sensors.
Orientative viewing angles based on focal length. In black the actual focal length of the lens, in parentheses the equivalent focal length on an APS-C sensor:
There are lenses specially designed for APS-C sensors (EF-S lenses on Canon, DX lenses on Nikon).
The only difference is that the area that illuminates the sensor is adjusted to the size of the APS-C sensor .
If a lens of this type is used in a camera with a full frame sensor, we will obtain an image with great vignetting (only the central part of the photograph will be illuminated, the edges will be darkened because they are not correctly illuminated.
Lens brightness
The lens aperture is the ratio of the focal length to the diameter of the diaphragm.
It gives us an idea of the amount of light that a lens lets through.
Lenses with a large maximum aperture (usually from f / 2.8) are called bright lenses.
There is also often talk of brightness, referring to the maximum aperture that a lens can achieve.
Intuitively, a brighter lens lets more light through (with the aperture fully open) than a less bright lens.
Brighter lenses tend to be bulkier as the lenses are larger in diameter, and they tend to be more expensive because a lens system that corrects for optical aberrations is also more difficult to come by.
The lens diaphragm is like an iris that opens and closes to let in more or less light.
The aperture of the diaphragm is measured in steps. Each complete step that closes the diaphragm is equivalent to letting through half the light as the previous step . Here you have more information about light steps in photography .
The opening is usually indicated by the number f. Here you have more information about light steps and aperture .
The lens is characterized by its maximum aperture (with the diaphragm fully open) and its minimum aperture (minimum gap that the diaphragm is capable of forming).
Normally the maximum opening is more important.
An f / 1 aperture means that the ratio of the focal length to the diaphragm diameter is 1 to 1, that is, a 50mm f / 1 lens would have a 50mm aperture diameter (this example would be a very very bright … and very expensive).
As we close the diaphragm steps, it would correspond mathematically to a geometric progression, in which each aperture lets through half the light as the previous one:
f / 1.4
f / 1.8
f / 2
f / 2.8
f / 4
f / 5.6
f / 8
f / 16
f / 22
f / 32
In practice, a lens is considered bright from a maximum aperture of f / 2.8
Most lenses (and cameras) allow the diaphragm to be opened and closed in fractions of a standard step. For example in halves of steps (f / 1.4 – f / 1.7 – f / 2 …) or thirds of steps (f / 1.4 – f1.6 – f1.8 …)
As the focal length of the lens increases, it is more difficult to achieve a certain maximum brightness, since the relationship between focal length and diaphragm diameter must be met.
For this reason, a bright telephoto lens is usually huge, heavy … and expensive .
The same goes for wide angle lenses, it is very difficult to build wide angles with a very large maximum aperture.
Zoom-type lenses can adjust their focal length between two values (minimum focal length – maximum focal length). In this case, the maximum aperture is indicated for each of the focal ends.
What usually happens in most zoom lenses is that the maximum brightness decreases as the focal length increases.
For example, kit lenses usually have a specification of 18-55mm f / 3.5-5.6 (that is, at 18mm we have a maximum aperture of f / 3.5 and at 55mm we have a maximum aperture of f / 5.6). They are not considered bright since they do not reach f / 2.8
Sometimes we talk about the speed of the target referring to its brightness.
This is so because other conditions being equal in the scene, a brighter lens allows you to shoot faster (shorter shutter time).
In many cases, it is a fundamental advantage to prevent the photo from being blurred when there is little light and moving objects in the scene.
Optical aberrations
Optical aberrations are unwanted effects that affect image fidelity to the actual scene.
There are basically two types of aberrations: geometric and chromatic.
The geometric aberrations are related to the trajectories of light rays assuming monochromatic (single color).
For example the rays that pass through the center of the lens tend to follow paths that converge at tighter points (smaller circles of confusion) with respect to the rays that pass through the outermost part of the lens.
The chromatic aberrations have to do with the fact that the refraction angle depends on the wavelength of light: red light will have a slightly different angle of refraction of light violet.
This effect is usually noticed especially on the edges of objects as small rainbows, although the color that stands out the most is usually violet ( purple fringing ) and green ( green fringing )
Objectives are built with different groups of optical elements including concave and convex lenses, constructed in such a way that attempts are made to compensate for and cancel out geometric aberrations.
Special coatings are also used that try to avoid chromatic aberrations.
Fixed focal lenses vs zoom lenses
In general, prime lenses offer higher optical quality than zoom type lenses.
The objectives of focal length varying include more optical elements (lens groups which move internally to achieve the transition between minimum and maximum focal). It is difficult to achieve optimal quality throughout the entire focal range.
The same goes for the maximum aperture. It is very difficult to maintain the maximum aperture throughout the entire focal range of a variable focal lens.
Normally, the maximum aperture decreases progressively as the focal length increases (remember that the brightness is determined by the relationship between focal length and diaphragm diameter)
As a general rule of thumb: a prime lens tends to offer better optical quality, and within zoom-type lenses the smaller the focal range (variation between minimum focal and maximum focal length) will usually offer higher optical quality throughout. that range.
Another issue is flexibility of use .
A prime lens is perfect for a certain type of photography, for example for studio portraits, or for a certain type of street photography.
They are ideal for controlled environments or if we plan to make a specific frame, or if we want to achieve a specific effect (eg blur-bokeh). This depends a lot on the type of photography and the needs or tastes of the photographer.
However, for certain types of photography more flexibility is needed, for example when we want to modify the frame (angle of view) without the possibility of physically moving towards the subject or scene.
For photography enthusiasts who cannot afford to carry two or more camera bodies with different fixed focal lengths, it may be more interesting to mount a variable focal lens.
Everyone has to find their own balance between optical quality, size and weight of the equipment, flexibility, price …
Many professionals work with variable focal lenses (usually high-end) because they need to prioritize flexibility, when capturing a single scene that will not be repeated for example.
In any case, there are fixed focal lenses that are worth having for their versatility (the 50mm or 35mm for example are widely used fixed focal lengths).
Lenses with image stabilizer vs without stabilizer
Image stabilizer lenses include an additional group of lenses that move internally to correct for involuntary photographer shake, shake, and more.
Stabilization on the lens is usually more effective than stabilization on the camera body (the sensor is moved slightly to correct for whole movement).
However, the fact of including additional optical elements implies that the optical quality of the assembly may be slightly affected. Some professional photographers prefer to use non-stabilized lenses for this reason.
Another factor to take into account is that automatic stabilization naturally has limits, which depend on many factors: distance to the subject, focal length, type of movement or vibration, shutter time, etc.
In difficult situations it is preferable to secure the shot using a tripod for example, since we cannot be sure that the stabilizer can do its job correctly.
But in general, for an amateur photographer it is advisable to use stabilized lenses because they can save us from more than one unforeseen situation, if we do not have a tripod at that time, there is no time to mount it, etc.
If we have a camera with stabilization on the body, we can use any type of lens, we can buy non-stabilized lenses that are also usually cheaper. Currently there are models with 3 and 5 axis stabilization that work very very well in the most common conditions of use.
Target diffraction
The diffraction is a physical phenomenon related to wave propagation .
When a wave encounters an obstacle in its direction of propagation, the wave splits and comes together again behind the obstacle.
Depending on the type of obstacle and the wavelength, etc. interference phenomena can occur (think that by dividing into several wave fronts, each front can travel slightly different distances before rejoining, and the phase difference sometimes causes them to add or subtract, forming interference patterns by example)
In the case of photography and light, diffraction occurs mainly at the edge of the diaphragm . The edge behaves like an obstacle and generates diffraction.
When the diaphragm is wide open the effect of diffraction is negligible. Imagine that this small effect ‘dissolves’ between the amount of light that enters through the aperture of the diaphragm without any type of diffraction.
However, as the diaphragm closes the effect of diffraction begins to become more apparent. When the aperture is very small, practically all the light that enters or a significant percentage is diffracted.
The effect of diffraction in the image is a kind of smoothing, loss of contrast, loss of sharpness in contours.
As the resolution of the sensors has increased, the effect of diffraction becomes more evident (when we analyze the image pixel by pixel on the computer) even without reaching the tighter diaphragms of the objective.
The fact of noticing the pixel-by-pixel effect does not mean that the image is useful, the one we print or the one we are going to use on the web, etc. this effect is noticeable. But with very closed diaphragms (above f / 16 in some cases) the loss of real image sharpness will be noticed.
We have to avoid as much as possible working with very closed diaphragms .
This can affect us, for example, in scenes with a lot of light in which we need long exposure times (silk effect in water …) or if we need a very large depth of field.
In the case of macro photography it is a problem because the depth of field is often so small due to the proximity of the subject that we need to close the diaphragm as much as possible.
Equivalent lens resolution
The lenses are not perfect from an optical point of view.
All lenses generate aberrations, either due to physical phenomena that cannot be fully corrected, or due to imperfections in the materials with which the optical elements are made or due to imperfections due to the manufacturing process itself.
Each lens has a certain spatial resolution
This resolution has no relation to the resolution of the sensor nor does it measure the same.
Spatial resolution is the ability of the lens to generate images in which small details can be distinguished.
Very fine parallel lines are commonly used to determine the degree of spatial resolution and this resolution is measured in pairs of lines per millimeter lp / mm
That is, how many pairs of lines per millimeter we are able to distinguish.
If the lines get narrower and closer together (higher spatial frequency) there will come a time when instead of separate lines there will simply appear a gray area in which we cannot distinguish individual lines.
Another parameter of a lens is contrast.
The contrast of the lens is the ability to differentiate the levels of black and white (differentiated light levels) between elements of the scene, that is, at the edges between objects.
A white should appear as white in the image, however the lenses are not completely transparent, there is some loss of light as it passes through the optics and the white will appear as a shade of gray.
A high contrast lens is one in which whites appear very close to white in contrast to pure blacks.
In the case of lenses, what is measured is the spatial resolution and contrast of the entire optical system (not of an individual lens)
Spatial resolution and contrast depend on many factors.
First of all, the build quality of the lens. High-end lenses use different groups of lenses (aspherical, concave, convex …) to try to compensate for geometric aberrations.
Second, it depends on the aperture of the diaphragm.
When the diaphragm is wide open, more geometric aberrations occur , some rays pass through the center of the lens, others pass through the ends … it is very difficult to make all these rays converge to produce perfectly sharp points (we are not talking about depth of field or blur, but points that should be perfectly in focus).
On the other hand, as we close the diaphragm, the phenomenon of diffraction becomes more important .
When we talk about variable focal lenses things get more complicated, because at each focal length the behavior can be slightly different.
You can know how a target behaves (at least roughly) through its MTF charts, which we will see below.
Apparent resolution (Mpx)
In some cases, the equivalent resolution of the objective in megapixels is given as guidance information .
Keep in mind that this data is a fairly large simplification, which mixes two characteristics that are not related to each other.
But hey, it can give us a basic idea of optical quality or how the lens affects image quality.
With today’s sensors, the megapixel resolution is so great that sometimes the final resolution of the image is limited by the lens. This means that when we see the image on the computer pixel by pixel we will notice a lack of sharpness at that level of detail.
It does not mean that the image is without contrast in its useful size (when we print it, even in large format, or when we use it on the web or in any other common electronic medium)
This is important: imperfections, lack of sharpness, contrast, optical quality in general must be evaluated in the final image , for example in a paper photo of a wedding photo report or a large format photo for an exhibition.
Evaluating pixel by pixel, especially with high-resolution sensors, can be misleading because at that level effects will appear that are then compensated for or disappear when resizing or printing.
Target sweet spot
This concept is totally related to the previous section.
For a certain objective, when we work with the diaphragm fully open we will notice a certain loss of sharpness due to different types of aberrations.
As we close the diaphragm, the light passes through the most central area of the lens, thus reducing the effects of aberrations (all the rays follow more similar paths)
But if we keep closing the diaphragm, the effect of diffraction has more and more influence.
There is a midpoint where the sharpness of that lens is maximum . This diaphragm aperture is called the lens’s sweet spot, where the sharpest images are obtained.
In many cases the difference is minimal, except for the tighter diaphragm positions, where the lack of sharpness due to diffraction is usually more noticeable.
All lenses have that sweet spot or zone, but it is not always at the same aperture. It is often said that it is 2-3 steps below the maximum aperture, but it depends on each objective.
MTF charts to see the quality of the target
The MTF ( Modulation Transfer Function ) graphs or curves of a target give us information about the resolution and contrast of said target from the point of view of the image reaching the sensor.
The graphs usually include several reference frequencies:
- 10 lp / mm (10 line pairs per millimeter)
- 30 lp / mm
- 50 lp / mm
For each frequency, two groups of lines are taken as reference, a group of horizontal lines (S-sagittal) and another of vertical lines (M-meridional).
Each group of frequencies generates a curve that shows the behavior of the target (contrast) along the sensor surface, from the optical center (sensor center) to one of its corners.
Here you have more information on how to read the MTF curves of a target
Basically and as a summary:
- The S10 / M10 curves give us an idea of the overall contrast and sharpness of the lens
- The higher the higher, the closer to 100%, the better the contrast (sharper)
- The flatter it indicates a more homogeneous behavior between the center and the corners of the image.
- The S30 / M30 curves (and S50 / M50 if included) give us an idea of the maximum resolution of the lens, that is, how they will behave with very small details of the scene
- The differences between the S and M curves give us an idea of the target’s astigmatism. If there is much difference, a circle from the scene will appear in the image as an oval. Occurs mostly towards corners.
- The differences between S and M also give us an idea of the quality of the blur (better or worse bokeh)