NEW Mtf Là Gì – Biểu Đồ Mtf (Mtf Chart)

Hello quý khách. Today, leephan xin góp chút kinh nghiệm cá nhân về mẹo vặt, kinh nghiệm không thể thiếu trong đời sống bằng bài chia sẽ Mtf Là Gì – Biểu Đồ Mtf (Mtf Chart)

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MTF (Modulation Transfer Function) Chart – roughly translated as Flowchart of the modulation transfer function (or simply called ) MTF chart ) is very important in evaluating the optical performance of a lens. By analyzing the MTF Histogram, we can capture key optical properties of a lens such as resolution, contrast, field curvature, and astigmatism. (astigmatism).

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Many people think that the MTF chart is not really necessary to judge the quality of a lens, it is just boring numbers. However, in reality, when evaluating the optical properties of the lens such as sharpness, contrast, each person’s assessment in different conditions is highly subjective, for example “high definition”. “” of one person is actually only equivalent to “average definition” of another, so the MTF still has an important place as an accurate data for people to refer to before they can use the lens themselves. However, you should always keep in mind that MTF charts made by companies always have certain differences due to the method of implementation and presentation, based on the same principle: measuring the ability to distinguish lines side by side of the lens at different positions on the sensor to evaluate the ability to “transfer” information from the actual image to the sensor/film surface.
First of all, we need to know that the MTF chart always consists of 2 axes: the horizontal axis represents the distance of the contrast/resolution measurement point from the sensor’s center, in mm (point 0 is the contrast value/ The focus is at the center of the image and the outermost point is the bottom corner of the image), and the vertical axis represents the contrast/resolution value of each measured point. Looking at the distance scale of the MTF histogram we can tell if it is the optical histogram of the lens on a full frame or crop sensor, for example the chart below is a Nikon full frame lens MTF because of the sensor size is 24 x 36mm.

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Contrast is the ability to distinguish between the bright and dark of two adjacent areas, as illustrated in the figure below: high contrast clearly shows the boundary between white and black lines (100% contrast), and when the contrast is reduced, the blacks are brighter, the whites darker until they are indistinguishable (0% contrast). This is also the unit scale for the vertical axis of the MTF chart (instead of a percentage scale, a decimal scale of 0 to 1 is used for brevity).

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To measure the contrast at different points, people take pictures of the MTF panel and analyze the resulting images. The MTF board is designed to consist of groups of closely spaced lines evenly distributed from the center to the corner of a rectangle corresponding to the sensor’s scale. In the picture below is Nikon’s MTF table for full frame lenses. At each evaluation point, two groups of lines are placed side-by-side: a group of 10 line pairs/mm (10 line pairs/mm or 10 lp/mm) and a group of 30 line pairs/mm (or 30 lp/mm) mm). It can be easily seen that when taking images of these two groups of lines, the group with high line density and small line size (30 lp/mm) will be more difficult to distinguish than the other group. Since most lenses these days can clearly distinguish straight lines of 10 lp/mm while 30 lp/mm are more difficult to distinguish, people use 10 lp/mm to measure contrast and 30 lp/mm to measure ability resolving power of the lens. Some camera manufacturers may use a slightly different line density group like 20 lp/mm, 40 lp/mm, even 50 lp/mm for lenses intended for larger sensor/film systems. frame 35mm. In addition, in the same group, the lines are also divided into 2 subgroups in the direction perpendicular to each other called Sagittal lines (parallel to the radial diagonal) and Meridonial lines (perpendicular to the radial diagonal). in some places called Tangential).

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Normally, a contrast value measured in lines of 10 or 20 lp/mm is higher than the value of resolution (or resolution for short) and is generally considered very high if it reaches 0.9 or higher, which is good in the range of 0.7 – 0.9, average in the range of 0.5 – 0.7 and below 0.5 is a lens with poor contrast and sharpness.

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The figure above is an example of the MTF chart of a typical lens. The upper red line is the contrast indicator (10 lp/mm) and the lower blue line is the resolution line (30 lp/mm). The contrast of this lens is high in the center of the frame but drops quickly to the middle of the frame and suddenly becomes sharper near the edges of the image, then drops very low at the edges/corners. The resolution of the lens is quite high in the center, gradually decreasing towards the edge, although it is a bit sharper in the outer 1/3 of the image but also very low at the edge / corner of the image. The curvilinear shape that suddenly emerges when facing outward represents a wavy field curvature (we’ll talk more about it below).

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Here is the MTF chart of a good lens. You can see that both the blue and red lines have values ​​from a higher range in the previous figure and when towards the edge the value is reduced very little, although the blue resolution line decreases rapidly at the edge but only accounts for the percentage. The ratio is very small throughout the frame while the contrast is still quite high at the edge of the image. This is also a typical typical MTF chart that includes 4 lines instead of 2 as shown in the table above. In this case each group of lines consists of a separate graph line, represented by a solid line (Sagittal line or radial line) and a separate line (Meridonial line, sometimes called Tangential or radial perpendicular). are S10, M10, S30 and M30 respectively. With this lens, there are no areas of the graph that increase dramatically, so a low curvature of the field can be expected. When comparing the S and M lines, we see that they are very close to each other in the center, which proves that this lens has almost no sign of astigmatism in the center, but up to 1/3 of the periphery, the difference is The deviation between the S and M lines is significant, so there will be astigmatism in this area.

Above, I mentioned field curvature and astigmatism, so in this section I will talk more specifically about these two optical errors and how to recognize them from the MTF chart.
Field curvature is a term that refers to the degree of curvature of the plane of focus due to the structural characteristics of each lens. Field curvature is very common, noticeable when shooting a flat object in a direction perpendicular to the surface, but only certain parts of the surface are sharp, while others are blurred as if out of focus. sharp. This phenomenon is caused by the nature of the lenses used in lenses with curved surfaces, so the field of focus cannot be completely flat (as shown in the figure below).

With a simple case of curvature like this, it’s easy to understand if your image is sharp in the center but blurry on the outside, since the main reason is that the outer area is not in the field of view of the lens. These carefully corrected modern lenses will have a closer image quality at the center and edges of the image as curvature is reduced to a very low level. However, the wavy field curvature as we mentioned in the first MTF plot is more difficult to correct, as shown in the figure below: only the middle and near the edges of the contour. , and near the center will be blurred or vice versa if the area near the center of focus, the center of the image and the edges will be blurred. Short focal length lenses often have a heavier field of view curvature than telephoto lenses, and sometimes the degree of curvature depends on the focus point, the image may be fine if the focus is close but the focus is far away.

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Astigmatism is a phenomenon very close to field curvature but with a slight difference. A lens has severe astigmatism when the details distributed in the direction of the image center have a different sharpness than the details in the perpendicular direction. To make it easier to imagine, you can refer to the swirling bokeh of the Helios 44-2 lens or the Petzval 85mm lens. As we mentioned on the MTF chart, there are two types of radial lines (S line) and radial perpendicular lines (M lines) that are used to evaluate the optical quality of lenses and why one should compare them. Comparing the image representation of the two groups of lines above is because they can behave differently when light passes through the lens system. In the above-mentioned case of field curvature, we do not refer to the two lines S and M because they have the same expression, represented by a single curve of the field of view. However, in practice, depending on the orientation of the detail on the image (near S or near M), the field curve can be different leading to astigmatism.
The figure below is a very good illustration of the behavior of astigmatism lenses: when focusing on the round wheel, the lens without astigmatism can focus in great detail, while the astigmatism tube focuses on the direction line. Sagittal focus, there is blur, swirling with details running in circles at the center of the shape and wheel rim. In contrast, when the astigmatism tube focuses on the circle (tangent focus), the radial lines are blurred.

And here are the consequences affecting the image’s bokeh in two cases: tangential astigmatism and sagittal astigmatism. This is the characteristic bokeh of two types of lenses that take advantage of optical errors to characterize artistic effects: Lensbaby (left) and Petzval / Helios (right). We can call the effect of radial astigmatism the zoom effect and the other effect the vortex effect.

Let’s analyze the MTF chart of the New Petzval 58mm f/1.9 lens to appreciate the lens’s swirling effect. The figure below is the MTF chart at f/2.8 provided by LOMO. In this case the MTF histogram has 4 lines (5, 10, 20, 40 lp/mm) and two low density lines are used to evaluate contrast, two high density lines are used to evaluate resolution. In general, the resolution of the lens is quite low and decreases very quickly towards the edge. If you pay attention to the middle of the frame, the S (solid) line drops very low while the M (discrete focus) line remains average, which means that the lens will be heavily swirled near the center and reduced level at the edge. This effect is enhanced by varying degrees of spin that can be adjusted right on the lens body.

1. Only compare the MTF histograms of different lenses from the same manufacturer and preferably at the same aperture, as the method of constructing histograms varies by manufacturer due to factors such as sensor type , light, line group distribution can be different.
2. Usually manufacturers can provide more complex MTF charts, including plot lines built with different apertures (such as maximum aperture and aperture with best resolution) so the MTF chart there can be 8 different lines or more.
3. S-line is usually shown by solid line but sometimes by dashed line, you should carefully read the note of the chart provider.
4. The unit lp/mm is commonly used with film cameras, but the unit lw/ph (line width/photo height) is often used for electronic sensors due to the sensor size. can vary widely by brand. The formula for calculating lw/ph = 2 x lp/mm x image height (in mm).
5. Some lens evaluation groups like ePHOTOzine do not use the graph above, but only measure the resolution when changing the lens aperture (in lw/ph) and show it as a bar chart with the value. The horizontal axis is the aperture, the vertical axis value is the resolution at that aperture, measured at various points such as the center, the mid-range, the edge of the image ( edge).
6. The limitation of the MTF histogram is that it does not reflect some important lens properties such as chromatic aberration, distortion, and flare resistance.

The article uses illustrations from photographylife.com, handprint.com, lomography.com, lensbaby.com and dpreview.com.

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