Resurrection of Crop. Bokeh

Many thanks for the idea of ​​the article Evgeny Kozhuhar... The article can be considered a continuation of the topic 'Crop identification'.

Bokeh, full frame and crop

Bokeh, full frame and crop. Rise CROP! Rise from the Dead! Reveal your BOKE!

Preamble

Full-frame cameras Nikon and Sony (maybe others) can work both in normal full-frame mode, when the entire camera sensor is used to obtain an image, and in crop mode. For example, you can use the APS-C crop mode (DX for Nikon). This mode uses only the center area of ​​the camera sensor. The size of this area exactly matches the size of the sensors on cropped APS-C cameras. To put it simply, full-frame cameras can be made to 'crop up'.

The ability to shoot in crop mode personally allows me to manipulate the equivalent focal lengths (EGFs) a bit. For me, this turned out to be a very nice feature when shooting on fixed lenses.

Example of using crop mode: I often shoot events with a fast-moving lens Nikon 50 / 1.4G and a full-frame camera Nikon D700... Sometimes I can't get close enough to the subject, then I turn on the crop mode. To do this, in the camera menu, just turn on the 'Image area' -> 'Select. image area 'and select' DX format 24 x 16 ′ there. I have set the AF Point Illumination to Off, which allows me to darken the unused area of ​​the image visible in the DX format 24 x 16 JVI. In fact, in the optical viewfinder, I see only the image that I will get after the shutter is released. Visually, it seems that a lens from a 50 mm fix turns into a 75 mm fix. Such a trick makes it easier to crop a future frame, to reach more distant shooting objects.

Of course, I understand very well that the exact same result can be obtained while cutting out the central part of the photo during processing (the result will be 100% similar to what I get with the 'DX format 24 x 16 ′ function). But psychologically it is much more convenient to line up the frame directly during shooting.

With electronic viewfinder Sony a7 even simpler - there you can immediately see the image obtained from the central part of the sensor without darkening areas in EVI.

Closer to the point

So, switching between FX <-> DX formats and shooting the same scenes with the same lens, I have noticed that at times the near and far blur in DX looks (visually looks) stronger than in full frame FX.

It should be just the opposite! We all know the tale that full-frame cameras blur the background more. How then to be?

Look at the next two pictures and note for yourself where the background blur is stronger. By blur, we can mean the size of the circles of confusion.

First shot:

Original, small image from Sony a7II camera

Original shot from Sony a7II camera. The picture has a lot of circles (disks) of blur

Second shot:

Original, small image from Sony a7II camera in APS-C mode

Original from the Sony a7II camera in APS-C mode (actually - a cropping of the central part of the previous image)

Visually, the blur zone in the second image is more pronounced, and the blur discs are larger. In this case, the second shot, roughly speaking, was taken on a crop. This happens if you shoot from the same distance without maintaining the proportions in the frame.

Let's take a separate pronounced disk (circle) of blur.

From full frame:

Single disc blur

Unsharp disk in full frame

From a cropped picture:

Pronounced blur disc

Pronounced blur disc

The selected blurriness disk in the pictures is the same size in pixels.

Selected Blur Disc

Selected Blur Disc

The full-frame Sony a7II is 6000 x 4000 pixels (24.000.000 pixels). The area of ​​the circle is Pi * D * D / 4 and equals 54.297 pixels. The size of the circle is 1/442 of the image of the entire image (0,23%).

The cropped image from the Sony a7II measures 3936 x 2624 pixels (10.328.064 pixels). The area of ​​the circle is Pi * D * D / 4 and is equal to the same 54.297 pixels. In this case, the size of the circle is 1/190 of the image of the entire image (0,53%).

When switching from full-frame to cropped, the ratio of the unsharp disk to the entire frame increased by approximately 2.3 times. The same number could be obtained thanks to the coefficient crop factor Kf = 1.5, squaring it.

A serious conclusion suggests itself.: if you shoot with crop and full frame cameras on the same lens, on the same aperture value and from the same distancethen the blur effect on the cropped camera will look stronger due to different proportions of the blur zones.

Spoiler 1: different cameras of the same type (crop or full frame) have different number of megapixels, but the ratio of the blur disk to the entire frame will be the same.

Spoiler 2: I was asked to do an experiment with a point source of light placed at infinity. I did not do this, because the experiment can be considered not 100% honest. You can conduct your investigation of circles of confusion at infinity yourself.

Spoiler 3: In this article, I show images reduced to the same pixel dimensions - 1200 pixels on the long side. This must be taken into account.

Spoiler 3.1: for comparison, the pictures from the crop and the full frame were adjusted to the same size. Pictures have the same aspect ratio of 2: 3, while down scale, pictures look the same.

Spoiler 4: The article is not about DOF. Do not confuse DOF and disc blur.

Spoiler 5: do not confuse the depth of field and the power of blurring the foreground / foreground. DOF may be the same for two shots, but the power of blurring the foreground / foreground will be radically different. Speaking very roughly, the depth of field depends most of all on the F number (aperture value), and the blurring of the foreground / foreground most depends on the focal length of the lens.

The trick is that the ratio of object size to frame size will change. To shoot the same object, in this case, a twig with berries, with the same scale (so that the size of the twig in the frame is the same on a full-frame camera and on a cropped camera), in the case of a cropped camera, you will have to move farther from the subject of shooting than in time using a full-frame camera.

Test. Get the same shots in full frame and crop using the same lens

To maintain the proportions of the subject in the frame with tfull frame and a cropped APS-C camera, the focusing distance should differ by 1.5 times. The difference in focusing distances is easy to calculate using my calculations here.

Very important: the difference in focus distance corresponds to a factor crop factor.

All pictures below are taken with the same ISO settings, excerpts and aperture, but with a different focusing distance and framing mode (anyway, that would be shot on a cropped and full-format camera at the same settings).

The first picture was taken in full frame mode (FX mode), the focusing distance is approximately 45 cm (data from EXIF):

Full frame shot

Full frame shot

The second picture was taken in crop mode (DX mode), the focusing distance is approximately 45 cm (data from EXIF) The picture was taken on the same camera, from the same position as the previous photo, just this time the 'DX 24 x 16 ′ format mode is turned on (full analogy if a cropped camera was used). You can see how much the shooting scale is increasing:

Crop shot

Crop shot

Move the camera away from the subject. The third picture was taken in full frame mode, the focusing distance is approximately 60 cm (data from EXIF):

Full frame shot

Full frame shot

The fourth picture was taken in crop mode, the focusing distance is approximately 60 cm (data from EXIF) The picture was taken on the same camera, from the same position as the previous photo, just this time the 'DX 24 x 16 ′ format mode is turned on (full analogy if a cropped camera was used). You can see how much the shooting scale is increasing:

Crop shot

Crop shot

Comparison of the image with a “full-frame” camera and a “cropped” camera:

Two shots

Two shots

It is clearly seen that the proportions of the subject in the frame remained the same (i.e., with the same scale), but the transmission of perspective has changed. In the case of the DX mode, the perspective has become narrower (visually felt as an influx of the background). The condensed perspective in DX is consistent with the 75mm lens used on a full-frame camera.

The change in perspective is clearly visible on the next GIF animation. Notice how in DX mode (ie crop) the far shot "zooms in", shrinking the perspective:

dx-fx-gif

A little remark. Although I pointed out that the difference in focusing distance should be 1.5 times to get the same shooting scale, you can see that in this case the difference is 60cm / 45cm = 1.33 times. A small error may be due to the fact that the data in EXIF may not be recorded exactly. This is indirectly confirmed by the fact that the lens Nikon 50 / 1.4G has an MDF equal to 45 cm, but I didn’t shoot on an MDF, since the focus ring was not fully screwed in, at the same time EXIF shows 45 cm. Also, the lens Nikon 50 / 1.4G has a Focus Breathing effect - changing the angle of view during focusing. And the pictures, nevertheless, are not quite alike due to lens distortion (distortion and vignetting are more noticeable at the edges of the full frame).

A small conclusion that everyone passes by: subject to the shooting scale (the subject has the same proportions in paired shots) on a full-frame camera and on a cropped camera, using the same focal length and the same F number (for example, the same prime lens with the same F number) visual blur (out of focus discs) will appear larger on crop than on full frame. Yes exactly! Crop will actually blur the far / foreground more. Believe it or not, just take a close look at the GIF above. Visually, you can see how much the DX camera's discs of confusion are larger than the FX discs. I believe that it is for this reason it’s so difficult to distinguish between full frame and crop using the same lens at the same aperture... Photographers psychologically expect more blur on a full frame camera, but the opposite is true. In this case, the radius of the disc of confusion increases by a factor of K, where K is the coefficient crop factor. Strange, but everyone passes by this conclusion.

Test. Get the same shots on a full frame and crop using different lenses (or a zoom lens)

In order for the images from the full frame and crop to be the same (or very, very very similar), you should use different focal lengths and aperture values.

For example, if you take a lens Tamron 28-75 / 2.8, then the same pictures on a full-frame and cropped camera should turn out, for example, in the following case:

  • 50 mm focal length and f / 2.8 aperture are used on the cropped camera
  • full frame camera uses 75mm focal length and f / 4 aperture

At the same time, the degree of blur, scale and perspective should be preserved.

The following pictures were taken from the same focus distance. The camera was always in the same place. Only the exposure and focal length settings were changed. Exposure (shutter speed / aperture) changed to compensate exposure and blur forces.

The first picture was taken in full frame mode:

Full frame

Full frame

The second shot was taken in full-frame mode, but with a pressed aperture:

Full frame

Full frame

The third shot was taken in crop mode from the same focus distance, but with a different focal length:

Crop shot

Crop shot

The similarity of the pictures is clearly seen in the following gif animation:

Similar shots

Similar shots

44 mm instead of 50 mm turned out, most likely, for several reasons:

  • perhaps Tamron 28-75 / 2.8 has not fair 75 mm at the long end, but 70 (like most lenses of this class)
  • possibly 44 mm of focal length inscribed in EXIF not entirely correct. Who knows how Tamron's chips are programmed
  • most likely, during the test, I still made a slight deviation in maintaining the similarity of the picture

Slightly different shots were due to:

  • different light
  • 2.8 * 1,5 = 4.2, but in the camera Nikon D700 you can not set the aperture value F / 4.2, you can only select F / 4.0 or F / 4.5, F / 4.0 is closer to the theoretical calculation
  • different distortions at different focal lengths and framing modes
  • different vignetting at different focal lengths and framing modes

All test materials in RAW + JPEG format can be download from this link and to dig deeper into the material from the article.

Results

  1. The most obvious result... If you shoot the same scene with cropped and full-frame cameras, using the lens with the same focal length, at the same aperture value, and at the same distance, then there will be change shooting scale.
  2. Not obvious result... If you shoot the same scene with cropped and full-frame cameras, using the lens with the same focal length, at the same aperture value, and at the same distance, then the blur effect on the cropped camera will look stronger (due to the different scale of the blur zone / disc, see pictures with blur discs). In numerical terms, the blur strength increases by a square crop factor. As a result, we can say that in such a situation, the crop camera blurs the background more strongly. I noticed this feature during a real shooting. It was this feature that caused this article to be written..
  3. The difference in focus distance between cameras with different matrix sizes, when using a lens with the same focal length and preserving the zoom scale, corresponds to the coefficient crop factor... For APS-C cameras (such as Nikon DX), compared to full frame cameras, you will have to increase the shooting distance in 1.5 times to maintain the same shooting scale.
  4. Difference in perspective... With the same prime lens on crop and full-frame cameras same pictures cannot be obtained due to different perspective transmission (see the first GIF animation).
  5. Identical frames (as far as possible due to different matrix resolutions and other conventions) from a cropped and full-frame camera can only be obtained with lenses with different focal lengths (see the second GIF animation). To make images from a cropped camera as close as possible to those from a full-frame camera, on a cropped camera you should use a focal length K times smaller than on a full frame, and the number of apertures K times less than on a full frame. K is the coefficient crop factor... In the case of Nikon DX crop, K = 1.5.

Read more interesting information on this topic in the section 'Crop identification'.

Thank you for attention. Arkady Shapoval.

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Comments: 184, on the topic: The Resurrection of Crop. Bokeh

  • Igor

    Good afternoon, experts. Please help with advice. I want to use my sprinkled Nikon D7000 to measure the exposure for Zenith ET. What is the exposure ratio between the cropped matrix and the full frame?

    • B. R. P.

      The size of the matrix does not affect this.

  • Alexander

    Good day! I can’t find a definite answer. For example, I put on the crop lens from the FF 70-300mm, will the focus be 105-450mm?
    and on many trading sites with photo equipment they write: approx. NIKON D7500 \ DX \ focal lens 18-200mm \ DX \, 35mm equivalent film will be 27-300mm.
    Nikon AF-S DX NIKKOR 18-105mm f / 3.5-5.6G ED VR Lens: Focal Length

    18 - 105 mm; Equivalent focal length (35 mm)

    27 - 157.5 mm. ???????????

    • Michael

      The focus will be the same. The focal length is written on the lenses - this is the characteristic of the lens. And by multiplication, an ether is obtained - an equivalent fr. Lenses 70-300 for crop and ff will show the same on the camera

    • Oleg

      The equivalent focal length is like indicating the price of one product in one currency. To have an understanding of how much this is, the price of 1 kg of bananas in Thai baht, Japanese yen, etc. If you write, 1 kg in Thailand is the equivalent of so many dollars (rubles), it will immediately become easier to calculate the costs, otherwise you will have to calculate everything with a calculator.

      That is, the equivalent focal angle is the angle of view that you get by shooting at the same focal length for a full frame. There is such a phone - Samsung Galaxy S4 zoom. Its lens has a focal length of 4,3-43 mm. And what does this information give us? It is necessary to find out the physical size of the matrix, calculate the equivalent through the calculator ... And so immediately in the review indicate the equivalent focal length - 24-240 mm. And immediately there is an understanding of what angle of view we will have during shooting.

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English-version of this article https://radojuva.com/en/2016/04/aps-c-bokeh-again/