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I have yongnuo 50mm f1.8 lens

In Av Mode When I m changing the fstop the diaphragm blades remains fully open and depth of field doesn't change

I have looked to user Guide and find that I can assign my Qset button to depth of field preview

It works perfectly but I want continuous DOF preview, Wonder if possible

I dont understand Why the heck diaphragm changes only at the moment when the picture is taken its nonsense

edit: When I'm in the outdoor or there is so much light in front of the camera diaphragm automatically closing but still I cant control it

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    This is to help the autofocus to get enough light to achieve critical focus – Kai Mattern May 8 '20 at 14:13
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    You most likely can't. Not sure if this is true of the 250D, but many DSLR bodies have a 'depth of field preview' button that will temporarily stop down the diaphragm so you can see the effect in the viewfinder. – twalberg May 8 '20 at 14:39
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    @Batu It is not "nonsense"... Decades of camera design evolution have determined that maintaining a wide open aperture until the moment of exposure is the best way to make photographs. – Mike Sowsun May 8 '20 at 17:09
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    Also have in mind that the viewfinder will darken when you close the diaphragm. It will be much harder to look for your composition. – Arjihad May 8 '20 at 17:32
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    I have an old lens where you have to move the diafragm directly. Stopping down more than one stop makes it very hard to judge focus, and around two or three is hard to compose, even in bright sunlight. The usual way to use is to stop up for composition and focus and quickly switching to take the shot, which the mechanism helps you do. – Davidmh May 9 '20 at 9:27
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Your camera is working correctly. Pretty much all modern SLRs for the past 60+ years have operated this way. There are several distinct advantages to doing it this way:

  • The viewfinder remains brighter during composition. As the aperture is stopped down, the viewfinder gets darker and darker as less and less light is allowed past the aperture blades.
  • The wider the lens' aperture is set, the easier it is for a person or automated system to focus. It's easiest to tell what is most in focus when the depth of field is narrower.
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  • A shallow DOF does not help the PDAF system... it's actually the opposite. But the PDAF system uses images that are so small, and from such small effective aperture areas, that the DOF/sharpness/contrast in those images is much greater than it is in the viewfinder or on sensor. That's why the PDAF system can still focus quickly/accurately even when the viewfinder image is quite out of focus. – Steven Kersting May 8 '20 at 19:37
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    @StevenKersting PDAF doesn't require a shallow DOF per se, but if a lens is stopped down beyond a certain point it simply stops working at all due to lack of coverage. – hobbs May 9 '20 at 0:46
  • @StevenKersting If the aperture blocks light to the PDAF sensors collecting light from each side of the lens, nothing gets through and it doesn't work. If the PDAF microlenses are pointed to edge areas that require f/5.6 or wider and the aperture is stopped down to f/8, all the microlenses see is the dark back of the aperture blades. It is easier for f/2.8 AF points to see fine differences from the very edge of lens' entrance/exit pupils than for f/8 AF points that see less difference from each side of the lens closer to the center of the ep. – Michael C May 9 '20 at 2:45
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    @MichaelC, I think I misinterpreted your comments; as the last two sentences being part of the same idea. Yes, a wide (enough) aperture helps the automated system (wider separation/greater potential accuracy). And yes, a shallower DOF helps a person judge where the focus plane actually is. – Steven Kersting May 9 '20 at 15:09
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If you open you manual and check on pages 452-453 there is instruction how you can set DISP or SET buttons to act as depth-of-field preview button.

On higher class EOS cameras there is dedicated button for this function, bot not on EOS 250D.

If you want permanent function you can't do this.

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I dont understand Why the heck diaphragm changes only at the moment when the picture is taken its nonsense

And I don't understand why you would want to handicap yourself by attempting to create a composition using anything less than 100% of the available light that you can get.

What would stopping down the lens actually do for you? At moderate apertures in midday (f/5.6 and below), you'd get a preview of the Depth of Field, possibly at the expense of autofocus. At f/8 or smaller, you'll still get the preview, but definitely at the expense of autofocus and in addition to the image being so dim that you may be unable to effectively compose from scene to scene.

If you are worried about lag time for when you actually take a shot - the aperture stopping down is not a big contributor to this. The mirror movement is.

What does leaving it open get you? Plenty of light to compose and functional autofocus - which is necessary in today's digital cameras because we no longer get focusing aids (like a split/microprism). Even if we had those aids, they work much better and are easier to use with more light.

Attempting to compose with the aperture stopped down is a handicap for no real benefit. If you want to preview the DoF, then use that button to see and then move on.

Note that most seasoned shooters are using the DoF to aid fast shooting (f/8 and go street shooting) where compositions must be fast - too fast for previewing DoF. Or taking advantage of hyperfocal distance - plenty of time to check but you're better off taking the picture and then going to a 100% view to check for sharpness where you want it. Or going for some sort of maximum bokeh, leaving the lens open or just slightly stopped down - in which case the resulting image is not that much different than what you see through the lens while wide open.

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Many cameras have the ability to show the image/exposure preview full time while in live view... which includes having the aperture stopped down to the exposure setting.

But IMO this is of little benefit; because DOF (depth of field) is a variable that changes with the relative image size (size of image and viewing distance).

I.e. trying to judge the final DOF using the viewfinder/DOF preview is extremely difficult; because the image is so much smaller and the DOF is much greater. Using the LCD/Live view is a little easier because the image is bigger and the DOF is somewhat shallower; closer to what it may be in the final image. But it is still probably nowhere near as relatively large as the final output image will be; so it's still not a great tool for judging sharpness/acceptable sharpness/DOF (unless you also zoom in to an appropriate magnification level).

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The mechanism of the SLR (single lens reflex) camera is to maintain the lens at maximum aperture diameter for comprising and focusing. This procedure ensures the brightest possible viewfinder view. Otherwise the image you see in the viewfinder is likely too dim to be of value. Additionally, when the lens is wide-open, depth-of-field is at its shallowest. This aids the camera's ability to find a tack sharp docus on the main subject. At the moment of actual exposure, the aperture quickly stops down to a likely smaller pre-set opening. This action greatly expands depth-of-field and this improves the odds that the resulting image will be in satisfactory focus.

You can deploy a smaller aperture during composition in an attempt to pre-view the range of the depth-of-field. Most find this procedure less than scarifying because the viewfinder image is super dim and the depth-of-field as seen is unlikely to truly replicate the picture that will ultimately displayed.

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