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Using the same setting, my 600d takes about 10 secs from flash to preview picture if I shoot with a high aperture of f/22. But if I shoot with the same settings at f/3.5, I can see the picture preview immediately.

Google has failed me.

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    You say "from flash". If you're using the flash, it may well run at higher power for the smaller aperture, increasing the recharge time until it's ready. Depending on what order the camera does things this could affect when the screen becomes available. Is there a difference between a fully charged and a nearly empty battery? – Chris H Feb 24 '16 at 17:43
  • Could you confirm what you mean by "the same settings"? Are you shooting with the same shutter speed and ISO? How are you controlling the flash power? – Philip Kendall Feb 24 '16 at 18:24
  • Same ISO, same shutter speed. Not controlling flash power, default, inbuilt flash. – Bar Akiva Feb 24 '16 at 19:01
  • What specific shutter speed are you using? – Michael C Feb 24 '16 at 19:25
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It's most likely to be because at f/22, the aperture size is much smaller, therefore the shutter speed is likely to be slowed down to allow the same amount of light to get in. At f/3.5, the aperture is wide open, therefore it doesn't take as long for the sensor to gather the same amount of light to produce an image exposed in the same way.

Take a look at the accepted answer to this question about the exposure triangle for how adjusting one setting (eg aperture) means you have to adjust another setting (either ISO or shutter speed) by an equivalent amount What is the "exposure triangle"?

  • In addition to this though its important to note after the shutter has finished it takes longer to process a slower shutter speed than a faster one. – RyanFromGDSE Feb 24 '16 at 17:15
  • @Ryan: why? Do you mean dark frame subtraction? – ths Feb 24 '16 at 17:23
  • @ths I don't honestly know the math and programming behind it entirely but one factor I've heard at work is noise reduction settings. I always just attributed it to the longer the exposure the more data the camera has to process. I haven't been able to find much "official" but here are people on another community discussing it: ephotozine.com/forums/topic/… – RyanFromGDSE Feb 24 '16 at 18:09
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    @ths - yes, darkframe subtraction. Most(?) DSLRs have this enabled by default for exposures longer than 1s, so it stands to reason that an aperture priority exposure at f/22 keeps the shutter open for (e.g.) 5s, followed by about 5s of darkframe subtraction, resulting in about 10s of delay between the shutter opeing and being able to interact with thw controls again. Easy way to test this theory is to set it to Manual mode, f/22 and a shutter speed of 100th and see if there's still a delay. – HamishKL Feb 24 '16 at 22:15
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You could be triggering the camera to apply Long Exposure Noise Reduction to the f/22 photo because of a lower signal-to-noise ratio. When LENR is set to Auto the camera bases the decision of whether to subtract a dark frame from the exposed frame based on several variables including not only shutter speed, but also ISO and the amount of light measured by the camera's meter. If LENR is set to "On", it will be applied to every image with a shutter time of one second or longer. LENR will double the shutter time because after the shutter closes a "dark" exposure with the same ISO and shutter time is made and subtracted from the exposure when the shutter was open. For more about LENR please see What's the best way to deal with hot/stuck pixels in long exposure night photographs?

Even if you have the built-in flash set to "automatic" (E-TTL?), the brightest a flash can illuminate a subject is at that flash's full power. If the camera computes that more power is needed than the maximum the flash can provide, there's not much it can do to add more light. If the camera computes the image will be dark even at full power it may apply LENR at certain shutter speeds.

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