Depth of Field — f-stop/aperture, speed, iso, exposure

There are quite a few things I want to mention here, but really want to focus on basically two of them.  This is about some of the technical aspects of taking pictures, and how they end up looking, so first off:

  • exposure – This is the overall level of light and dark, though it can also refer to parts of a picture in some cases.  Overexposed means stuff is too light, and underexposed means that things are too dark.

I start there because everything else I want to talk about is what determines the exposure.  Those are:

  • f-stop/aperture – the degree that the opening in the lens is open
  • shutter speed – how quickly the camera opens and closes (this is a bit of an oversimplification)
  • iso/asa – what used to be called “film speed”.  It is the sensitivity of the sensor/film.

When these are in balance, the picture has a balanced exposure (though if it is high contrast it you can have areas of overexposure or underexposure, and sometimes both).

That is the brief overview of the different aspects of these.  We going to focus on f-stop and depth of field.  With some information about how changing that ends up affecting the other aspects.

Here are some shots that I took today.  They are all the same subject, but there are environmental changes, and changes with how I’m holding the camera (I maybe should have shot with a tripod).  The only thing which I consciously changed with these photographs is the f-stop, and it should be at “full stop” intervals if I did it right (I don’t know that I did).

Yellow flower with green leaves beside it

Rudbeckia shot at f6.4

This one is shot at f6.4 which on this lense (at this focal length) is as wide open as the aperture can be.

This gives us our narrowest depth of field and this is probably where things are a little bit complicated/confusing as this low number indicates that the aperture is more open, than a larger number.

I’m not sure how to explain it better without getting very technical, so for now I am going to leave this at this level of explanation.

There are also a couple of other things to note with this (which will maybe become more clear as we go through this).

The focal length of these pictures is 200mm so I am not going to mention that much further.  The other two things to mention are the two values which I mentioned above, ISO and shutter speed.  I left these for the camera to decide, so they end up changing:

  • ISO: 125
  • Shutter speed: 1/332 s

To me I would say that the shutter speed the camera “found” is a bit faster than I would have liked, as I would have thought that 1/200 s would have been sufficiently fast, and possibly (other than flower movement) passable as low as 1/50 s.  This doesn’t really matter all that much for this one, as the ISO is really what I would say is at the low (ie. slow sensor) end of the value range.  I would probably be happy up to about an ISO of about 400.

The reason that you (usually) want a low ISO is that the lower the ISO the less “noise” that you see.  I’m not sure that it will show up enough in the post itself, but these pictures when viewed at full resolution will show the dramatic difference that this can make.

Yellow flower with green leaves around it, background starting to show some details

Rudbeckia shot at f9.1

This picture is taken at an f9.1 setting (quick calculation) is one stop (rounded from just over 9.05) smaller aperture (and larger f-number).  The background leaves are just starting to get some very limited detail, but are still very much out of focus.

The picture has slightly different values for shutter speed, and ISO:

  • ISO 250
  • Shutter speed: 1/332 s

This is a full stop difference in the ISO, and is still well within what I think most cameras are able to produce very fine-grained pictures.

The two values changing by one stop, in opposite directions means that the exposure remains the same.

Yellow flower with green leaves around it, background starting to show some more details

Rudbeckia shot at f13

Here we have taken the f-stop to f13.  Again we have managed to have a full stop change with the f-stop.  The details in the background are starting to become a little clearer.

As with the other picture, we have some changes to the ISO and shutter speed:

  • ISO: 500
  • Shutter speed: 1/332 s

When you look at this picture (as I was expecting) you are starting to get some visible noise/grain with the ISO getting higher than 400.  It’s not that dramatic yet, but it will become more so if the camera continues this trend.  I would say that this would be the upper end if I want fine grain at looking at it at 100% zoom factor.  It probably could go higher at display (or print) resolutions higher than my screen at ~94 pixels/inch.  I’m not sure I would want to for important pictures, unless the camera really “needed” to.

Yellow flower with green leaves around it, background starting to have slightly blurry green plants behind it

Rudbeckia shot at f18

Here we have gone to f18.  The depth of field is much deeper, and the background is starting to show some details that were totally absent in the first picture.  Again, I managed to take this at one full stop higher f-stop.  The reason I think I might not be getting it right on is that my camera has 1/3 stop increments, and I know at least one is off.

Again, we have a similar change in terms of ISO and shutter speed:

  • ISO: 1000
  • Shutter speed: 1/332 s

The noise at this resolution is starting to be noticeable even at display at this size (which is actually bigger than it is when it is displayed on the blog).  This probably is the highest ISO that is practical except in low light situations.  Which we aren’t in at all here.

Yellow flower with green leaves around it, background starting to have slightly less blurry green plants behind it

Rudbeckia shot at f26

Here we are at f26 which probably is a “rounding error” to correct the previous ones, and from f6.4 it looks to be perfect, but from f19 it looks like a significant rounding error (oops f19 was what I calculated based on, but it should have been f18 which makes it make sense).

We are getting enough detail in the background that the veins on the leaves are starting to show up clearly (out of focus, but clearly enough).

We have a similar change in terms of ISO and shutter speed:

  • ISO: 2000
  • Shutter speed: 1/332 s

In parts at this resolution I am seeing the noise showing up fairly heavily.  Personally I don’t like it, and if I wanted it for an “effect” I could add it in post production.  And still, I feel the shutter speed is faster than we really need here.

yellow flower with brown centre with green leaves around and behind it

Rudbeckia shot at f40

This is the last one of the series.  I have gone more than one stop, it looks like it is 1 ⅓ stops.  Some of the background is fully in focus in this, and it is a very different photo than the first one in the series.

The ISO and the shutter speed ended up changing slightly differently here:

  • ISO: 4000
  • Shutter speed: 1/256 s

I’m not quite sure if this is being done because of the 1/3 stop difference and it is putting that change in the shutter speed, rather than ISO?  Or because it is starting to see that it is better to increase the ISO less.

I have changed some settings which should lead to allowing for slower shutter speeds, thus lower ISO values.  I may have taken it far enough to lead to low enough shutter speeds that camera shake and motion blur are a bit more of a possibility.  I am still working on getting the settings on the camera the way that I want them, and keep trying new ways of shooting to let that happen.

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