Friday, June 24, 2011

ISO, fstop, EV from a different perspective

more light please...

there are lots of entries/articles out there explaining fstop, ISO, shutter speed, and the like. They all are speaking from the camera's perspective. Well, today I want to allude to these from the photographers perspective.

The scenario... My boy is in the dining room having breakfast, alone. The rest of the kids are asleep or otherwise occupied.  I grab the camera for some shots because the light from the window (no other ambient light source) is inviting.

I am using the 70-200mm lens... because it's attached and I really like it for portraits.  I try to compose the shot with mainly his face and his breakfast.  I also want to get some darks just behind his head because I want his exposed face to really stand out. I want to contrast with focus as well, hence the long zoom and wide aperture (more to come).

And mind you, I am not thinking all of the following items prior to first shot. I am thinking, shooting, looking at image, then repeating the process until I start to see what I want... often called "chimping".

First, I put the camera in A mode (aperture priority)- this way I can adjust the fstop, the ISO, and the EV. So ISO first. I have recently set my ISO to full stops... This means when I move my ISO selector, it moves in these increments 100, 200, 400, 800, 1600. If I had a cropped sensor, I would be a little more conservative and put 1/2 steps. This is because the full size sensor looks just fine at ISO 1600. So, ISO 800 is what I choose... [could have gone 1600 in retrospect to allow for faster shutter speed, because about half of my images weren't crisp clear.]  By the way, I have also changed my fstop to full stops too.  This is because when I am working with my Manual powered flashes, it's simpler math.  So ISO 800

Next, fstop. When I adjust fstop in A mode, I am wanting to see what the camera gives my in shutter speed. So I choose f2.8 (wide open). This will give me a really narrow depth of focus, but I may be willing to take it. I try 2.8 and I try 4. I end up using 4 because it gives me a little more playing room of focus distance. See previous blog of depth of field, or depth of focus  If I chose f2.8,  point my camera at my subject, then it gives a shutter speed of 1/125 - that's ok. But I want deeper focus, so I change to f4 and figure the 1/60 will work out.

I end up shooting my subject at 160mm so that my subject more fully fills the frame.
***The rule of thumb to avoid blur due to large zoom is to have the shutter speed match the zoom. So my shutter speed should be around 1/160***   Having said that, vibration reduction allows me to use slower speeds and not get the blur.

So, I've set my ISO to 800 and my fstop to 4.  I turn on the VR (vibration reduction) - don't use this if you are on a tripod.  I shoot off a couple of shots, and my subject is too bright for my taste.  What can I do to underexpose the picture a little, yet not have to change to manual mode and dial in f4 and 1/60th?  The answer:  adjust the EV.  Exposure value (EV) is a nice little tool.  I find I use it when my camera is trying to expose for the darker part of the image, and my subject is overexposed.  This usually occurs when the majority of the frame is dark and I am using matrix metering.
IE:  yesterday I was scouting out a place for some engagement shots I am doing for a neighbor and I had a barn in the semi-shade and the cloudy/blue sky as well.  It appeared that I was overexposing the sky to get the barn in correct exposure.  So I dialed my EV down 1 full stop (to -1.0) and those highlights were no longer blown out.  I can easily go back and add highlights in post-processing.

To recap... ISO 800 due to low lighting
f4 to maintain large aperture but still have good depth of focus
EV -1.0 to allow a slightly underexposed image
Result:  good lighting, not so good composition.

What did the subject think of the image?  --"I am really small... I have volcano hair."
Analysis... my subject is smack in the middle... fix: crop
Also, I am not liking the blue chair in the background... it is distracting.  fix: bw or desaturate just the blue
if bw, then increase the contrast to pop the whites

So, the blue chair becomes gray and is still distracting... bummer.  But I like the image other than that.

Pic of the day:  I really like this pose and composition... in fact most all of her images are impeccable.

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