Eyes with a 50mm f1.8 - What am I doing wrong?

[theantiquetiger]

These are 4 images I tried to get of my daughter watching TV with her pet gorilla. Either one eye is in focus, or it is under exposed

shot #1 1/50 f/1.8 iso 1600 - right eye in focus

http://farm8.staticflickr.com/7155/6650219295_d699376d59_b.jpg

shot #2 1/50 f/2.0 iso 1600 - right eye in focus

http://farm8.staticflickr.com/7025/6650220671_82dd6a7028_b.jpg

shot #3 1/50 f/3.2 iso 1600 - left eye and under exposed

http://farm8.staticflickr.com/7018/6650222061_d44a9abb42_b.jpg

shot #4 1/40 f/3.2 iso 1600 - left eye and under exposed

http://farm8.staticflickr.com/7017/6650223351_80f8ebb47d_b.jpg

[kat]

The focus has to do with depth of field.

Understanding Depth of Field in Photography (http://www.cambridgeincolour.com/tutorials/depth-of-field.htm)

Reading your info and seeing the photo..it really is too dark. You need to add light.

[Mad Aussie]

The more open your aperture is (the smaller the f-stop number value), in this case wide open at f1.8, the more narrow your depth of field (amount of image in focus) is going to be.

When the DOF is this narrow you are going to have to work within the available focus, especially with a prime lens. If it was a zoom lens you could try to zoom out much more and would gain a bit more DOF.

So, the reason you can't get both eyes in focus in this case is because the eyes are not the same distance from the camera sensor when you take the photo. In other words, you would need to shoot her pretty much straight on to achieve this.

[theantiquetiger]

The more open your aperture is (the smaller the f-stop number value), in this case wide open at f1.8, the more narrow your depth of field (amount of image in focus) is going to be.

When the DOF is this narrow you are going to have to work within the available focus, especially with a prime lens. If it was a zoom lens you could try to zoom out much more and would gain a bit more DOF.

So, the reason you can't get both eyes in focus in this case is because the eyes are not the same distance from the camera sensor when you take the photo. In other words, you would need to shoot her pretty much straight on to achieve this.

I understand all this, but if you look at shot #3, both eyes are pretty much in focus, but the image is under exposed. I could not go any slower on the speed because it was handheld, could not go any higher on ISO, and if I opened the aperture, I would narrow the DOF. Seems if I am damned if I do, damned if I don't!!

[Mad Aussie]

You're not quite understanding at all. In Shot 3 her eyes are still different distances from the sensor, although the difference is less than the other shots. Hence, the closer the focus is on the out of focus eye.
Trying to speed up the possible shutter speed by ramping up the ISO is one way to try to get more light to the sensor, but it won't help with DOF and it can only do so much with the available light.
And that's the key here to your underexposure problem, available light. You don't have enough. You need at least enough light to suit your least acceptable settings. More light so you can use better settings is obviously preferable.
In this case, you need to add light yourself, not try to work with the limited light available. Whether you use flash, reflectors, torches, candles, open more blinds, or the shine off your teeth, you need to control the light by adding more.

[kat]

Personally, I don't like to use an ISO higher than 800 on my camera. I find the noise (the colored dots or dots) is pretty dominant if I go higher. So if I can't take a shot with at least 1/60 ISO 800, I add light (and most times I don't shoot over 400). Too each their own of course.

As far as depth of field. A little reading on it will help big time to understand the f -stop and what will be in focus.

There is a podcast from Marko on it
http://www.photography.ca/blog/2006/09/14/photographyca-podcast-2/

[vyeko]

I agree with MA and Kat.You need a more light if you like to change aperture for bigger DOF. I do some test with my 50 mm F:1.7 and ISO 1600 which I never use.For even more light I set camera on +2EV and WB on tungsten.Aperture on 2.8 to 4 with manual focus.Image looks perfectly exposed and DOF is just enough wide for sharp eyes.My shutter speed is 1/15 sec.I not use tripod.I take image of one word on door from angle of 45 degrees.
It is possible do it this way but for better quality (noise) and for better DOF use some lights.

[Andrew]

This is pretty basic stuff. The elementary photography triangle covers it all. You can't adjust one without compensating with the rest.

[Iguanasan]

This is an pretty good set of videos to give you a good idea about DOF and the different factors that affect it.

Part 1: DOF - Depth Of Field Part 1 - YouTube (http://www.youtube.com/watch?v=a-9rbFF3cWg)
Part 2: DOF - Depth of field part 2 - YouTube (http://www.youtube.com/watch?v=m10f_HgdfiQ)
Part 3: DOF - Depth of Field part 3 - YouTube (http://www.youtube.com/watch?v=QgYo0APlgV8)
Part 4: DOF - Depth of Field part 4 - YouTube (http://www.youtube.com/watch?v=CJj7VRb4JfM)