Trouble focusing in low light...using tighter aperture?

Big Mike · Aug 23, 2011

According to that, if I used my 18-200mm lens and kept it at 18mm at f/16, assuming I could have gotten my exposure time to be long enough, I would have been able to get the entire field of view into focus, pretty much. If I understand that page correctly (big assumption!) I could have used a flashlight to manually focus on something about four feet away. At 18mm and f/16, that should mean everything I wanted to be in focus last night would have been in focus. I'll need to re-read that information to make sure I understand it, though.

Sounds like you are getting it. The DOF calculator is a great tool. You could print one off (for your camera & lens combo) or could could use it in the field. I have a DOF calculator App for my iPod, for Example.

Older lenses (and higher end new ones) would have a distance scale right on the barrel...so you could focus to a distance just with that.

Neiby · Aug 23, 2011

Big Mike said:
According to that, if I used my 18-200mm lens and kept it at 18mm at f/16, assuming I could have gotten my exposure time to be long enough, I would have been able to get the entire field of view into focus, pretty much. If I understand that page correctly (big assumption!) I could have used a flashlight to manually focus on something about four feet away. At 18mm and f/16, that should mean everything I wanted to be in focus last night would have been in focus. I'll need to re-read that information to make sure I understand it, though.

Click to expand...

Sounds like you are getting it. The DOF calculator is a great tool. You could print one off (for your camera & lens combo) or could could use it in the field. I have a DOF calculator App for my iPod, for Example.

Older lenses (and higher end new ones) would have a distance scale right on the barrel...so you could focus to a distance just with that.

Great. Just another reason I need to go spend some more money on a better lens.

Thanks!

Neiby · Aug 23, 2011

I had actually found that DOF calculator earlier today because I was trying to solve a mystery from a couple of days ago. I knew a little about depth of field, but I didn't really know the details. I took some pictures of someone with my 50mm at a really low aperture and it seemed oddly out of focus and I think I figured out why. First, I have a 50mm on a 1.6 crop body with no IS. My shutter speed was at 1/50, which is too slow for that. Second, the low aperture game me a REALLY short depth of field, which meant that even though one part of her face might have been in focus, a part a few inches away would already be going out of focus. After consulting the DOF calculator, I think I should have used something more like f/11 and 1/80 shutter speed.

Big Mike · Aug 23, 2011

Yes, DOF can be a delicate dance.

The lens aperture is the main way that we control it...but you also have to consider the focal length and the distance to your subject. Using the DOF calculator is a great way to learn how these thing affect your DOF. Before long, you'll be able to estimate the DOF without the calculator.

Neiby · Aug 23, 2011

Big Mike said:
Yes, DOF can be a delicate dance.

The lens aperture is the main way that we control it...but you also have to consider the focal length and the distance to your subject. Using the DOF calculator is a great way to learn how these thing affect your DOF. Before long, you'll be able to estimate the DOF without the calculator.

I learned quickly that the DOF is VERY shallow with a 50mm lens on a 1.6 crop body at f/1.8 and a subject about 4 or 5 feet away. I'll use that effect on purpose, I'm sure, but I wasn't meaning to do it this time.