Low light, wide aperture and focus.

[Nige]

Hello,

I have been thinking about this for quite a while, how do you get nice crisp images with a wide aperture? It might be that I have been reading to much about depth of fields and focal panels, but my understanding is as follows:

Wide apertures are used to isolate subjects because of the shallow depth of field but it may allow you to shoot 1 over the focal length to make sure the what ever is focused is sharp. If I have a fast lens f2.8 and I shoot at f2.8 it will make my depth of field shallow. This is problem when trying to capture image of two people and they fill the frame. The nose may be in focus but the eyes will be out of focus and if the second person is not on the same focus plane, then the results will vary.

The reason for the wide aperture are as follows:

• I don't have a tripod with me
• I cant use a flash
• My shutter speed is 1 over my focal length
• Its a low light scene

Below is an image with what I am talking about except I used a flash. In the unedited version the Father was very much out of focus. Lots of PPing has made it reasonable but I would prefer to avoid this sort of situation. I know I used a macro lens (100mm EF 2.8 L) for this shot which dramatically affects the DOF.


Maddison and Dan by nige_mar, on Flickr


Just curious, how do you get around these problems? I suspect the answer will have something to do with distance from subject as well.
Cheers

-Nigel

[soulman]

I know I used a macro lens (100mm EF 2.8 L) for this shot which dramatically affects the DOF.

I think you've nailed all the basic issues there, but using a macro lens doesn't have any effect on DoF. It's only that they can get closer to subjects that it can seem like they have less.

There is no magic bullet for dealing with low light and the wide apertures &/or slow shutter speeds it forces you to use. The only other element you can easily control is ISO. If you want good performance at high ISO then you need to select the right camera - I know the 5DII is great, I hear the 30D was also very good - and learn to use your software well so you can deal with the noise without losing too much sharpness.

I also tend to use the shortest focal length I can for the subject, because it means you're more likely to get a sharper result with a longer shutter speed and it helps to minimise shallow DoF issues generally.

Focusing is just generally challenging in low light I find. Auto focus tends to hunt more and take longer, plus you have to take more care to make sure you're getting it on the right spot because an inch can make all the difference with a close up portrait at wide apertures. My keeper rate is dramatically reduced with moving subjects in low light because I miss the right focus point.

Like everything else, you get better with practice, so do as much of it as you can and you'll find yourself missing less shots. I also just try to get as many elements going my way as possible - a slight improvement in a few areas adds up - and I find that just being mindful of all the issues helps quite a lot. I can shoot in almost complete darkness with my 5DII + 50 f/1.4 @ 6400 ISO and get usable results. It's amazing what you can do if you get everything working in your favour.

[arthurking83]

G'day Nigel.

the most important element of photography in your dilemma is the word compromise.
All lenses seem to perform differently, so where you've used a 100mm macro lens at it's widest setting, Canon may have made a decision with this particular lens to not allow it to be as super sharp as possible at f/2.8 and design it to perform better at smaller aperture settings.. it is a macro lens afterall, and with that designation, there is a higher probability that the user would be making a concerted effort to use it for that purpose.
So a more traditional 'portrait lens' such as a 100mm f/2(or whatever else Canon make) would be more appropriate if wide open shooting is a requirement.

So for the user, the requirement to compromise is still important as well. In this example you've chosen, you would have various alternative methods to capture the scene as you would have preferred too, and some of those alternatives would be shorter focal length, longer subject distance, smaller aperture value .. and because it's your unique set of requirements, only you can determine which compromise would have worked best for you.

So, for me.. one of my first alternatives, if I were to find myself in your situation, is to use a shorter focal length and reframe to compensate.
The other alternative I'd have used is to make sure that the child and the father were along the same plane of focus, by setting them up in such a manner.
Choosing the 'correct' lens for the purpose can be an important part of taking good photographs as you require too. Do some research as to how well each of your lenses perform for a given aperture(for use as a guide) which will then help you determine which one will work best for your purpose.

eg. I have two 'portrait' zoom lenses(both Tammy, 17-50 and 28-75) and they both perform very well at f/2.8.. in general. But the 28-75mm is softer at the edges at f/2.8 than the 17-50 is, so if I know I want people at the edges of the frame at 35-50mm, then I would definitely use the 17-50 over the 28-75mm. Even if I wanted that 75mm focal length, I'd still prefer the 17-50mm and then either get closer to the subjects or crop later in post.
But if the subjects are to be closer to the centre of the frame, then the 28-75 will always win hands down every time, even tho I expect a slightly lower ultimate sharpness quality in the image. The bokeh of the 28-75 is so much nicer than the 17-50 lens, it almost always wins as my preferred portrait lens(and even as a general purpose lens).

If ultimate DOF(eg nose to eyes distances) are a problem, then your options in getting that slightly deeper DOF are a smaller aperture value(say to f/4). At f/4 the image will naturally have better DOF, but more importantly greater contrast(from the lens) so that any pp steps such as USM will have a greater effect on the pixels you choose to edit.