Basic Lens Choice Question

[abarickman]

I have two primary lenses that I shoot portraits with a Canon 50mm 1.8 and a 70-200mm f4. I have shot a lot but am not that technically knowledgeable. Most of my shots are shot by simply switching to App Priority and then opening the lens all the way up and firing. The results have been good but I want to know a little more about depth of field. With the 50mm at 1.8 I get alot of background blur and a shallow depth of field which i can adjust as needed by stopping down.
Why do I see alot of pros using a very long lens for portraits? I believe that open ing the lens all the way up and zooming in will decrease my depth of field and increase the blur for the background, correct?

I know the obvious about which lens to pick (low light and flexibility of distance) but assuming that I can stand as close or as far away as needed and am shooting in good lighting, when I should use a telephoto for portrait versus my 50mm lens?

[wulf]

Putting the subject further away will increase the depth of field. It may be that those photographers want some of the quality of picture they get with quite a wide aperture but without a visible loss of sharpness on the subject, hence chosing a longer lens. If you are down at f/1.8 and close to, you can end up getting somebody's eyes in focus and the tip of their nose slightly blurred (an extreme example but you are certainly going to lose detail on what they are sitting on or standing in front of).

It may also be that they like other characteristics about the longer lens. One exercise to help is to take a series of pictures with both your lenses, making a note of how you adjusted the distance and seeing what effect it has.

Wulf

[jdepould]

Telephoto lenses tend to flatten things, which is helpful for portraiture. It may also be that the pro-grade lenses are just bigger. I know the Nikkor 17-55 LOOKS like a pretty big lens, but is really a standard range zoom.

[thingy]

Im with Wulf on this. As far as I am aware, the larger lenses are used because they naturally decrease the depth of field without having to open up your aperture to a point where you lose sharpness. Im sure I remember Dean Collins saying something along those lines :)

G

[inkista]

Also, lens performance is better on telephotos. The smaller field of view just makes it easier for a lens design to have less distortion, better corner-to-corner sharpness, contrast, and smoother bokeh. Take a look at the MTF charts (http://www.luminous-landscape.com/tutorials/understanding-series/understanding-mtf.shtml) for two top-of-the-line Canon L primes:

50mm f/1.2L: http://www.usa.canon.com/app/images/EOS_2006/EF50_1_2LUSM/mtf_tele.gif135mm f/2L: http://www.usa.canon.com/app/images/lens/ef_135_2mtf.gif

[abarickman]

Thanks everyone. This may actually answer another of my question that I was going to ask. I did not realize that you lose resolution with a larger aperture since the 50mm 1.8 is by far the fastest lens I have owned and I just got it.

I assume this explains this picture I took of my son with the nifty 50 and 1.8. It might be hard to see in the smaller size.
<a href="http://picasaweb.google.com/ajbarickman/ColeJuly/photo?authkey=EtPN542pDXU#5086260174204932850"><img src="http://lh4.google.com/ajbarickman/RpYGriazkvI/AAAAAAAACGQ/uDZCrG3BBug/s400/july-11-07-036.jpg" /></a>

I was wondering why it looks almost like it is on a canvas. The photo is completely untouched. Does the loss of resolution explain this?

[thingy]

I would be careful with the use of the word 'resolution' as this really relates to the pixels x pixels size of the shot. As you mentioned, at 1.8 your not leaving much depth of field to be in focus and so are loosing sharpness. With a 50mm at 1.8 and say about 1m away from your subject, your depth of field would only be around 3cm. That doesn't leave much depth to keep a face in focus, not unless your subject is looking straight at you and even then you could loose the nose and ears (assuming your focusing on the eyes). I had the same problem recently when shooting with my 50 and opened up the aperture to blur the background (and forgetting about the effect on my subject).

I would recommend playing around with this site http://www.dofmaster.com/dofjs.html and familiarise yourself with your lenses capabilities. Select your camera, focal length, aperture and distance then after clicking calculate, check out the depth of field total on the right.

Hope that helps

G

[wulf]

The site Thingy mentioned is the same one I used to check before I posted - a great resource.

The most important thing is to know your lenses. There might be occasions where the characteristics of the f/1.8 wide open (speed and softening) are exactly what you need; there is no fixed rule to say that X is always better than Y.

Wulf

[kevinv033]

The site Thingy mentioned is the same one I used to check before I posted - a great resource.

The most important thing is to know your lenses. There might be occasions where the characteristics of the f/1.8 wide open (speed and softening) are exactly what you need; there is no fixed rule to say that X is always better than Y.

Wulf

I agree with Wulf. Nothing will help you more than knowing your lenses. Then when you're out on a shoot you can pre-visualize what you want to do and immediately know your lens choice.