BetterPhoto Q&A
Category: To Be Categorized

Looking for a Faster Lens

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January 16, 2001

A portrait lens is usually considered about 135mm, give or take a few. 105 is quite acceptable, and probably a very good lens. The Macro features might be good if you want to do some expressive shots (Lips, eyes etc)

180... I feel this is a little too long, but that's opinion, not based off any facts. I just feel you need to be too far away, and you get no rapport when working with your model. This is especially true if you want to do full body shots. I guess that's fine though, if you do lots of candids.

Just a few things about optics to help you make your decision.

1) The longer the lens, the shorter the depth of field at the same aperture.
At 100mm f/2.8 you get 7cm DOF at 2m, but at 200mm you get less than 2cm.

2) A longer lens compresses a photo - for example, line up 2 people, one 5m behind the other. Take the photo with a 300mm lens and it looks like the people are pretty close together. Move closer and take it with a 17mm lens and it looks like the second person is halfway across the world.

Personally, I don't have any problems toting around a 1.5kg 70-200/2.8 lens. I've found it a very versatile lens, and since it's also a very picky lens, it's taught me to be much more attentive before hitting the shutter button.

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January 17, 2001

What he said is true if the camera-to-subject distance is kept the same (2 meters in his example). However, in terms of practical image management, by maintaining the same distance and changing from a 100mm to 200mm focal length, the magnification (size) of the subject at 2 meters has doubled. To keep the magnification (size of subject) the same, one would have to move to 4 meters and refocus. If you do the DOF mathematics, you will find when keeping constant magnification (same subject size), the DOF essentially stays the same regardless of focal length (the difference is on the order of a few millimeters). It wasn't until I worked DOF numbers in a spreadsheet for constant subject magnification using different focal lengths with the same aperture that I discovered this. The constancy of DOF for constant magnification starts to break down when the subject distance is extremely close . . . at about four to six times the focal length or less. This is in the realm of macro-photography, not general photography. (4X 100mm is about 40 cm, about 16 inches).

What does change? How out-of-focus objects *far* behind the subject appear. As the focal length increases, the circle of confusion grows larger for objects far behind the subject making them appear more out of focus. Using a longer focal length works to isolate the subject better from a distant background.

Point 2 is well made. Indeed, I once switched from a 135mm to a 200mm telephoto and moved back to keep the subject the same size. The reason was to get some very distant background bluffs taller than the subject for a better backdrop.

The overall conclusion is good; for what you want to do . . . candids of kids while not interfering or attracting their attention . . . go for the longer focal length . . . keeping in mind 200mm is about the limit of hand held. At that length you need to be well braced in a very steady hold and be shooting at 1/250th shutter speed or faster. The rule of thumb is shutter speed no slower than 1/(focal length).

-- John

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January 23, 2001

I'm still not 100% sure what that means. Does it mean on a 200mm f/2.8 converted to 400mm f/5.6 you get 1/2 the DOF on a prime (No converter) 400mm f/5.6? Or does it mean that you get 1/2 the DOF of a 200mm f/2.8 (which could well be the same numbers)

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January 25, 2001