Long Lenses and DOF

[Steve M.]

I like a 90 lens to be pretty fast for portraits. Maybe 1.8 to 2.8, so I can blur the background. W/ some lenses you can get just the eyes and front of the face sharpish, and have everything further back soft. I like this look. But is this necessarily true if I go to a longer focal length (within reason)?

I'm thinking of getting a 135mm f3.5 lens for my one Canon FD body, which would normally be far too slow for portraits. However, since it's a 135 and not my usual 90, maybe this isn't that important? I know that longer lenses compress space, which shouldn't really have a large effect w/ portraits, but is my reasoning sound....namely, that the longer the lens the less need for ultimate speed to get the same shallow DOF? Or is f3.5 going to be f3.5, more or less, no matter what?

[goffer]

Drop your numbers into this and it will tell you your DoF:
http://www.dofmaster.com/dofjs.html

According to this, at 10' you will get a slightly smaller DoF with the 135@3.5 than the 90 @1.8

135/3.5 @ 10' = .32ft
90/1.8 @ 10' = .39ft

Although, for the same composition you will need to stand closer to the subject with the 90mm which will decrease the DoF more.

[Steve M.]

Thanks goffer. That's a great chart! W/o the chart, it's tricky for me to previsualize this because, as you said, one lens would be shot at say 10', the other further away. At 10' for the 90 2.5, and 15' for the 135 3.5 (just my swag on the distance), the 90 2.5 has less DOF by the chart. In the end I may have to buy the 135 lens and see how it goes.

[charjohncarter]

.32 feet is 4 inches, so the tip of the nose and the ears will both be sightly out of focus. If this is the look you are trying to achieve fine. But if you move your subject away from the background you can still isolate your portrait subject without losing facial features.

[Steve Bellayr]

For me the rule of thumb has been f4 or f5.6 for portraits ergo f3.5 is not too slow.

[TXForester]

Originally Posted by goffer

Drop your numbers into this and it will tell you your DoF:
http://www.dofmaster.com/dofjs.html

According to this, at 10' you will get a slightly smaller DoF with the 135@3.5 than the 90 @1.8

135/3.5 @ 10' = .32ft
90/1.8 @ 10' = .39ft

Although, for the same composition you will need to stand closer to the subject with the 90mm which will decrease the DoF more.

Or, back away with the 135mm which will increase the DoF. However with the 135mm, your focal length increases and it's a slower lens. Both of those combine to affect image sharpness if it's hand held. So, a faster film and/or faster shutter speed is necessary under the same light conditions. I think that is going to be a bigger factor than Dof.

Also 135mm does come in faster models. Olympus had a f3.5 and f2.8. Third party lenses also came in f2.8 (I have one in OM mount coming in a couple days). KEH.com has some 135mm f2.5 and f2.8 for FD.