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09-21-2010, 09:29 AM   #1
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I never completely understood why longer lenses make it easier to isolate a subject

Can anyone explain the reason for this? I have a 28-75mm f2.8 lens and it's hard to get 2 people in focus if I want a shallow depth of field so I'm thnking of getting a 50-135mm f2.8 to help with this but want to see if it would help with isolating the subjects and keeping them both in focus while throwing the background out of focus.

09-21-2010, 10:25 AM - 2 Likes   #2
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Without getting deeper into techical stuff, the depth of field is depends on some parameters.

One is distance to subject. Get closer, you get more blur on the background. The limit is your lens minimum focusing distance.
One is focal length. Wide angle = more depth of field (less blur) -> narrow angle (70-100 mm+ lenses) less depth of field (more blur).
The third is aperture. Wider aperture = more blur.

The size of the sensor also counts, but not in the way most of the people think. The same lens projects the same image into the viewing plane (take for instance your 28-75), it don't cares about that you're using a 35mm film or an APS-C sensor. What does count is the size of the photosites in your sensor - smaller they are, the more blur you recognize at really small sizes. But it does not affect your image in big way, like the 3 thing I mentioned above.

Also when you compose, you have to think about your subject, and its distance from the background. With a close background (like a wall behind your model) you will get less separation in DOF. You can get separation by colors and texture, but that is another view of the whole thing.

So if you place your 2 people correctly, you can get nice results with the 28-75 on 2.8-4.0 and with 50-75mm.
09-21-2010, 10:43 AM   #3
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Thank you for the response. I understand the basics but kept reading that long lenses help seperate the subject from the background easier which is why I wanted the 50-135mm. You're saying I'd still run into similar problems of out of focus shots with the 50-135mm if I'm not careful with my focal lengths, distance, and aperture? It seems really hard to get the background blurred a lot with both people even if it is further away. Someone is mostly a little out of focus and that's not good
09-21-2010, 10:57 AM   #4
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One major thing is the change in perspective. With a longer lens, you will be seeing less of the background, and the part you will be seeing is further away.



09-21-2010, 01:15 PM   #5
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Getting two people in focus is a DOF problem - you need enough DOF and/or you need to consider your relative angle to the two (i.e. if they are parallel to the camera, it's easier to get both in focus). At the same aperture, a shorter lens will have greater DOF.

However, with a longer lens, and shooting stopped down a bit (to get DOF back to where it covers your subject) the compression of the scene works for that isolation you speak of - the backgoround goes to blur quicker than with a wider lens.

Try your current lens at the long end, and try it at f/4 or even 5.6 while reasonably close. As long as the background isn't right on top of them, your people should separate in focus from a blurry background. For f/2.8 to work better, you'd need to back up away from the people so they remain in focus front to back.
09-21-2010, 02:01 PM   #6
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tmtke is right on. And it's well explained.

If you want to see calculation results for DOF, visit dofmaster.com and play with the tool there.
09-21-2010, 04:39 PM   #7
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QuoteOriginally posted by crossover37 Quote
Thank you for the response. I understand the basics but kept reading that long lenses help seperate the subject from the background easier which is why I wanted the 50-135mm. You're saying I'd still run into similar problems of out of focus shots with the 50-135mm if I'm not careful with my focal lengths, distance, and aperture? It seems really hard to get the background blurred a lot with both people even if it is further away. Someone is mostly a little out of focus and that's not good
You're looking for two contradictory things: two people in focus instead of one (which means slightly greater depth of field) and more background blur (which means less depth of field). No easy answer - you just need enough DOF to get the things you want in focus but no more. You can't have it both ways.

Longer focal length means less DOF; but greater distance away means more DOF.
Shorter focal length means more DOF; but closer to the subject means less DOF.
Smaller aperture (higher f-number) means more DOF no matter what.

It's a whole series of trade-offs whatever you do. Best thing, as bdery said, to do is to go to dofmaster and play around with the different parameters for the online DOF calculator. It's quite instructive.
09-21-2010, 05:01 PM   #8
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There are two ways to get two people in focus and isolate the background. The first as described above is to stop down enough to get both in the range of focus and then focus between the two, the second is to have them both the same distance away

Many times in two person photos one person is deliberately blurred but if you want them both in focus try them facing each other. You can cheat a little on shooting at an angle but not much

09-22-2010, 06:44 PM   #9
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thanks everyone for the advice and info!
09-22-2010, 09:23 PM   #10
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Check this page: Depth of field

Long story short, DoF is the same if the subject is identically framed (same magnification, same size in the sensor) and the f-number is the same, and because the angle of view is narrower, less area of the background is seen with longer focal lengths. Problem is, the longer the focal length the farther you need to be to have the same magnification.

Try the lens at 50-75 @f/2.8. If you can have the subject in focus but you want a more "blurred out" background, you'll need a faster lens (or a longer focal length, unless you can't use 75 easily because of distance to subject).

I would recommend a fast prime before a longer focal length (that's how i like it, most of the time I can't get far enough for the framing I want).

Lastly two examples:

A 35mm at f/3.5 (I was around 2.5 meters apart)



A 77mm @ f/2.0 (I was around 4-6 meters apart)

09-23-2010, 05:04 AM   #11
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summonbaka, why are those nice ladies dressed that way?
09-23-2010, 01:24 PM   #12
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Not the one you asked, but I'd guess Cosplay. Mostly based on Video games and/or Manga-characters.
See good old Wikipedia: Cosplay - Wikipedia, the free encyclopedia
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